In Situ Growth Of Cobalt And Nickel Phosphate On Nickel Foam For Electrocatalytic Oxidation

With the development of human economy and society,the traditional fossil energy coal,oil and natural gas,is increasingly exhausted,and the consumption of fossil energy has caused serious environmental problems.Therefore,it is urgent to seek clean and renewable energy.Hydrogen energy is regarded as an ideal energy carrier due to its advantages such as abundant reserves,high calorific value and no pollution.Electrolysis of water is expected to be one of the most promising ways for hydrogen production in the future.Electrolysis of water mainly includes two half reactions,the hydrogen evolution reaction(HER)at the cathode and the oxygen evolution reaction(OER)at the anode.Compared with electrolysis of water,electrolysis of urea has a lower thermodynamic voltage,which can not only greatly reduce the cost of hydrogen production,but also degrade urea in wastewater to prevent water eutrophication.Electrolysis of urea also consists of two half reactions,the HER at the cathode and the urea oxidation reaction(UOR)at the anode.In addition to a series of the basic processes,OER and UOR also contain a four-electron and six-electron transfer process,respectively,which limits the overall reaction efficiency of water and urea electrolysis.Therefore,it is necessary to develop the efficient and cheap OER and UOR electrocatalysts.In this thesis,two transition metal phosphate electrocatalysts in-situ grown on nickel foam(NF)were prepared and their OER and UOR performance were studied.The main work included the following parts:(1)A series of electrocatalysts of cobalt phosphate in situ grown on NF substrates(CoPO/NF)were prepared by the hydrothermal/solvothermal method in different solvents.Electrochemical tests revealed that the CoPO/NF catalyst prepared in the mixed solvent of water and ethylene glycol(CoPO/NF-H2O/EG)exhibited the excellent OER activity with a tafel slope of 65.6 mV dec-1 in 1.0 M KOH electrolyte solution and an overpotential of 164 mV at the current density of 50 mA cm-2.In addition,CoPO/NF-H2O/EG catalyst had an ultra-high electrochemical stability.Compared with the initial current density,CoPO/NF-H2O/EG remained 96.7%after 48 h of current-time test.There was no obvious decrease in activity after cyclic voltammetry(CV)scanning to 20000 cycles.The results show that CoPO/NF-H2O/EG is a highly efficient and stable three-dimensional self-supporting electrocatalyst for OER.(2)The previous work showed that the binderless three-dimensional selfsupporting catalyst in situ grown on NF substrate exhibited excellent performance for electrocatalytic water oxidation.On the basis of the above mentioned results,nickel cobalt phosphate evenly grown on NF substrate(NiCoPO/NF)was also synthesized by the hydrothermal method.Electrochemical tests showed that the NiCoPO/NF catalyst not only had the prominent OER performance but also had the prominent UOR performance.At the current density of 100 mA cm-2,the potentials of OER and UOR were 1.51 V and 1.34 V vs.reversible hydrogen electrode,respectively.In the 1.0 M KOH electrolyte containing 0.33 M Urea or not,the current density of NiCoPO/NF catalyst did not apparently decrease after the continuous electrolysis for 48 h.The results showed that NiCoPO/NF is a highly efficient and stable bifunctional electrocatalytic oxidation material that can be used for OER and UOR.