Preparation And Performance Optimization Of Oxygen Evolution Catalysts And Their Application In Hematite Photoanode To Improve Photoelectrochemical Water Splitting Performance

Oxygen evolution reaction?OER?is a bottleneck transversion process involved in many energy conversion and storage processes due to its sluggish kinetics.Developing effective catalysts to reduce the high overpotential could accelerate the water oxidation process with an enhanced efficiency.Photoelectrochemical?PEC?water splitting can convert solar energy into chemical energy,which may provide a possible avenue to relieve the problem of energy crisis and environmental pollution.Among various photoanode for water splitting,hematite??-Fe₂O₃?is one of the most promising candidates.However,suffered from its short carrier diffusion length and sluggish OER kinetics,the?-Fe₂O₃ photoanode performance is still far from practical requirement.This research tried to explore cost-effective OER catalysts,which were then applied onto?-Fe₂O₃ photoanode to optimize its PEC water splitting performance.Here,metal foam electrodes,trace Au activated CoFe₂O₄ catalysts?Au-CoFe₂O₄?and Ag loaded Fe-Co-S/N-doped carbon composites?Ag-CISC?were designed,prepared and systematically characterized for oxygen evolution reaction.The influences of trace precious metals such as Au and Ag on Co-Fe based oxygen evolution catalysts were investigated.We further tried to apply Au-CoFe₂O₄ and Ag-CISC catalysts on?-Fe2O3photoanode to further enhance its PEC water splitting performance.This research has accumulated valid experimental data in selecting transition metal-based materials for OER catalysts and?-Fe₂O₃ photoanode cocatalysts.The details are as follows:1.By introducing Fe species,cobalt foam with high-active double metal hydroxide surface was prepared as catalytic oxygen evolution electrode?Co-BA-Fe?through simple dipping method.This electrode requires a low overpotential of 262 mV at 10mA cm^-2,showing 82 mV of decrease than that of pristine cobalt foam.Benefited from the good electrical conductivity of metal foam,the Tafel slope of Co-BA-Fe is reduced to 20.2 mV dec^-1.The catalytic electrode for oxygen evolution presents remarkable stability at operation current density of 20 mA cm^-2 for more than 70 h.This surface modification method can also be extended to nickel foam.The catalytic nickel foam electrode obtained by similar method delivers current density of 10 mA cm^-2 at 239 mV,with a Tafel slope of 16.9 mV dec^-1.Especially,this simple surface engineering strategy with mild preparation condition but without other special thermal equipment and external energy can be easily scaled up,thus endowing it possible practical application.2.Introducing of trace Au on CoFe₂O₄ surface by electrochemical deposition highly improves the OER activity,decreasing the overpotential of 374 mV at 10 mA cm^-2 to 312 mV and showing a smaller Tafel slope of 35 mV dec^-1 in 1 M KOH and an enhanced stability.The loaded trace Au largely increases the electrochemical active specific area.The electronic interactions between Au and the adjacent CoFe₂O₄modulate the electron structure of metal sites,which further facilitate the transform of rate-limiting step.When loaded onto?-Fe₂O₃ photoanode as cocatalyst,Au-CoFe₂O₄can help the photocurrent density increase from 0.082 to 0.56 mA cm^-2 at 1.23 vs RHE.The loading of Au-CoFe₂O₄ cocatalyst promotes the consumption of holes on?-Fe₂O₃photoanode,thus suppressing the recombination of electron-hole pairs and keeping high stability during water splitting process.3.Ag species can be introduced into Fe-Co-S/N-doped carbon composites?CISC?by simple dipping method.With appropriate amount of Ag loading,the overpotential to achieve current density of 10 mA cm^-2 decreased of 37 mV in 1 M KOH.It is demonstrated that loading of Ag not only improves the activity of catalytic site,but also causes a bigger electrochemically-active surface area.The optimized OER catalyst was then covered on a hematite photoanode.It was found that the catalyst can improve the photocurrent density with 7 times of enhancement at potential of 1.23 V vs RHE and induce a negative shift of 330 mV of onset potential by accelerating the consumption of surface photogenerated holes.In addition,the amperometric i-t curves show the good stability of Ag-CISC catalyst and catalyst-modified?-Fe₂O₃.This study demonstrates a surface decoration route to improve the catalytic activity for OER.