Recent Advances of Transition Metal Basic Salts for Electrocatalytic Oxygen Evolution Reaction and Overall Water Electrolysis

Abstract

Electrocatalytic oxygen evolution reaction (OER) has been recognized as the bottleneck of overall water splitting, which is a promising approach for sustainable production of H₂. Transition metal (TM) hydroxides are the most conventional and classical non-noble metal-based electrocatalysts for OER, while TM basic salts [M²⁺(OH)_2-x(A^m-)_x/m, A = CO₃^2-, NO₃^-, F^-, Cl^-] consisting of OH^- and another anion have drawn extensive research interest due to its higher catalytic activity in the past decade. In this review, we summarize the recent advances of TM basic salts and their application in OER and further overall water splitting. We categorize TM basic salt-based OER pre-catalysts into four types (CO₃^2-, NO₃^-, F^-, Cl^-) according to the anion, which is a key factor for their outstanding performance towards OER. We highlight experimental and theoretical methods for understanding the structure evolution during OER and the effect of anion on catalytic performance. To develop bifunctional TM basic salts as catalyst for the practical electrolysis application, we also review the present strategies for enhancing its hydrogen evolution reaction activity and thereby improving its overall water splitting performance. Finally, we conclude this review with a summary and perspective about the remaining challenges and future opportunities of TM basic salts as catalysts for water electrolysis.

Keywords: Electrocatalytic; Overall water electrolysis; Oxygen evolution reaction (OER); Transition metal basic salts.

Fig. 1

Transition metal basic salts for OER and overall water splitting

Fig. 2

Crystal structure of a Co₂(OH)₃NO₃, b Co(OH)F, c Co₂(OH)₂CO₃, and d Co₂(OH)₃Cl

Fig. 3

a Crystal structure of Co₂(OH)₃Cl. b CV curves of 100 cycles in 1.0 M KOH with a scan rate of 100 mV s⁻¹. c Optical photos for the detection of Cl⁻ in electrolyte with the increase of the CV cycles. d LSV curves with the increase of the CV cycles. Scan rate: 5 mV s⁻¹. e STEM image of the AC-Co₂(OH)₃Cl. f Fourier-transformed Co K-edge EXAFS spectra and corresponding fitting curves for the activation derived-Co₂(OH)₃Cl. Reproduced with permission from Ref. [124]. Copyright 2019, Wiley–VCH Verlag GmbH & Co. KGaA, Weinheim. g Schematic illustration of the Fe modification to Co₂(OH)₃Cl. h LSV curves of oxygen reduction reaction before (solid line) or after 500 CV cycles (dashed line) in O₂-saturated 1 M KOH (negative scan). i LSV curves of the Fe modified Co₂(OH)₃Cl. Reproduced with permission from Ref. [125]. Copyright 2020, Elsevier Inc

Fig. 4

a Schematic of the growth and substructures, b SEM image of the hierarchical 3D Co(OH)F microspheres. c CVs of the Co-based pre-catalysts. d CV current–potential responses of the Co(OH)F sample at different scan rates: 0.5 V s⁻¹ (azure), 1 V s⁻¹ (green), 2 V s⁻¹ (red), 3 V s⁻¹(purple), 4 V s⁻¹ (blue), and 5 V s⁻¹ (brick red). e Calculated densities of states of Co(OH)F. Reproduced with permission from Ref. [105]. Copyright 2017, Wiley–VCH Verlag GmbH & Co. KGaA, Weinheim

Fig. 5

a Schematic representation of the growth process of the 3D needlelike N:Co(OH)F array structure on CFP. b LSV curves of the N:Co(OH)F and series comparison samples. Reproduced with permission from Ref. [130]. Copyright 2018, Royal Society of Chemistry. c LSV curves of Co(OH)F, P-Co(OH)F, YP-Co(OH)F, and IrO₂ for OER in 1 M KOH. Reproduced with permission from Ref. [131]. Copyright 2019, Wiley–VCH Verlag GmbH & Co. KGaA, Weinheim. d Schematic diagram of the sulfur atom replacing the fluorine atom. e The formation energy of Co_1−xS/Co(OH)F and Co(OH)F from DFT calculation. f DOS curves of Co(OH)F/CC and Co_1−xS/Co(OH)F/CC. g Schematic of Co_1−xS/Co(OH)F sample improving the OER activity. h OER LSV curves of Co_1−xS/Co(OH)F sample with a scan rate of 20 mV s⁻¹ and 80% iR correction. Reproduced with permission from Ref. [132]. Copyright 2022, American Chemical Society. i LSV curves of the as-prepared β-Co(OH)₂, Co(OH)F, and β-Co(OH)₂/Co(OH)F hybrid. Reproduced with permission from Ref. [133]. Copyright 2018, Elsevier Ltd

Fig. 6

a Schematic illustration of Co_xMn_yCH pre-catalyst. b The number of electrons in the 3d orbital per Co atom in CoCH and CoMnCH. c OER polarization curves of Co_xMn_yCH samples. Reproduced with permission from Ref. [96]. Copyright 2017, American Chemical Society. d Schematic illustration of psCoFeCH. e HRTEM image (inset: enlarged HRTEM image) of psCoFeCH. f OER LSV curves of CoFeCH, psCoFeCH, osCoFeCH, and the NF substrate. Reproduced with permission from Ref. [150]. Copyright 2019, Royal Society of Chemistry. g Schematic illustration of the hierarchical Cu(OH)₂@CoNiCH core/shell NTs grown on CF. Reproduced with permission from Ref. [98]. Copyright 2018, Royal Society of Chemistry. h SEM image of NiFeCHs-CNT/G. i Polarization curves of NiFeCHs-CNT/G and related samples. Reproduced with permission from Ref. [152]. Copyright 2022, Wiley–VCH GmbH

Fig. 7

a Schematic illustration of the fabrication procedure of MNH/NF through molten salt decomposition method for OER. b SEM image of NiNH/NF. c LSV curves of MNH/NF samples and NF (after iR correction). d Raman spectra in comparison with NiNH/NF before and after OER. Reproduced with permission from Ref. [154]. Copyright 2018, Wiley–VCH Verlag GmbH & Co. KGaA, Weinheim. e Schematic illustration of the synthesis of NiNH@Fe(OH)₃/NF by immersing NiNH/NF in Fe(NO₃)₃·9H₂O ethanol solution. f OER electrocatalytic properties of NiNH@Fe(OH)₃/NF materials in 1 M KOH solution. Reproduced with permission from Ref. [160]. Copyright 2020, American Chemical Society. g Schematic illustration of the fabrication procedure of Mo-NiNH on MNF substrate through molten salt decomposition strategy. h Polarization curves of Mo-NiNH @/MNF samples in 1 M KOH. Reproduced with permission from Ref. [161]. Copyright 2022, Elsevier Inc

Fig. 8

Schematic of structural evolution and anion effects for transition metal basic salts

Fig. 9

Polarization curves of various FeCoCH/NF samples for a HER, b OER and c overall water splitting. d The number of electrons in the 3d orbital per Co atom in CoCH and FeCoCH of Mo-NiNH @/MNF samples in 1 M KOH. Reproduced with permission from Ref. [118]. Copyright 2018, Wiley–VCH Verlag GmbH & Co. KGaA, Weinheim. Backward CV of sheet-like NiFeCH and Rh³⁺-NiFeCH toward e HER and f overall water splitting. Reproduced with permission from Ref. [169]. Copyright 2020, Royal Society of Chemistry

Fig. 10

a Schematic illustration for the fabrication procedures and b synergistic effect of CF@Ru-CoCH samples. c Free energy diagram of the Volmer-Tafel and Volmer-Heyrovsky pathways for HER in alkaline electrolyte for CF@Ru-CoCH. Reproduced with permission from Ref. [170]. Copyright 2019, Elsevier Itd. d ΔGH* values at different sites on surface 2D NiHN and Pt_SA-2D NiHN models towards HER at different sites on their surface. e ΔGH* comparison between Pt_SA-2D NiHN and Pt_SA-2D NiH models (inset: corresponding catalyst models). Reproduced with permission from Ref. [171]. Copyright 2022, Elsevier B.V. f Gibbs free energy diagram at 1.23 V for OER over Cu₃N and Cu₃N@CoNiCHs. Reproduced with permission from Ref. [173]. Copyright 2021, Elsevier B.V. g Schematic illustration of the fabrication procedure of NiCoS_x@CoCH NAs/NF. h Calculated adsorption energies of H* and H₂O* for CoCH, NiCoS_x, and NiCoS_x@CoCH. i Polarization curves of NF, Ni@CoCH NAs/NF, NiCoS_x@ CoCH NAs/NF, and Pt/C (20 wt%). Reproduced with permission from Ref. [166]. Copyright 2021, American Chemical Society

Fig. 11

a Schematic illustration for the synthetic process of MnCo-CH@NiFe-OH pn junction. b Energy diagrams of the MnCo-CH and NiFe-OH (left) and the MnCo-CH@NiFe-OH pn junction (right). c Free energetic pathway of water oxidation over MnCo-CH@NiFe-OH. d IR-corrected LSV curves of MnCo-CH@NiFe-OH toward overall water splitting. Reproduced with permission from Ref. [175]. Copyright 2021, Elsevier B.V. e Schematic illustration for the step-wise fabrication of the Co(OH)F@CoFe-LDH heterostructure supported on NF. f Digital photograph of the setup for water splitting driven by one battery. g Polarization curves of various Co(OH)F@CoFe-LDH sample. Reproduced with permission from Ref. [95]. Copyright 2022, Royal Society of Chemistry