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    28 September 2022, Volume 28 Issue 9
    Special Issue on Water Electrolysis for Hydrogen Production
    Preface to Special Issue on Water Electrolysis for Hydrogen Production
    Li Li, Jin-Song Hu, Zi-Dong Wei
    2022, 28(9):  22214000.  doi:10.13208/j.electrochem.2214000
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    Author Spotlight
    2022, 28(9):  2214111.  doi:10.13208/j.electrochem.2214111
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    Perovskite-Type Water Oxidation Electrocatalysts
    Xiao Liang, Ke-Xin Zhang, Yu-Cheng Shen, Ke Sun, Lei Shi, Hui Chen, Ke-Yan Zheng, Xiao-Xin Zou
    2022, 28(9):  2214004.  doi:10.13208/j.electrochem.2214004
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    The development of energy conversion/storage technologies can achieve the reliable and stable renewable energy supply, and bring us a sustainable future. As the core half-reaction of many energy-related systems, water oxidation is the bottleneck due to its sluggish kinetics of the four-concerted proton-electron transfer (CPET) process. This necessitates the exploitation of low cost, highly active and stable water oxidation electrocatalysts. Perovskite-type oxides possess diverse crystal structures, flexible compositions and unique electronic properties, enabling them ideal material platform for the optimization of catalytic performance. In this review, we provide a comprehensive summary for the crystal structures, electronic structures and synthetic methods of perovskite-type oxides in their application background of water oxidation electrocatalysis. Then, we summarize the recent research advances of perovskite-type water oxidation electrocatalysts in alkaline and acidic media, and highlight the significance of their structure-activity relationship and activation/deactivation mechanism. Finally, challenges and the corresponding solutions for the perovskite-type electrocatalysts are highlighted, which is expected to open the opportunities to their practical applications.

    Recent Development of Low Iridium Electrocatalysts toward Efficient Water Oxidation
    Jing Ni, Zhao-Ping Shi, Xian Wang, Yi-Bo Wang, Hong-Xiang Wu, Chang-Peng Liu, Jun-Jie Ge, Wei Xing
    2022, 28(9):  2214010.  doi:10.13208/j.electrochem.2214010
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    Developing high-performance and low-cost electrocatalysts for oxygen evolution reaction (OER) is the key to implementing polymer electrolyte membrane water electrolyzer (PEMWE) for hydrogen production. To date, iridium (Ir) is the state-of-the-art OER catalyst, but still suffers from the insufficient activity and scarce earth abundance, which results in high cost both in stack and electricity. Design low-Ir catalysts with enhanced activity and stability that can match the requirements of high current and long-term operation in PEMWE is thus highly desired, which necessitate a deep understanding of acidic OER mechanisms, unique insights of material design strategies, and reliable performance evaluation norm, especially for durability. With these demand in mind, we in this review firstly performed a systematic summary on the currently recognized acidic OER mechanism on both activity expression (i.e. the adsorbate evolution mechanism, the lattice oxygen mediated mechanism and the multi-active center mechanism) and inactivation (i.e. active species dissolution, evolution of crystal phase and morphology, as well as catalyst shedding and active site blocking), which can provide guidance for material structural engineering towards higher performance in PEMWE devices. Subsequently, we critically reviewed several types of low-Ir OER catalysts recently reported, i.e. multimetallic alloy oxide, supported, spatially structured and single site catalysts, focusing on how the performance has been regulated and the underlying structure-performance relationship. Lastly, the commonly used indicators for catalyst stability evaluation, wide accepted deactivation characterization techniques and the lifetime probing methods mimicking the practical operation condition of PEMWE are introduced, hoping to provide a basis for catalyst screening. In the end, few suggestions on exploring future low-Ir OER catalysts that can be applied in the PEMWE system are proposed.

    Surface Structure Engineering of FeNi-Based Pre-Catalyst for Oxygen Evolution Reaction: A Mini Review
    Jia-Xin Li, Li-Gang Feng
    2022, 28(9):  2214001.  doi:10.13208/j.electrochem.2214001
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    Oxygen evolution reaction (OER) is a significant half-reaction for water splitting reaction, and attention is directed to the high-performance non-precious catalysts. Iron nickel (FeNi)-based material is considered as the most promising pre-catalyst, that will be transferred to the real active phase in the form of high valence state metal species. Even so, the catalytic performance is largely influenced by the structure and morphology of the FeNi pre-catalysts, and lots of work has been done to optimize and tune the structure and chemical environment of the FeNi- based pre-catalysts so as to increase the catalytic performance. Herein, based on our work, a mini review is proposed for the surface structure engineering of FeNi-based pre-catalyst for OER. The reaction mechanism of alkaline OER is firstly presented, and then the strategies in surface engineering of FeNi-based pre-catalyst for improving OER performance are discussed in terms of heteroatom doping, surface composition modification, selective structural transformation, surface chemical state regulation, heterostructure construction, and support effect. It can be concluded that the surface structure, morphology, and the chemical states of Fe/Ni in the system will significantly influence the final catalytic performance, though all of them were transferred into the active phase state of high valence state metal species. In other words, the catalytic performance of FeNi-based catalysts is also determined by the property of their pre-catalysts. To carefully design and maximize the synergistic effect of Fe and Ni is necessary to boost the catalytic performance. We hope this topic will be a good and timely complement to the study of FeNi-based catalysts for OER in the water-splitting technique.

    Self-Supporting NiFe LDHs@Co-OH-CO3 Nanorod Array Electrode for Alkaline Anion Exchange Membrane Water Electrolyzer
    Dan-Dan Guo, Hong-Mei Yu, Jun Chi, Zhi-Gang Shao
    2022, 28(9):  2214003.  doi:10.13208/j.electrochem.2214003
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    The development of efficient and durable electrodes for anion exchange membrane water electrolyzers (AEMWEs) is essential for hydrogen production. In this work, 2D NiFe layered double hydroxides (NiFe LDHs) nanosheets were grown on the 1D cobaltous carbonate hydroxide nanowires array (Co-OH-CO3) and the unique 3D layered self-supporting nanorod array (NiFe LDHs@Co-OH-CO3/NF) electrode was obtained. Importantly, we demonstrated an efficient and durable self-supporting NiFe LDHs@Co-OH-CO3/NF electrode for oxygen evolution reaction (OER) and as the anode of the AEMWE. In a three-electrode system, the self-supporting NiFe LDHs@Co-OH-CO3/NF electrode showed excellent catalytic activity for OER, with an overpotential of 215 mV at a current density of 20 mA·cm-2 in 1 mol·L-1 KOH, and the promising AEMWE performance upon using as the anode, with a current density of 0.5 A·cm-2 at 1.72 V in 1 mol·L-1 KOH at 70 oC. The experimental results further revealed the outstanding performance of the electrode with the special morphological structure. The 3D layered structure of nanorod array electrode could effectively prevent the agglomeration of nanosheets, which is conducive to electron transfer and provides a large number of edge active sites for water electrolyzer.

    The Rapid Preparation of Efficient MoFeCo-Based Bifunctional Electrocatalysts via Joule Heating for Overall Water Splitting
    Ao Zhou, Wei-Jian Guo, Yue-Qing Wang, Jin-Tao Zhang
    2022, 28(9):  2214007.  doi:10.13208/j.electrochem.2214007
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    Water electrolysis is an available way to obtain green hydrogen. The development of highly efficient electrocatalysts is a current research hotspot for water splitting, but it remains challenging. Herein, we demonstrate the synthesis of a robust bifunctional multi-metal electrocatalysts toward water splitting via the rapid Joule-heating conversion of metal precursors. The composition and morphology were well regulated via altering the ratio of metal precursors. In particular, the trimetal MoC/FeO/CoO/carbon cloth (CC) electrode revealed the outstanding bifunctional electrocatalytic performance due to the unique composition and large electrochemical active surface area. Typically, the MoC/FeO/CoO/CC catalyst needed low overpotentials of 121 and 268 mV to reach 10 mA·cm-2 toward HER and OER in 1 mol·L-1 KOH solution, respectively. When used as both cathode and anode, a small potential of 1.69 V was required to achieve 10 mA·cm-2 for overall water splitting and an impressive stability for 25 h was observed. This facile and rapid Joule heating strategy offers guideline for rational manufacture of bimetal or multi-metal electrocatalysts toward diverse application.

    A Co Porphyrin with Electron-Withdrawing and Hydrophilic Substituents for Improved Electrocatalytic Oxygen Reduction
    Hong-Bo Guo, Ya-Ni Wang, Kai Guo, Hai-Tao Lei, Zuo-Zhong Liang, Xue-Peng Zhang, Rui Cao
    2022, 28(9):  2214002.  doi:10.13208/j.electrochem.2214002
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    Understanding factors that influence the catalyst activity for oxygen reduction reaction (ORR) is essential for the rational design of efficient ORR catalysts. Regulating catalyst electronic structure is commonly used to fine-tune electrocatalytic ORR activity. However, modifying the hydrophilicity of catalysts has been rarely reported to improve ORR, which happens at the liquid/gas/solid interface. Herein, we report on two Co porphyrins, namely, NO2-CoP (Co complex of 5,10,15,20-tetrakis(4-nitrophenyl)porphyrin) and 5F-CoP (Co complex of 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin), and their electrocatalytic ORR features. By simultaneously controlling the electronic structure and hydrophilic property of the meso-substituents, the NO2-CoP showed higher electrocatalytic activity than the 5F-CoP by shifting the ORR half-wave potential to the anodic direction by 60 mV. Compared with the 5F-CoP, the complex NO2-CoP was more hydrophilic. Theoretical calculations suggest that NO2-CoP is also more efficient than 5F-CoP to bind with an O2 molecule to form CoIII-O2·-. This work provides a simple but an effective strategy to improve ORR activity of Co porphyrins by using electron-withdrawing and hydrophilic substituents. This strategy will be also valuable for the design of other ORR molecular electrocatalysts.