Preparation Of Transition Metal Hydroxides At Room Temperature And Their Electrocatalytic Oxygen Evolution Performance

The excessive consumption of traditional fossil fuels and the rapid deterioration of the environment have forced people to vigorously develop safe,clean,and high-energy-density renewable energy sources.Hydrogen production by electrolysis of water is one of the effective ways to produce clean energy.Since the oxygen evolution reaction（OER）of water electrolysis involves the transfer process of four electrons,the efficiency of hydrogen production by electrolysis of water is limited.Although some metal oxides（Ru O₂,Ir O₂）have high electrocatalytic OER performance,their high price and scarce reserves limit their wide application.Transition metal（Fe,Co,Ni,etc.）hydroxides have large reserves on the earth,are cheap,and have high OER activity and stability.Therefore,they have attracted the attention of researchers.In this paper,the goal is to develop highly efficient and inexpensive transition metal hydroxide electrocatalysts that can be prepared at room temperature.The non-covalent phosphorylated CoCrLDHs（Layered Double Hydroxides）and the new non-noble metal Cd（OH）₂ are obtained through the co-precipitation method.The“sandwich”structure CoFeCo-OH was obtained by electrodeposition method and their OER electrocatalytic performance was studied.The main research contents are as follows:1.First,CoCrLDHs were synthesized by co-precipitation at room temperature,and then phosphate and CoCrLDHs were blended to obtain non-covalent phosphorus-modified P-CoCrLDHs.The overpotential of the synthesized P-Co₂Cr₁ LDHs is 264 m V at 10 m A/cm²,the Tafel slope is 45 m V/dec,the double-layer capacitance(C_dl)is 69.6 m F/cm²,and the charge transfer resistance(R_ct)is 11.3Ω,far better than unmodified Co₂Cr₁ LDHs and commercial Ru O₂.Various characterization results showed that the modification of phosphate ester increased the content of Co³⁺and the number of active sites,and improved the conductivity of CoCrLDHs,making the electrocatalytic activity of P-CoCrLDHs much higher than that of CoCrLDHs.2.A novel non-noble metal and non-first-period transition metal hydroxide Cd（OH）₂ OER electrocatalyst was develpoed,Cd（OH）₂ nanosheets were easily prepared by adding sodium hydroxide solution to the cadmium salt solution by co-precipitation method at room temperature.The theoretical calculation results show that the energy barrier of the rate-limiting step of Cd（OH）₂ is 2.41 e V,which is lower than that of the commonly used OER catalyst Co（OH）₂（2.46 e V）.The free energy of formation of the key active substance（*OOH）is 0.98e V,which is much lower than the free energy of Co（OH）₂（2.41 e V）,which proves that the reaction kinetics and intrinsic activity of Cd（OH）₂ are excellent.The overpotential of Cd（OH）₂nanosheets is 266 m V at 10 m A/cm²,the Tafel slope is 47 m V/dec,the double-layer capacitance（Cdl）is 57.6 m F/cm²,and the charge transfer resistance（Rct）is 6.4Ω,far better than commercial Ru O₂.After 100 hours of constant current test,it is found that the overpotential does not have a significant shift,which indicates that the Cd（OH）₂ sample has good stability.Compared with the representative first-period transition metal hydroxide OER electrocatalyst Co（OH）₂,Cd（OH）₂ exhibits more excellent OER activity and lower adsorption free energy of active intermediates.3.At room temperature,a new type of“sandwich”structure CoFeCo-OH OER electrocatalyst was prepared by electrodeposition.The overpotential of CoFeCo-OH is 234 m V at 10 m A/cm²,the Tafel slope is 39.3 m V/dec,the double-layer capacitance(C_dl)is 43.9 m F/cm²,and the charge transfer resistance is 3.67Ω,far better than commercial Ru O₂.The mass loss of CoFeCo-OH was only 3.7%after the chronopotentiometry test for 24 hours.The SEM morphology before and after the test also proved that the porous sheet structure of Co（OH）₂provides a stable support structure for Fe（OH）₃ nanoparticles and prevents the Fe（OH）₃nanoparticles from falling off.This shows that the“sandwich”structure of CoFeCo-OH is conducive to the high activity of Fe（OH）₃ nanoparticles and the stable main role of Co（OH）₂.At the same time,the high-conductivity Co layer contacts Fe（OH）₃ nanoparticles on both sides,so that Co and Fe have a strong synergistic effect.Further improve the conductivity and electrocatalytic OER activity of the CoFeCo-OH electrocatalyst.