A Review of Technical Advances, Barriers, and Solutions in the Power to Hydrogen (P2H) Roadmap

Guoping Hu , Chao Chen , Hiep Thuan Lu , Yue Wu , Congmin Liu , Lefu Tao , Yuhan Men , Guangli He , Kevin Gang Li

Engineering ›› 2020, Vol. 6 ›› Issue (12) : 1364 -1380.

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Engineering ›› 2020, Vol. 6 ›› Issue (12) :1364 -1380. DOI: 10.1016/j.eng.2020.04.016
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A Review of Technical Advances, Barriers, and Solutions in the Power to Hydrogen (P2H) Roadmap
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Abstract

Power to hydrogen (P2H) provides a promising solution to the geographic mismatch between sources of renewable energy and the market, due to its technological maturity, flexibility, and the availability of technical and economic data from a range of active demonstration projects. In this review, we aim to provide an overview of the status of P2H, analyze its technical barriers and solutions, and propose potential opportunities for future research and industrial demonstrations. We specifically focus on the transport of hydrogen via natural gas pipeline networks and end-user purification. Strong evidence shows that an addition of about 10% hydrogen into natural gas pipelines has negligible effects on the pipelines and utilization appliances, and may therefore extend the asset value of the pipelines after natural gas is depleted. To obtain pure hydrogen from hydrogen-enriched natural gas (HENG) mixtures, end-user separation is inevitable, and can be achieved through membranes, adsorption, and other promising separation technologies. However, novel materials with high selectivity and capacity will be the key to the development of industrial processes, and an integrated membrane-adsorption process may be considered in order to produce high-purity hydrogen from HENG. It is also worth investigating the feasibility of electrochemical separation (hydrogen pumping) at a large scale and its energy analysis. Cryogenics may only be feasible when liquefied natural gas (LNG) is one of the major products. A range of other technological and operational barriers and opportunities, such as water availability, byproduct (oxygen) utilization, and environmental impacts, are also discussed. This review will advance readers' understanding of P2H and foster the development of the hydrogen economy.

Keywords

Power to hydrogen / Renewables / Hydrogen / Energy

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Guoping Hu, Chao Chen, Hiep Thuan Lu, Yue Wu, Congmin Liu, Lefu Tao, Yuhan Men, Guangli He, Kevin Gang Li. A Review of Technical Advances, Barriers, and Solutions in the Power to Hydrogen (P2H) Roadmap. Engineering, 2020, 6(12): 1364-1380 DOI:10.1016/j.eng.2020.04.016

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1. Introduction

1.1. Analysis of current energy supply status

Energy is essential to human beings, and its stable supply is crucial. Traditional energy sources such as coal, gas, and oil are still playing significant roles in the energy sector, with an ongoing expansion to other sources such as renewables and biomass [1]. The demand for energy will continue to increase (Fig. 1), with a forecasted value of 16.2 gigatonnes of oil equivalent (Gtoe) in the year 2030 by the International Energy Agency (IEA) [2]. There are obvious gaps between the source of energy supply and its increasing demand. Thus, investment in the energy sector is maintained at a very high level worldwide, with a reported record of 6.9 billion USD (venture capital investment) from energy technology companies in 2018, mostly in transport areas (5.5 billion USD) [3].

Fig. 1. Global primary energy demand (new policies scenario) in 2015, 2017, and forecasted until 2040. Mtoe: million tonnes of oil equivalent.

Meanwhile, environmental problems including air pollution, global warming, and climate change are attracting increasing attention. These problems have been largely attributed to the combustion of fossil fuels and the emissions of carbon dioxide (CO2) and other greenhouse gases. It has been reported that the greenhouse gas level in the atmosphere is greater than 480 parts-per-million (ppm) carbon dioxide equivalent (CO2e)—a measure that comprises CO2 (> 400 ppm) and other greenhouse gases including methane (CH4), nitrous oxide (N2O), fluorinated gases, and so forth [4]. Carbon capture and storage (CCS) from coalfired power plants has been a popular topic for decades[5–7], and it must be recognized that CCS can reduce the emission of CO2 and many other pollutants, including fly ash, mercury, SOx, and NOx. However, CCS is still high in energy consumption and cost. The fuel consumption and land and water requirements of CCS net delivered energy are about 25% greater than those without CCS [8]. In addition, the supply chain of fossil fuels has attracted concern regarding energy security, due to the increasing difficulty of fossil fuel extraction and its nonrenewable nature [8].

 According to the IEA [2], the shares of fossil fuels have been changing over the years. We have observed an obvious decline in the share of coal [2] due to the competitive price of oil and gas, availability of renewables, and stricter regulations on pollutant emissions. Natural gas is playing more important roles in the energy sector due to the shale gas revolution in the United States and increased supply in other major natural gas exporters such as Russia and Australia. It has been forecasted that the portion of natural gas will further increase from 22% to 24% in the global energy market by 2035 [9].

1.2. Roles of renewables in the energy sector

Renewables, especially solar and wind, have been developed significantly over the past decades. As shown in the 2018 IEA renewable energy market report [2], the share of renewables is expected to increase to a historically high rate of 12.4% of global energy consumption in 2023. More specifically, the share of renewables in the electricity sector will increase from 24% in 2017 to 30% in 2023 [10]. Although these numbers may vary depending on the assumptions and configurations of different models, it is noteworthy that the deployment of renewables (Fig. 2), especially solar and wind, is playing an increasing role in energy supply [11], as well as in reducing greenhouse gas emissions.

Fig. 2. Electricity generation by technology (new policies scenario) in 2015, 2017, and forecasted until 2040 (other renewables include solar, wind, marine, geothermal, concentrated solar, and thermal).

The employment of renewables greatly relies on the revolution of science and technology. Breakthroughs such as materials innovation, energy conversion efficiency improvements, and costs reduction are all crucial to the increased adoption and use of renewables. Although the distribution of renewables may be a limitation, the source of renewables (i.e., the world as a whole) is not a concern. Taking wave energy as an example, although its distribution is greatly restricted geographically, its potential is huge. The net wave resource was estimated to be about 3 TW globally [12], which is abundant compared with the global electricity consumption of 2.3 TW in 2018 [13]. Although there are still technical barriers (e.g., fluid corrosions, energy transmission and fluctuations) to overcome before large-scale deployment, the potential of wave energy has been manifested through many active demonstrations by companies such as Carnegie Clean Energy Ltd. (formerly Carnegie Wave Energy, Australia) and Wavestar® Energy (Denmark).

As a secondary energy, electricity is a flexible way to distribute energy from renewables. However, as the voltage values of electricity grids are strictly restricted to ±10% of the nominal value, the intermittent supply of renewables such as solar and wind present a difficulty in plugging renewable electricity into existing grids [14]. Therefore, appropriate energy storage and management methods must be developed to enable continual and wide employment of renewables.

As one of the leading countries in both renewable technology development and its implementation, Germany is playing a significant role in the renewable energy sector. It was reported that renewables (wind and solar only) contributed more than 51% (Fig. 3) to electricity generation in Germany in 2018 [15]. This value is expected to be further enhanced in 2019 and onward. It should be noticed that fossil fuels are still an important contributor to electricity generation. That is because coal- and gas-fired power plants are most effective in complementing the intermittent renewable energy to maintain a stable and reliable electricity supply (Fig. 4). Although Germany is an electricity exporter (gross) [16], it also imports electricity as a supplement in certain areas; these numbers are not reflected in Fig. 4.

The availability of renewables strongly depends on weather conditions and seasonal changes (Fig. 4). The supply of solar-sourced electricity (Fig. 4(a)) is limited to the daytime and peaks in summer (Fig. 4(c)). Wind-sourced electricity (Figs. 4(a) and (b)) may shift significantly on a daily or even hourly basis, while its capacity peaks in winter. The gap between the highest and lowest electricity supply for solar and wind combined can be as large as 40 GW, which is about 57% of the annual electricity consumption in Germany. These huge shifts lead to significant problems due to the lack of appropriate energy storage solutions. As shown in Fig. 5, Germany has utilized negative electricity prices to encourage consumers to use electricity in off-peak hours. On the other hand, surplus electricity during these times may be curtailed due to the limitation of energy storage methods [17].

In another scenario, countries such as Australia have abundant renewables, which are much greater than their domestic energy demand. Thus, an effective and smart energy management system must be implemented to ensure better storage, transportation, and international trade of variable renewable energy while satisfying the domestic need [18].

Fig. 3. Net electricity generation in Germany in 2018.

Fig. 4. Power generation and consumption in Germany. (a) Summer (8–11 June 2019); (b) winter (9–12 December 2018); (c) year round (June 2018 to June 2019). Source: Agora Energiewende (https://www.agora-energiewende.de/en/service/recent-electricity-data/), with permission.

Fig. 5. Price of electricity in Germany (May 2019). Source: Agora Energiewende (https://www.agora-energiewende.de/en/service/recent-electricity-data/), with permission.

1.3. Why hydrogen?

As the share of renewables continues to grow rapidly, energy storage systems are crucial to the successful integration of intermittent renewable energy into existing energy supply networks. Hydrogen (H2) is an ideal medium for the storage of renewable energy at various scales for a few reasons. First, hydrogen has the highest gravimetric energy density (120 MJ·kg-1 ), even though it has a lower volumetric energy density (2.7 MJ·L-1 for 350 bar (1 bar = 105 Pa) compressed hydrogen, 4.7 MJ·L-1 for 700 bar compressed hydrogen, and 2.36 MJ·L-1 for liquid hydrogen), compared with other common liquid fuels such as ethanol, propane, and gasoline (Fig. 6 [19]). Second, hydrogen can store energy almost permanently, if proper storage methods are provided [8], compared with energy storage by batteries. Finally, hydrogen can be used in various industries.

Fig. 6. Gravimetric energy density and volumetric energy density (based on lower heating values) of fuels [19]. JP-8: jet propellant 8; E-10: ethanol-blended fuel; liq: liquid. Credit: US Department of Energy Fuel Cell Technologies Office.

The traditional pathways for hydrogen production include hydrocarbon reforming, gasification [20], hydrocarbon pyrolysis, biomass, and water splitting (through electrolysis, solar thermolysis, or photocatalysis) [21]. Current annual hydrogen production is 0.1 Gt, which is mainly consumed onsite [21] for refinery and metal treatment. The current emerging demand of hydrogen for fuel cells requires a significant enhancement in its production. Fig. 7 provides a new pathway to produce hydrogen from renewables, which can contribute to both an increased supply of hydrogen and the continual reduction of greenhouse gases caused by the consumption of fossil fuels via traditional processes.

Hydrogen is an important industrial chemical that is used in applications such as ammonia synthesis, refining (i.e., converting crude oil into diesel and jet fuels), and gas purification (i.e., removing sulfur and nitrogen from fuel) [18]; it is also used to replace carbon monoxide in iron making [18]. Furthermore, hydrogen is an important clean energy that is technically feasible, economically acceptable, and environmentally friendly [22] for multiple purposes such as heating[17,23], power generation, fuel cells, and transportation (trucks and trains)[18,24]. Taking California, USA, as an example, it was reported that more than 3000 fuel cell electric vehicles were sold or leased as of September 2017, 40 hydrogen refill stations were operating as of 22 May 2019 [25], and over 17 million passengers took hydrogen fuel cell buses in 2017. There has also been evidence showing that the addition of hydrogen to traditional fuels may benefit energy efficiency, fuel consumption, and carbon emission reduction [26]. The prolonged lifetime of fuel cells (60 000–90 000 h, which remains to be proved [27], and is targeted at 60 000–80 000 h set by the US Department of Energy (DOE) [28]) may also add confidence to the overall deployment of fuel cells, with a range of automobile companies stepping into this sector such as Hyundai, Toyota, Honda, and Mercedes-Benz [29].

Due to its diverse sources and utilization pathways, as well as its low levels of carbon/pollutant emissions, hydrogen may play an important role in a clean and secure energy future. As an energy carrier for short (hourly), intermediate (daily), and long-term (seasonal) energy storage, the importance of hydrogen is also very well recognized in maintaining the energy supply stability from inherently variable and seasonal renewables (especially solar and wind) [27]. Therefore, many countries, including Germany and other European countries[30–32], the United States[18,33,34], and Australia[35,36], have already proposed strategic plans in the hydrogen sector.

1.4. Objectives of this review

In the power to hydrogen (P2H) process (Fig. 7) using electrolysis [37], surplus electricity is converted into hydrogen [38] by consuming electricity using water electrolysis to reduce the peak loads on the grid [17,18,39,40]. Thereafter, hydrogen can be used for heating, transportation, the chemical industry, and so forth [41]. With an extremely short response time (4 min from start up to maximum loading [32]), P2H could act as an important connection between the inter-seasonal surplus electricity and hydrogen production.

P2H is a very popular research topic and a great deal has been published in the literature on its technical details such as electrolysis, hydrogen distribution, and hydrogen purification. However, to the best of our knowledge, there is still a lack of a review of its research advances, barriers, and solutions. The present review aims to fill this literature gap from the perspective of process engineering. In the following parts of this paper, we will review the status and research advances of P2H and hydrogen distribution, analyze the related technical barriers and solutions, and propose potential opportunities for future research and industrial demonstrations.

Fig. 7. The roadmap of power to hydrogen (P2H) technology. HENG: hydrogen-enriched natural gas.

2. P2H demonstration projects

Germany is one of the leading countries in the area of P2H. It sources a large portion of its electricity from renewables (mainly solar and wind), with strong commitment and support from the government and efforts from a range of high-tech companies. Its P2H strategy platform, named the German Energy Agency (Deutsche Energie-Agentur GmbH, DENA), was launched in 2011 and is behaving as a significant window showing progress in stabilizing the fluctuating power supply (both seasonal and spatial) from renewables. There are more than 30 active P2H pilot projects in Germany at present, with an electrolysis capacity of about 25 MW. Furthermore, an important online database [31] displays active P2H projects worldwide. Some of the P2H demonstration projects are listed in Table 1 [42].

Table 1 Examples of P2H demonstration projects.

P2G: power-to-gas; Nm3 : the volume of the gas under standard condition (0 °C, 1.01 × 105 Pa).

A feasibility study [14] on installing an electrolyzer to produce hydrogen as an alternative to a network expansion in a German electrical distribution grid showed that hydrogen production from P2H (4–7.5 EUR·kg-1 ) was cost competitive for the hydrogen supply to refueling stations in Germany (9.5 EUR·kg-1 ) as of 2011. A case study in South Carolina, USA [43] also proved that electrolysis can be profitable for supplying hydrogen for fuel-cell-powered vehicles, with a hydrogen price of 12.5–16 USD·kg-1 (production and delivery), as reported by Sutherland and Joseck [44] in 2015. The US DOE Fuel Cell Technologies Office further set up a target of 7 USD·kg-1 by 2025 [45], with about 5 USD of this coming from delivery/dispensing [46] and 2 USD coming from production [47].

The capital costs of P2H facilities are mainly from the electrolyzers, while the operating costs are mainly from the electricity [48]. When P2H is used as an energy storage solution, the cost of electricity may be disregarded, as excessive renewable electricity must be curtailed when appropriate energy storage methods are absent. The investment costs of polymer electrolyte membrane (PEM) electrolyzer (3100–6600 USD per kilowatt hydrogen ( ) and alkaline electrolyzer (2100–5700 ) have been reported to vary widely with the scale of plants (by as much as 3–4 times)[11,43]. It was also concluded that alkaline electrolyzer is more mature and cost effective, albeit slightly lower in efficiency than the PEM electrolyzer from a long-term perspective [43]. Robinius et al. [14] recently summarized the reported data in the literature and provided an equation (Eq. (1)) as a reference for the calculation of investment costs (in EUR·kW-1 ).

where  is the flowrate of hydrogen (Nm3 ·h-1 ) (Nm3 : the volume of the gas under standard condition (0 °C, 1.01 × 105 Pa) and I is the current density (kA·m-2 ).

To date, the scale of P2H pilots is very small (in terms of MW). Many questions remain to be answered before P2H can take on an even larger share of the energy market, including the availability of cheap renewable power, the need for low capital costs of electrolyzers, efficiency shifts with the scale of the electrolyzers and loading, the feasibility of high-pressure electrolysis, and so forth. It can be seen from Table 2 [32,37,49–51] that the overall longterm efficiency of P2H energy is still quite low, regardless of the end user of the hydrogen. It has been suggested that the integration of electricity generation and heat generation (known as combined heat and power (CHP)) from hydrogen fuel cells may lead to a much higher total energy efficiency (80%–95%) [27]. However, this solution is not always applicable. Thus, many researchers are dedicated to improving the overall efficiency of P2H.

Table 2 Energy efficiency by employing P2H for different purposes.

HHV: higher heating value.

3. Discussion

Due to the mismatch between the supply and demand of hydrogen, hydrogen’s distribution methods and infrastructures are crucial for increasing the overall size of the hydrogen market [52]. However, the volumetric energy density of hydrogen is relatively low and the energy requirement for hydrogen liquefaction is extremely high. Thus, the storage and transportation of hydrogen are recognized as the most difficult phases for the redistribution of hydrogen [29].

3.1. Hydrogen conversion and redistribution

A range of technologies have been investigated to convert hydrogen produced from P2H into other forms for hydrogen storage and redistribution, such as CO2 hydrogenation into fuels[53,54] (e.g., methanol, ethanol, formic acid, or even gasoline)— which was favored due to its low modification requirement for existing fuel systems—as well as ammonia synthesis [55] and methanation.

Hydrogen production from renewable energy sources paves the way for many storage systems and for the downstream synthesis of valuable chemicals. For example, Morgan et al. [55] investigated ammonia fuel production from wind power, with H2 from electrolysis as a key intermediate in the process. Recently, Nayak-Luke et al. [56] reported the concept of "green” ammonia produced from variable renewable energy, in which the intermittency of energy sources is largely mitigated by ramping the electrolyzer for hydrogen production. Hydrogen also plays an important role in CO2 hydrogenation, as demonstrated by the studies of Van-Dal and Bouallou [57], Pérez-Fortes et al. [58], and Chen et al. [53]. CO2 methanation from hydrogen provides a promising alternative for reducing carbon emissions and generating fuels. The materials for catalytic CO2 methanation include noble metals, such as rhodium-, ruthenium-, and nickel-based catalysts [59].

The synthesis of derivatives for reversible H2 uptake and release—namely, liquid organic hydrogen carriers (LOHC)[60,61] such as ethylene glycol [62], dibenzyltoluene [63], and 1,2- dimethylindole [64]—can be considered as a cascade storage of renewable energy to meet different requirements of storage size, duration, and management. The synthesized high-energy chemicals can be converted back into hydrogen to meet energy demand when required. As an example, the Marquardt Group (Germany) has proposed a conceptual design of an energy storage system with pressurized reversible solid-oxide fuel cells for power conversion, coupled with external ammonia synthesis/decomposition processes and a steam power cycle, resulting in a round-trip efficiency of 72% [65]. Although LOHC technology is still in its early stage [60], it has demonstrated significant potential for industrial implementation due to its mild transport conditions (atmospheric temperature and moderate pressure) and high hydrogen uptake, with a reported value of 6.5 wt% using ethylene glycol and 5.23 wt% using 1,2-dimethylindole [64].

3.2. Direct distribution of hydrogen

A range of hydrogen-transportation pathways are available, including cryogenic liquid tankers (moderate to large stations), pressurized hydrogen tube trailers (early market and small stations), gas pipelines (mature market and large stations) [66], and other storage forms such as hydrogen carriers [67]. However, these methods have different shortcomings [67], such as high operating costs for large-quantity transportation, energy inefficiency for liquefaction, and high capital and time costs for pipeline construction. The construction costs of hydrogen delivery pipelines can vary significantly from one case to another, with some estimations reported at 329–590 USD per meter for 2 and 4 in (1 in = 2.54 cm) pipelines (69 bar) [68]—a mean value of 854 USD·m-1 for 30 cm (in diameter) pipelines [69], which is around 10%–20% more expensive than natural gas pipelines [70]. The US DOE Fuel Cell Technologies Office Multi-Year Research, Development, and Demonstration (MYRD & D) Plan set a 2020 target of 432 USD·m-1 for hydrogen pipelines at a transport pressure of 100 bar for a lifetime of 50 years [71].

As the existing natural gas pipelines are very well established (Fig. 8) [9], another strategy is to combine existing natural gas pipelines, with supplemented new pipelines as needed, to transport pure hydrogen [70]. However, there are still difficulties in using natural gas pipelines (which are mostly composed of ferritic stainless steel[72,73], with a small portion using plastic and cast or wrought iron [74]) for hydrogen transport, especially when dealing with high-pressure hydrogen gases. These difficulties include hydrogen blistering, hydrogen embrittlement, and hydrogen-induced fracture[29,75].

Fig. 8. Existing natural gas pipelines on a country/region basis in 2016. CIS: Commonwealth of Independent States.

Mixing hydrogen with natural gas and using existing natural gas pipelines for transportation has been demonstrated and discussed in various aspects [76]. In 2010, Pinchbeck and Huizing [77] suggested different safe hydrogen addition limits into existing natural gas facilities, such as 6% and 10% for natural gas pipelines at 40 and 8 bar, respectively; 12% for burners; and 18% for in-house appliances. Later, in 2013, Altfeld and Pinchbeck [78] summarized that a hydrogen mixture of 10% in natural gas is possible in some parts, but has different requirements in different areas. Therefore, they stated that case-by-case studies for specific projects are essential. Melaina et al. [79] concluded that a hydrogen concentration of 5%–15% (vol) seems to bring no significant risks to household appliances, public safety, and existing natural gas pipelines in delivering renewables (hydrogen) to the market. However, it should be noted that the heating value of the gas mixture (British thermal unit (BTU) value) will decrease with the addition of hydrogen due to its low BTU value, such that modification of natural gas appliances may be required when the addition of H2 is greater than 11% [75]. Another very important reason for encouraging the transportation of hydrogen using existing natural gas pipelines is that the future of natural gas pipelines must be revisited. The uncertainty of the depletion of natural gas wells makes investors extremely cautious to invest in new facilities, due to the uncertainty of natural gas storage. If existing natural gas pipelines could be modified (through coating, etc.) and converted into hydrogen pipelines, it might help investors to decide to build better developed pipeline networks, which will benefit the transportation of hydrogen.

In summary, the addition of hydrogen into existing natural gas pipelines is a feasible short-term solution for the following reasons. First, an addition of about 10% into natural gas pipelines is technically feasible, with minor ignition risks [79], no increase in pipeline fatigue [23], and no increased leakage risks[79,80]. Second, transporting hydrogen via existing natural gas lines is economically acceptable, as it can be treated as an alternative way to decrease the overall high construction cost and to save time during the early market-development phase [79]. Third, the transportation of flammable gases through existing natural gas pipelines is already accepted by the public. Last but not least, the operating practice of such cases will be important in order to explore the further utilization of existing natural gas pipelines. On that basis, the transportation of hydrogen using existing gas pipelines may be an interim solution, and the downstream hydrogen/natural gas mixtures may be used for direct heating, electricity generation, or pure hydrogen sources via the downstream separation of hydrogen from natural gas.

3.3. End-user purification

The transportation and redistribution of bulk and decentralized hydrogen produced by P2H projects are currently facing practical issues due to a lack of extensive hydrogen delivery networks. Building large-scale and dedicated hydrogen infrastructures is currently regarded as economically inviable; thus, injecting a low concentration of hydrogen into existing natural gas pipelines is an interim method. Based on research conducted by various institutions to date, blending about 10% hydrogen based on gas volume would barely induce minor issues, depending on the specific conditions of the pipelines and the composition of the carrying natural gas [79]. In such cases, the hydrogen blended with natural gas in the pipelines, sometimes called hydrogen-enriched natural gas (HENG), can be directly used as a fuel for boilers or furnaces in order to extract the heating value and for a gas turbine to generate electricity [17]. An ongoing two-year trial of P2H (Jemena Powerto-Gas Demonstration) in Australia and the HyDeploy project in the United Kingdom are injecting part of the hydrogen into the local natural gas network for household appliances[81,82].

Aside from direct utilization, hydrogen-blended gases could be an effective way to move bulk hydrogen from production sites to the potentially vast amount of renewable energy end users, if downstream separation techniques to obtain pure hydrogen from HENG are proved to be technologically feasible and economically viable. One of the key issues here is the separation of lowconcentration H2 from CH4 mixtures to produce pure hydrogen [83].

However, most current hydrogen separation technologies are customized for steam methane reforming (SMR), SMR off-gas (SMROG), or syngas, which may contain up to 80% hydrogen, balanced with CO2, carbon monoxide (CO), and methane[17,79,84], under high temperatures. These streams have different compositions from HENG, with less than 15% hydrogen. A comparison of the feed stream specification for these applications is shown in Table 3[17,79,84].

Table 3 Typical compositions of gas mixtures[17,79,84].

It has been reported that current separation costs are comparatively high (2–7 USD·m-1 for 100–1000 kg·d-1 ) [85]. The separation of hydrogen from low-concentration mixture sources suitable for HENG at low temperatures is not yet well investigated. Therefore, developing inexpensive, highly efficient separation techniques suitable for low concentrations of hydrogen in natural gas at low temperatures is of great significance for ensuring that overall hydrogen prices are competitive.

Many technologies are available to achieve the target of efficient, affordable separation technologies with compact facilities to acquire pure hydrogen (≥ 99.99%, as set by the US DOE [86]). In this section, we provide an overview of the technologies for separating H2 from CH4.

3.3.1. Membranes

Membrane technology has been commercialized for decades and is widely implemented in natural gas sweetening [87], hydrogen recovery from ammonia purge gas [88], and carbon capture[89,90]. Hydrogen transport across membranes may follow one or a combination of the following mechanisms: viscous flow, Knudsen diffusion, molecular sieving, solution–diffusion, and surface diffusion. The description and application of each mechanism are well established in the literature[91–93], although they are out the scope of this work and will be discussed in another paper [76].

The membrane performance for H2/CH4 separation can be expressed via the permeability (P) of H2 and the selectivity (α) of H2 versus CH4 (see Eqs. (2) and (3)). The driving force for hydrogen transport across the membrane barrier relies on the partial pressure difference of hydrogen on the two sides of the membrane. The partial pressure of hydrogen on the feed side of the membrane must be higher than that on the permeate side in order to support the permeate flow. As a result, the hydrogen separation efficiency (i.e., H2 concentration in the permeate/H2 concentration in the feed) is limited by the pressure ratio of the feed side to permeate side (see Eq. (4)). This suggests that a high pressure ratio is required to obtain high-purity hydrogen from a dilute hydrogen feed gas mixture. However, this limit is not the case when separating H2/CH4 in transmission pipelines, which commonly operate at around 68 bar [79].

where Qi is the gas permeance of the gas component i; l is the membrane thickness; J is the gas flux; and is the partial pressure difference of the gas component on two sides of the membrane.

Membrane studies in the literature are mostly aimed at mixtures with high hydrogen concentration (≥ 50%) to obtain a pure hydrogen product (≥ 99.99%) at high temperatures (≥ 300 °C). A range of membranes have been investigated for performance in hydrogen separation (Table 4 [92,94–98]), including dense metallic membranes [79,92,99,100], porous inorganic membranes [79,92,101,102], metal organic framework (MOF) membranes [103,104], and polymeric membranes [105–124]. Among these, dense metallic membranes and porous inorganic membranes are probably the closest to industrialization for hydrogen purification. Palladium membranes are able to separate hydrogen from methane with 99.9999999% purity; however, they also show low recovery and need a high operation temperature of 300 °C, which is not suitable for the purpose of separating hydrogen from natural gas pipelines [79]. Molecular sieve (MS) membranes [86,125,126] have generally low costs, show high permeability, and can be operated at low temperatures [127], but can only achieve a limited hydrogen purity. For example, a carbon molecular sieve membrane was reported to achieve a hydrogen purity of 98% [79] for the separation of hydrogen (~20%) from methane. Recently, the fabrication of graphene-based carbon molecular sieve (CMS) membranes has drawn attention due to its low thickness (single-atom thickness), high stability, and high H2 separation performance (e.g., 1 mol·m-2 ·s-1 ·Pa-1 H2 permeance and 108 H2/CH4 selectivity) [102]. Polyimide-based membranes such as 4,4' -(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and Matrimid® have also been introduced at the industrial scale, and showed better H2 transport than conventional polysulfone and cellulose acetate membranes[87,92]. However, a significant limitation in polymer membranes is the tradeoff between gas permeability and selectivity, also known as the Robeson’s upper bound (Fig. 9) [97,98,105–124]. Several approaches have been applied to overcome this upper bound, including fabricating ultrathin membranes, synthesizing new polymers, and incorporating a polymeric membrane with other materials such as MOF materials. Many laboratory-scale membranes (Table 5 [97,105–124]) were reported to cross the upper bound; examples include polymer of intrinsic microporosity (PIM) membranes [105–107] and thermal rearranged (TR) membranes [108]. However, membrane-based hydrogen separation is still in the research and development (R&D) stage and more research in this area is expected before large-scale deployment.

Table 4 Common membranes for hydrogen separation [92,94–98].

a H2/CH4 selectivity unless otherwise specified;

b H2/N2 selectivity.

Fig. 9. Robeson’s upper bound (2008) for H2/CH4 separation, and some recent advanced membranes. PIM: polymer of intrinsic microporosity; TR: thermal rearranged; Barrer: unit for gas permeability through membranes, 1 Barrer = 3.35 × 10-16 mol·(m2 ·s·Pa)-1 .

Table 5 A shortlist of polymeric membranes for hydrogen/methane separation.

3.3.2. Sorbents

Hydrogen purification was the very first large-scale application of pressure swing adsorption (PSA) technology (Fig. 10), and hydrogen purification via PSA is a well-established technology in the industry, with most examples servicing SMR processes [128]. Most researchers are currently focusing on further improving the performance of hydrogen PSA for the SMR process. Activated carbons and zeolites are the most frequently used adsorbents for H2 purification as equilibrium separation processes, with a reported selectivity (CH4/H2) of 10 and 13.5 for active carbon and zeolite 5A, respectively [129]. It is difficult to achieve high purity and recovery of hydrogen with the use of activated carbon as the sole adsorbent. However, 99.999% hydrogen can be obtained easily through PSA with a high recovery using zeolite 5A as the adsorbents through multiple columns and inter-column purging steps [129]. An earlier approach [130] to separate a lower concentration of hydrogen from a gas mixture via PSA was intended to recover hydrogen from the waste gas of SMR, which is known as gas polishing. Zeolites 5A are also known for their broadband adsorption capability for various impurities in hydrogen such as CO2, CO, H2O, N2, and so forth. Sircar [130] managed to use a threestage PSA process to improve the hydrogen recovery of the SMR process from 87% to 95%; in this process, the third stage of PSA with two adsorption columns was designed to treat feed gas with a lower hydrogen concentration. However, to the best of our knowledge, no published study has had the precise objective of extracting dilute hydrogen from methane via PSA thus far.

Fig. 10. Schematic diagram of a PSA. Images courtesy of CO2CRC.

Adsorbents are key to the production of high-purity hydrogen via PSA. Several researchers have focused on the development of novel materials for CH4/H2 separation, such as MOF materials. A range of (CH4/H2) selectivity data have been reported, such as 20 for copper (Cu)–benzene-1,3,5-tricarboxylic acid (BTC), 5 for MOF-5 [131], 2.5 for zeolitic imidazolate framework (ZIF)-70, 4 for ZIF-68 [132], 15 for ZIF-3 [133], and 10.5 (500 kPa) for MIL101_R7-benzene dicarboxylic acid (BDC) (by adding one more benzene ring into each carboxylate ligands of MIL-101) [134]. Kinetic separation processes have also been investigated for the separation of CH4 and H2, as the latter has a much faster mass transfer rate through MS beds. On strontium-modified sodium exchanged Engelhard titanosilicate (NaETS-4), the kinetic selectivity of H2/ CH4 was as high as 8.91 [135]. Most of these materials are at a very early stage of utilization, however, due to their high manufacturing costs.

PSA requires a large facility and more frequent adsorption– regeneration cycles due to the high concentration of methane in HENG (because sorbents are often methane-selective); thus, it has high overall costs. Because PSA technology is an intensive site-specific process, the gas feed composition and product specification may impact many design factors. One important factor to consider is the PSA column size against the tolerable level of methane. With more impurities in the gas feed, more adsorbents and larger PSA columns are needed in order to achieve high purity and recovery of hydrogen. Thus, a PSA bed for extracting pure hydrogen from HENG will be much larger than that in SMR processes. Any other differences in the process parameters, such as source gas pressure, sub-product gas pressure, and specification, will inevitably change the PSA design accordingly. Therefore, the experience of designing PSA for SMR cannot be directly adopted for the scenario of HENG purification.

In conclusion, there are two issues to be overcome in separating a low concentration of hydrogen from natural gas with PSA. First, as PSA requires a large facility and more frequent adsorption– regeneration cycles due to the high concentration of methane, it has high overall costs. Second, recovering hydrogen from a PSA unit is likely to involve the recovery of the main constituent at a pressure as low as a few bars; thus, huge recompression costs will be needed for further transportation.

Furthermore, an integrated membrane-adsorption process (Fig. 11) may be considered to produce high-purity hydrogen in order to overcome the shortcomings of low selectivity for membranes at low temperatures and the low methane-adsorption capacity of most adsorbents.

Fig. 11. An integrated membrane-adsorption process for the separation of H2 from CH4.

3.3.3. Hydrogen pump

Electrochemical separation is another choice for selectively capturing and pressurizing hydrogen from bulk methane mixtures. Pressure (i.e., partial pressure) is not the driving force in this technology (Fig. 12); instead, the electric potential drives the process by imposing voltage to dissociate H2 to H+ at a protonconducting anode (step 1, oxidation), transporting H+ through proton-conducting membranes/materials (step 2, transportation), and then re-associating protons to H2 at another protonconducting cathode (step 3, reduction) [136,137]. Recently, Wagner et al. [138] reported an electrochemical cell for the selective extraction of hydrogen from its mixtures with methane at a concentration as low as 5%. The advantage of the hydrogen pump is that it can separate hydrogen gas from methane and other gases [139] (CO2, H2O, CO, etc.) and has a very high hydrogen product purity, so that no further purification is needed. Although various electrolytes including ceramics [137] and polymers [139] have been reported as proton conductors, it is critical to find suitable materials with higher proton conductivity and lower mass transport resistance. The minimum electrical work to be supplied to the cell is the theoretical work of compression, as described by the Nernst equation [136]. The hydrogen pump process, however, is not yet ready for large-scale industrial deployment.

Fig. 12. Schematic showcase of the hydrogen pump process.

3.3.4. Solvents

The separation of hydrogen from methane using solvents has been a huge challenge during past decades, in comparison with other effective methods such as the use of membranes and adsorbents to separate gas mixtures. Suitable absorbents require a low solvent power for hydrogen but a high solvent power for methane in order to achieve high selectivity of methane over hydrogen. Solvents with a high solvent power for hydrogen could result in a significant loss of hydrogen and make hydrogen difficult to regenerate from rich absorbents, whereas absorbents with a low solubility of methane could have a negative impact on the purity of the final products. In addition, low vapor pressure and degradation of absorbents are required to avoid the loss of solvent.

Considering that both methane and hydrogen are non-polar and inert, their interactions with most solvents are very weak. Physical absorption under harsh operating conditions such as high pressures and sub-cooling temperatures is likely to be the only way to separate these two gases via absorption. For example, Palazzo and Schreiner [140] filed a US patent for the separation of methane and hydrogen using liquefied C2 and C3 hydrocarbons at –180 to –160 °C and about 7 bar. This process is not cost effective due to its high energy consumption. In addition, a portion of purified hydrogen was injected to regenerate the rich solvent, compromising the productivity of the hydrogen recovery.

Kim et al. [141] evaluated the solubility of methane in common liquid solvents in the form of the Henry’s constant using a molecular simulation method named the conductor-like screening model for realistic solvents (COSMO-RS) [142,143]. The solubility is calculated based on the chemical potentials between the solute and solvent, which are expressed by the σ-profile (Eq. (5)).

where x is the mole fraction of the solute; (kJ·mol-1 ) is the pseudo-chemical potential of the solute when x = 1, a pure liquid state; (kJ·mol-1 ) is the pseudo-chemical potential of the solvent; kB is the Boltzmann constant; and T (K) is the temperature.

The simulation was conducted at 300 K and 1 bar, with (methane) = –26.8 kJ·mol-1 [144,145]. Calculation details are provided in Ref. [141].

Trinh et al. [146] investigated the solubility of hydrogen in 42 organic solvents at different temperatures, including alcohols, carboxylic acids, esters, ethers, aldehydes, glycols, n-alkanes, and water. They calculated the Henry’s constant of hydrogen using the following rigorous equation:

where Hi (MPa-1 ) is the Henry’s constant of the solute; (MPa) is the vapor pressure of the solvent; and is the fugacity coefficient of the solute in the liquid mixture at an infinite dilution state, which is evaluated using the Peng–Robinson (PR) equation of state (EoS) with binary parameters.

In total, 838 experimental data points from these 42 organic solvents were used to regress the binary parameters in the PR EoS with a deviation of 5%–10%. The regressed results were further used to validate the simulation using a Monte Carlo molecular simulation via an anisotropic united atom (AUA4) force field [147–151]. It was found that hydrogen has the highest solubility in n-alkanes, followed by ethers, aldehydes, esters, and alcohols, with diols having the lowest solubility.

To compare the selectivity of hydrogen over methane, we chose the Henry’s constants of hydrogen and methane using identical solvents from the above resources—namely, n-hexane, n-octane, methanol, ethanol, 1-propanol, 1-butanol, acetone, diethyl ether, acetic acid, and water—at 300 K; the results are shown in Fig. 13.

Fig. 13. CH4/H2 molar selectivity versus CH4 Henry’s constant in different liquid solvents at T = 300 K.

It was found that all these liquid solvents have relatively high selectivity (≥ 5) of CH4/H2 at 300 K except for water, with 1-butanol showing the highest selectivity at 8.34. However, the Henry’s constants for both gases are extremely low at this temperature, which means that there is very limited solubility even for the best solvent, 1-butanol. For example, if the partial pressure of CH4 is 1 MPa, the mole fraction of CH4 loaded in n-octane is only 0.035. Optimizing the operating conditions (temperature and pressure) will require high energy input, resulting in economic concerns. Therefore, the use of solvents for separating hydrogen and methane is not favorable for industry.

3.3.5. Cryogenic separation

Cryogenic technology (Fig. 14) is one of the commercial techniques for gas separation [152], particularly for H2/CH4 mixtures. The recovery and purity of hydrogen can be as high as 95% and 90%–98%, respectively, with a feed gas H2 composition of 30%– 80% using cryogenic technology [153]. With an extremely low boiling point of 20.4 K, hydrogen is typically separated out of other gases as the gaseous phase in cryogenic processes [154]. Although it is applied widely in industrial processes, cryogenic technology is a costly and energy-intensive process [92,155]. Baker [87] recommended that the appropriate gas capacity for a cryogenic plant to be economical was in the range of 1.8 × 106 –8.8 × 106 m3 ·h-1 . This suggests that cryogenics may not be suitable for small-scale separation facilities in the downstream of the H2/CH4 transmission process. Another technical concern for the cryogenic processing of H2/ CH4 mixtures is the formation of methane clathrate at high pressures and low temperatures, which might block the processing pipelines [156]. Furthermore, insulation of the diffraction columns and liquefied gas containers are factors that affect the energy and economic efficiency of the cryogenic process [155,157]. Recently, hybrid systems that combine membrane and cryogenic systems [87,158] have been introduced and hold promise for reducing the land and economic footprints of the cryogenic process. It is worth noting that cryogenics may be an ideal option when the end user requires liquefied natural gas (LNG) on a relatively large scale (millions of cubic meters per hour).

Fig. 14. Schematic graph of a single-column cryogenic process for the separation of H2 from CH4. LNG: liquefied natural gas.

3.4. Other barriers and alternatives

Although there are still barriers for the P2H technology, we are quite confident that the development of this technology will be able to contribute to a clean and secure future. Other than the above technological barriers, other barriers also need to be addressed in order to ensure the large-scale deployment of P2H; these include the availability of water, government regulations, environment impacts, and so forth.

3.4.1. Supply of water for electrolysis

Availability of water is a key to P2H technologies, as water is essential for electrolysis and hydrogen generation. However, there may be a mismatch between the availability of renewable power and a source of clean water. Taking solar power as an example, drought areas are often indicative of a large amount of solar energy, whereas the availability of water is poor. In this case, either the transmission of electricity or the transportation of a large amount of water will be necessary. Therefore, it is worth investigating the local availability of water and analyzing the economic tradeoff between the transmission of electricity and the transportation of water and hydrogen.

3.4.2. Utilization of oxygen

Oxygen is a byproduct when producing hydrogen via electrolysis. However, the utilization of this byproduct is still not well explored. As the production of hydrogen increases, the quantity of oxygen produced will become significant as well. It is worth revisiting the possibility of utilizing high-concentration oxygen in industry. The following list provides some directions that may be considered:

(1) Using oxygen in fuel cells to enhance the overall efficiency and initiation speed;

(2) Using oxygen in combustion to enhance the temperature and fuel efficiency;

(3) An oxy-fuel combustion process followed by CCS with low costs;

(4) Steel or oxygen acetylene welding.

3.4.3. Environmental impacts

Many research publications have assessed the environmental effects of hydrogen utilization [26,73,159–163]. Bicer and Dincer [72] compared the effects of different vehicles (hydrogen, methanol, and electric) from the perspective of environmental impact ranging from global warming potential and ozone layer depletion, to human toxicity indicators, and covering the whole industry chain including vehicle production, operation, maintenance, and disposal. They concluded that hydrogen is the most environmentally friendly option. However, this conclusion can differ if the source of the hydrogen is taken into consideration. The amount of greenhouse gases emitted during the hydrogen production phase can vary significantly, depending on what type of fuel or energy is consumed. Renewable hydrogen may lead to fewer greenhouse gas emissions than hydrogen derived from fossil fuels. Other environmental impacts of different hydrogen production technologies, especially the consumption of resources (e.g., the large amount of water consumed by electrolysis or shifting processes), are yet to be investigated.

3.4.4. Safety, regulations, and public acceptance

Regulation is another factor that needs to be considered [75]. Taking China as an example, hydrogen is defined as dangerous (chemical) goods. Therefore, hydrogen refill stations must be operated in chemical plant areas or far away from urban areas. This makes the large-scale deployment of hydrogen in in-house appliances, self-used fuel cell cars, and so forth almost impossible. Whether this situation can be changed soon is still a big question. However, we have also observed some positive signs related to hydrogen deployment very recently. On 14 June 2019, the IEA released a special report [164] on hydrogen for the upcoming Group of Twenty (G20) meeting, followed by a joint statement issued by the Ministry of Economy, Trade, and Industry (METI) of Japan, the European Commission (EC) Directorate-General for Energy (ENER), and the US DOE to promote the development of hydrogen and fuel cells. This will accelerate the speed of hydrogen deployment, with potential deregulation of hydrogen in certain aspects.

3.4.5. Alternatives/competitors

P2H may not be competitive with fossil fuels from the perspective of current overall costs yet, and its cost reduction in large-scale deployment and in the long run will be highly dependent on the availability of excessive power, the scale of the electrolyzer, and the number of electrolyzer units [11]. Meanwhile, there are many competitors in the renewable energy sector, such as energy storage batteries, the conversion of renewables into fuels, and more, which can also contribute to a clean and secure energy future.

Converting renewable energy into electricity is one of many pathways for utilizing renewables in the energy sector. Various projects and research are underway, such as concentrated solar thermal energy and the production of hydrogen via direct photolysis or thermolysis of water [165–169]. Although electrolysis is a relatively mature technology, there is as yet no winner in the competition among solutions for renewables. Innovation in technology and engineering solutions will be the key for the growth of such technologies in the market, and it may take a few decades or even longer to make final decisions.

The storage of electricity using large-capacity batteries and supercapacitor systems [8] provides another choice for energy storage. Although hydrogen is much better in terms of energy storage density (volumetric/gravimetric) compared with current lithium (Li)-ion batteries [49], it is very likely that the energy density of batteries will be improved over time, given the large increase in battery R&D, and their price will be further reduced. It can be seen from Fig. 15 that the costs of batteries have dropped significantly over the last decade, and this decline in costs is believed to be continuing [170]. In addition, although problems such as the availability of resources and the economical recycling of used batteries remain unsolved, battery storage is a relatively mature technology with a vast manufacturing capacity (~70 GW·h in 2018) [3]. Furthermore, there are very strong indications that big corporations are willing to push battery storage development, with about 0.2 billion USD invested in batteries R&D in 2018 and a planned increase in the production capacity of batteries to about 400 GW·h by the mid-2020s [3]. Andrews and Shabani [8] recommended that hydrogen should play a role, together with electricity and other energy storage options, in future sustainable energy systems. Indeed, batteries are playing an important role in the energy storage sector, and it is believed that they will continue to do so in the future.

Fig. 15. Price of lithium batteries by year.

4. Summary and outlook

4.1. Summary

This article reviewed the overall process of the P2H value chain, from energy demand and power generation from renewables, to hydrogen production from electricity via electrolysis, and eventually to hydrogen redistribution, utilization, and re-separation from low-concentration hydrogen–methane mixtures. We showed that the share of renewables in the energy sector has increased dramatically in recent years and has been recognized as an important source of energy supply in the future. Energy storage pathways are essential, and P2H provides a promising solution to this issue. The redistribution of hydrogen using existing natural gas pipelines is well studied, and the recommendation of approximately 10% addition into natural gas pipelines has been shown to have negligible effects on pipelines and utilization appliances; this may also provide a solution for natural gas pipelines after natural gas has been depleted. Current research on end-user hydrogen purification mainly focuses on SMROG or syngas, often with higher hydrogen concentration and other impurities besides methane.

We reviewed the progress of a range of techniques for the separation of hydrogen from natural gas mixtures, including solvent absorption, membranes, adsorption, hydrogen pumping, and cryogenics. Solvents are not recommended due to their low absorption capacity. Cryogenics are too energy intensive to be cost effective, unless LNG is one of the final products. Membranes, adsorption, and hydrogen pumping are potential candidates with different advantages and degrees of maturity.

4.2. Future R&D outlook

P2H is a relatively new area in the energy sector. This review discussed its technical advances and barriers, along with potential solutions in the P2H roadmap. With this review, we hope to promote promising areas for future R&D in order to advance this field. By overcoming these challenges, P2H will contribute significantly to a clean and secure energy future.

A key barrier to the large-scale deployment of P2H is its low overall energy efficiency and high cost. In the value chain of P2H, electrolysis, compression, and liquefaction are relatively mature in terms of technological readiness, but there is still a great deal of room for technological improvement in the following areas: hydrogen redistribution, transmission, and transportation; hydrogen purification and separation; conversion between hydrogenloaded and unloaded carriers; hydrogen utilization; and cost reduction.

4.2.1. Gas separation

Membranes and adsorption are key technologies in separating hydrogen from HENG. Due to the current limitations, the development of novel membranes and adsorbent materials with high selectivity and capacity, the advance of an integrated membraneadsorption process, and the feasibility study of hydrogen pumping utilization on a large scale are both necessary and promising. In Fig. 15. Price of lithium batteries by year. addition, research on the removal of low-concentration gas odorant blends (parts-per-billion (ppb) to ppm) such as tetrahydrothiophene, dimethyl sulfide, methyl ethyl sulfide, tetrahydrothiophene, and other mercaptans remains very rare, yet is crucial to the future deployment of hydrogen separation from HENG by end users.

4.2.2. Energy efficiency assessment

This review provides a rough energy efficiency assessment in Table 2 [32,37,49–51]. A more rigorous energy efficiency model will be essential for the comparison of new technologies and in order to determine the cost threshold of P2H.

4.2.3. Others

The conversion of hydrogen into liquid fuels (e.g., methanol, ethanol, methane, formic acid, or even gasoline) will remain a popular research topic, as these hydrogen-sourced fuels require little modification of existing fuel systems. This approach may play a significant role in promoting the progress of P2H in light of carbon dioxide utilization. Other aspects, such as the cost reduction of electrolyzers, the redistribution and utilization of oxygen, environmental impacts, the availability of water, and the readiness of regulations are still in their early stages and require considerable efforts to explore.

Acknowledgements

The authors acknowledge the support of Global Innovation Linkage (GIL) awarded by Department of Industry, Innovation and Science entitled "Development of Unconventional Gas Technologies for Sustainable Energy Security” (GIL54444), Early Career Researcher Grants Scheme awarded by the University of Melbourne entitled "Production of HighPurity Hydrogen from Mixed Pipeline Gases” (1858821), and Future Fuels Cooperative Research Centre (CRC) "Novel Separation Technology development forhydrogen and future fuels systems” (RP3.2-08). G. Hu also would like to acknowledge Mr. J. Liu for preparing the table of art.

Compliance with ethics guidelines

Guoping Hu, Chao Chen, Hiep Thuan Lu, Yue Wu, Congmin Liu, Lefu Tao, Yuhan Men, Guangli He, and Kevin Gang Li declare that they have no conflict of interest or financial conflicts to disclose.

References

Publishing order | Descend order by publishing year | Descend order by cited within
[1]

Better energy, greater prosperity: achievable pathways to low-carbon energy systems [Internet]. Hawthorn: Energy Transitions Commission; 2017 Apr 25 [cited 2020 Mar 26]. Available from: https://apo.org.au/node/76074.
[2]

World energy outlook 2018 [Internet]. Paris: International Energy Agency; 2018 Nov [cited 2020 Mar 26]. Available from: https://www.iea.org/ reports/world-energy-outlook-2018.
[3]

World energy investment 2019 [Internet]. Paris: International Energy Agency; 2019 May [cited 2020 Mar 26]. Available from: https://www.iea. org/reports/world-energy-investment-2019.
[4]

Chu S, Cui Y, Liu N. The path towards sustainable energy. Nat Mater 2017;16 (1):16–22.
[5]

Hu G, Nicholas NJ, Smith KH, Mumford KA, Kentish SE, Stevens GW. Carbon dioxide absorption into promoted potassium carbonate solutions: a review. Int J Greenhouse Gas Control 2016;53:28–40.
[6]

Chu S. Carbon capture and sequestration. Science 2009;325(5948):1599.
[7]

Hu G. Novel promoters for carbon dioxide absorption in potassium carbonate solutions [dissertation]. Melbourne: The University of Melbourne; 2018.
[8]

Andrews J, Shabani B. Re-envisioning the role of hydrogen in a sustainable energy economy. Int J Hydrogen Energy 2012;37(2):1184–203.
[9]

Belsito E, Dewar A, Marten I. Global gas report 2018. Technical report. Boston: The Boston Consulting Group; 2018.
[10]

Renewables 2018: analysis and forecasts to 2023 [Internet]. Paris: International Energy Agency; 2018 Oct [cited 2020 Mar 26]. Available from: https://www.iea.org/reports/renewables-2018.
[11]

Linnemann J, Steinberger-Wilckens R. Realistic costs of wind-hydrogen vehicle fuel production. Int J Hydrogen Energy 2007;32(10–11):1492–9.
[12]

Pecher A, Kofoed JP, editors. Handbook of ocean wave energy. Cham: Springer; 2017.
[13]

Global energy statistical yearbook 2018 [Internet]. New York: Enerdata; 2018 [cited 2020 Mar 26]. Available from: https://yearbook.enerdata.net/ 2017/.
[14]

Robinius M, Raje T, Nykamp S, Rott T, Müller M, Grube T, et al. Power-to-gas: electrolyzers as an alternative to network expansion—an example from a distribution system operator. Appl Energy 2018;210:182–97.
[15]

Net installed electricity generation capacity in Germany in 2018 [Internet]. Freiburg: Fraunhofer institute for solar energy systems (ISE); 2019 Jan 4 [cited 2020 Mar 26]. Available from: https://www.ise.fraunhofer. de/content/dam/ise/en/documents/News/Stromerzeugung_2018_2_en.pdf.
[16]

Recent electricity data: power generation and consumption [Internet]. Berlin: Agora Energiewende; 2020 Mar 26 [cited 2020 Mar 26]. Available from: https://www.agora-energiewende.de/en/service/recent-electricity-data/chart/ power_generation/30.03.2020/02.04.2020/.
[17]

Maroufmashat A, Fowler M. Transition of future energy system infrastructure; through power-to-gas pathways. Energies 2017;10(8):1089.
[18]

Pivovar B. 2017 H2@SCALE workshop report. Report. Golden: National Renewable Energy Laboratory; 2016. Report No.: HT12.IN53.
[19]

Hydrogen storage [Internet]. Washington, DC: Office of Energy Efficiency and Renewable Energy; c2020 [cited 2020 Mar 26]. Available from: https://www. energy.gov/eere/fuelcells/hydrogen-storage.
[20]

Pereira EG, da Silva JN, de Oliveira JL, Machado CS. Sustainable energy: a review of gasification technologies. Renewable Sustain Energy Rev 2012;16 (7):4753–62.
[21]

Nikolaidis P, Poullikkas A. A comparative overview of hydrogen production processes. Renewable Sustain Energy Rev 2017;67:597–611.
[22]

Fúnez Guerra C, Jaén Caparrós M, Nieto Calderón B, Sendarrubias Carbonero V, Nieto Gallego E, Reyes-Bozo L, et al. Viability analysis of centralized hydrogen generation plant for use in mobility sector. Int J Hydrogen Energy 2018;43(26):11793–802.
[23]

Penev M, Melaina M, Bush B, Muratori M, Warner E, Chen Y. Low-carbon natural gas for transportation: well-to-wheels emissions and potential market assessment in California. Technical report. Golden: National Renewable Energy Laboratory; 2016.
[24]

Wilberforce T, El-Hassan Z, Khatib FN, Al Makky A, Baroutaji A, Carton JG, et al. Developments of electric cars and fuel cell hydrogen electric cars. Int J Hydrogen Energy 2017;42(40):25695–734.
[25]

cafcp.org [Internet]. West Sacramento: California fuel cell partnership; c2020 [cited 2020 Mar 26]. Available from: https://cafcp.org/.
[26]

De Almeida LQ, Sales LCM, Sodré JR. Fuel consumption and emissions from a vehicle operating with ethanol, gasoline and hydrogen produced on-board. Int J Hydrogen Energy 2015;40(21):6988–94.
[27]

Andrews J, Shabani B. Dimensionless analysis of the global techno-economic feasibility of solar-hydrogen systems for constant year-round power supply. Int J Hydrogen Energy 2012;37(1):6–18.
[28]

Multi-year research, development and demonstration plan: 2016 fuel cells section [Internet]. Washington, DC: Fuel Cell Technologies Office; 2017 May [cited 2020 Sep 26]. Available from: https://www.energy.gov/sites/prod/files/ 2017/05/f34/fcto_myrdd_fuel_cells.pdf.
[29]

Sinigaglia T, Lewiski F, Santos Martins ME, Mairesse Siluk JC. Production, storage, fuel stations of hydrogen and its utilization in automotive applications—a review. Int J Hydrogen Energy 2017;42(39):24597–611.
[30]

hydrogeneurope.eu/ [Internet]. European Hydrogen and Fuel Cell Association; c2020 [cited 26 March 2020]. Available from: https://hydrogeneurope.eu/.
[31]

Power to gas strategy platform [Internet]. Berlin: German Energy Agency; c2016 [cited 2020 Mar 26]. Available from: https://www.dena.de/en/topicsprojects/projects/energy-systems/power-to-gas-strategy-platform/.
[32]

Vlap H, van der Steen A, Knijp J, Holstein J, Grond L. An overview of the technical assumptions and results of the power-to-gas demonstration project in Rozenburg, the Netherlands Technical report. Groningen: Det Norske Veritas (DNV) Germanischer Lloyd (GL) Oil & Gas; 2015.
[33]

H2@Scale CRADA projects: 2018 annual progress report. Technical report. Washington, DC: US Department of Energy; 2018.
[34]

Hydrogen and fuel cells program [Internet]. Washington, DC: US Department of Energy; c2020 [cited 2020 Mar 26]. Available from: https://www. hydrogen.energy.gov/.
[35]

Palmer G. Australia’s hydrogen future: a research report Technical report. Canberra: Energy Transition Hub; 2018.
[36]

Maniatopoulos P, Andrews J, Shabani B. Towards a sustainable strategy for road transportation in Australia: the potential contribution of hydrogen. Renew Sustain Energy Rev 2015;52:24–34.
[37]

Penev M. Hybrid hydrogen energy storage [Internet]. Golden: National Renewable Energy Laboratory (NREL); 2013 May 22 [cited 2020 Mar 26]. Available from: http://www.h2fcsupergen.com/wp-content/uploads/2013/ 06/Hybrid-Hydrogen-Energy-Storage-Michael-Penev-National-Energy-ResearchLaboratory.pdf.
[38]

Smolinka T. Water electrolysis: status and potential for development [Internet]. Freiburg: Fraunhoder Fraunhofer institute for solar energy systems (ISE); 2014 Mar 4 [cited 2020 Mar 26]. Available from: https:// www.fch.europa.eu/sites/default/files/2%20Water%20Electrolysis%20Status% 20and%20Potential%20for%20Development.pdf.
[39]

Schmidt O, Gambhir A, Staffell I, Hawkes A, Nelson J, Few S. Future cost and performance of water electrolysis: an expert elicitation study. Int J Hydrogen Energy 2017;42(52):30470–92.
[40]

Lehner M, Tichler R, Steinmüller H, Koppe M. The power-to-gas concept. In: Lehner M, Tichler R, Steinmüller H, Koppe M, editors. Power-to-gas: technology and business models. Cham: Springer; 2014. p. 7–17.
[41]

Waidhas M. Electrolyzer technology—the Siemens view [Internet]. Sandviken: Hydrogen and Fuel Cell (HFC) Nordic; 2016 Oct 26 [cited 2020 Mar 26]. Available from: http://www.scandinavianhydrogen.org/wp-content/ uploads/2016/11/2_Manfred-Waidhas.pdf.
[42]

Otten R. The first industrial PtG plant—Audi e-gas as driver for the energy turnaround [Internet]. Bruxelles: Community Economic Development and Employability Corporation (CEDEC); 2014 May 27 [cited 2020 Mar 26]. Available from: http://www.cedec.com/files/default/8-2014-05-27-cedecgas-day-reinhard-otten-audi-ag.pdf.
[43]

Felgenhauer M, Hamacher T. State-of-the-art of commercial electrolyzers and on-site hydrogen generation for logistic vehicles in South Carolina. Int J Hydrogen Energy 2015;40(5):2084–90.
[44]

Sutherland E, Joseck F. DOE hydrogen and fuel cells program record (15011): low volume production and delivery cost [Internet]. Washington, DC: US Department of Energy; 2015 Aug 20 [cited 2020 Mar 26]. Available from: https://www.hydrogen.energy.gov/pdfs/15011_low_volume_production_ delivery_cost.pdf.
[45]

Joseck F. Technology acceleration and hydrogen infrastructure R&D overview [Internet]. Washington, DC: US Department of Energy; 2018 [cited 2020 Mar 26]. Available from: https://www.hydrogen.energy.gov/pdfs/progress18/ tahi_overview_2018.pdf.
[46]

Rustagi N, Elgowainy A, Vickers J. DOE hydrogen and fuel cells program pecord (18003): current status of hydrogen delivery and dispensing costs and pathways to future cost reductions [Internet]. Washington, DC: US Department of Energy; 2018 Dec 17 [cited 2020 Mar 26]. Available from: https://www.hydrogen.energy.gov/pdfs/18003_current_status_hydrogen_ delivery_dispensing_costs.pdf.
[47]

Weil S, Dillich S, Joseck F, Ruth M. DOE hydrogen and fuel cells program pecord (12001): H2 production and delivery cost apportionment [Internet]. Washington, DC: US Department of Energy; 2012 Dec 14 [cited 2020 Mar 26]. Available from: https://www.hydrogen.energy.gov/pdfs/12001_h2_pd_cost_ apportionment.pdf.
[48]

James B, Colella W, Moton J, Saur G, Ramsden T. PEM electrolysis H2A production case study documentation [Internet]. Washington, DC: US Department of Energy; 2013 Dec 31 [cited 2020 Mar 26]. Available from: https://www.nrel.gov/docs/fy15osti/61387.pdf.
[49]

Zidar M, Georgilakis PS, Hatziargyriou ND, Capuder T, Škrlec D. Review of energy storage allocation in power distribution networks: applications, methods and future research. IET Gener Transm Distrib 2016;10(3):645–52.
[50]

Doddathimmaiah A, Andrews J. Theory, modelling and performance measurement of unitised regenerative fuel cells. Int J Hydrogen Energy 2009;34(19):8157–70.
[51]

Shabani B, Andrews J. An experimental investigation of a PEM fuel cell to supply both heat and power in a solar–hydrogen RAPS system. Int J Hydrogen Energy 2011;36(9):5442–52.
[52]

De Falco M, Basile A, editors. Enriched methane: the first step towards the hydrogen economy. Cham: Springer; 2016.
[53]

Chen C, Lu Y, Banares-Alcantara R. Direct and indirect electrification of chemical industry using methanol production as a case study. Appl Energy 2019;243:71–90.
[54]

Marques Mota F, Kim DH. From CO2 methanation to ambitious long-chain hydrocarbons: alternative fuels paving the path to sustainability. Chem Soc Rev 2019;48(1):205–59.
[55]

Morgan E, Manwell J, McGowan J. Wind-powered ammonia fuel production for remote islands: a case study. Renewable Energy 2014;72:51–61.
[56]

Nayak-Luke R, Bañares-Alcántara R, Wilkinson I. ‘‘Green” ammonia: impact of renewable energy intermittency on plant sizing and levelised cost of ammonia. Ind Eng Chem Res 2018;57(43):14607–16.
[57]

Van-Dal ES, Bouallou C. Design and simulation of a methanol production plant from CO2 hydrogenation. J Cleaner Prod 2013;57:38–45.
[58]

Pérez-Fortes M, Schöneberger JC, Boulamanti A, Tzimas E. Methanol synthesis using captured CO2 as raw material: techno-economic and environmental assessment. Appl Energy 2016;161:718–32.
[59]

Frontera P, Macario A, Ferraro M, Antonucci P. Supported catalysts for CO2 methanation: a review. Catalysts 2017;7(2):59.
[60]

Modisha PM, Ouma CNM, Garidzirai R, Wasserscheid P, Bessarabov D. The prospect of hydrogen storage using liquid organic hydrogen carriers. Energy Fuels 2019;33(4):2778–96.
[61]

Makepeace JW, He T, Weidenthaler C, Jensen TR, Chang F, Vegge T, et al. Reversible ammonia-based and liquid organic hydrogen carriers for highdensity hydrogen storage: recent progress. Int J Hydrogen Energy 2019;44 (15):7746–67.
[62]

Zou YQ, von Wolff N, Anaby A, Xie Y, Milstein D. Ethylene glycol as an efficient and reversible liquid-organic hydrogen carrier. Nat Catal 2019;2:415–22.
[63]

Shi L, Qi S, Qu J, Che T, Yi C, Yang B. Integration of hydrogenation and dehydrogenation based on dibenzyltoluene as liquid organic hydrogen energy carrier. Int J Hydrogen Energy 2019;44(11):5345–54.
[64]

Dong Y, Yang M, Li L, Zhu T, Chen X, Cheng H. Study on reversible hydrogen uptake and release of 1,2-dimethylindole as a new liquid organic hydrogen carrier. Int J Hydrogen Energy 2019;44(10):4919–29.
[65]

Wang G, Mitsos A, Marquardt W. Conceptual design of ammonia-based energy storage system: system design and time-invariant performance. AIChE J 2017;63(5):1620–37.
[66]

Elgowainy A, Reddi K, Brown D. Hydrogen delivery infrastructure analysis Technical report. Chicago: Argonne National Laboratory; 2016.
[67]

Demir ME, Dincer I. Cost assessment and evaluation of various hydrogen delivery scenarios. Int J Hydrogen Energy 2018;43(22):10420–30.
[68]

Florisson O. NATURALHY: assessing the potential of the existing natural gas network for hydrogen delivery. Brussels: GERG Academic Network Event; 2010.
[69]

Van der Zwaan BCC, Schoots K, Rivera-Tinoco R, Verbong GPJ. The cost of pipelining climate change mitigation: an overview of the economics of CH4, CO2 and H2 transportation. Appl Energy 2011;88(11):3821–31.
[70]

Dodds PE, Staffell I, Hawkes AD, Li F, Grünewald P, McDowall W, et al. Hydrogen and fuel cell technologies for heating: a review. Int J Hydrogen Energy 2015;40(5):2065–83.
[71]

Ronevich J, Feng Z, Slifka A, Amaro R, Song EJ, Wang Y. Fatigue performance of high-strength pipeline steels and their welds in hydrogen gas service Technical report. Washington, DC: US Department of Energy; 2018.
[72]

Bicer Y, Dincer I. Comparative life cycle assessment of hydrogen, methanol and electric vehicles from well to wheel. Int J Hydrogen Energy 2017;42 (6):3767–77.
[73]

Yang C, Ogden J. Determining the lowest-cost hydrogen delivery mode. Int J Hydrogen Energy 2007;32(2):268–86.
[74]

Schmura E, Klingenberg M. Existing natural gas pipeline materials and associated operational characteristics. Technical report. Washington, DC: US Department of Energy Hydrogen Program; 2005.
[75]

Reitenbach V, Ganzer L, Albrecht D, Hagemann B. Influence of added hydrogen on underground gas storage: a review of key issues. Environ Earth Sci 2015;73:6927–37.
[76]

Lu HT, Li W, Miandoab ES, Kanehashi S, Hu G. The opportunity of membrane technology for hydrogen purification in the power to hydrogen (P2H) roadmap: a review. Front Chem Sci Eng 2020. In press.
[77]

Pinchbeck D, Huizing R. Preparing for the hydrogen economy by using the existing natural gas system as a catalyst (overview of the NATURALHYproject) [Internet]. Luxembour: The Community Research and Development Information Service (CORDIS); c2010 [cited 2020 Mar 26]. Available from: https://cordis.europa.eu/project/id/502661.
[78]

Altfeld K, Pinchbeck D. Admissible hydrogen concentrations in natural gas systems. Gas Energy 2013;2103(3):1–2.
[79]

Melaina MW, Antonia O, Penev M. Blending hydrogen into natural gas pipeline networks: a review of key issues Technical report. Golden: National Renewable Energy Laboratory; 2013.
[80]

Dodds PE, Demoullin S. Conversion of the UK gas system to transport hydrogen. Int J Hydrogen Energy 2013;38(18):7189–200.
[81]

Hydrogen to be trialled in NSW gas networks [Internet]. Canberra: Australian Renewable Energy Agency; 2018 Oct 22 [cited 2020 Mar 26]. Available from: https://arena.gov.au/news/hydrogen-to-be-trialled-in-nsw-gas-networks/.
[82]

Isaac T. HyDeploy: the UK’s first hydrogen blending deployment project. Clean Energy 2019;3:114–25.
[83]

Ibeh B, Gardner C, Ternan M. Separation of hydrogen from a hydrogen/ methane mixture using a PEM fuel cell. Int J Hydrogen Energy 2007;32 (7):908–14.
[84]

Todd DM. Gas turbine improvements enhance IGCC viability. In: Proceedings of the 2000 Gasification Technologies Conference; 2000 Oct 8–11; San Francisco, CA, USA; 2020.
[85]

Weeks B. Hydrogen delivery in the natural gas pipeline network [Internet]. Washington, DC: US Department of Energy’s Hydrogen Energy Storage for Grid and Transportation Service Workshop; 2014 May 14 [cited 2020 Sep 26]. Available from: https://www.energy.gov/sites/prod/files/2015/01/f19/fcto_ 2014_h2_energy_storage_grid_transportation_svcs_wkshp_weeks.pdf.
[86]

Ockwig NW, Nenoff TM. Membranes for hydrogen separation. Chem Rev 2010;110(4):2573–4.
[87]

Baker RW. Future directions of membrane gas separation technology. Ind Eng Chem Res 2002;41(6):1393–411.
[88]

Ghosal K, Freeman BD. Gas separation using polymer membranes: an overview. Polym Adv Technol 1994;5(11):673–97.
[89]

Merkel TC, Lin H, Wei X, Baker R. Power plant post-combustion carbon dioxide capture: an opportunity for membranes. J Membr Sci 2010;359(1– 2):126–39.
[90]

Kentish SE, Scholes CA, Stevens GW. Carbon dioxide separation through polymeric membrane systems for flue gas applications. Recent Pat Chem Eng 2008;1(1):52–66.
[91]

Koros WJ, Fleming GK. Membrane-based gas separation. J Membr Sci 1993;83 (1):1–80.
[92]

Zornoza B, Casado C, Navajas A. Chapter 11—advances in hydrogen separation and purification with membrane technology. In: Gandía LM, Arzamendi G, Diéguez PM, editors. Renewable hydrogen technologies. Amsterdam: Elsevier; 2013. p. 245–68.
[93]

Li P, Wang Z, Qiao Z, Liu Y, Cao X, Li W, et al. Recent developments in membranes for efficient hydrogen purification. J Membr Sci 2015;495:130–68.
[94]

Lubitz W, Tumas W. Hydrogen: an overview. Chem Rev 2007;107 (10):3900–3.
[95]

Peters TA, Kaleta T, Stange M, Bredesen R. Development of thin binary and ternary Pd-based alloy membranes for use in hydrogen production. J Membr Sci 2011;383(1–2):124–34.
[96]

Peters TA, Kaleta T, Stange M, Bredesen R. Hydrogen transport through a selection of thin Pd-alloy membranes: membrane stability, H2S inhibition, and flux recovery in hydrogen and simulated WGS mixtures. Catal Today 2012;193(1):8–19.
[97]

Robeson LM. Correlation of separation factor versus permeability for polymeric membranes. J Membr Sci 1991;62(2):165–85.
[98]

Robeson LM. The upper bound revisited. J Membr Sci 2008;320(1– 2):390–400.
[99]

Kamakoti P, Morreale BD, Ciocco MV, Howard BH, Killmeyer RP, Cugini AV, et al. Prediction of hydrogen flux through sulfur-tolerant binary alloy membranes. Science 2005;307(5709):569–73.
[100]

Yun S, Ted OS. Correlations in palladium membranes for hydrogen separation: a review. J Membr Sci 2011;375(1–2):28–45.
[101]

Shafie AH, An W, Hejazi SAH, Sawada JA, Kuznicki SM. Natural zeolite-based cement composite membranes for H2/CO2 separation. Sep Purif Technol 2012;88:24–8.
[102]

Jiang DE, Cooper VR, Dai S. Porous graphene as the ultimate membrane for gas separation. Nano Lett 2009;9(12):4019–24.
[103]

Huang A, Dou W, Caro J. Steam-stable zeolitic imidazolate framework ZIF-90 membrane with hydrogen selectivity through covalent functionalization. J Am Chem Soc 2010;132(44):15562–4.
[104]

Zhang F, Zou X, Gao X, Fan S, Sun F, Ren H, et al. Hydrogen selective NH2-MIL53 (Al) MOF membranes with high permeability. Adv Funct Mater 2012;22 (17):3583–90.
[105]

Esposito E, Mazzei I, Monteleone M, Fuoco A, Carta M, McKeown N, et al. Highly permeable Matrimid/PIM-EA (H2)-TB blend membrane for gas separation. Polymers 2019;11(1):46.
[106]

McKeown NB, Budd PM. Polymers of intrinsic microporosity (PIMs): organic materials for membrane separations, heterogeneous catalysis and hydrogen storage. Chem Soc Rev 2006;35(8):675–83.
[107]

Li FY, Xiao Y, Chung TS, Kawi S. High-performance thermally self-cross-linked polymer of intrinsic microporosity (PIM-1) membranes for energy development. Macromolecules 2012;45(3):1427–37.
[108]

Kim S, Lee YM. Rigid and microporous polymers for gas separation membranes. Prog Polym Sci 2015;43:1–32.
[109]

Zornoza B, Téllez C, Coronas J, Esekhile O, Koros WJ. Mixed matrix membranes based on 6FDA polyimide with silica and zeolite microsphere dispersed phases. AIChE J 2015;61(12):4481–90.
[110]

Safak Boroglu M, Yumru AB. Gas separation performance of 6FDA-DAM-ZIF11 mixed-matrix membranes for H2/CH4 and CO2/CH4 separation. Sep Purif Technol 2017;173:269–79.
[111]

Kim E, Kim H, Kim D, Kim J, Lee P. Preparation of mixed matrix membranes containing ZIF-8 and UiO-66 for multicomponent light gas separation. Crystals 2019;9(1):15.
[112]

Weng TH, Tseng HH, Wey MY. Preparation and characterization of multiwalled carbon nanotube/PBNPI nanocomposite membrane for H2/CH4 separation. Int J Hydrogen Energy 2009;34(20):8707–15.
[113]

Sanders DF, Smith ZP, Guo R, Robeson LM, McGrath JE, Paul DR, et al. Energyefficient polymeric gas separation membranes for a sustainable future: a review. Polymer 2013;54(18):4729–61.
[114]

Stern SA, Mi Y, Yamamoto H, Clair AKS. Structure/permeability relationships of polyimide membranes. Applications to the separation of gas mixtures. J Polym Sci Pol Phys 1989;27(9):1887–909.
[115]

Hayes RA, inventor; E. I. Du Pont de Nemours and Company, assignee. Polyimide gas separation membranes. United States patent US4705540. 1987 Nov 10.
[116]

Min KE, Paul DR. Effect of tacticity on permeation properties of poly(methyl methacrylate). J Polym Sci Pol Phys 1988;26(5):1021–33.
[117]

Macchione M, Jansen JC, De Luca G, Tocci E, Longeri M, Drioli E. Experimental analysis and simulation of the gas transport in dense Hyflon AD60X membranes: influence of residual solvent. Polymer 2007;48(9):2619–35.
[118]

Takada K, Matsuya H, Masuda T, Higashimura T. Gas permeability of polyacetylenes carrying substituents. J Appl Polym Sci 1985;30 (4):1605–16.
[119]

Toy LG, Nagai K, Freeman BD, Pinnau I, He Z, Masuda T, et al. Pure-gas and vapor permeation and sorption properties of poly[1-phenyl-2-[p- (trimethylsilyl)phenyl]acetylene] (PTMSDPA). Macromolecules 2000;33 (7):2516–24.
[120]

Nagai K, Higuchi A, Nakagawa T. Gas permeability and stability of poly(1- trimethylsilyl-1-propyne-co-1-phenyl-1-propyne) membranes. J Polym Sci Pol Phys 1995;33(2):289–98.
[121]

Yang L, Fang J, Meichin N, Tanaka K, Kita H, Okamoto K. Gas permeation properties of thianthrene-5,5,10,10-tetraoxide-containing polyimides. Polymer 2001;42(5):2021–9.
[122]

Tanaka K, Okano M, Toshino H, Kita H, Okamoto KI. Effect of methyl substituents on permeability and permselectivity of gases in polyimides prepared from methyl-substituted phenylenediamines. J Polym Sci Pol Phys 1992;30(8):907–14.
[123]

Tanaka K, Islam MN, Kido M, Kita H, Okamoto KI. Gas permeation and separation properties of sulfonated polyimide membranes. Polymer 2006;47 (12):4370–7.
[124]

Pinnau I, Toy LG. Gas and vapor transport properties of amorphous perfluorinated copolymer membranes based on 2,2-bistrifluoromethyl-4,5- difluoro-1,3-dioxole/tetrafluoroethylene. J Membr Sci 1996;109(1):125–33.
[125]

Koresh JE, Soffer A. The carbon molecular sieve membranes: general properties and the permeability of CH4/H2 mixture. Sep Purif Technol 1987;22:973–82.
[126]

Hong M, Li S, Falconer JL, Noble RD. Hydrogen purification using a SAPO-34 membrane. J Membr Sci 2008;307(2):277–83.
[127]

Cacho-Bailo F, Seoane B, Téllez C, Coronas J. ZIF-8 continuous membrane on porous polysulfone for hydrogen separation. J Membr Sci 2014;464:119–26.
[128]

Basile A, Dalena F, Tong J, Veziroglu TN, editors. Hydrogen production, separation and purification for energy. London: The Institution of Engineering and Technology; 2017.
[129]

Yang RT. Adsorbents: fundamentals and applications. Hoboken: John Wiley & Sons, Inc.; 2003.
[130]

Sircar S. Production of hydrogen and ammonia synthesis gas by pressure swing adsorption. Sep Sci Technol 1990;25:1087–99.
[131]

García-Pérez E, Gascón J, Morales-Flórez V, Castillo JM, Kapteijn F, Calero S. Identification of adsorption sites in Cu-BTC by experimentation and molecular simulation. Langmuir 2009;25(3):1725–31.
[132]

Liu J, Keskin S, Sholl DS, Johnson JK. Molecular simulations and theoretical predictions for adsorption and diffusion of CH4/H2 and CO2/CH4 mixtures in ZIFs. J Phys Chem C 2011;115(25):12560–6.
[133]

Guo HC, Shi F, Ma ZF, Liu XQ. Molecular simulation for adsorption and separation of CH4/H2 in zeolitic imidazolate frameworks. J Phys Chem C 2010;114(49):21891.
[134]

Liu D, Lin YS, Li Z, Xi H. Adsorption and separation of CH4/H2 in MIL-101s by molecular simulation study. Chem Eng Sci 2013;98:246–54.
[135]

Liu Y, Xiao S, Bai P, Hu H, Jin L. Adsorption separation performance of H2/CH4 on ETS-4 by concentration pulse chromatography. J Energy Chem 2014;23 (2):213–20.
[136]

Casati C, Longhi P, Zanderighi L, Bianchi F. Some fundamental aspects in electrochemical hydrogen purification/compression. J Power Sources 2008;180(1):103–13.
[137]

Iwahara H. Hydrogen pumps using proton-conducting ceramics and their applications. Solid State Ionics 1999;125(1–4):271–8.
[138]

Wagner KK, Tiwari P, Swiegers GF, Wallace GG. An electrochemical cell with Gortex-based electrodes capable of extracting pure hydrogen from highly dilute hydrogen–methane mixtures. Energy Environ Sci 2018;11(1):172–84.
[139]

Abdulla A, Laney K, Padilla M, Sundaresan S, Benziger J. Efficiency of hydrogen recovery from reformate with a polymer electrolyte hydrogen pump. AIChE J 2011;57(7):1767–79.
[140]

Palazzo DF, Schreiner WC, inventors; Kellogg M W Co, assignee. Separation of hydrogen and methane. United States patent US3026683. 1962 Mar 27.
[141]

Kim J, Maiti A, Lin LC, Stolaroff JK, Smit B, Aines RD. New materials for methane capture from dilute and medium-concentration sources. Nat Commun 2013;4:1694–701.
[142]

Klamt A. COSMO-RS: from quantum chemistry to fluid phase thermodynamics. In: Friedl A, Klemeš JJ, Radl S, Varbanov PS, Wallek T, editors. Computer aided chemical engineering. Amsterdam: Elsevier; 2018. p. 9.
[143]

Klamt A. Conductor-like screening model for real solvents: a new approach to the quantitative calculation of solvation phenomena. J Phys Chem 1995;99 (7):2224–35.
[144]

Suzuki K, Sue H, Itou M, Smith RL, Inomata H, Arai K, et al. Isothermal vapor– liquid equilibrium data for binary systems at high pressures: carbon dioxide– methanol, carbon dioxide–ethanol, carbon dioxide–1-propanol, methane– ethanol, methane–1-propanol, ethane–ethanol, and ethane–1-propanol systems. J Chem Eng Data 1990;35(1):63–6.
[145]

Ukai T, Kodama D, Miyazaki J, Kato M. Solubility of methane in alcohols and saturated density at 280.15 K. J Chem Eng Data 2002;47(5):1320–3.
[146]

Trinh TKH, de Hemptinne JC, Lugo R, Ferrando N, Passarello JP. Hydrogen solubility in hydrocarbon and oxygenated organic compounds. J Chem Eng Data 2016;61(1):19–34.
[147]

Ferrando N, Lachet V, Teuler JM, Boutin A. Transferable force field for alcohols and polyalcohols. J Phys Chem B 2009;113(17):5985–95.
[148]

Ferrando N, Lachet V, Boutin A. Transferable force field for carboxylate esters: application to fatty acid methylic ester phase equilibria prediction. J Phys Chem B 2012;116(10):3239–48.
[149]

Ferrando N, Lachet V, Boutin A. Monte Carlo simulations of mixtures involving ketones and aldehydes by a direct bubble pressure calculation. J Phys Chem B 2010;114(26):8680–8.
[150]

Ferrando N, Gedik I, Lachet V, Pigeon L, Lugo R. Prediction of phase equilibrium and hydration free energy of carboxylic acids by Monte Carlo simulations. J Phys Chem B 2013;117(23):7123–32.
[151]

Ferrando N, Lachet V, Pérez-Pellitero J, MacKie AD, Malfreyt P, Boutin A. A transferable force field to predict phase equilibria and surface tension of ethers and glycol ethers. J Phys Chem B 2011;115(36):10654–64.
[152]

Rufford TE, Smart S, Watson GCY, Graham BF, Boxall J, Diniz da Costa JC, et al. The removal of CO2 and N2 from natural gas: a review of conventional and emerging process technologies. J Pet Sci Eng 2012;94–95:123–54.
[153]

Grashoff GJ, Pilkington CE, Corti CW. Purification of hydrogen. Platin Met Rev 1983;27(4):157–69.
[154]

Rufford TE, Chan KI, Huang SH, May EF. A review of conventional and emerging process technologies for the recovery of helium from natural gas. Adsorpt Sci Technol 2014;32(1):49–72.
[155]

Zhang F, Zhao P, Niu M, Maddy J. The survey of key technologies in hydrogen energy storage. Int J Hydrogen Energy 2016;41(33):14535–52.
[156]

Eslamimanesh A, Mohammadi AH, Richon D, Naidoo P, Ramjugernath D. Application of gas hydrate formation in separation processes: a review of experimental studies. J Chem Thermodyn 2012;46:62–71.
[157]

Aceves SM, Petitpas G, Espinosa-Loza F, Matthews MJ, Ledesma-Orozco E. Safe, long range, inexpensive and rapidly refuelable hydrogen vehicles with cryogenic pressure vessels. Int J Hydrogen Energy 2013;38(5):2480–9.
[158]

Hopkins JA, Dimartino SP, Nicholas DM, inventors; Air Products and Chemicals, Inc., assignee. Hybrid membrane/cryogenic process for hydrogen purification. United States patent US4654047. 1987 Mar 31.
[159]

Bartolozzi I, Rizzi F, Frey M. Comparison between hydrogen and electric vehicles by life cycle assessment: a case study in Tuscany, Italy. Appl Energy 2013;101:103–11.
[160]

Granovskii M, Dincer I, Rosen MA. Life cycle assessment of hydrogen fuel cell and gasoline vehicles. Int J Hydrogen Energy 2006;31(3):337–52.
[161]

Granovskii M, Dincer I, Rosen MA. Economic and environmental comparison of conventional, hybrid, electric and hydrogen fuel cell vehicles. J Power Sources 2006;159(2):1186–93.
[162]

Hacatoglu K, Rosen MA, Dincer I. Comparative life cycle assessment of hydrogen and other selected fuels. Int J Hydrogen Energy 2012;37 (13):9933–40.
[163]

Suleman F, Dincer I, Agelin-Chaab M. Environmental impact assessment and comparison of some hydrogen production options. Int J Hydrogen Energy 2015;40(21):6976–87.
[164]

International Energy Agency. The future of hydrogen: seizing today’s opportunities Technical report. Paris: International Energy Agency; 2019.
[165]

Yun S, Qin Y, Uhl AR, Vlachopoulos N, Yin M, Li D, et al. New-generation integrated devices based on dye-sensitized and perovskite solar cells. Energy Environ Sci 2018;11(3):476–526.
[166]

Zhang H, An P, Zhou W, Guan BY, Zhang P, Dong J, et al. Dynamic traction of lattice-confined platinum atoms into mesoporous carbon matrix for hydrogen evolution reaction. Sci Adv 2018;4(1):1–9.
[167]

Abdulrahman MW, inventor; Abdulrahman MW, assignee. Direct contact heat transfer in the thermolysis reactor of hydrogen production Cu–Cl cycle. United States patent US20170015552. 2017 Jan 19.
[168]

Marques JGO, Costa AL, Pereira C. Na–O–H thermochemical water splitting cycle: a new approach in hydrogen production based on sodium cooled fast reactor. Int J Hydrogen Energy 2018;43(16):7738–53.
[169]

Wajda T, Gabriel K. Thermolysis reactor scale-up for pilot scale Cu-Cl hybrid hydrogen production. Int J Hydrogen Energy 2019;44(20):9779–90.
[170]

Goldie-Scot L. A behind the scenes take on lithium-ion battery prices. London: BloomburgNEF; 2019 Mar 5 [cited 2020 Mar 26]. Available from: https://about.bnef.com/blog/behind-scenes-take-lithium-ion-battery-prices/.

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