Electrochemical Overall Water Splitting Based On 3d Transition Metal Alloy Layered Double Hydroxides

As the 3d transition metal ions contain unfilled 3d electron orbits,i.e.the uncoupled 3d electrons,it is easy for the 3d transition metal ions to form the chemical bondings with H+ or OH-in aqueous solution.What is more,the chemical bondings are neither too strong for the produced H₂/O₂ to release from the surface nor too weak for the H+/OH-to be located around the metal ions.As a result,the 3d transition metals exhibit superior hydrogen?HER?and oxygen evolution?OER?activity simultaneously.Therefore,3d transition metal alloy layered double hydroxides are supposed to show excellent catalyst activity for overall water splitting due to there layer structure of larger specific surface area and more active sites.And above all,the interlayer ions and water molecules can guarantee sufficient contact between the catalyst electrode and the electrolyte to further accelerate the electrochemical reaction process.Hence,to this day,the 3d transition metal alloy layered double hydroxides?LDHs?are widely studied by domestic and foreign scientists.So,for the above reasons,the work of this paper is carried out based on 3d transition metal alloy LDHs as catalyst electrodes.In this paper,to further improve the performance of electrochemical catalysts for overall water splitting,we simply synthesized three kinds of 3d transition metal alloy LDHs,i.e.NiCo,NiFe@Ni foam and NiFeCo LDHs@Ni foam,as the HER and OER catalysts through hydrothermal method.Firstly,by adjusting the ion ratios of Ni/Co,Ni/Fe and Ni/Fe/Co of the catalyst electrodes,the performances for both hydrogen?HER?and oxygen evolution reaction?OER?are dramatically enhanced.And the most appropriate ratios are Ni_0.74Co_0.36,Ni_0.75Fe_0.25 and Ni_0.49Fe_0.3Co_0.21,respectively,which is subsequently proved by various characterization methods.Shown in the Scanning electron microscopy?SEM?images,with the contents of Co,Fe and Ni atoms inside the LDHs varying,the surface topographies significantly change,like nanoplates,nanofibers,nanowires,nanorobs and so on.And so do the surface area and the electroconductivity of the electrodes.When electrochemically tested in a three-electrode system,with Pt foil as counter electrode,Ag/AgCl as reference electrode and LDHs sample as working electrode,the samples Ni_0.68Co_0.32,Ni_0.75Fe_0.25 and Ni_0.49Fe_0.3Co_0.21 LDHs have overpotentials ?10?to reach the current density of 10 mA·cm-2?of respectively 179 mV?HER?and 150 mV?OER?,169 mV?HER?and 140 mV?OER?,200 mV?HER?and 150 mV?HER?.And then,using a two-electrode system to evaluate the real activity for OER and HER simultaneously?the anode and cathode are both the same LDHs sample?,the hydrogen evolution rates are 148 umol·h-1·cm-2,170 umol·h-1·cm-2 and 202 umol·h-1·cm-2,respectively.And the ratios of H₂/O₂ are all nearly 2:1.What is more important,boldly,we applied a new novel method,i.e.analyzing the XPS spectra especially the O1s spectra,to confirm the exact active sites on the surface of the electrode.As a result,the Ni3+ in the ?-NiOOH-like structure is verified to be the active site for OER.When the content of Co²⁺ or Fe³⁺ substitutes of Ni²⁺ increases,the amount of active sites rises to a maximum and then decreases.Although the X-ray diffraction?XRD?spectra do not exhibit any distinct peaks except the three diffraction peaks of Ni foam,the High revolution transmission electron microscope?HRTEM?proves the nanoflakes to be consisted of weakly crystallized nanoparticles between the noncrystallized domains,which assures the sufficient transparency of electronics and holes.To summarize,the 3d transition metal alloy LDHs are highly active towards both OER and HER.And by increasing the active sites during synthesis process,we can significantly improve the catalytic activity,as well to further promote the commercial application.The electrochemical activities for overall water splitting are listed in this order:NiFeCo LDHs>NiFe LDHs>NiCo LDHs.