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Achieving a future sustainable carbon-neutral society requires the development of efficient systems for the chemical conversion of solar light energy to CO2-free fuels such as H2as alternatives to fossil fuel. In this case, water splitting driven by solar energy is one of the most promising approaches for sustainable H2production. However, the critical bottleneck in establishing efficient water splitting systems is the sluggish oxygen evolution reaction (OER) with high overpotentials (ηO2). Thus, the development of highly efficient OER electrocatalysts to minimize ηO2and thus attain efficient water splitting is a challenging key task. Recently, there has been prominent progress in the development of efficient Ni-based electrocatalysts with extremely low ηO2. Herein, the aspects and mechanisms of state-of-the-art Ni-based OER electrocatalysts reported in the last three years from 2020 to 2023 are reviewed. Furthermore, we discuss the perspectives to develop efficient Ni-based OER electrocatalysts based on the comprehensive understanding of the catalysts introduced in this review.

distal-[Ru(EtOtpy)(pynp)Cl]+(d-EtO1Cl) (EtOtpy = 4′-ethoxy-2,2′:6′,2″-terpyridine, pynp = 2-(2-pyridyl)-1,8-naphthyridine), and distal/proximal-[Ru(EtOtpy)(pynp)OH2]2+(d/p-EtO1H2O) complexes were newly synthesized to investigate the synergistic influence of the geometric configuration coupled with substituent introduction of an ethoxy (EtO) group on the physicochemical properties and reactions of the Ru(II) complexes. Configurationally nonselective aquation of d-EtO1Clwas uniquely observed to form d/p-EtO1H2Oisomers in water, in contrast to configurationally selective aquation of distal-[Ru(tpy)(pynp)Cl]+(d-1Cl, tpy = 2,2′:6′,2″-terpyridine) without the EtO group [Yamazaki, H.. J. Am. Chem. Soc.2011, 133, 8846–8849].The kinetic profiles of the aquation reactions of d-EtO1Clwere well analyzed using a sequential reversible reaction model assuming the reversible interconversion between d/p-EtO1H2Oisomers via d-EtO1Cl. The observed equilibrium constant (Kiso) of isomerization between p/d-EtO1H2Owas calculated from the kinetic analysis as Kiso= 0.45, which is consistent with the final concentration ratio (1:0.43) of p/d-EtO1H2Ogenerated in the aquation reaction of d-EtO1Cl. The irreversible photoisomerization from d-EtO1H2Oto p-EtO1H2Owas observed in water with an internal quantum yield (Φ) of 0.44% at 520 nm. Electrochemical measurements showed that d-EtO1H2Oundergoes a 2-step oxidation reaction of 1H+-coupled 1e–processes of RuII–OH2/RuIII–OH and RuIII–OH/RuIV═O at pH 1.3–9.7, whereas p-EtO1H2Oundergoes a 1-step oxidation reaction of a 2H+-coupled 2e–process of RuII–OH2/RuIV═O in the pH range of 1.8–11.5. Any redox potential of d/p-EtO1H2Oisomers was decreased by the electro-donating EtO substitution, compared with distal/proximal-[Ru(tpy)(pynp)OH2]2+(d/p-1H2O). The turnover frequency (kO2= 1.7 × 10–2s–1) of d-EtO1H2Ofor water oxidation catalysis is higher than that (3.5 × 10–4s–1) of p-EtO1H2Oby a factor of 48.6. The kO2value (1.7 × 10–2s–1) for d-EtO1H2Ois 4.5-fold higher than those of d-1H2O(3.8 × 10–3s–1). The higher kO2value of d-EtO1H2Ocompared with d-1H2Ocould be explained by the fast oxidation rate from RuIV═O to RuV═O involved in the rate-determining step due to the electron-donating EtO group.

Geometric isomers of mononuclear ruthenium(II) complexes, distal-/proximal-[Ru(tpy)(dpda)Cl]+(d-/p-RuCl, tpy = 2,2′:6′,2″-terpyridine, dpda = 2,7-bis(2-pyridyl)-1,8-diazaanthracene), were newly synthesized to comprehensively investigate the geometric and electronic structures and distinctive aspects in various reactions between isomers. The ultraviolet (UV)–visible absorption spectra of d-/p-RuClisomers show intense bands for metal-to-ligand charge transfer (MLCT) at close wavelengths of 576 and 573 nm, respectively. However, time-dependent density functional theory (TD-DFT) calculations suggest that the MLCT transition of d-RuClinvolves mainly single transitions to the π* orbital of the dpda ligand in contrast to mixing of the π* orbitals of the dpda and tpy ligands for p-RuCl. The aquation reaction (1.5 × 10–3s–1) of p-RuClto yield proximal-[Ru(tpy)(dpda)(OH2)]2+(p-RuH2O) is faster than that (5.3 × 10–6s–1) of d-RuClin D2O/CD3OD (4:1 v/v) by three orders of magnitude, which resulted from the longer Ru–Cl bond by 0.017 Å and the distorted angle (100.2(3)°) of Cl–Ru–N (a nitrogen of dpda, being on a tpy plane) due to the steric repulsion between Cl and dpda for p-RuCl. Electrochemical measurements showed that d-RuH2Oundergoes a 2-step oxidation reaction of 1H+-coupled 1e–processes of RuII–OH2/RuIII–OH and RuIII–OH/RuIV═O at pH 1–9, whereas p-RuH2Oundergoes a 1-step oxidation reaction of a 2H+-coupled 2e–process of RuII–OH2/RuIV═O in the pH range of pH 1–10. The irreversible photoisomerization from d-RuH2Oto p-RuH2Owas observed in aqueous solution with an internal quantum yield (Φ) of 5.4 × 10–3% at 520 nm, which is lower compared with Φ = 1.1–2.1% of mononuclear Ru(II) aquo complexes with similar bidentate ligands instead of dpda by three orders of magnitude. This is possibly ascribed to the faster nonradiative decay rate from the excited 3MLCT state to the ground state for d-RuH2Odue to the lower π* level of dpda ligands according to the energy-gap law: the rate decreases exponentially with the increasing energy gap.

A dinuclear Ru complex, proximal,proximal-[Ru2L(C8Otpy)2(OH)(OH2)]3+(C8Otpy = 4′-octyloxy-2,2’; 6′,2″-terpyridine) (1) with long alkoxyl chains, was synthesized to be immobilized on a carbon paper (CP) electrode via hydrophobic interactions between the long alkoxyl chains and the CP surface. The 1/CP electrode demonstrated efficient electrocatalytic water oxidation with a low overpotential (ηonset) of 0.26 V (based on the onset potential for water oxidation) in an aqueous medium at pH 7.0, which is compared advantageously with those of hitherto-reported molecular anodes for water oxidation. The active species of RuIIIRuIII(μ-OO) with a μ-OO bridge was involved in water oxidation at 0.95 V versusAg/AgCl. As the applied potential increased to 1.40 V, water oxidation was promoted by participation of the more active species of RuIIIRuIV(μ-OO), and very durable electrocatalysis was gained for more than 35 h without elution of 1into the electrolyte solution. The introduced long alkoxyl chains act as a dual role of the linker of 1on the CP surface and decrease the η value. Theoretical investigation provides insights into the O–O bond formation mechanism and the activity difference between RuIIIRuIII(μ-OO) and RuIIIRuIV(μ-OO) for electrocatalytic water oxidation.

Facile and scalable fabrication of α-Fe2O3photoanodes using a precursor solution containing FeIIIions and 1-ethylimidazole (EIm) in methanol was demonstrated to afford a rigidly adhered α-Fe2O3film with a controllable thickness on a fluorine-doped tin oxide (FTO) substrate. EIm ligation to FeIIIions in the precursor solution brought about high crystallinity of three-dimensionally well-interconnected nanoparticles of α-Fe2O3upon sintering. This is responsible for the 13.6 times higher photocurrent density (at 1.23 V vs reference hydrogen electrode (RHE)) for photoelectrochemical (PEC) water oxidation on the α-Fe2O3(w-α-Fe2O3) photoanode prepared with EIm compared with that (w/o-α-Fe2O3) prepared without EIm. The w-α-Fe2O3photoanode provided the highest charge separation efficiency (ηsep) value of 27% among the state-of-the-art pristine α-Fe2O3photoanodes, providing incident photon-to-current conversion efficiency (IPCE) of 13% at 420 nm and 1.23 V vs RHE. The superior ηsepfor the w-α-Fe2O3photoanode is attributed to the decreased recombination of the photogenerated charge carriers at the grain boundary between nanoparticles, in addition to the higher number of the catalytically active sites and the efficient bulk charge transport in the film, compared with w/o-α-Fe2O3.

[Ru(Rtpy)(bpy)(H2O)]2+(1R; bpy = 2,2′-bipyridine, and Rtpy = 2,2′:6′,2″-terpyridine derivatives) complexes with a variety of 4′-substituent groups on Rtpy were synthesized and characterized to reveal the effects of substituents on their structures, physicochemical properties, and catalytic activities for water oxidation. The geometric structures of 1Rare not considerably influenced by the electron-donating ability of the 4′-substituent groups on Rtpy. Similar multistep proton-coupled electron transfer reactions were observed for 1R, and the redox potentials for each oxidation step tended to decrease with an increase in the electron-donating ability of the substituent, which is explained by the increased electron density on the Ru center by electron-donating groups, stabilizing the positive charge that builds up upon oxidation. This is consistent with the red-shift of the absorption bands around 480 nm assigned to the metal-to-ligand charge transfer transition for 1Rdue to the increased d orbital energy level of the Ru center. The turnover frequency (kO2) of 1Rfor water oxidation catalysis, however, depended greatly on the Rtpy ligands, varying from 0.05 × 10–2to 44 × 10–2s–1(as the highest kO2was observed for R = ethoxy) by a factor of 880. A critical electron-donating ability of the 4′-substituent groups with a narrow range of Hammett constants (σp= −0.27 to −0.24) found for the highest kO2values is valuable for understanding the great difficulty in the search for efficient water oxidation catalysts. On another front, the kO2values increased with a decrease in the redox potentials of RuIV═O/RuV═O for 1R, indicating that the potential of formation of RuV═O species for 1Ris crucial for water oxidation catalysis under the employed conditions.

A unique transformation of WO3nanowires (NW-WO3) into hexagonal prisms (HP-WO3) was demonstrated by tuning the temperature of the (N2H4)WO3precursor suspension prepared from tungstic acid and hydrazine as a structure-directing agent. The precursor preparation at 20 °C followed by calcination at 550 °C produced NW-WO3nanocrystals (ca.<100 nm width, 3–5 μm length) with anisotropic growth of monoclinic WO3crystals to (002) and (200) planes and a polycrystalline character with randomly oriented crystallites in the lateral face of nanowires. The precursor preparation at 45 °C followed by calcination at 550 °C produced HP-WO3nanocrystals (ca.500–1000 nm diameter) with preferentially exposed (002) and (020) facets on the top-flat and side-rectangle surfaces, respectively, of hexagonal prismatic WO3nanocrystals with a single-crystalline character. The HP-WO3electrode exhibited the superior photoelectrochemical (PEC) performance for visible-light-driven water oxidation to that for the NW-WO3electrode; the incident photon-to-current conversion efficiency (IPCE) of 47% at 420 nm and 1.23 V vs RHE for HP-WO3was 3.1-fold higher than 15% for the NW-WO3electrode. PEC impedance data revealed that the bulk electron transport through the NW-WO3layer with the unidirectional nanowire structure is more efficient than that through the HP-WO3layer with the hexagonal prismatic structure. However, the water oxidation reaction at the surface for the HP-WO3electrode is more efficient than the NW-WO3electrode, contributing significantly to the superior PEC water oxidation performance observed for the HP-WO3electrode. The efficient water oxidation reaction at the surface for the HP-WO3electrode was explained by the high surface fraction of the active (002) facet with fewer grain boundaries and defects on the surface of HP-WO3to suppress the electron–hole recombination at the surface.