Naoto Todoroki,Kensaku Nagasawa,Hayato Enjoji,Shigenori Mitsushima
Suppression of Catalyst Layer Detachment by Interfacial Microstructural Modulation of the NiCo2O4/Ni Oxygen Evolution Electrode for Renewable Energy-Powered Alkaline Water Electrolysis
alkaline water electrolysis,oxygen evolution reaction,catalyst layer detachment,power fluctuation,renewable energy,nickel cobalt oxide
Alkaline water electrolysis (AWE) is a large-scale hydrogen production technology. A major degradation mode of AWE when using fluctuating power derived from renewable energies is the detachment of the catalyst layer (CL). Here, this study investigates the CL detachment mechanism of NiCo2O4-CL-coated Ni (NCO/Ni) electrodes under an accelerated durability test (ADT) simulating a fluctuating power and the effect of post-annealing on detachment behavior. Microstructural analysis reveals that detachment begins at the nanoscale gaps between the stacked CLs and between CL and the substrate. Post-annealing at 400 °C removes the degradation starting point in CL, and a composition gradient Co-doped NiO interlayer and NiO(111)/Ni(111) epitaxial interface form between CL and the Ni substrate, nearly suppressing CL detachment. Although the electrode performance of the annealed sample is initially lower than that of the as-prepared sample, the overpotential is significantly reduced during ADT due to the formation of the NiCo hydroxide active surface layer. These results demonstrate that interfacial microstructural modulation by post-annealing is a powerful approach to realizing durable electrodes for green hydrogen production by renewable energy-powered AWE.
Suppression of Catalyst Layer Detachment by Interfacial Microstructural Modulation of the NiCo2O4/Ni Oxygen Evolution Electrode for Renewable Energy-Powered Alkaline Water Electrolysis

Alkaline water electrolysis (AWE) is a large-scale hydrogen production technology. A major degradation mode of AWE when using fluctuating power derived from renewable energies is the detachment of the catalyst layer (CL). Here, this study investigates the CL detachment mechanism of NiCo2O4-CL-coated Ni (NCO/Ni) electrodes under an accelerated durability test (ADT) simulating a fluctuating power and the effect of post-annealing on detachment behavior. Microstructural analysis reveals that detachment begins at the nanoscale gaps between the stacked CLs and between CL and the substrate. Post-annealing at 400 °C removes the degradation starting point in CL, and a composition gradient Co-doped NiO interlayer and NiO(111)/Ni(111) epitaxial interface form between CL and the Ni substrate, nearly suppressing CL detachment. Although the electrode performance of the annealed sample is initially lower than that of the as-prepared sample, the overpotential is significantly reduced during ADT due to the formation of the NiCo hydroxide active surface layer. These results demonstrate that interfacial microstructural modulation by post-annealing is a powerful approach to realizing durable electrodes for green hydrogen production by renewable energy-powered AWE.

Green hydrogen production by renewable energy-powered water electrolysis is a key technology for carbon neutrality. Generally, alkaline water electrolysis (AWE) is a large-scale hydrogen production technology due to its relatively low stack cost compared to that of a proton exchange membrane water electrolyzer (PEMWE). However, its insufficient energy efficiency makes green hydrogen production costs of AWE much higher than that of the conventional steam reforming method of natural gases. In particular, the slow kinetics of electrode reactions hinders the performance improvement of AWE. Consequently, active and durable catalysts have been intensively researched both for anodes and cathodes. Highly active nanostructured catalysts have been developed as anode catalysts for the oxygen evolution reaction (OER), represented by NiFe-layered double hydroxide (LDH). Additionally, NiCo2O4 (NCO) is a widely investigated OER catalyst for AWE due to its high activity, durability, and electronic conductivity. For practical use, the NCO catalyst layer (CL) is generally formed by the coating-pyrolysis method on a high-surface-area Ni electrode such as mesh due to its low manufacturing cost and high scalability. The performance of AWE electrodes is sensitive to power fluctuations when electricity is supplied by renewable energy. For example, the reverse current, which stems from the startup–shutdown conditions, degrades the AWE anode and cathode. Therefore, commercial AWEs powered by renewable energy usually require a protective current to maintain the electrode potential near the onset for the water electrolysis reaction (HER at the cathode and OER at the anode) when the electricity stops. However, because auxiliaries for the protective current increase the total AWE system’s cost, direct use of renewable energy with fluctuating power has been explored as a cost reduction measure for green hydrogen. The accelerated durability test (ADT) protocol, which reflects the practical conditions of renewable energy-powered AWE, is crucial to conduct a fair comparative study of various electrode materials. Recently, Haleem et al. proposed and applied a novel ADT protocol, which simulates the start and shutdown environments of AWE, to an NCO-CL-coated Ni-mesh electrode. The electrode performance drastically degraded due to CL detachment during ADT. Because CL detachment is a major degradation mode of various water electrolyzers, the detachment mechanism has been widely investigated. However, the CL detachment mechanism on AWE electrodes under power fluctuation conditions is unclear. Consequently, an effective strategy to suppress detachment has yet to be proposed. Since CL detachment should occur regardless of the type of material for the catalyst and current collector electrode, clarifying the mechanism and developing an effective strategy to inhibit CL detachment should improve the durability of renewable energy-powered AWE. In this study, we investigate the CL detachment mechanism under ADT simulating power fluctuation conditions using an NCO-CL-coated Ni plate electrode. Cross-sectional structural analysis for the CL-detached electrode after ADT reveals two origins for CL detachment: (1) the repeated stress cycles of electrolyte penetration into the nanoscale defects in the stacked NCO-CL and CL/Ni substrate interface and (2) oxygen bubble generation by electrolysis in the defects. Considering the above detachment behaviors, post-annealing on the NCO/Ni electrode almost completely suppresses CL detachment under severe ADT conditions. The microstructural interfacial modulation by post-annealing effectively diminishes the nanoscale defects in CL and the CL/substrate interface, improving structural stability against ADT.

The NCO-coated Ni plate (NCO/Ni) electrode was prepared by the thermal decomposition method. First, a blasted nickel plate (10 × 10 × 1.0 mm3) was etched in boiling 18.5 wt % hydrochloric acid for 12 min and rinsed with pure water. Second, the etched Ni plate was coated with the precursor solution composed of Ni(NO3)2·6H2O (>98.0%, Kanto Chemical), Co(NO3)2·6H2O (>98.0%, Kanto Chemical), and ultrapure water. The total metal concentration and molar ratio of Ni:Co in the precursor solution were fixed to 50 g/L and 1:2, respectively. Third, the plate was dried and subsequently annealed at 450 °C for 12 min to thermally decompose the precursor. The above process was repeated 12 times to prepare NCO-CL.

The as-prepared NCO/Ni electrode (as-NCO/Ni) was subjected to post-annealing. Annealing was conducted using a tube furnace (TF1 11/32/150, VERDER Scientific) in air at 100–700 °C for 24 h. The temperature rise and fall rates were set to 5 °C min–1 for all the annealing temperatures.

All electrochemical measurements were performed with a potentiostat/galvanostat (SP-50ez, Biologic). A polytetrafluoroethylene (PTFE)-made cell, Ni wire, and Hg/HgO (7 M KOH) were employed as the EC cell, counter electrode, and reference electrode, respectively. The KOH electrolyte was prepared using KOH pellets (ACS reagent, > 85%, Aldrich) and ultrapure water (Milli-Q). Prior to the electrochemical measurements, the geometrical surface area of the working electrode (NCO/Ni) was confined to 0.5 cm2 by insulating glue and the electrolyte was purged with N2 for at least 30 min. In this study, the electrochemical potentials were converted to RHE and calibrated by the experimentally estimated potential difference between the Hg/HgO electrode and RHE. The linear sweep voltammetry (LSV) curves for OER were measured in 7 M KOH at 20 °C with a potential-sweep rate of 1 mV s–1, and they were corrected using 90% iR compensation. The estimated solution resistance (Rs) was ca. 1.5 Ω from the ac impedance spectra collected at frequencies ranging from 10 mHz to 200 kHz at 1.5 V. This study defined the OER overpotential as the potential at which the current density reached 100 mA cm on the corresponding LSV curve. The Tafel slopes were estimated from Tafel plots constructed from the LSV curves based on the Tafel equation (eq 1).where η is the overpotential, i is the current density, i0 is the exchange current density, and b is the Tafel slope. The cyclic voltammograms were collected at a scan rate of 50 mVs–1 between 0.93 and 1.49 V vs RHE. Electrode durability was evaluated using the partially modified ADT protocol proposed by Haleem et al. to simulate the power fluctuations in renewable energy. The protocols included cycles of constant current density electrolysis at 600 mA cm–2 for 60 s and a constant potential holding at 0.3 V vs RHE for 60 s (Figure 1a, inset). The modified protocol used in this study omitted the open circuit potential (OCP) holding step and the potential sweep by LSV from the OCP to 0.3 V vs RHE. One ADT cycle simulated a single startup and shutdown of AWE. When photovoltaic cells supply the electrical power for AWE, the startup/shutdown cycle occurs once a day. The operating time of commercial AWE is between 60,000 and 80,000 h, which is approximately 2500 to 3300 ADT cycles. Accordingly, the cycle numbers (2000 and 4000) used in this study simulated practical operating conditions. On the other hand, the total electrolysis times at 600 mA cm–2 during 2000 and 4000 cycles were ca. 33 and 66 h, respectively.

The crystal structures of the NCO/Ni samples were analyzed using X-ray diffraction (XRD; SmartLab, Rigaku). Surface resistivity was evaluated by a resistance meter (HIOKI, RM3544) with a four-terminal probe (SR4-SS, Astellatech). The cross-sectional microstructures were observed using scanning electron microscopy (SEM; SU-70, Hitachi Hi-Tech) with energy-dispersive X-ray spectroscopy (EDS) for samples subjected to ion milling pre-processing (IM-4000, Hitachi Hi-Tech). The interfacial nanostructure for the 400-NCO/Ni sample was analyzed by scanning transmission electron microscopy (STEM; ARM200F, JEOL) with EDS after focused ion beam (FIB) processing (JIB-4600F, JEOL). The surface chemical bonding state was analyzed using X-ray photoelectron spectroscopy (XPS; Theta Probe, Thermo Fisher Scientific) with a monochromatized Al X-ray source.

Figure 1 depicts the electrochemical properties of the as-NCO/Ni electrode. After 2000 ADT cycles, the LSV curves shift to the higher potential side by ca. 40–60 mV (Figure 1a), indicating that the OER activity drastically decreases. This is consistent with the previous report by Haleem et al. The peak positions on the anodic and cathodic sides shift slightly after 2000 cycles in the CV curves. This change corresponds to the oxidation–reduction properties of the Ni and Co sites in NCO (Figure 1b). The Cole–Cole plots recorded at 1.5 V show a single semi-arc both before and after ADT (Figure 1c), suggesting that the film resistance of CL is negligible and unaffected by ADT. Comparing an enlarged semi-arc after ADT with the initial one shows an increase in the charge transfer resistance for OER (i.e., a decrease in OER activity of the electrode). Figure 2 shows cross-sectional SEM images of the as-NCO/Ni electrode before and after 2000 ADT cycles. The ca. 1 μm-thick NCO-CL contains 10–200 nm-wide voids (Figure 2a,b). Additionally, nanoscale interstitial gaps are partly present in the NCO layer (Figure 2b, inset). The gaps observed in the SEM images are defects generated between each NCO layer during the thermal decomposition process, which was repeated 12 times to prepare CL. After 2000 ADT cycles, most of the NCO layer disappears (Figure 2c), the residual KOH on the electrode surface appears between layers of NCO and between NCO and the Ni substrate (Figure 2d), and the gap expands (Figure 2b and Figure S3). The results clearly reveal that the detachment of the NCO layer originates from the interstitial gaps present within the stacked NCO layers as well as those between the NCO layer and the Ni substrate. Figure 2e depicts the proposed CL detachment mechanism. First, the electrolyte penetrates into the interstitial gap of the stacked CL (Figure 2c) when 0.3 V vs RHE, which corresponds to the “stop” condition of AWE, is applied to the electrode for 60 s. Second, the oxygen bubbles actively formed in the gap induce mechanical stress between the stacked CLs. Third, CL detaches from the electrode surface into the electrolyte when it can no longer withstand the stress induced by the oxygen bubbles. The CL detachment process continues during ADT. Finally, the Ni substrate surface, which shows a lower OER activity than NCO, is exposed to the electrolyte, increasing the overpotential. Hence, CL detachment can be suppressed by decreasing the gap or improving the structural durability against mechanical stress derived from oxygen bubbles. Next, we attempted to suppress CL detachment by post-annealing-induced microstructural modulation. Figure 3a presents the ex situ XRD patterns of as- and post-annealed NCO/Ni electrodes. The as-NCO/Ni sample shows peaks at 36.3° and 44.2°, which correspond to NiCo2O4(111) and NiCo2O4(400), respectively. As the annealing temperature increases, the NiCo2O4 peaks gradually intensify. The weak peak corresponding to NiO(200) appears at 42.9° in the 400 °C-annealed sample, while only the peaks of the substrate Ni and NiCo2O4 are present for the as-NCO/Ni and 100–300 °C-annealed samples. The NiO peaks of the other crystal planes, (111), (220), and (311), appear from 500 to 700 °C, and the NiO peak intensity becomes larger than that of NiCo2O4 at 700 °C. Furthermore, the relatively weak peaks of (333) and (440) planes of Co3O4 at 58.8° and 64.8°, respectively, appear at 500–700 °C. The generation of NiO by post-annealing could arise from thermal oxidation of the Ni substrate or thermal decomposition of NCO. NCO exhibits semi-metallic conductivity. In contrast, NiO, which has a rock salt-type structure, is a p-type semiconductor and has very low electric conductivity compared to NCO. Thus, we evaluated the changes in the surface resistivity of CL as a function of the annealing temperature. Figure 3b shows the surface resistivity and XRD area intensity ratio of NiO(200) and NCO(400) peaks as functions of the annealing temperature. Although the 100 °C-annealed sample has 10-fold higher resistivity compared to the as-prepared one, the resistivity value is constant from 100 to 300 °C. However, the resistivity significantly increases with the annealing temperature above 400 °C. Since there is a good correlation between the XRD intensity ratio and the change in resistivity, the increase in resistivity with increased annealing temperature is attributed to the increased NiO content. Zhen et al. reported the influence of post-annealing on the resistivity of epitaxial NCO(111) thin films. The resistivity of a NCO(111) thin film decreases relative to the as-deposited one by annealing at 500 °C in air due to the increased spinel inversion. Above 600 °C, the resistivity drastically increases with temperature because the NCO thin film decomposes into NiO and Co3O4. In this study, the XRD peak of Co3O4 appears above 500 °C (Figure 3a), suggesting that NCO decomposes above 500 °C for polycrystalline NCO-CL. Therefore, the increased resistivity at 400 °C is derived from the slight amount of NiO generated at the CL/Ni substrate interface, as discussed below. Figure 4 summarizes the electrochemical properties of the as-NCO/Ni and annealed electrodes. The LSV curves for OER show that the current density decreases as the annealing temperature increases (Figure 4a). Specifically, 500-NCO/Ni displays negligible OER current because the resistivity is 10 times higher than that of 300- and 400-NCO/Ni (Figure 3b). Figure 4b shows the CV curves corresponding to the redox reactions of the Ni and Co sites in NCO. Compared with the as-prepared sample, the redox charges of 300- and 400-NCO/Ni significantly decrease, suggesting that post-annealing reduces the surface area. Judging from the resistivity results (Figure 3b) and the XPS results (Figure S5), the decrease in the OER current of 400-NCO relative to that of 300-NCO is attributed to the increased surface resistivity (Figure 3b) since both 300- and 400-NCO/Ni have nearly identical features for the surface chemical compositions and CV redox charges. Figure 4c shows the changes in the OER overpotential at 100 mA cm–2 before and after 2000 ADT cycles, which are estimated from the corresponding LSV curves. Although ADT increases the overpotential in the as-prepared and 300 °C-annealed samples, that in the 400 °C-annealed one decreases by 40 mV after 2000 cycles. Furthermore, the overpotential after 4000 cycles decreases by 10 mV relative to that after 2000 cycles. The OER currents of the 2000- and 4000-cycled samples increase significantly compared with that of the initial sample (Figure 4d). The Tafel slope gradually decreases during the ADT cycles (Figure 4d, inset), suggesting that the OER kinetics of the NCO surface improve. After 2000 and 4000 ADT cycles, new redox peaks appear in the CV curves at 1.23 V at the anodic side and 1.1 V at the cathodic side (Figure 4e). The redox peak positions agree well with those of NiCo hydroxides. Furthermore, as for the Cole–Cole plots, the semi-arc after the ADT cycles in the higher frequency regions for the charge transfer resistance becomes smaller than that of the initial one, although the frequency regions corresponding to the film resistance remain the same. Consequently, the improved OER kinetics after ADT mainly stem from the changes in the chemical state of the topmost surface regions in the NCO layers. Figure 5a summarizes the XPS spectra for the Ni2p, Co2p, and O1s bands of 400-NCO/Ni before and after 4000 ADT cycles. After 4000 ADT cycles, the band intensity of Ni2p increases, while that of Co2p decreases relative to the initial one. The surface composition of Ni and Co in the NCO layers was estimated from the XPS results (Figure 5b). The initial atomic ratio of Ni:Co is 3:7, but it is 1:1 after 4000 ADT cycles, suggesting that the surface Co species selectively elutes into the KOH electrolyte during ADT. As for the O1s band, the OH– components become dominant after ADT. Therefore, the results of the electrochemical and XPS measurements indicate that NiCo (oxy)hydroxide layers form during the ADT cycles, leading to the increased OER activity of 400-NCO/Ni. Microstructural analysis was conducted for the 400-NCO/Ni electrode to clarify the origin of the high durability against the ADT cycles. Figure 6 shows the cross-sectional SEM images and SEM-EDS mappings of 400-NCO/Ni before and after 4000 ADT cycles. The CL thickness of the initial 400-NCO/Ni is ca. 1 μm (Figure 6a), which is almost the same as as-NCO/Ni. A 200–300 nm-thick dense interlayer forms between the NCO layer and the Ni substrate. Furthermore, the interstitial gap presented in as-NCO/Ni almost completely disappears from the NCO layer, while the nanoscale voids remain in the layer, forming nanoporous-like interconnected structures (Figure 6b, inset, and Figure S6 for the STEM image). The SEM images for the ADT-cycled sample indicate that the CL layer thickness does not change even after 4000 cycles (Figure 6c,d), which is double the number of cycles applied to as-NCO/Ni. The corresponding SEM-EDS mappings indicate that Ni is enriched in the dense oxide interlayer, while ADT has a negligible impact on the EDS signal distribution (Figure 6e,f). To reveal the microstructural changes of the CL/Ni substrate, cross-sectional STEM observations were conducted for the 400-NCO/Ni sample before ADT. Figure 7a,b shows the high-angle annular dark field (HAADF)-STEM images and EDS line profiles, respectively. The EDS signal intensity of Co gradually decreases from the NCO layer (Figure 6a, point A) to the Ni substrate (point B) through the Ni-rich oxide interlayer, whereas the Ni intensity increases in the same region. Figure 7c–e shows high-resolution STEM images for regions (c), (d), and (e) indicated in Figure 7a,b. The lattice fringe spacings in (c) correspond to the (111) and (311) planes of spinel NiCo2O4. In (d) and the upper region of (e), the spacings are almost the same as that for the (111) plane of NaCl-type NiO. Accordingly, the STEM images suggest that the Ni-rich oxide interlayer is mainly composed of Co-doped NiO, and the doping amount decreases from point A to B. Figure 7e shows that the lattice fringes of NiO(111) in the interlayer and Ni(111) in the Ni substrate overlap at the interface, suggesting that epitaxial interfaces of NiO(111)/Ni(111) are partially formed. The electron diffraction structural analysis for this region strongly supports the presence of the epitaxial interface (Figure S10). Han et al. investigated the thermodynamic stability of the Ni-Co-O ternary phase. The spinel NiCo2O4 is metastable at 400 °C when the Co:Ni ratio is higher than 2:1, while NaCl-type NiO and spinel Co3O4 are stable when the ratio is below 2:1. Therefore, the NCO layer hardly decomposes to NiO and Co3O4 at 400 °C. In fact, the STEM images do not show decomposed species in the NCO layer (Figure S6). Therefore, the Co-doped NiO layer should be generated through an interfacial reaction between NCO and the Ni substrate while post-annealing at 400 °C. According to the Ellingham diagram, the Gibbs free energy of NiO is more negative than the spinel structure. This suggests that the Ni substrate in contact with the NCO layer reduces the interfacial NCO. Then, the Ni surface is gradually oxidized to NiO, and Co diffuses from interfacial reduced NCO to the NiO interlayer. The diffusion coefficient of Co in CoO is about three orders of magnitude greater than that of Ni in NiO. This well explains the composition gradient Co-doped NiO layer generated by post-annealing. Furthermore, the Co-doped NiO layer epitaxially grows on the Ni substrate surface by forming a coherent interface of the (111) planes since the crystal lattices are nearly aligned with each other and exhibit 7-fold (NiO) and 8-fold (Ni) periodicities (Figure 7f, ball model). Such an epitaxial interface should structurally stabilize the CL/Ni substrate interface. The cross-sectional SEM and STEM analyses clearly reveal that the interfacial microstructural change should help suppress CL detachment and improve the durability of the NCO/Ni electrode against power fluctuations in renewable energy applications. The structural changes can be summarized as follows. First, the formation of the interconnected structures in the NCO layer diminishes the interstitial gap, causing CL detachment within the stacked NCO layer. Second, the composition gradient Co-doped NiO interlayer between the NCO layer and the Ni substrate can mitigate the structural mismatch of the NCO catalyst layer and Ni substrate. In addition, doped Co may suppress a fatal increase in the interfacial resistance due to NiO generation since Co doping can improve the conductivity of NiO. Third, partially presenting the NiO(111)/Ni(111) epitaxial interface should stabilize the oxide/metal interface. This work demonstrates that interfacial microstructural modulation by post-annealing successively suppresses CL detachment. This simple but highly effective technique has vast potential for use in various electrocatalysts and current collector substrates.

This study investigated the CL detachment mechanism on the NCO/Ni oxygen evolution electrode for AWE and examined the post-annealing effect on the detachment. The electrode performance of as-NCO/Ni is drastically degraded after 2000 ADT cycles due to CL detachment. The cross-sectional SEM observations revealed that CL detachment begins in the nanoscale interstitial gaps of the stacked CL and between CL and the Ni substrate. Post-annealing increases the crystallinity of the NCO layer. Furthermore, NiO is generated above 400 °C, and the generated fraction increased with the annealing temperature, resulting in dramatically higher surface resistivity. Although the initial OER overpotential of 400 °C-annealed NCO/Ni is higher than that of the as-prepared one, it significantly decreases over the 4000 ADT cycles because an OER-active NiCo hydroxide surface is generated. However, the CL thickness in the post-annealed sample remains relatively constant even after 4000 cycles, indicating that post-annealing at 400 °C almost completely suppresses CL detachment. Cross-sectional SEM and STEM analyses revealed that the excellent durability against the CL detachment is due to the generation of a gap-less interconnected structure of the NCO layer, composition gradient Co-doped NiO interlayer, and NiO/Ni epitaxial interface. These findings provide a novel strategy to suppress CL detachment on water electrolysis electrodes under power fluctuation conditions.