1. Electrochemical engineering: From heat-driven to electron-driven chemistry
The application of chemical engineering to produce fuels and chemicals through heat-driven reactions under high temperatures and pressures has long been a major driving force of modern industrial civilization [
1]. Although traditional chemical engineering has supported numerous large-scale industrial processes for more than a century, its strong dependency on fossil fuels has resulted in severe energy and environmental challenges associated with greenhouse-gas emissions, air pollution, and climate change. As the global demand for sustainable development continues to increase, alternative routes for chemical engineering processes are highly desirable.
Electrochemical engineering provides a promising alternative to traditional chemical engineering by replacing thermal energy with electricity to drive chemical/energy transformations [
2]. Unlike traditional thermochemical systems (
Fig. 1), electrochemical processes often involve electron transfer at electrode interfaces under ambient or near-ambient conditions. When powered by renewable electricity from solar or wind sources, electrochemical engineering enables direct energy-chemical conversions to produce value-added chemicals with minimal carbon emissions. Thus, the rapid expansion of renewable electricity generation is accelerating the transition from heat-driven chemistry to electron-driven chemistry, thereby significantly promoting the development and implementation of electrochemical technologies.
2. Electrochemical engineering for the resurgence of the H2 economy: Critical roles of carbon-based metal-free electrocatalysts (C-MFECs)
The hydrogen (H
2) economy, based on renewable-electricity-driven H
2 production from water, along with H
2 storage and transportation for energy-chemical conversions, is widely regarded as one of the most promising pathways toward global carbon neutrality. Driven by the rapid reduction in renewable electricity cost, increasing demand for decarbonization, and substantial advances in electrolyzers and fuel cells, green H
2 is evolving from a conceptual energy carrier or chemical precursor into an emerging industrial reality. Gigawatt-scale H
2 projects are now being deployed worldwide, signaling the arrival of a new era for a sustainable H
2 economy [
3]. Electrochemical engineering plays a pivotal role in this shift by enabling direct energy-chemical conversion under mild operating conditions [
2], [
3], thereby allowing efficient and sustainable energy and chemical transformations for the H
2 economy. These conversions include—but are not limited to—water electrolysis for H
2 production, nitrogen (N
2) and nitrate (NO
3−) reduction, hydrogen peroxide (H
2O
2) production, carbon dioxide (CO
2) reduction, urea synthesis, alcohol synthesis from H
2 and CO
2, and methane conversion (
Fig. 2). Such processes not only enable renewable energy storage in chemical bonds but also generate value-added products such as ammonia (NH
3), H
2O
2, alcohols, and urea. Moreover, these value-added chemicals can function as hydrogen carriers or carbon-neutral fuels for easy storage and transportation, thereby broadening the technological scope of the H
2 economy. For H
2 utilization, fuel cells remain among the most efficient technologies for converting H
2 into electricity with high energy-conversion efficiency and near-zero pollutant emissions [
4], [
5].
Catalysts are the key to these conversion processes. Unlike critical minerals or conventional noble-metal catalysts (
Fig. 3), C-MFECs are earth-abundant and can be engineered through heteroatom-/defect-doping with multidimensional porous architectures [
4]. Through heteroatom doping (e.g., with nitrogen (N), boron (B), sulfur (S), phosphorus (P), and/or fluorine (F)), defect engineering, and electronic structure modulation, C-MFECs can be tailored to exhibit remarkable electrocatalytic performance for key electrochemical reactions associated with H
2 production and utilization. The design strategy fundamentally relies on induced charge transfer and spin redistribution. Accordingly, a variety of synthetic routes, including bottom-up methods (e.g., chemical vapor deposition) and post-synthetic treatments (e.g., plasma treatment, ball milling, and thermal annealing), have been developed to fabricate C-MFECs with tunable structures and functionalities. In addition to their advantages in cost and scalability, C-MFECs possess excellent electrical conductivity, controllable surface chemistry, electronic structure, and three-dimensional architectures, making them highly attractive as multifunctional electrocatalysts for next-generation electrochemical systems and the H
2 economy. Nevertheless, carbon materials may experience some durability loss during prolonged operations under harsh electrochemical conditions. In particular, C-MFECs are susceptible to accelerated carbon corrosion, oxidative degradation, and/or protonation, especially in acidic media. This issue can be effectively mitigated through rational structural-engineering strategies, such as increasing the degree of graphitization via high-temperature thermal treatment (typically above 900 °C) and/or employing stable heteroatom and defect engineering approaches, including edge-selective F-doping to reduce the overpotential and hence enhance the stability.
2.1. Electrochemical engineering for chemical conversion with C-MFECs
The combination of advanced C-MFECs and scalable electrochemical engineering technologies could provide low-cost, durable, and ecofriendly pathways for various chemical conversions. At present, electrochemical engineering with C-MFECs enables a broad range of important chemical transformations. It is envisioned that C-MFECs can play increasingly important roles in scalable and sustainable chemical-conversion technologies for the H
2 economy (
Fig. 4), including H
2 production from water/seawater electrolysis [
6], [
7] and electrochemical alcohol synthesis from H
2 and CO
2 [
4]. Electrochemical NH
3 synthesis has emerged as a promising alternative to the conventional Haber-Bosch process [
4], which is highly energy-intensive and heavily dependent on fossil fuels. The direct electrochemical reduction of N
2 under ambient conditions offers an attractive pathway for decentralized and sustainable NH
3 production powered by renewable electricity. Recent studies have shown that defective C-MFECs can effectively activate the chemically inert N≡N bond and significantly improve catalytic performance. Notably, F-doped porous carbon exhibited a high Faradaic efficiency of 54.8% and an NH
3 yield rate of 197.7 μg of NH
3 per milligram of catalyst per hour at −0.3 V (vs reversible hydrogen electrode (RHE)) [
8]. To date, the selectivity of electrochemical NH
3 synthesis remains far below the level required for practical applications. However, regulating the local electronic structure of C-MFECs could be an effective approach to promote N
2 adsorption/activation and suppress competing H
2 evolution reaction, consequently improving NH
3 production efficiency and selectivity. It has also been found that C-MFECs are effective for the catalytic two-electron oxygen reduction reaction (ORR) for H
2O
2 production [
9], the electrochemical reduction of CO
2 into value-added products [
10], and electrochemical urea synthesis [
11]. Nevertheless, a major challenge is to maintain high selectivity at industrially relevant current densities while preventing electrode flooding.
2.2. Electrochemical engineering for energy conversion and storage with C-MFECs
The significance of C-MFECs extends beyond the chemical conversions described above. For instance, fuel cells have attracted significant attention for electric-vehicle applications because they can directly convert chemical energy into electricity with high efficiency and low emissions (
Fig. 5(a)). However, the widespread commercialization of fuel cells is still limited by the high cost and insufficient durability of noble-metal catalysts, especially platinum (Pt)-based cathodes for the ORR. Through the rational control of electrode structure, pore distribution, conductivity, and catalyst-electrolyte interfaces, the intrinsic activity of C-MFECs can be effectively translated into practical device performance. For example, hierarchically porous C-MFECs with a graphitic-N regulated defect structure have been demonstrated to show exceptional performance in anion-exchange membrane fuel cells (AEMFCs), with a peak power density as high as 913 mW·cm
−2 [
12], which approaches the performance of certain Pt-based systems, and an excellent stability with only about 25% voltage decay after 100 h at 250 mA·cm
−2. Moreover, zinc (Zn)-air batteries are considered promising next-generation energy-storage systems because of their high theoretical energy density, low cost, and intrinsic safety (
Fig. 5(b)). Zhang et al. [
13] were the first to demonstrate an N, P-codoped carbon foam as an efficient bifunctional C-MFEC for both primary and rechargeable Zn-air batteries. The foam exhibited an open-circuit voltage of 1.48 V, a high specific capacity of 735 mA·h per gram of Zn, and a peak power density of 55 mW·cm
−2. The fabricated rechargeable Zn-air batteries also showed remarkable cycling stability, operating steadily for 180 charge-discharge cycles. Lithium (Li)-CO
2 batteries are also attracting significant attention because they simultaneously store energy and utilize CO
2, thereby offering a potential pathway toward carbon-neutral energy systems and advanced energy technologies for deep-space exploration in locations such as Mars, where the atmosphere is >95% CO
2 (
Fig. 5(c)). Nevertheless, the electrochemical reactions in Li-CO
2 batteries are intrinsically complex and often suffer from high overpotentials and poor cycling stability. C-MFECs can significantly improve the reaction kinetics by regulating electron transfer and stabilizing reaction intermediates [
14].
3. Challenges and perspectives
Electrochemical engineering is increasingly recognized as a key enabling discipline for the emerging H2 economy, which seeks to establish H2 as a clean energy carrier and chemical precursor for sustainable industrial systems. Unlike conventional thermochemical engineering processes, electrochemical engineering enables direct energy-chemical conversion through interfacial electron-transfer reactions. This transition from thermochemical to electrochemical engineering represents not only a technological advancement but also a fundamental shift in the production of energy, fuels, and chemicals, allowing renewable electricity from solar and wind sources to be utilized with minimal carbon emissions. As the share of renewable energy continues to grow, electrochemical technologies, such as electrolyzers, fuel cells, and electrochemical reactors, are expected to play a central role in energy conversion/storage, grid stabilization, and carbon-neutral chemical production, further advancing the development and application of electrochemical engineering.
Compared with traditional chemical engineering, electrochemical engineering offers several strategic advantages, including lower operating temperatures and pressures, higher reaction selectivity, modular and distributed reactor configurations, and direct compatibility with intermittent renewable electricity. Beyond H
2 production, electrochemical platforms are increasingly being explored for chemical conversion and integrated energy-conversion/storage systems, positioning the field at the intersection of energy, materials, and environmental engineering. Nevertheless, significant challenges remain, including catalyst degradation, charge-transfer resistance, ion-transport limitations, reactor scale-up, and overall system efficiency. Addressing these challenges requires advances in electrode materials, reactor architecture, multiphysics modeling, and large-scale process integration [
15].
While most electrochemical engineering processes involve expensive (noble) metal-based catalysts, the development of C-MFECs and related electrochemical systems further broadens the scope of electrochemical engineering by integrating advanced catalysts with electrochemical energy-chemical conversion technologies. C-MFECs represent an emerging frontier that combines renewable energy conversion and storage, circular resource recovery, and carbon neutralization and utilization technologies. Despite these advantages, there is still a lack of effective approaches to precisely identify and regulate the active sites of C-MFECs in order to maximize their catalytic activity. Achieving industrial-level current density, selectivity, and stability simultaneously remains challenging for many electrochemical reactions. Further advancement in C-MFECs should support the broader evolution of chemical engineering toward electrochemical engineering for energy-chemical conversions under ambient conditions and the sustainable development of the H2 economy.
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