Design,Synthesis,and Evaluation Of Spinel Transition-Metal-Based Electrocatalysts For Water-Splitting

Currently the energy crisis presents major challenges and alternative sources to natural resources should be sought to ensure sustainable development while reducing energy consumption.Currently,noble-metal-based catalysts have predominantly been used,but the scarcity and high cost of the noble metals hinder their large-scale applications.Significant progress has been made over the past a few years in developing single-transition-metal?STM?-based sulfides,selenides,nitrides,and phosphides for both hydrogen evolution reaction?HER?and oxygen evolution reaction?OER?.Nevertheless they were reported to be thermodynamically less stable and have a lower electrocatalytic activity than notable-metals.Therefore,it still remains a significant challenge to develop efficient,durable,and low-cost new-generation electrocatalysts for electrochemical water-splitting.Among mixed-transition-metal?MTM?-based electrocatalysts,spinel?AB₂X₄?,which form an interesting class of compounds,have emerged as efficient and durable electrocatalysts for water splitting,owing to their desirable electrical conductivity,synergistic effect of bimetallic atoms,and structural stability.In this paper,the study revolves around how to obtain a highly efficient and stable spinel structure catalyst to achieve catalyst modification and specific functions.The performance is continuously improved by controlling the morphology,structure,and composition distribution to change the surface state and electronic structure of the catalysts.Combined with theoretical guidance and experimental exploration,reveal the nature of catalysis.The main research contents are as follows:?1?The morphology plays critical role on catalytic properties,i.e.,the ultrathin NiCo₂O₄ nanosheets?NCO-NSs?and NiCo₂O₄ nanowires?NCO-NWs?as efficient electrocatalyst for HER and OER in alkaline solution,respectively.With the help of DFT calculation on the d-band center energy and the adsorption energy of intermediates on different shaped NiCo₂O₄,we have successfully identified spinel nickel cobaltite catalysts of two distinctly different morphologies with highly catalytic capacity for water splitting.We demonstrated the adsorption of H^*and O^*on NiCo₂O₄?311?surface is the rate-determining-step of the HER and OER process in alkaline solution,respectively,in which the Ni_Td³⁺(Ni³⁺occupied at the tetrahedral site)acts as the active site.Furthermore,a NCO-NSs||NCO-NWs two-electrode alkaline electrolyzer exhibits excellent electricity to chemical power conversion efficiency with a low bias of 1.70 V to achieve a current density of 20?mA?cm^-2 for overall water-splitting.These results clearly demonstrate the benefits for improvements in performance and stability through the use of highly proficient morphology of catalysts in the water splitting system.Higher power conversion efficiencies will most certainly be obtained in the future by optimization of catalyst hierarchical nanoporous morphology.?2?The sea coral-like NiCo₂O₄@?Ni,Co?OOH heterojunction exhibited high catalytic activity and excellent durability,achieving a current density of 10 mA?cm^-2 at a low overpotential of 120 mV for HER and 220 mV for OER,respectively.A NiCo₂O₄@?Ni,Co?OOH||NiCo₂O₄@?Ni,Co?OOH two-electrode alkaline electrolyzer achieved a value of 100 mA?cm^-2 at a low cell bias of 1.83 V,Furthermore,the high current density approached 0.6 A?cm^-22 at 2.1 V along with 10 h stability.DFT calculations demonstrate that the activation energy of water-splitting can be lowered by forming the NiCo₂O₄@?Ni,Co?OOH heterojunctions than that of NiCo₂O₄.The FPMD results revealed the different water-splitting mechanisms:e.g.,for NiCo₂O₄@?Ni,Co?OOH heterojunction,it was the stretching vibrations of the metallic bonds to open the hydrogen bonding of water.We believe that NiCo₂O₄@?Ni,Co?OOH heterojunctions meet the stringent requirements of high current density from industrial water electrolysis applications.The cations effects of the substituted AB₂X₄ help to the catalytic behavior for water-splitting.?3?ACo₂O₄ phases?A=Mn,Fe,Co,Ni,Cu,Zn?were successfully synthesized using a facile hydrothermal method,and they show a unique flower-like morphology,with uniform mesopores and good crystallinity.They have good electrocatalytical activities for OER.The FeCo₂O₄ MMOFs showed excellent catalytic activities with a current density of10 mA·cm^-2 at a bias of 1.39 V in the alkaline media.The effects of substitution for A cations and the catalytic behavior for OER of the substituted ACo₂O₄ have been analyzed in detail.We further investigated the cations diffusion in the ACo₂O₄ for the OER step of water-splitting as an example to illustrate the fundamental principles of cations diffusion in these metal oxides.The spinel crystal field was identified as the essential parameter to control the charge transport and valence states of cations in the ACo₂O₄.?4?This work presents the NiCo₂X₄?X=O,S,Se,Te?nanocluster catalysts for efficient overall water splitting application.The introduction of electronegative elements to the NiCo₂-based compounds can significantly affect?G_H values at the catalyst surface.Furthermore,the excellent electrocatalytic activities of NiCo₂Te₄/PTCDA could be attributed to the perfect tuning of the energy distributions of valence electrons of NiCo₂Te₄ by induced by surface ligand PTCDA.The newly designed catalyst of NiCo₂Te₄/PTCDA exhibited an overpotential of 43 mV at 10 mA·cm^-2?vs.RHE?during the HER electrocatalytic activity in 1 M PBS.Additionally,this catalyst showed a high catalytic activity towards OER with a low overpotential of 120 mV at 10 mA·cm^-2.A two-electrode water electrolysis cell using the NiCo₂Te₄/PTCDA only needed a low bias voltage of 1.40 V to acquire a current density of 10 mA·cm^-2 for overall water splitting.This is remarkably superior to those reported electrochemical activities of the transition metal catalysts,including the noble metal-based Pt/C||RuO₂ cell.