Improved Oxygen Evolution Kinetics and Surface States Passivation of Ni-Bi Co-Catalyst for a Hematite Photoanode

Ke Dang; Tuo Wang; Chengcheng Li; Jijie Zhang; Shanshan Liu; Jinlong Gong

doi:10.1016/J.ENG.2017.03.005

Engineering ›› 2017, Vol. 3 ›› Issue (3) :285 -289. DOI: 10.1016/J.ENG.2017.03.005

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Improved Oxygen Evolution Kinetics and Surface States Passivation of Ni-Bi Co-Catalyst for a Hematite Photoanode

Ke Dang ^a^,^b
, Tuo Wang ^a^,^b
, Chengcheng Li ^a^,^b
, Jijie Zhang ^a^,^b
, Shanshan Liu ^a^,^b
, Jinlong Gong ^a^,^b

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Abstract

This paper describes the combinational surface kinetics enhancement and surface states passivation of nickel-borate (Ni-B_i) co-catalyst for a hematite (Fe₂O₃) photoanode. The Ni-B_i-modified Fe₂O₃ photoanode exhibits a cathodic onset potential shift of 230 mV and a 2.3-fold enhancement of the photocurrent at 1.23 V, versus the reversible hydrogen electrode (RHE). The borate (B_i) in the Ni-B_i film promotes the release of protons for the oxygen evolution reaction (OER).

Keywords

Nickel-borate / Hematite / Oxygen evolution reaction / Co-catalyst

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Ke Dang, Tuo Wang, Chengcheng Li, Jijie Zhang, Shanshan Liu, Jinlong Gong. Improved Oxygen Evolution Kinetics and Surface States Passivation of Ni-Bi Co-Catalyst for a Hematite Photoanode. Engineering, 2017, 3(3): 285-289 DOI:10.1016/J.ENG.2017.03.005

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Introduction

Since Fujishima and Honda’s seminal report on photoelectrochemical (PEC) hydrogen evolution [ 1], extensive research has been devoted toward various semiconductors that are capable of meeting the strict requirements for solar-to-fuel devices. Despite theoretically high efficiencies, most electrodes based on oxide semiconductors suffer from rapid charge recombination on the surface and/or in the bulk. Hematite (Fe₂O₃), with a suitable bandgap of 2 eV, for example, has low charge-separation efficiency and slow oxygen evolution kinetics.

To overcome such challenges, oxygen evolution catalysts (OECs) have often been coupled with oxide semiconductors to catalyze water oxidation [2], which is considered to be the rate-limiting step for both PEC water splitting and CO₂ reduction. In addition to the high-cost platinum group metal oxides, earth-abundant OECs (e.g., metal oxides and hydroxides) [3] have been investigated as a way to accelerate the surface reaction kinetics of semiconductors. In particular, cobalt-phosphate (Co-P_i) complexes have been demonstrated to enhance the oxygen evolution reaction (OER) performance of oxide semiconductors such as Fe₂O₃ [ 4] and WO₃ [ 5]. Nickel-borate (Ni-B_i) has also been demonstrated by Dincă et al. [ 6] to have electrochemical catalytic properties that are similar to those of Co-P_i. The formula of Ni-B_i has been suggested to be NiO(OH)_2/3(H₂BO₃)_1/3·1.5H₂O by elemental analysis [ 6]. A further dramatic increase in catalytic performance can be achieved by anodic activation of the electrodeposited Ni-B_i films [ 7]. It has been shown that the efficiency of proton and electron transfer for Ni-B_i OEC is the same as for IrO_x, and higher than for Ni(OH)₂ [ 6]. Thus, Ni-B_i is a very promising OEC for photoanodes. Ni-B_i has been demonstrated to successfully catalyze OER over a BiVO₄ photoanode, for which the photocurrent at 1.23 V versus the reversible hydrogen electrode (vs. RHE) is enhanced by a factor of 3.8 [8]. Gan et al. [ 9] reported that the loading of Ni-B_i on a BiVO₄ photoanode enables a 350 mV cathodic shift of onset potential for oxygen evolution at pH 9. However, the role of Ni-B_i as a co-catalyst is not yet fully understood in PEC applications.

This paper describes the coupling of Ni-B_i OEC with an Fe₂O₃ photoanode for solar oxygen evolution. By comparing the OER kinetics and electrode/electrolyte interface of Ni-B_i/Fe₂O₃ with those of bare Fe₂O₃, it is demonstrated that Ni-B_i is a dual-function co-catalyst that improves oxygen evolution kinetics and realizes surface states passivation simultaneously for an Fe₂O₃ photoanode. The specific role of B_i in Ni-B_i is further explored by comparing the catalytic performance of Ni-B_i and NiOOH using solutions with different proton concentrations.

Results and discussion

Nanostructured Fe₂O₃ photoanodes were synthesized via a hydrothermal approach [ 10], and the loading of the Ni-B_i co-catalyst was carried out using a modified electrodeposition technique [ 6] (experimental details are provided in Supplementary Information). The obtained Fe₂O₃ shows a one-dimensional nanorod structure in the field-emission scanning electron microscopy (SEM) image (Fig. 1(a)). The film thickness is 310±76 nm (see Fig. S1 in Supplementary Information). The Ni-B_i-modified photoanode shows a similar nanorod feature with a slightly smoother surface (Fig. 1(b)). The existence of the Ni-B_i layer was verified by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images of Ni-B_i/Fe₂O₃ (Figs. 1(c) and (d)). The Fe₂O₃ in the HRTEM image displays high crystallinity with a lattice spacing of 0.27 nm (Fig. 1(d)) corresponding to the (104) plane of crystallized Fe₂O₃. The selected area electron diffraction (SAED) pattern (Fig. 1(d) inset) shows the polycrystalline structure of the Fe₂O₃ nanorod. The Ni-B_i amorphous layer is relatively uniform, with a thickness of approximately 10 nm. The absence of additional X-ray diffraction patterns upon the loading of Ni-B_i (see Fig. S2 in Supplementary Information) also suggests the amorphous nature of Ni-B_i, which is consistent with previous research [ 6]. It is clear from the electron energy-loss spectroscopy mapping of the nanorod (see Fig. S3 in Supplementary Information) that the bulk of each nanorod is enriched with iron (Fe), while the outer layer is rich in nickel (Ni) and boron (B). X-ray photoelectron spectroscopy (XPS) characterizations (see Fig. S4 in Supplementary Information) also corroborate the Ni-B_i overlayer on the Fe₂O₃. The ratio of Ni to B was determined to be about 1:2 by employing inductively coupled plasma mass spectrometry after dissolving the entire film of Ni-B_i/Fe₂O₃. The ultraviolet (UV)/visible light-absorbance spectra (see Fig. S5 in Supplementary Information) indicate that the Ni-B_i overlayer has a negligible influence on the light absorption of the Fe₂O₃ photoanode.

With optimized Ni-B_i loading (see Fig. S6 in Supplementary Information), photocurrent density versus applied potential (J-V) measurements were performed for Fe₂O₃ and Ni-B_i/Fe₂O₃ photoanodes (Fig. 2(a)). The bare Fe₂O₃ photoanode exhibits the typical response of a hydrothermal-prepared Fe₂O₃ electrode, with a photocurrent onset potential of 0.85 V vs. RHE, and a photocurrent of 0.48 mA·cm⁻² at 1.23 V vs. RHE. Upon the loading of Ni-B_i, the onset of the photocurrent significantly shifts cathodically by 230 mV to 0.62 V vs. RHE. The cathodic onset potential of 0.62 V (vs. RHE) enables the use of Ni-B_i/Fe₂O₃ to construct overall water-splitting devices with suitable photocathodes, such as Cu₂O-based photocathodes (onset potential of 1.02 V vs. RHE) [ 11], CuBi₂O₄-based photocathodes (onset potential of ~1.0 V vs. RHE) [ 12], and Cu(In, Ga)Se-based photocathodes (onset potential of 0.65 V vs. RHE) [ 13]. In addition, the photocurrent increases substantially from 0.48 mA·cm⁻² to 1.12 mA·cm⁻² at 1.23 V vs. RHE. This performance is comparable to that of some of the best Fe₂O₃ photoanodes with OECs [ 10, 14– 16]. The improved incident-photo-to-current efficiency (IPCE) of Ni-B_i/Fe₂O₃ (Fig. 2(b)) is also consistent with the photocurrent. The applied-bias photo-to-current conversion efficiency (APCE) of the Ni-B_i/Fe₂O₃ photoanode is 0.16% at 0.97 V vs. RHE, which is 5.3 times greater than that of the bare Fe₂O₃ photoanode (0.03% at 1.03 V vs. RHE) (see Fig. S7 in Supplementary Information). Although an anodic activation process is necessary after the loading of Ni-B_i to enhance its activity (see experimental section in Supplementary Information), photocurrent enhancement due to the anodization of Fe₂O₃ itself was ruled out (see Fig. S8 in Supplementary Information). The Ni-B_i reduced the photogenerated holes that had accumulated on the Fe₂O₃ surface, as illustrated by the reduced transient spikes in the transient photocurrent measurements at 1.23 V vs. RHE under chopped illumination (see Fig. S9 in Supplementary Information). The stability of the Fe₂O₃ electrode was almost unchanged after the loading of the Ni-B_i co-catalyst (see Fig. S10 in Supplementary Information).

Electrochemical impedance spectroscopy (EIS) in Nyquist plots under conditions of darkness and illumination (Fig. 2(c) and (d), respectively) were obtained to further explore the enhancing mechanism of the Ni-B_i film. Fig. 2(c) indicates that the residual double-layer charge transfer resistance under darkness, which is characterized by the semidiameter of the semicircle, decreased significantly for Fe₂O₃ upon the deposition of the Ni-B_i overlayer, suggesting improved charge transfer across the electrode/electrolyte interface. Two clear semicircles are visible in Fig. 2(d) for the Fe₂O₃ and Ni-B_i/Fe₂O₃ photoanodes under illumination, indicating the existence of two capacitances [ 17]. A typical equivalent circuit model was used to fit the EIS data (Fig. 2(d) inset). The fitting results (see Table S1 in Supplementary Information) suggest that the series resistance (R_s) is almost unchanged after modification with Ni-B_i, which is reasonable because the interface between Fe₂O₃ and fluorine-doped tin oxide (FTO) is unchanged. A significant decrease in the charge transfer resistance at the electrode/electrolyte interface (R_ct) from 321.3 Ω to 166.9 Ω demonstrates that the photogenerated holes transfer into the electrolyte more easily than those on the bare Fe₂O₃ surface, due to the enhanced OER kinetics with Ni-B_i. This finding aligns with the EIS analysis under darkness. The enhanced OER kinetics with Ni-B_i is also evidenced by the Tafel slope (see Fig. S11 in Supplementary Information). The Tafel slope of the Ni-B_i/Fe₂O₃ electrode (~46 mV·decade⁻¹) is much smaller than that of the bare Fe₂O₃ electrode (~138 mV·decade⁻¹). The charge transfer resistance in the bulk electrode (R_trap) drops significantly, from 128.1 Ω to 90.2 Ω upon the loading of the Ni-B_i catalyst. The R_trap is relevant to the charge mobility in Fe₂O₃ (conductivity) and to the lifetime of the electron/hole (charge separation). Since the conductivity of the bulk Fe₂O₃ could not change upon catalyst loading, the enhanced charge transfer may be due to improved charge separation, suggesting that Ni-B_i may play another role in addition to its surface catalytic effect. This result led us to further explore the combined effects of the Ni-B_i co-catalyst on the PEC performance.

With regard to Fe₂O₃, the surface states and the pinning effects of the Fermi level have been investigated theoretically and experimentally [ 18, 19]. In addition to passivation layers of, for example, Al₂O₃[ 20], Du et al. have demonstrated that a surface catalytic NiFeO_x layer can passivate surface states and minimize or eliminate the potential drop within the Helmholtz layer, resulting in a much higher photovoltage [ 21]. A higher photovoltage suggests an improvement in the thermodynamics of the system upon catalyst loading. Thus, the cathodic shift of the onset potential of an Fe₂O₃ electrode may be attributed to two aspects: kinetics and thermodynamics.

In order to identify the thermodynamics-enhancing effects of Ni-B_i, the steady-state open-circuit potential (OCP) was measured with and without illumination (Fig. 3(a)) to minimize the influence of kinetics factors. The difference in OCPs of the bare Fe₂O₃ electrode under darkness and illumination indicates a photovoltage of 0.21 V. A large photovoltage of 0.72 V is obtained upon the loading of the Ni-B_i catalyst, due to the significantly more positive OCP in the dark. The OCP in the dark usually reflects the extent of upward band bending of the photoanode in equilibrium with the electrolyte. The significantly more positive OCP (1.45 V vs. RHE) that was obtained with Ni-B_i in the dark suggests a more pronounced upward band bending, which is likely due to the fact that the surface states of Fe₂O₃ are passivated to unpin the Fermi level. Therefore, the improved photovoltage of Ni-B_i/Fe₂O₃ compared with Fe₂O₃ could be attributed to passivation of the surface states. In addition, compared with the bare Fe₂O₃ sample, the Ni-B_i-loaded sample exhibits a shift in the cathodic direction of 80 mV of effective flat-band potential (E_FB), as indicated by the Mott-Schottky plots of Fe₂O₃ and Ni-B_i/Fe₂O₃ (Fig. 3(b)). This cathodic shift also indicates a more pronounced band bending upon the loading of Ni-B_i, due to the passivation of surface states by the Ni-B_i overlayer that alleviates the Fermi-level pinning [ 15]. The photo-oxidation of an H₂O₂ scavenger was further studied (see Fig. S12 in Supplementary Information). The comparable onset potential of dark currents of Fe₂O₃ and Ni-B_i/Fe₂O₃ (see Fig. S12(a) in Supplementary Information) suggests equally fast H₂O₂ oxidation kinetics on the surface of Fe₂O₃ and Ni-B_i, where the surface catalytic effect of Ni-B_i can be excluded. It has been demonstrated that the photocurrent onset potential for a sacrificial agent with fast oxidation kinetics should be very close to E_FB[ 22]. Thus, the cathodic shift of the photocurrent onset of H₂O₂ oxidation (see Fig. S12(b) in Supplementary Information), which is consistent with our E_FB results, can be attributed to the enhanced thermodynamics factors owing to the passivated surface upon the loading of Ni-B_i[ 23, 24]. The slight increase of the photocurrent (see Fig. S12(b) in Supplementary Information) is the result of reduced charge transfer resistance in the bulk electrode, which is consistent with the reduced R_trap from EIS.

To further verify the surface states passivation of the Ni-B_i layer, Photoluminescence (PL) tests were conducted. Whereas the recombination between the valance band and the conduction band is radiative, the recombination at the surface state is non-radiative [ 25]. Since the numbers of photogenerated electrons and holes are the same in the Fe₂O₃ photoanode under the same light stimulation, an increase in PL intensity, which indicates increased recombination between the valence and the conduction bands, may be the result of passivated surface states, as reported previously [ 26, 27]. Thus, the strengthened PL intensity of Ni-B_i/Fe₂O₃ compared with Fe₂O₃ (Fig. 3(c)) suggests that the Ni-B_i layer is able to passivate the surface states of Fe₂O₃.

The specific role of B_i in Ni-B_i was also investigated. In the process of water oxidation via a Ni-B_i co-catalyst, it has been suggested that Ni²⁺/Ni³⁺/Ni⁴⁺ are the active sites for oxygen evolution from water [ 27]. Ye et al. demonstrated that in NiOOH for a BiVO₄ photoanode, Ni²⁺/Ni³⁺/Ni⁴⁺ are the active sites for hole transfer from BiVO₄ to the electrolyte and water oxidation [ 28]. Our XPS results (see Fig. S4(b) in Supplementary Information) also reveal the existence of Ni²⁺/Ni³⁺ states in our Ni-B_i OEC. Thus, we propose that Ni²⁺/Ni³⁺ sites are the main active sites for water oxidation in Ni-B_i. On the other hand, Trześniewski et al. [ 29] demonstrated the importance of the deprotonation of NiOOH for OER. Because B_i is a proton-accepting electrolyte, it is speculated that the B_i may facilitate the release of protons from water, and thus enhance OER in this study.

To verify this assumption, we compared the influence of protons on our Ni-based co-catalysts, with and without B_i (Ni-B_i and NiOOH), in different proton concentrations. NiOOH was loaded on a freshly prepared Fe₂O₃ electrode using a photo-assistant electrodeposition method [ 17]. To obtain different proton concentrations, we prepared 0.1 mol·L⁻¹ KB_i aqueous electrolytes buffered to different pHs (pH 8–12), in which the overpotentials for OER on Ni-B_i/Fe₂O₃, NiOOH/Fe₂O₃, and bare Fe₂O₃ electrodes were measured (Fig. 3(d)). A higher pH of the electrolyte reflects a lower proton concentration, which is favorable to promote the release of protons from water molecules. It is generally considered that the OER in an alkaline solution is the oxidation of OH^– after the release of a proton from a water molecule [ 30, 31]. Thus, a decreasing overpotential of OER was obtained for Fe₂O₃ and NiOOH/Fe₂O₃ electrodes with higher pH (Fig. 3(d)). In comparison, Ni-B_i/Fe₂O₃ exhibits much lower overpotentials for water oxidation over the entire tested pH range, which is owing to the catalytic effect of Ni-B_i. More importantly, the linear decreasing dependency on pH is absent in the case of Ni-B_i; in this case, the electrolytes with lower pH (i.e., higher proton concentrations) also exhibit lower overpotentials. Although the release of protons tends to occur less in electrolytes with higher proton concentrations, B_i would facilitate the release of protons from water molecules in the electrolyte [ 8]. It has also been demonstrated in Dincă’s work that Ni-B_i loses one electron accompanied by the transfer of ~1.5 protons, compared with the one-electron-one-proton oxidation of water for NiOOH in neutral electrolyte [ 6]. Since the morphology of NiOOH and of Ni-B_i are almost the same (see Fig. S13 in Supplementary Information) [ 17], a different overpotential for the OER caused by a different surface area can be ruled out. Thus, the B_i favors the release of protons for oxygen evolution in electrolytes with high proton concentration. In addition, when the pH is higher than 9–9.5, the oxidation overpotentials of Ni-B_i/Fe₂O₃ decrease again. This phenomenon could be attributed to the limited performance of B_i for proton release. In neutral electrolytes, the OER mechanism has been suggested to be a proton-coupled electron-transfer process [ 32], or a direct oxidation of water molecules followed by the release of protons [ 33]. In these cases, B_i would also be able to promote the transfer of protons for improved OER kinetics.

Fig. 4 illustrates the kinetics and thermodynamics dual function of a Ni-B_i co-catalyst. In a bare Fe₂O₃ electrode, a hole may be trapped in surface states and then slowly recombine with an electron instead of oxidizing water. The surface state pins the Fermi level and results in lowered band bending. In Ni-B_i-decorated Fe₂O₃, the enhanced water oxidation can be attributed to both kinetics and thermodynamics factors. The surface states passivation via Ni-B_i leads to the unpinning of the Fermi level and to a more profound upward band bending, improving the charge separation in Fe₂O₃. In addition to surface states passivation, Ni-B_i kinetically promotes OER. The B_i in the Ni-B_i is proven to promote the release of protons, thus enhancing OER on a Ni-B_i/Fe₂O₃ photoanode.

Conclusion

In summary, we have demonstrated that Ni-B_i co-catalyst can be coupled with an Fe₂O₃ photoanode to enhance oxygen evolution. Upon the loading of Ni-B_i, the onset of the photocurrent shifts cathodically by 230 mV to 0.62 V vs. RHE, and the photocurrent increases from 0.48 mA·cm^-2 to 1.12 mA·cm^-2 at 1.23 V vs. RHE. The role of Ni-B_i can be attributed to two aspects: kinetics and thermodynamics. The enhanced OER kinetics are evidenced by the significant drop of charge transfer resistance at the electrode/electrolyte interface, as well as by the decreased Tafel slope upon the loading of Ni-B_i. The passivation of surface states is verified by a much larger photovoltage and by the cathodic shift of the effective flat-band potential, which is due to a more pronounced upward band bending with alleviated Fermi-level pinning. PL intensity also reveals the passivation of surface states. In the process of water oxidation, the B_i in the Ni-B_i film promotes the release of protons from water molecules to improve OER.

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2017 THE AUTHORS. Published by Elsevier LTD on behalf of the Chinese Academy of Engineering and Higher Education Press Limited Company. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).