Alkaline oxygen evolution reaction (OER) is critical for green hydrogen production from water electrolysis but encounters great challenges when operated at industrial-required ampere-scaled current densities, such as insufficient mass transfer, reduced catalytic activity, and limited lifetimes. Here we developed a one-step seed-assisted heterogeneous nucleation (HN) method (25 °C, 24 h) for producing a nickel iron-based electrocatalyst (CAPist-L1) for robust OER at ≥ 1000 mA cm-2. Based on the insoluble nanoparticles in the HN system (generated from the solubility difference of salts in water and organic solvents), a dense interlayer was formed and anchored the catalyst layer tightly on the substrate, ensuring stable long-term durability of over 14000 h (> 20 months) in 1 M KOH at 1000 mA cm-2. When applying CAPist-L1 as the anode catalyst in practical anion exchange membrane water electrolysis (AEM-WE), it delivered high activity of 7350 mA cm-2 at 2.0 V and good stability at 1000 mA cm-2 for 1500 h at 80 °C. The low cost and simplicity characteristics make the HN strategy a valuable approach for developing stable OER catalysts for the industrialization of AEM-WE.
Article
Seed-assisted Formation of Robust Anode Catalysts for Anion Exchange Membrane Water Electrolysis with Industrial-Scaled Current Density
https://doi.org/10.21203/rs.3.rs-3926103/v1
This work is licensed under a CC BY 4.0 License
Journal Publication
published 13 Aug, 2024
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Anion exchange membrane water electrolysis (AEM-WE), as an emerging energy conversion technology that can produce green hydrogen (H2) from water at low temperatures (e.g., 20–80°C), theoretically combines the advantages of traditional water electrolysis technologies, such as the low cost of alkaline water electrolysis (AWE) 1–3 and the high energy efficiency, large current density and high H2 purity of proton exchange membrane water electrolysis (PEM-WE) 4–8. However, the practical application of AEM-WE is still challenged largely by sluggish reaction kinetics and poor long-term performance at high current densities (≥ 1000 mA cm− 2) 9,10, particularly for the water oxidation (i.e., oxygen evolution reaction, OER) over the anode 11–13. Therefore, developing easy-made and inexpensive electrocatalysts with efficient and robust OER performance at ampere-scaled current densities is crucial to promoting the industrialization of AEM-WE 14.
Up to date, many OER catalysts have achieved substantial advancements in alkaline electrocatalysis, such as noble metal oxides 15, first row (3d) transition metal (e.g., nonprecious Fe, Co, and Ni) based oxides 16–18, layered double hydroxides (LDHs) 19, (oxy)hydroxides 20–22, sulfides 23,24, phosphides 25, and molecular complexes 26. Among them, state-of-the-art NiFe-LDHs have been identified as one of the most promising non-noble OER catalyst candidates because of their unique lamellar structures, abundant active sites, and facile synthesis 27. Although lots of LDH-type catalysts have been reported every year, only a few were able to replace existing metal electrodes (e.g., Raney nickel) in industry and be utilized as industrial OER electrocatalysts because of either less efficiencies or limited operational stabilities (e.g., commonly lower than 1500 h), especially at ampere-scaled current densities.
The main challenges that must be overcome to achieve ampere-scaled OER electrocatalysts include the following: (i) insufficient mass transfer (gas release and electrolyte ions diffusion) at gas-liquid-solid interface under fierce electrochemical reaction which limits the smooth interaction between active sites and reaction intermediates 28,29; (ii) low mechanical stability leads to catalyst detachment during violent gas collisions and superheating of catalyst interface with increased local pressure 30–32; and (iii) high chemical dissolution of catalyst caused by harsh surface adsorption/desorption processes 33,34. As such, it is imperative to develop existing electrocatalysts in the field of oxygen evolution electrolysis, particularly at ampere-scaled current densities, for promoting the industrialization of AEM-WE.
Herein, we report a one-step heterogeneous nucleation (HN) strategy to produce the nickel iron-based OER electrocatalyst (CAPist-L1) which achieved highly active and stable water oxidation of more than 14000 h (> 20 months) at an industrial needed current density of 1000 mA cm− 2 in 1 M KOH. The HN process is initiated by directly soaking a commercial nickel foam (NF) into an HN liquid containing insoluble NiFe-nanoparticles, which are deliberately created due to the solubility difference of metal salts (e.g., FeSO4) in water and organic solvents (e.g., isopropanol). The nanoparticles have proved to be crucial for the formation of a dense interlayer between the NF substrate and the lamellar catalytic layer. The interlayer anchors the catalytic layer tightly on the NF surface, ensuring fast mass transfer and gas evolution pathway for enhanced OER activity and stability at industrial-required ampere-scaled current densities (e.g., ≥ 1000 mA cm− 2). In the practical AEM-WEs, the CAPist-L1 catalysts-based electrolyzers exhibit high water electrolysis performance of 7350 mA cm− 2 in 1 M KOH at 2.00 V, surpassing the commercial IrO2 catalyst (2410 mA cm− 2 at 2.00 V), and can operate stably for 1500 h at 1000 mA cm− 2.
The HN liquid used in this work was prepared by slowly adding FeSO4 solution (in H2O) into Ni(NO3)2 solution (in isopropanol) (Fig. 1a), and the insoluble nanoparticles were formed due to the poor solubility of metal salts (e.g., sulfates) in organic solvents. High-angle annular dark-field imaging scanning transmission electron microscopy (HAADF-STEM) images demonstrate that the insoluble nanoparticles were widely and evenly distributed, with an average diameter of 1.57 ± 0.49 nm (Fig. 1b and Supplementary Fig. 1). Energy-dispersive X-ray (EDX) results showed the uniform distribution of Fe, Ni, O, and S in the nanoparticles (Supplementary Fig. 2), with a Ni/Fe atomic ratio of 3.96 characterized by inductively coupled plasma mass spectrometry (ICP-MS). Then, a one-step soaking of NF in the above HN liquid at 25°C for 24 h was conducted to obtain the CAPist-L1 catalyst (Fig. 1a and Supplementary Fig. 3). The nanoparticles adsorb on the surface of NF to reduce the interfacial energy and act as the crystal nucleus (i.e., seeds), without the fundamental nucleation process (e.g., common LDH catalysts generated in homogeneous solution), to facilitate the formation of the catalyst structure. The seed-assisted HN process could easily prepare the CAPist-L1 catalyst with a large size of 20 cm × 20 cm over nickel-based substrates, such as NF (Fig. 1c) or nickel mesh (Supplementary Fig. 4).
X-ray diffraction (XRD) patterns for prepared CAPist-L1 were consistent with the refined Ni2Fe(SO4)0.5(OH)6(H2O)3.85 (Fig. 1d and Supplementary Fig. 5). The large SO42− ions in the interlayer resulted in a small diffraction peak corresponding to (001) at 8.1°. The Raman peak at 980 cm− 1 also confirmed the presence of SO42− in CAPist-L1 (Supplementary Fig. 6). Both X-ray absorption near-edge structure (XANES) spectra of the Fe K-edge and X-ray photoelectron spectra of Fe 2p confirmed that the oxidation state of Fe in CAPist-L1 was similar to that in a typical NiFe-LDH catalyst (Supplementary Fig. 7a and Fig. 8). Fourier transform extended X-ray absorption fine structure (FT-EXAFS) data showed a larger Fe-O scattering path for CAPist-L1 (1.56 Å) than that for NiFe-LDH (1.51 Å), while their Fe-O-Fe/Ni scattering paths were similar to each other (2.75 Å) (Supplementary Fig. 7b). Scanning electron microscopic images (SEM) of CAPist-L1 showed micron-scale hydrangea-shaped structures grew vertically and evenly distributed on a honeycomb-like catalyst base (Fig. 1e), different from the stacked nanosheets of NiFe-LDH (Supplementary Fig. 9). As demonstrated below, this hierarchical structure of CAPist-L1 can effectively increase the electrochemically active area and facilitate the release of oxygen bubbles from inter-channels to the surface before ultimate detachment. ICP-MS results indicated a Ni/Fe atomic ratio of 2.21 in CAPist-L1. Energy-dispersive X-ray (EDX) transmission electron microscopy (TEM) images indicated that Ni, Fe, O, and S were uniformly distributed in the catalyst structure (Supplementary Fig. 10).
The OER performance of CAPist-L1 was evaluated in 1 M KOH at 25°C in a three-electrode system, compared with those of typical NiFe-LDH (self-synthesized, one of the best LDH-type OER catalysts as shown in Supplementary Fig. 11), commercial IrO2, and blank NF catalysts. The iR-corrected polarization curves at an ampere-scaled current density of 1000 mA cm− 2 show that the overpotential of CAPist-L1 was only 220 ± 4.5 mV, much lower than those of NiFe-LDH (337 ± 4.8 mV) and IrO2 (594 ± 4.2 mV) (Fig. 2a). Furthermore, CAPist-L1 exhibited an overpotential of only 283 ± 12.7 mV even at a higher current density of 5000 mA cm− 2. These results indicated that CAPist-L1 had a high OER activity. Noticeably, we also found that the activity of CAPist-L1 had high repeatability and uniformity (Supplementary Fig. 12). The OER Faradaic efficiency of CAPist-L1 was nearly 100% (Supplementary Table 1), which together with facile synthesis temperature and ultra-low material cost (Fig. 2b) makes it a great promise for industrial applications.
To understand the intrinsic reasons that dominate the OER activity of CAPist-L1, the in-situ Raman measurements were conducted in 1 M KOH to explore the active sites of the catalyst. With increasing anodic potentials, especially over 1.35 V (vs. RHE, same as below unless otherwise mentioned), two new peaks at 475 cm− 1 and 556 cm− 1 corresponding to NiIII-O 35 appeared (Supplementary Fig. 13). The result showed that OER processes were mainly related to the NiIII-O for both CAPist-L1 and NiFe-LDH. Besides, because of the strong hierarchy of CAPist-L1 catalyst layers, abundant interspaces existed in the hierarchical sheet array, which was capable of contributing to the decrease of ions diffusion distance and thus the increase of OER activity 36.
Note that mass transfer becomes a key factor determining electrochemical performance when the current density is increased. At an ampere-scaled current density, sluggish detachment of the oxygen bubbles (i.e., bubble clogging) from the catalyst surface inevitably contributes to a decrease in the surface area, inhibition of electrolyte ion diffusion, an increase in the local pressure, and thus shedding and stripping of the catalyst. Therefore, the OER activity is significantly limited at high current densities, as indicated by crooking of the polarization curve for NiFe-LDH at approximately 1500 mA cm− 2 (relative to the dashed line in Fig. 2a). In contrast, CAPist-L1 exhibited good mass transfer, and no obvious OER suppression was observed even up to 5000 mA cm− 2. The good mass transfer of CAPist-L1 was further evidenced by the statical analysis of the formed bubbles at the electrode surface at 1000 mA cm− 2, which demonstrated an average bubble diameter of ~ 39 µm much smaller than that of NiFe-LDH (~ 66 µm) (Supplementary Fig. 14). Interestingly, the size of the bubbles generated by CAPist-L1 decreased as the current density increased, which is the opposite of the trend seen for NiFe-LDH (Supplementary Fig. 15). It is plausible that the difference in mass transfer rates resulted mainly from the different material morphologies; that is, the hierarchical structure of the CAPist-L1 catalyst favored the bubble detachment because of the increase in the interlayer spaces from the inner catalyst layer to the outer layer 37. In addition, O2 bubble rupture may result in fluctuating OER performance 38, especially for large bubbles generated at high current densities.
Apart from the OER activity, the operation lifetime is a key parameter for the industrial utilization of catalysts. The durability of the CAPist-L1 catalyst was measured in a three-electrode system employing chronopotentiometry at a current density of 1000 mA cm− 2 in 1 M KOH at 25°C, comparing with those of NiFe-LDH and IrO2 (Fig. 2c). The anodic potential of CAPist-L1 was stable for over 14000 h (> 20 months). In contrast, the NiFe-LDH showed an obvious increase in the potential of approximately 400 mV after 1500 h, and the degradation of IrO2 was more severe as the potential increased from ~ 1.80 to ~ 2.50 V within 510 h. Considering the different synthesis technologies of these electrocatalysts, the CAPist-L1 (H2O) catalyst was further synthesized via soaking NF in a homogeneous metal solution (i.e., nickel and iron ions in water) for comparison. It also showed an obvious potential increase within 1700 h at 1000 mA cm− 2, similar to that of NiFe-LDH (Fig. 2c). The results confirmed that the unique seed-assisted HN process contributed to constructing a CAPist-L1 catalyst with enhanced stability.
Mechanical stability, structural stability, and chemical stability are key factors that substantially affect the lifetime of a catalyst 39–42. As observed in a cross-sectional SEM image (Figs. 3a and 3b), NiFe-LDH showed typically a single stacked nanosheet layer on the NF substrate. The CAPist-L1, however, was composed of two catalytic layers (i.e., hydrangea layer and honeycomb layer) and one dense interlayer on the surface of NF (Figs. 3c and 3d). The size of the catalyst sheet in each layer of CAPist-L1 increased from tens of nanometers for the interlayer to hundreds of nanometers for the honeycomb layer and to micrometers for the hydrangea layer (Supplementary Fig. 16). Accordingly, the interspaces in multilayers of CAPist-L1 catalyst gradually increased from the interlayer to the hydrangea layer as well. This hierarchical structure facilitated ion diffusion and gas release and prevented the decrease in mechanical stability. Besides, the dense interlayer anchored the hydrangea layer and honeycomb layer tightly on the surface of the NF, even covering the irregular surfaces (Supplementary Fig. 17), thus reinforcing the mechanical stability of the catalyst. Nano-scratch measurements were employed to determine the critical binding force of CAPist-L1, and the obtained value of 65.4 ± 6.0 mN was higher than that of NiFe-LDH (35.2 ± 8.2 mN) (Supplementary Fig. 18). The results confirmed a stronger adhesion force between the CAPist-L1 catalyst layer and the NF than that between the NiFe-LDH stacked nanosheet layer and the NF. To prove this, we conducted a short-term OER measurement (at 500 mA cm− 2 for 50 h), observing partial peeling-off of NiFe-LDH catalyst structure from NF substrate after the OER test (Supplementary Fig. 19). In contrast, CAPist-L1 exhibited good maintenance of the catalyst morphology based on the SEM images (Supplementary Fig. 20). Therefore, the interlayer would not participate in the OER process directly, but it mechanically fixed the catalyst layer against the adhesive force of oxygen bubbles and possible catalyst detachment 31.
To investigate the growth process of CAPist-L1, the ex-situ SEM was conducted to observe the catalyst hierarchical structures from the top view (Supplementary Fig. 21) and the cross-sectional view (Fig. 3e and Supplementary Fig. 22). The results showed that the catalyst structure appeared and grew rapidly within the first two hours. The dense interlayer, between the NF and catalytic layers, would hardly be observed until after four hours of soaking probably due to its slow growth kinetics. Then, the thickness of the interlayer gradually increased from hundreds of nanometers (soaking time of 4–8 h) to several micrometers (soaking time of 16–96 h). Based on the grazing incidence X-ray diffraction analysis, the interlayer exhibited the same LDH-type structure as that of the catalytic layer in CAPist-L1 (Supplementary Fig. 23). We anticipated the underlying role of insoluble nanoparticles in HN liquid that contributed to the formation of the interlayer structure. Therefore, it was evaluated by increasing the H2O proportion in the HN liquid to eliminate the nanoparticles (Supplementary Fig. 24). For example, the CAPist-L1 (H2O), synthesized in homogeneous water solution, exhibited typical lamellar catalyst structure but without interlayer structure. Furthermore, interchanging soaking systems between HN liquid and homogeneous solution confirmed that the insoluble nanoparticles played a vital role in inducing the formation of the interlayer especially at the initial soaking stage (Supplementary Fig. 25). The insoluble nanoparticles were thus believed to absorb on the surface of NF substrate and then participate in the formation of interlayer with slow kinetics.
The structural stability of the catalyst was evaluated by exploring the retention of the crystal structure. The in-situ XRD results indicated that the crystallinity of CAPist-L1 was well preserved during electrocatalysis at 1.20–1.60 V, different from that of NiFe-LDH (Supplementary Fig. 26). Moreover, the aforementioned in-situ Raman spectroscopy indicated the formation of oxyhydroxide species during electrocatalysis for CAPist-L1 (Supplementary Fig. 13), which may explain why the interlamellar spacing of the unit cell, obtained from in-situ XRD, decreased from 0.7590 Å (at 1.30 V) to 0.7532 Å (at 1.60 V). Therefore, the presence of an amorphous layer on the surface of the micro-sheet did not damage the main LDH structure even at 1.60 V (Supplementary Fig. 26). Additionally, the aforementioned short-term OER test indicated that the crystal structure of the CAPist-L1 catalyst in KOH persisted after 50 h of the OER at 500 mA cm− 2 (Supplementary Fig. 27a), unlike the amorphization of NiFe-LDH (Supplementary Fig. 27b).
The chemical stability of the catalyst was estimated by measuring the concentrations of dissolved metal ions in the electrolyte. Even though the normalized activity of CAPist-L1 was higher than that of NiFe-LDH, the concentrations of dissolved Fe and Ni ions were much lower (Supplementary Fig. 28a). The stability number of noxygen/nions, proposed by Geiger et al. 43, was used to determine the chemical stability of a catalyst, where noxygen is the number of oxygen molecules, nions is the number of dissolved cations and the high S-number indicates good chemical stability of the catalyst. After 50 h short-term OER catalysis, the S-number of CAPist-L1 (noxygen/nNi+Fe = 24627) was approximated four-fold higher than that of NiFe-LDH (noxygen/nNi+Fe = 5893), indicating that the CAPist-L1 has superior electrochemical stability (Supplementary Fig. 28b). The fitted EXAFS results showed that the coordination numbers (CNs) of the Fe-O and Fe-O-Fe/Ni species in CAPist-L1 are 5.7 and 2.9, respectively, after 50 h of OER, and these values were higher than those of the NiFe-LDH (CN = 5.0 for Fe-O, CN = 2.5 for Fe-O-Fe/Ni) (Supplementary Fig. 29 and Supplementary Table 2), consistent with the aforementioned results of the dissolution experiment. Therefore, the enhanced mechanical, structural, and chemical stabilities of CAPist-L1 enabled long-term measurements for over 14000 h at 1000 mA cm− 2 without an obvious activity decline.
We then explored the practical utilization of CAPist-L1 as an anode catalyst in a 1 cm2 (electrode area: 1 cm × 1 cm) AEM-WE system (Fig. 4a and Supplementary Fig. 30) and compared this anode to the NiFe-LDH and IrO2 anodes. In the AEM-WE electrolyzer, a terphenyl-based poly (aryl piperidinium) polymer (PAP-TP-85, dry thickness of 45.0 ± 0.6 µm) was used as the anion exchange membrane, and noble metal-free Ni4Mo/MoO2 known as a superior HER catalyst (detail information are described in the experimental section of S.I.) was used to catalyze H2O into H2. At 80°C, the current density of the CAPist-L1-catalyzed AEM-WE (CAPist-L1//Ni4Mo/MoO2) system was 7350 mA cm− 2 at a cell voltage of 2.0 V, approximately 1.6-fold and 3.0-fold higher than those of the NiFe-LDH (4450 mA cm− 2) and IrO2 (2410 mA cm− 2) systems, respectively (Fig. 4b and Supplementary Fig. 31). The Faradaic efficiency in the AEM-WE was 99.29 ± 0.59% in a short-term measurement (Supplementary Fig. 32). The CAPist-L1//Ni4Mo/MoO2 system showed one of the best performance levels reported to date for water electrolysis (e.g., AEM-WE and PEM-WE as shown in Supplementary Table 3 and Table 4, respectively) and demonstrated the potential of replacing noble metal-based catalysts with CAPist-L1 to produce hydrogen cost-effectively in alkaline solutions.
The long-term durability of the CAPist-L1//Ni4Mo/MoO2 electrolyzer (1 cm2) was evaluated in 1 M KOH at 1000 mA cm− 2, achieving stable operation for over 1700 h at 25°C and over 1500 h at 80°C (Fig. 4c). In addition, the electrolyzer was also stable at 2000 mA cm− 2 for 1200 h (Supplementary Fig. 33). These results indicated the high stability of CAPist-L1 catalyst at industrial-scale current densities in practical utilization, much better than most reported anode electrocatalysts.
Based on the 1 cm2 electrolyzer measurements, we further tested a 25 cm2 CAPist-L1//Ni4Mo/MoO2 electrolyzer (electrode area: 5 cm × 5 cm) in 1 M KOH. The 25 cm2 electrolyzer exhibited a high current density of 2730 mA cm− 2 (60°C) at a cell voltage of 1.80 V, surpassing the U.S. DOE target for alkaline water electrolysis systems (2000 mA cm− 2 at 1.80 V) 44 (Fig. 4d). Impressively, the water electrolysis performance of the electrolyzers remains similar when increasing the electrode area from 1 cm2 to 25 cm2. For example, at 1000 mA cm− 2, the cell voltage of the 1 cm2 electrolyzer was approximately 1.622 V at 60°C, which was very similar to that of the 25 cm2 electrolyzer (1.623 V). The results demonstrated the good scale-up feasibility of CAPist-L1 as an AEM-WE anode. In addition, the pilot CAPist-L1//Ni4Mo/MoO2 electrolyzer was operated stably at 25000 mA (i.e., 1000 mA cm− 2) for 1500 h at 25°C (Fig. 4e). Overall, the pilot-scale CAPist-L1//Ni4Mo/MoO2 electrolyzer showed the expected durability and strikingly good performance for water electrolysis.
In summary, we have developed a simple seed-assisted heterogeneous nucleation (HN) strategy which produces a CAPist-L1 catalyst with high scalability, mild operation conditions, and good repeatability. The CAPist-L1 catalyst displayed high OER activity with low overpotentials of 220 ± 4.5 mV at 1000 mA cm− 2 and 283 ± 12.7 mV at 5000 mA cm− 2, respectively, and good long-term durability for over 14000 h at 1000 mA cm− 2. A dense interlayer in CAPist-L1, formed in the presence of insoluble nanoparticles in the HN system, anchored the catalyst layer tightly on the nickel substrate, which together with the unique hierarchical structure of CAPist-L1 ensured efficient mass transfer and gas evolution with excellent structural, mechanical, and chemical stabilities, making it an ideal anode catalyst for water electrocatalysis. We also used CAPist-L1 as the anode catalyst in AEM-WEs and confirmed high water electrolysis performance (7350 mA cm− 2 for 1 cm2 electrolyzer, at 2.0 V and 80°C) and good stability (1500 h for either 1 cm2 or 25 cm2 electrolyzer, at 1000 mA cm− 2).
The high OER activity and good stability of CAPist-L1 catalyst represent a significant achievement in developing noble-metal free OER catalysts, with wide applicability in AEM-WE and other electrolyzers (e.g., photoelectrochemical water splitting electrolyzers 45,46 and CO2 reduction electrolyzers 47). Also, the proposed HN processes are inspiring for developing other electrocatalysts due to the solution processability and ease of large-scale production.
Acknowledgments
This work was financially supported by the National Key R&D Program of China (2022YFA0911902), National Natural Science Foundation of China (22088102), the Research Center for Industries of the Future (RCIF) at Westlake University, China Postdoctoral Science Foundation (2022M712837, 2023M733175). The authors would like to thank Dr. Xiaohe Miao at the Instrumentation and Service Center for Physical Science (ISCPS) at Westlake University for her unselfish help in the In-situ XRD analysis, Dr. Yunxuan Ding at the Center of Artificial Photosynthesis for Solar Fuels and the Department of Chemistry at Westlake University for his help in verifying the structure of CAPist-L1 from the point of view of theoretical calculation. We also acknowledge the support from the BL11B station in Shanghai Synchrotron Radiation Facility (SSRF) for the XAS measurements.
Competing interests
The authors declare no competing interests.
Author contributions
Conceptualization and methodology: Z. Li and L. Sun. Material – synthesis, characterization, and test: Z. Li, G. Lin, L. Wang, G. Ding, R. Ren, X. Cao, S. Ding, W. Ye, and W. Yang. AEM – assemble and test: Z. Li, H. Lee, J. Du, T. Tang, and W. Li. Data analysis: Z. Li and G. Lin. Supervision: L. Sun. Writing – original draft: Z. Li and G. Lin. Writing – review and editing: Z. Li, G. Lin, W. Yang, and L. Sun.
Chemicals. Iron (II) sulfate [FeSO4·7H2O, Aladdin Industrial Co., ≥99.0 %, Cas No. 7782-63-0], Iron (III) nitrate [Fe(NO3)3·9H2O, Aladdin Industrial Co., ≥98.5 %, Cas No. 7782-61-8], nickel (II) nitrate [Ni(NO3)2·6H2O, Aladdin Industrial Co., ≥99.0 %, Cas No. 13478-00-7], iridium oxide [IrO2, Sigma-Aldrich, 99%], ammonium molybdate [(NH4)6Mo7O24·4H2O, Macklin reagent, 99.9 %, Cas No. 12054-85-2], urea [CO(NH2)2, Macklin reagent, 99.0 %, Cas No. 57-16-3], isopropanol [i-PrOH, Sinopharm Chemical Reagent Co. Ltd., ≥99.7 %, Cas No. 67-63-0], ethanol [EtOH, Sinopharm Chemical Reagent Co. Ltd., ≥99.7 %, Cas No. 64-17-5], acetone [Sinopharm Chemical Reagent Co. Ltd., ≥99.2 %, Cas No. 67-64-1], potassium hydroxide [KOH, Macklin reagent, 95 %, Cas No. 1310-58-3], hydrochloric Acid [HCl, Sinopharm Chemical Reagent Co. Ltd., 37 %, Cas No. 7647-01-0] and Nafion 117 solution [Sigma-Aldrich, 5.0 % in EtOH] were used without any pre-treatment or purification. Ni foam [NF, Suzhou Jiashide Co. Ltd., ≥99.99 %, thickness of 1.0 cm] was pre-washed with HCl (3 M), EtOH, and acetone solutions to remove the nickel oxide and hydrocarbons on the surface. Terphenyl-based poly (aryl piperidinium) polymer (PAP-TP-85, dry thickness of 45.0 ± 0.6 μm) was purchased from Suzhou Sinero Technology Co., Ltd.
Catalyst preparation. CAPist-L1: 142.5 g Ni(NO3)2·6H2O and 19.5 g FeSO4·7H2O were dissolved in 2400 mL i-PrOH and 800 mL deionized water, respectively. The above solutions were then mixed under vigorous stirring to form the heterogeneous nucleation (HN) liquid. A pre-washed NF (20 × 20 cm) was then totally immersed into the HN liquid at 25 °C for 24 h (see Figure S1), followed by the washing with deionized water, and drying in a vacuum oven at 25 °C for 12h to generate CAPist-L1. The loading mass of CAPist-L1 is 3.5 mg cm-2. NiFe-LDH 48,49: 0.350 g Ni(NO3)2·6H2O, 0.162 g Fe(NO3)3·9H2O and 0.32 g urea were dissolved in 30 mL deionized water under vigorous stirring, followed by the immersion of a pre-washed NF (1.5 × 2.0 cm) into the solution and reaction in a Teflon-lined autoclave at 120 °C for 12 h. After washing with deionized water and drying in a vacuum oven at 25 °C for 12h, the NiFe-LDH was obtained. IrO2/NF: 10 mg IrO2 powder and 40 μL Nafion 117 solution (5.0 % in EtOH) were mixed in 2.0 mL EtOH and i-PrOH mixture (volume ratio of 1:1) with ultrasound to form a uniform slurry. The as-prepared slurry was then sprayed evenly on the washed NF with an N2-borne spray gun, followed by a thorough drying in a vacuum oven at 25 °C for 12 h. The loading mass of IrO2 was manipulated for 3.5 mg cm-2 which is comparable to CAPist-L1. Ni4Mo/MoO2@NF 50: 0.419 g Ni(NO3)2·6H2O and 0.445 g (NH4)6Mo7O24·4H2O were mixed in 30 mL deionized water to form a solution, followed by the immersion of a pre-washed NF (2.5 × 3.0 cm) into the solution and then the reaction in a Teflon-lined autoclave at 150 °C for 6 h to form a pre-catalyst. After washing with deionized water and drying in a vacuum oven at 25 °C for 12 h, the pre-catalyst was calcinated at 500 °C for 2 h in a H2/Ar (volume ratio of 5:95) atmosphere to obtain the Ni4Mo/MoO2@NF catalyst.
Catalyst characterization. X-ray diffraction (XRD, Bruker D8 advance) and in-situ XRD patterns were carried out on a Philips powder diffractometer using Cu-Kα radiation at 40 kV with a current of 40 mA. Scanning electron microscopy (SEM) was performed on a Zeiss Gemini 450 microscope system. High-resolution transmission electron microscope (HRTEM) and energy dispersive X-ray (EDX) analysis were carried out using a Thermo Fisher (Talos F200X G2) system at an acceleration voltage of 200 kV. In-situ Raman spectra were recorded by a HORIBA (XploRA plus) micro-Raman spectrometer system. Inductively coupled plasma mass spectrometry (ICP-MS, iCAP RQ, ThermoFisher) was used to determine the content of Ni and Fe ions in liquid. Before measurement, the liquid was purified by the membrane to remove the insoluble particles. Nuclear magnetic resonance (NMR) spectroscopy was obtained by using the Bruker Biospin (Advance NEO) device.
AEM-WE assemble. The PAP-TP-85 acted as the commercial AEM-WE membrane was pre-prepared by the ion exchange in 1 M KOH for 12 h. The CAPist-L1 and Ni4Mo/MoO2@NF catalysts were cut into a square with the size of 1.0 × 1.0 cm (or 5.0 × 5.0 cm) for further utilization. The single cell, with a serpentine flow field area of 1 cm2 (or 25 cm2), was pre-washed with EtOH and acetone to remove the hydrocarbon contaminants. Then, the single cell, membrane, OER catalyst, and HER catalyst were assembled, together with polytetrafluoroethylene (PTFE) gaskets for insulation.
Electrochemical measurements. The electrochemical OER activity and stability were measured on the Autolab Vionic workstation by using a typical three-electrode system in 1 M KOH. One Hg/HgO, calibrated as 0.110 V, was used as the reference electrode and a Pt mesh was used as the counter. The geometric area of the CAPist-L1 catalyst was 1.0 × 1.0 cm. The linear sweep voltammetry measurements were conducted at a scan rate of 5 mV s-1, and the electrolyte resistance (R) was determined by the impedance test at 0.6 V vs. Hg/HgO. The stability tests were assisted by online deionized water injection in the three-electrode system to keep a constant KOH concentration. All the measurements were conducted at 25 °C, except for the ones that are specifically marked. The iR compensation was calculated by the following equation 51:
where E(Hg/HgO) is the measured potential, R is the electrolyte resistance, i is the current, and the value of 0.110 V is the difference between the Hg/HgO reference electrode and the RHE in 1 M KOH at 25 °C.
The 1 cm2 AEM-WE (electrode area: 1 cm × 1 cm) was measured on the Autolab PGSTAT302 (with 20 A booster) workstation, with the KOH flow of 50 mL min-1. The temperature of the KOH solution was controlled by two temperature sensors in the bipolar plates (very near to the flow field). The linear sweep voltammetry measurements were conducted at a scan rate of 5 mV s-1. The stability tests were measured at the constant current density with a reduced KOH flow of 20 mL min-1. An online water injection was connected to the 1 cm2 AEM-WE system to keep a constant KOH concentration.
The 25 cm2 AEM-WE (electrode area: 5 cm × 5 cm) was measured on the Solartron Analytical (with 100 A booster) workstation, with the KOH flow of 350 mL min-1. The temperature of the KOH solution was also controlled by two temperature sensors in the bipolar plates (very near to the flow field). The linear sweep voltammetry measurements were conducted at a scan rate of 5 mV s-1. The stability tests were measured at the constant current density with a reduced KOH flow of 300 mL min-1. An online water injection was connected to the 25 cm2 AEM-WE system to keep a constant KOH concentration.
Faradaic efficiency (FE) measurement. The FE was measured by a water drainage method 52 of O2 production to reflect the electrocatalytic performance in a three-electrode H-cell system, according to the following equation 53:
where n is the molar number of electrons that participate in the O2 production (1 mol) during the electrolysis, F is the Faraday constant (96485 C mol-1), VO2 is the O2 volume measured by the water drainage method, Q is the total electric charge (C), and Vm is molar O2 volume (24.5 L mol-1 at 25 °C). Similarly, the FE of H2 was also tested.
For comparison, the gas chromatography method was also used to measure the FE of O2 and H2 production in a three-electrode H-cell system and a single-cell system 54,55. Ar balanced H2 (0.9999 %, 4.9999 %, 9.9999 %, 14.9999 %, and 19.9999 %) and O2 (0.9999 %, 4.9999 %, 9.9999 %, 14.9999 %, and 19.9999 %) were applied as the standard gases to obtain the external standard curve. The Fuli gas chromatograph (GC 9790 Plus) with a 5 A molecular sieves column (2 m × 4 mm, 60-80 meshes) was used to analyze the H2 and O2 contents in the gases. Ar gas (99.9999 %) was applied as the carrier gas (20.0 mL min-1) with a split ratio of 100:1 and the sweep gas (50 mL min-1). The temperature of the thermal conductivity detector (TCD) was 130 °C.
Material cost calculation. Only the raw materials that were required for preparing catalysts are considered for the calculation of material cost. Other inputs, such as heating, solvents, and gases, were not involved in the material cost. For example, the preparation of IrO2 catalyst required IrO2 [~ RMB ¥ 2,792 per gram, Macklin reagent, 95 %, Cas No.: 12030-49-8], nickel metal [~ US $ 18050 per ton, London Metal Exchange, date: 9th November, 2023], and Nafion 117 solution [RMB ¥ 738 per 5 mL, Aladdin reagent, Cas No.: 31175-20-9], and the cost of IrO2 catalyst (sprayed on 25 cm2 Ni foam, 3.5 mg cm-2) was approximately RMB ¥ 293.9. Considering the lifetime of IrO2 for 510 h at 1000 mA cm-2 (Figure 3C), the cost of the IrO2 catalyst was approximately RMB ¥ 574.5 per 1000 h. Similarly, the material costs of NiFe-LDH and CAPist-L1 were approximately RMB ¥ 0.377 and RMB ¥ 0.059 (< ¥ 0.1) per 1000 h, respectively.
- Li, D. et al. Highly quaternized polystyrene ionomers for high performance anion exchange membrane water electrolysers. Nat. Energy 5, 378–385, (2020).
- Wang, J. et al. Non-precious-metal catalysts for alkaline water electrolysis: operando characterizations, theoretical calculations, and recent advances. Chem. Soc. Rev. 49, 9154–9196, (2020).
- Niether, C. et al. Improved water electrolysis using magnetic heating of FeC–Ni core–shell nanoparticles. Nat. Energy 3, 476–483, (2018).
- Chong, L. et al. La- and Mn-doped cobalt spinel oxygen evolution catalyst for proton exchange membrane electrolysis. Science 380, 609–616, (2023).
- Xu, J. et al. IrOx·nH2O with lattice water–assisted oxygen exchange for high-performance proton exchange membrane water electrolyzers. Science Advances 9, eadh1718, (2023).
- King, L. A. et al. A non-precious metal hydrogen catalyst in a commercial polymer electrolyte membrane electrolyser. Nat. Nanotechnol. 14, 1071–1074, (2019).
- Wu, Z.-Y. et al. Non-iridium-based electrocatalyst for durable acidic oxygen evolution reaction in proton exchange membrane water electrolysis. Nat. Mater. 22, 100–108, (2023).
- Lin, C. et al. In-situ reconstructed Ru atom array on α-MnO2 with enhanced performance for acidic water oxidation. Nat. Catal. 4, 1012–1023, (2021).
- Liu, Y. et al. Corrosion engineering towards efficient oxygen evolution electrodes with stable catalytic activity for over 6000 hours. Nat. Commun. 9, 2609, (2018).
- Zou, X. et al. Ultrafast Formation of Amorphous Bimetallic Hydroxide Films on 3D Conductive Sulfide Nanoarrays for Large-Current-Density Oxygen Evolution Electrocatalysis. Adv. Mater. 29, 1700404, (2017).
- Seh, Z. W. et al. Combining theory and experiment in electrocatalysis: Insights into materials design. Science 355, eaad4998, (2017).
- Liang, C. et al. Exceptional performance of hierarchical Ni–Fe oxyhydroxide@NiFe alloy nanowire array electrocatalysts for large current density water splitting. Energy Environ. Sci. 13, 86–95, (2020).
- Suntivich, J., May, K. J., Gasteiger, H. A., Goodenough, J. B. & Shao-Horn, Y. A Perovskite Oxide Optimized for Oxygen Evolution Catalysis from Molecular Orbital Principles. Science 334, 1383–1385, (2011).
- Seitz, L. C. et al. A highly active and stable IrOx/SrIrO3 catalyst for the oxygen evolution reaction. Science 353, 1011–1014, (2016).
- Du, K. et al. Interface engineering breaks both stability and activity limits of RuO2 for sustainable water oxidation. Nat. Commun. 13, 5448, (2022).
- Zhang, B. et al. Homogeneously dispersed multimetal oxygen-evolving catalysts. Science 352, 333–337, (2016).
- Zhuang, L. et al. Ultrathin Iron-Cobalt Oxide Nanosheets with Abundant Oxygen Vacancies for the Oxygen Evolution Reaction. Adv. Mater. 29, 1606793, (2017).
- Merrill, M. D. & Dougherty, R. C. Metal Oxide Catalysts for the Evolution of O2 from H2O. J. Phys. Chem. C 112, 3655–3666, (2008).
- Luo, J. et al. Water photolysis at 12.3% efficiency via perovskite photovoltaics and Earth-abundant catalysts. Science 345, 1593–1596, (2014).
- Corrigan, D. A. & Bendert, R. M. Effect of Coprecipitated Metal Ions on the Electrochemistry of Nickel Hydroxide Thin Films: Cyclic Voltammetry in 1M KOH J. Electrochem. Soc. 136, 723, (1989).
- Li, X., Walsh, F. C. & Pletcher, D. Nickel based electrocatalysts for oxygen evolution in high current density, alkaline water electrolysers. Phys. Chem. Chem. Phys. 13, 1162–1167, (2011).
- Hall, D. E. Ni(OH)2-Impregnated Anodes for Alkaline Water Electrolysis. J. Electrochem. Soc. 130, 317, (1983).
- Ding, G. et al. Highly Efficient and Durable Anion Exchange Membrane Water Electrolyzer Enabled by a Fe–Ni3S2 Anode Catalyst. Adv. Energy Sustainability Res. 4, 2200130, (2023).
- Karunadasa, H. I. et al. A Molecular MoS2 Edge Site Mimic for Catalytic Hydrogen Generation. Science 335, 698–702, (2012).
- Wu, L. et al. Heterogeneous Bimetallic Phosphide Ni2P-Fe2P as an Efficient Bifunctional Catalyst for Water/Seawater Splitting. Adv. Funct. Mater. 31, 2006484, (2021).
- Cheng, W. et al. Lattice-strained metal–organic-framework arrays for bifunctional oxygen electrocatalysis. Nat. Energy 4, 115–122, (2019).
- Zhai, P. et al. Engineering single-atomic ruthenium catalytic sites on defective nickel-iron layered double hydroxide for overall water splitting. Nat. Commun. 12, 4587, (2021).
- Angulo, A., van der Linde, P., Gardeniers, H., Modestino, M. & Fernández Rivas, D. Influence of Bubbles on the Energy Conversion Efficiency of Electrochemical Reactors. Joule 4, 555–579, (2020).
- Iwata, R. et al. Bubble growth and departure modes on wettable/non-wettable porous foams in alkaline water splitting. Joule 5, 887–900, (2021).
- Spöri, C., Kwan, J. T. H., Bonakdarpour, A., Wilkinson, D. P. & Strasser, P. The Stability Challenges of Oxygen Evolving Catalysts: Towards a Common Fundamental Understanding and Mitigation of Catalyst Degradation. Angew. Chem. Int. Ed. 56, 5994–6021, (2017).
- Liu, H. et al. Dual interfacial engineering of a Chevrel phase electrode material for stable hydrogen evolution at 2500 mA cm– 2. Nat. Commun. 13, 6382, (2022).
- Xiao, X., Wang, L., Wang, Z. & Wang, Z. Superheating of grain boundaries within bulk colloidal crystals. Nat. Commun. 13, 1599, (2022).
- Chen, R. et al. Layered Structure Causes Bulk NiFe Layered Double Hydroxide Unstable in Alkaline Oxygen Evolution Reaction. Adv. Mater. 31, 1903909, (2019).
- Wu, Y.-j. et al. Evolution of Cationic Vacancy Defects: A Motif for Surface Restructuration of OER Precatalyst. Angew. Chem. Int. Ed. 60, 26829–26836, (2021).
- Li, Y. et al. Operando spectroscopies unveil interfacial FeOOH induced highly reactive β-Ni(Fe)OOH for efficient oxygen evolution. Appl. Catal. B-Environ. 318, 121825, (2022).
- Li, X. et al. Ultrafast Room-Temperature Synthesis of Self-Supported NiFe-Layered Double Hydroxide as Large-Current–Density Oxygen Evolution Electrocatalyst. Small 18, 2104354, (2022).
- Xu, X. et al. Highly Efficient All-3D-Printed Electrolyzer toward Ultrastable Water Electrolysis. Nano Lett. 23, 629–636, (2023).
- Yang, F., Kim, M. J., Brown, M. & Wiley, B. J. Alkaline Water Electrolysis at 25 A cm– 2 with a Microfibrous Flow-through Electrode. Adv. Energy Mater. 10, 2001174, (2020).
- Howarth, A. J. et al. Chemical, thermal and mechanical stabilities of metal–organic frameworks. Nat. Rev. Mater. 1, 15018, (2016).
- Mayerhöfer, B. et al. Electrochemical- and mechanical stability of catalyst layers in anion exchange membrane water electrolysis. Int. J. Hydrogen Energy 47, 4304–4314, (2022).
- Liang, C. et al. Highly Conductive and Mechanically Robust NiFe Alloy Aerogels: An Exceptionally Active and Durable Water Oxidation Catalyst. Small 18, 2203663, (2022).
- Ding, Z. et al. High Entropy Intermetallic–Oxide Core–Shell Nanostructure as Superb Oxygen Evolution Reaction Catalyst. Adv. Sustain. Syst. 4, 1900105, (2020).
- Geiger, S. et al. The stability number as a metric for electrocatalyst stability benchmarking. Nat. Catal. 1, 508–515, (2018).
- Kim, Y. S. Scalable Elastomeric Membranes for Alkaline Water Electrolysis US Department of Energy, (2019).
- Kenney, M. J. et al. High-Performance Silicon Photoanodes Passivated with Ultrathin Nickel Films for Water Oxidation. Science 342, 836–840, (2013).
- Khaselev, O. & Turner, J. A. A Monolithic Photovoltaic-Photoelectrochemical Device for Hydrogen Production via Water Splitting. Science 280, 425–427, (1998).
- García de Arquer, F. P. et al. CO2 electrolysis to multicarbon products at activities greater than 1 A cm– 2. Science 367, 661–666, (2020).
- Abellán, G., Coronado, E., Martí-Gastaldo, C., Pinilla-Cienfuegos, E. & Ribera, A. Hexagonal nanosheets from the exfoliation of Ni2+-Fe3+ LDHs: a route towards layered multifunctional materials. J. Mater. Chem. 20, 7451–7455, (2010).
- Saiah, F. B. D., Su, B.-L. & Bettahar, N. Removal of Evans Blue by using Nickel-Iron Layered Double Hydroxide (LDH) Nanoparticles: Effect of Hydrothermal Treatment Temperature on Textural Properties and Dye Adsorption. Macromol. Symp. 273, 125–134, (2008).
- Zhang, J. et al. Efficient hydrogen production on MoNi4 electrocatalysts with fast water dissociation kinetics. Nat. Commun. 8, 15437, (2017).
- Yang, L. et al. Vertical Growth of 2D Amorphous FePO4 Nanosheet on Ni Foam: Outer and Inner Structural Design for Superior Water Splitting. Adv. Mater. 29, 1704574, (2017).
- Fan, L. et al. Atomically isolated nickel species anchored on graphitized carbon for efficient hydrogen evolution electrocatalysis. Nat. Commun. 7, 10667, (2016).
- You, B., Liu, X., Jiang, N. & Sun, Y. A General Strategy for Decoupled Hydrogen Production from Water Splitting by Integrating Oxidative Biomass Valorization. J. Am. Chem. Soc. 138, 13639–13646, (2016).
- Sonoyama, N. & Sakata, T. Electrochemical Continuous Decomposition of Chloroform and Other Volatile Chlorinated Hydrocarbons in Water Using a Column Type Metal Impregnated Carbon Fiber Electrode. Environ. Sci. Technol. 33, 3438–3442, (1999).
- Liu, C. et al. Oxygen evolution reaction over catalytic single-site Co in a well-defined brookite TiO2 nanorod surface. Nat. Catal. 4, 36–45, (2021).
- Anantharaj, S. et al. Precision and correctness in the evaluation of electrocatalytic water splitting: revisiting activity parameters with a critical assessment. Energy Environ. Sci. 11, 744–771, (2018).
There is NO Competing Interest.