Cobalt-based Highly Efficient Electrocatalysts For Oxygen Evolution Reaction

Electrocatalytic water-splitting is pivotal for efficient and economical production of hydrogen,the device efficiency is mainly determined by the oxygen evolution reaction?OER?process at the anode.Thus,it is imperative to develop inexpensive,highly active OER catalysts.By examining the underlying critical factors for OER performance,this paper presents general design principles for nanosized OER catalysts including enhancing the intrinsic activity of active site and maximizing the quantity of accessible active sites.Guided by these strategies,a series of highly efficient OER catalysts were developed as exemplified.1.Electronic and morphological dual modulation of cobalt carbonate hydroxides by Mn doping towards highly efficient and stable OER electrocatalystCo-Mn carbonate hydroxide?CoMnCH?nanosheet arrays with controllable morphology and composition were developed on nickel foam?NF?as such bifunctional electrocatalysts was reported in this section.It is discovered that Mn doping in CoCH can simultaneously modulate the nanosheet morphology to significantly increase the electrochemical active surface area for exposing more accessible active sites and tune the electronic structure of Co center to effectively boost its intrinsic activity.As a result,the optimized Co₁Mn₁CH/NF electrode exhibits unprecedented OER activity with an ultralow overpotential of 294 mV at 30 mA cm^-2,compared with all reported metal carbonate hydroxides.Benefited from 3D open nanosheet array topographic structure with tight contact between nanosheets and NF,it is able to deliver a high and stable current density of 1000 mA cm^-2 at only an overpotential of 462 mV with no interference from high-flux oxygen evolution.Despite no reports about effective HER on metal carbonate hydroxides yet,the small overpotential of 180 mV at 10 mA cm^-2 for HER can be also achieved on Co₁Mn₁CH/NF probably due to the dual modulation of Mn doping.This offers a two-electrode electrolyzer using bifunctional Co₁Mn₁CH/NF as both anode and cathode to perform stable overall water splitting with a cell voltage of only 1.68 V at 10 mA cm^-2.These findings may open up opportunities to explore other multimetal carbonate hydroxides as practical bifunctional electrocatalysts for scale-up water electrolysis.2.Kinetically controlled synthesis of metal hydroxide nanosheets/graphene composites towards efficient OER electrocatalystA kinetically controlled room-temperature coprecipitation was developed as a general strategy to produce a variety of sandwich-type metal hydroxide/graphene composites was reported in this section.Specifically,well-defined?-phase nickel cobalt hydroxides nanosheets are vertically assembled on the entire graphene surface?NiCo-HS@G?to provide plenty of accessible active sites and enable facile gas escaping.The tight contact between NiCo-HS and graphene promises effective electron transfer and remarkable durability.It is discovered that Ni doping adjusts the nanosheet morphology to augment active sites and effectively modulates the electronic structure of Co center to favor the adsorption of oxygen species.Consequently,NiCo-HS@G exhibits superior electrocatalytic activity and durability for OER with a very low overpotential of259 mV at 10 mA/cm².Furthermore,a practical water electrolyzer demonstrates a small cell voltage of 1.51 V to stably achieve the current density of 10 mA/cm²,and 1.68 V to 50 mA/cm².Such superior electrocatalytic performance indicates that this facile and manageable strategy with low energy consumption may open up opportunities for the cost-effective mass production of various metal hydroxides/graphene nanocomposites with desirable morphology and competing performance for diverse applications.3.Constructing single-layered cobalt-iron hydroxide nanosheets/graphene composites and modulating their catalytic activity for oxygen evolution reactionA successful construction of a sandwich-type CoFe hydroxide nanosheets/graphene hybrid electrocatalyst with proper hydroxide nanosheets?HS?loading was reported in this section.By adjusting the feeding amount,the whole graphene matrix was fully covered by single-layered CoFe HS.This unique and integrated structure are in favor of the high exposure of electrochemical active sites,the inward diffusion of electrolyte ions,and still maintained the fast electron transfer during the electrochemical reaction process.Benefits from these advantages,the as-made CoFe-0.28@G nanocomposites demonstrated a lower OER overpotential of 260,294 and312 mV at current densities of 10,50 and 100 mA cm^-2.Furthermore,a three-dimmensional CoFe-0.28@G architecture were further fabricated to deliver 20 mA cm^-2 at 246 mV for OER.This architecture sturcture was then measured under industrialized electrolytic condition and could deliver 20,100,1000 and 2000 mA cm^-2 under extremely low potentials of 1.406,1.431,1.480and 1.507 V?vs.RHE?,respectively.Futhermore,a practical water electrolyzer demonstrates a small cell voltage of1.44,1.52 and 1.58 V to deliver 10,50 and 100 mA cm^-2,respctively under industrialized electrolytic condition.Such superior catalytic performance enables the application of novel CoFe-0.28@G/NF composites in practical water electrolysis.