Development Of Cobalt-Based Transition Metal Electrocatalysts For Water Electrolysis Application

As a clean and renewable energy,the proportion of hydrogen energy will be gradually increased in the future energy structure.Water electrolysis powered by renewable energy sources is an important technological approach to achieve low carbon emissions.The cathodic hydrogen evolution reaction（HER）and the anodic oxygen evolution reaction（OER）are integral parts of water electrolysis and the active components at anode and cathode are electrocatalysts.Thus,highly-efficient and stable electrocatalysts are essential for practical application of water electrolysis.At present,the electrocatalysts used for commercial hydrogen production are noble metal-based catalysts,thereby the exorbitant price restricts its large-scale applications in water electrolysis system.This dissertation is concentrated on the objective of constructing economic and energy-efficient system for hydrogen production from water electrolysis,which is base on the investigation strategy of the modulation of the microstructure and active component of HER and OER electrocatalysts,the exploration of different anodic reactions;their integrated application is carried out in the solar photovoltaic（PV）-water electrolysis system.The contents of studies are as following:（1）The cobalt phosphides embedded on nitrogen-doped carbon matrix nanoarrays（Co P_x@N-C NAs）with controllable structure and component was successfully prepared through phosphating process of Co-based metal organic frameworks（MOFs）material,which was as the catalytic material for HER.The experimental results demonstrated that the nanoarrays structure was beneficial for the rapid detachment of gas bubbles from the surface of electrocatalysts;the appropriate content of P optimizes the hydrogen adsorption free energy.The reduced overpotentials induced by the surface microstructure and the active component of the optimal electrocatalyst（Co₂P&Co P@N-C NAs）were 40 m V and 28 m V at 10 m A cm^-2,respectively.Integrating with a solar cell,the PV-water electrolysis system was constructed and achieved a solar-to-hydrogen（STH）conversion efficiency of 14.5%.（2）Cobalt/iron-molybdenum oxides with ultrathin nanosheets loaded on Ni foam(NF/Co_1-xFe_xMo O)was fabricated by modulating the proportion of cobalt/iron and the preparation parameters,which were applied to activate HER and OER reactions.In H₂/Ar atmosphere,the NF/Co Mo O-H was obtained,delivering HER catalytic activity;in Ar atmosphere,the NF/Co_1-xFe_xMo O-A was fabricated,showing OER catalytic activity.For the HER process,the electron-rich Mo of NF/Co Mo O-H presents low valence states,which was effective in promoting HER activity(J=10 m A cm^-2,η=15m V).For the OER process,the strong electron-withdrawing of Mo induce Co and Fe atoms to remain high valance states,but the electrochemical oxidation of Co to high valance state of reactive intermediate（Co OOH）was suppressed with Fe content being increased.When the Fe content reached 60%(NF/Co_0.4Fe_0.6Mo O-A),an improved OER activity and a low overpotential was achieved(J=10 m A cm^-2,η=190 m V).Furthermore,NF/Co_0.4Fe_0.6Mo O-A（+）||NF/Co Mo O-H（-）demonstrated a low voltage of 1.45 V at the current density of 10 m A cm^-2 for overall water electrolysis.The PV-water electrolysis system was constructed and delivered a hydrogen generation rate of0.47 L m_cell^-2 h^-1 and no performance degradation was observed during 160 h,in conjunction with a STH efficiency of 15.1%.（3）Based on the above work,cobalt/iron-molybdenum oxide（NF/p-Co Fe Mo O₄）with ultrathin and porous nanosheets was obtained by further optimization of surface microstructure.The reduced anodic OER overpotential(J=10 m A cm^-2,η=176 m V)could be ascribed to the increased specific surface area and active sites.The replacement of OER with urea oxidation reaction（UOR）induced the significant operation voltage decrease of water electrolysis(decreased by 0.15 V at 100 m A cm^-2).Hydrogen generation rates of 0.75 L m_cell^-2 h^-1 and 0.64 L m_cell^-2 h^-1 were achieved on PV-water electrolysis and PV-urea hybrid seawater electrolysis systems,respectively.The highest STH efficiency of 20.3%was obtained from PV-water electrolysis system.Energy-efficient PV-seawater electrolysis for hydrogen production was realized without any external energy input.