has received considerable attention as a promising energy storage media owing to its high gravimetric energy density (142 MJ kg −1 ) and near-zero carbon emission power generation. [7][8][9][10][11][12] Currently, both alkaline and protonconducting polymer electrolyte membrane water electrolysis (PEMWE), also known as proton exchange membrane water electrolysis, are well established as commercial or near-commercial electrochemical water-splitting technologies. [13,14] Compared to alkaline water electrolysis, PEMWE has the advantages of faster dynamics owing to high ionic conductivity, operation at higher current density (maximum 2-3 A cm −2 ), and high-purity H 2 production. [15,16] However, PEMWE necessitates the use of noble-metal-based electrocatalysts on both cathode and anode. Various kinds of electrocatalysts have been developed to replace platinum/ carbon (Pt/C) for the hydrogen evolution reaction (HER) at the cathode. [17][18][19][20][21] However, currently available electrocatalysts for the oxygen evolution reaction (OER) at the anode still depend on iridium (Ir), ruthenium (Ru), or their oxide forms. The high cost and scarcity of Ir and Ru limit the large-scale application of PEMWE. [22,23] Although some noblemetal-free materials show OER activity in strongly acidic solutions, many of them are still unstable in acidic solutions and exhibit inferior electrocatalytic OER activity than noble-metalbased materials. [24,25] Furthermore, especially in acidic media, oxygen evolution is even more limited in the water-splitting process owing to the slow kinetics of water oxidation originating from the complex four-proton-coupled electron transfer reactions as well as additional water dissociation processes. [26][27][28] As such, it is mandatory to develop noble-metal-reduced OER electrocatalysts with a high electrocatalytic activity that are well tolerated in acidic solutions. Some OER electrocatalysts which could be tolerated in acidic solutions were reported before 2010. However, since 2015, intensive research has been carried out to address the other aforementioned issues by developing alternative electrocatalytic materials for the OER in acidic solutions.Recently, Ir-or Ru-based perovskite oxides have emerged as promising novel OER electrocatalysts in acidic solutions, owing to their multimetal-oxide nature that can reduce the usage of noble metal. Perovskite oxides have various structures, i.e., single perovskite structure with the general formula ABO 3 , double perovskite structure with the formula A 2 BB′O 6 (double Proton-conducting polymer electrolyte membrane water electrolysis (PEMWE) is a promising technology for generating clean and sustainable hydrogen fuels from water. However, PEMWE requires the use of expensive electrocatalysts; the currently available electrocatalysts for the oxygen evolution reaction (OER) depend on noble metals (Ir, Ru). Since noble metals are expensive, commercialization of PEMWE remains elusive. In addition, PEMWE suffers from the very slow kinetics of the OER in acidic medi...

For mass production of hydrogen fuel by electrochemical water splitting, seawater electrolysis is preferred over freshwater electrolysis because of the abundance of seawater in nature. However, the electrochemically active anions in seawater can cause the corrosion of electrodes or undesirable side reactions during the anodic reaction at the anode, thus degrading the overall system efficiency. Hence, it is imperative to develop highly active and stable oxygen evolution reaction (OER) electrocatalysts for efficient seawater electrolysis. In this study, carbon-coated sodium cobalt−iron pyrophosphate (Na 2 Co 1−x Fe x P 2 O 7 /C, 0 ≤ x ≤ 1) nanoparticles (NCFPO/C NPs) and NCFPO/C NPs loaded on a carbon cloth (NCFPO/C@CC) are fabricated as a promising OER electrocatalyst for alkaline seawater electrolysis. The electrocatalytic OER performance of the NCFPO/C NPs is optimized by controlling their Co/Fe ratio. NCFPO/C@CC acts as an efficient OER electrocatalyst during the OER test in an alkaline saline solution without corroding the electrode and generating reactive chloride species. It also exhibits long-term stability and durability with continuous oxygen generation. In addition, NCFPO/C@CC shows the electrocatalytic OER activity in alkaline seawater, demonstrating that NCFPO/C is a promising candidate as an OER electrocatalyst for realistic alkaline seawater electrolysis.

Developing low-cost, highly active, and stable bifunctional electrocatalysts is a challenging issue in electrochemical water electrolysis. Building on 3D architectured electrocatalysts through structural and compositional engineering is an effective strategy to enhance catalytic activities as well as stability and durability. Herein, 3D architectures of quaternary Co-Ni-S-P compounds coupled with graphene ((Co 1−x Ni x )(S 1−y P y ) 2 /G) electrocatalysts are proposed, in which nanosheets are self-assembled to form 3D architectures with round and flat doughnut-like shapes, toward overall water splitting. Benefiting from the 3D architectures and Ni, P substitution, (Co 1−x Ni x )(S 1−y P y ) 2 /G exhibits superior electrocatalytic activities with low overpotentials of 117 and 285 mV at 10 mA cm −2 and Tafel slopes of 85 and 105 mV dec −1 for hydrogen and oxygen evolution reactions, respectively, in alkaline media. In addition, minimal increases in overpotential are observed, even after the 10 000th voltammetric cycle and continuous chronopotentiometric testing over 50-100 h, confirming the high stability and durability of (Co 1−x Ni x )(S 1−y P y ) 2 /G. When used as both cathode and anode, (Co 1−x Ni x )(S 1−y P y ) 2 /G achieves excellent overall water splitting performance with a cell potential as low as 1.65 V, reaching a current density of 10 mA cm −2 with no obvious decay after 50 h, demonstrating that (Co 1−x Ni x )(S 1−y P y ) 2 /G is an efficient bifunctional electrocatalyst for overall water splitting.

The development of efficient electrocatalysts is important to produce clean and sustainable hydrogen fuel on a large scale. With respect to cathodic reactions, Pt exhibits an overwhelming electrocatalytic capability in the hydrogen evolution reaction (HER) in comparison with other earth‐abundant electrocatalysts, despite its rarity and high cost. So, a hybrid catalyst that combines a low‐cost electrocatalyst with Pt would balance cost‐effectiveness with catalytic activity. Herein, α‐phase molybdenum carbide (MoC 1− x ) nanoparticles (NPs) decorated with a small amount of Pt (MoC 1− x /Pt‐NPs) are designed to achieve high‐performance hydrogen production in acidic and alkaline media. MoC 1− x ‐NPs exhibit good electrocatalytic HER activity as well as stability and durability. They show favorable catalytic kinetics in an alkaline medium, suggesting an active water dissociation process. After Pt decoration, Pt‐NPs that are 2–3 nm in diameter are well incorporated with MoC 1− x ‐NPs. MoC 1− x /Pt‐NPs with a small amount of Pt (2.7–3 wt%) and are able to perform superior electrocatalytic HER activity, and possess stability and durability that is comparable to that of commercial Pt/C. Notably, they exhibit a higher intrinsic catalytic activity compared to that of Pt/C in an alkaline medium, indicating that they promote the sluggish catalytic kinetics of Pt in alkaline medium.

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