Electrodes Based On Non-noble Metals (Iron,Cobalt,Nickel,and Copper) For Electrocatalytic Water Oxidation

The conversion of solar energy into hydrogen energy through water splitting is an effective way to provide clean energy sources for production and living of human society.Water oxidation is considered to be the key part of the full water splitting,because it need high thermodynamic barriers and undergoes the complex transfer process of four electrons and protons.Therefore,it is crucial to develop highly efficient.stable and and low cost electrocatalysts to accelerate the reaction kinetics of water oxidation reaction.Firstly,the cobalt-based heterogeneous electrocatalyst(Co-Hi)was prepared on fluorine doped tin oxide(FTO)substrate via electrodeposition method from the neutral 2-[4-(2-hydroxyethyl)piperazin-l-yl]ethanesulfonic acid(HEPES)buffer solution dissolved Co2+.The as-prepared Co-Hi/FTO electrode can reach a catalytic current density of 1.5 mA/cm2 for water oxidation at 1.39 V vs.NHE in phosphate buffer solution(0.1 M,pH = 7.0)during 20 h,and the Faraday efficiency is 98%.Secondly,the copper-based heterogeneous electrocatalyst(Cu-Tris)was electrodeposited on indium doped tin oxide(ITO)substrate from the precursor formed in-suit based on Tris(hydroxymethyl)aminomethane(Tris)and Cu2+.The as-prepared Cu-Tris/ITO electrode showed a significantly low overpotential of 390 mV at a current density of 1.0 mA/cm2 for electrocatalytic water oxidation in phosphate buffer solution(0.2 M,pH = 12.0).Simultaneously,this electrode also exhibited a high and stable current density of 7.5 mA/cm2 at 1.15 V vs.NHE during 10 h,and the Faraday efficiency is 97%.Furthermore,the nickel-based heterogeneous electrocatalysts(NiOx,-aqua and NiOx-Tris)were formed on FTO substrate from the different precursors of Ni2+ and Ni-Tris complex(obtained in-situ from Tris and Ni2+)through electrodeposition and electrolytic polarization treatment.The NiOx-Tris and NiOx-aqua electrocatalysts achieved the stable catalytic current density of 5 and 4 mA/cm2 for 20 h at a potential of 1.3 V vs.NHE in the borate buffer solution(0.6 M,pH = 9.2),respectively,and Faraday efficiency is also close to 100%.Results of inductively coupled plasma atomic emission spectra(ICP-AES)and X-ray photoelectron spectra(XPS)indicate that a small amount of Fe element was introduced into the nickel-based electrocatalysts after the polarization treatment,and the synergy effect between the Ni and Fe elements lead to the enhancement of catalytic activity of catalysts.In addition,the different electrocatalytic activities of these two electrocatalysts for water oxidation are mainly due to their diverse morphologies.Finally,the seaweed-like Cu2O nano wires,fabricated on copper foam(CF)through the simple anodic oxidation and high temperature calcination methods,were loaded with NiFe electrocatalyst to form a highly efficient electrode(NiFe/Cu2O-NWs/CF).The NiFe/Cu2O NWs/CF electrode only required an overpotential of 215 mV to achieve a catalytic current density of 10 mA/cm2 in 1 M KOH solution(pH = 13.6).Moreover,the electrode exhibited a steady catalytic current density of 100 mA/cm2 at an overpotential of 260 mV during 50 h,and the Faraday efficiency is 99%.