Construction Of Highly Efficient Electrocatalysts Based On Metal Oxalates And Their Performances For Electrocatalytic Water Splitting

The energy and environment has become a problem for the sustainable development of human society. Expoitment of clean, green and sustainable new energy has attracted increasing attention nowadays. Water is the most widespread material. Splitting water to product H2 would alleviate energy problem and the pollution associated with fossil fuel. Oxygen evolution reaction(OER) process is the bottleneck of water electrolysis, because of its slow kinetics and high overpotencial, lowering the efficiency of water splitting. Noble-metal electrocatalysts could improve the reaction efficiency, but their high cost limits the large-scale application. It is of great significance to develop nonprecious electrocatalysts to lower overpotencial and obtain ideal anodic and cathodic current density. In fact, recent research has demonstrated transition-metal materials show comparable catalytic activity to those of noble metal electrocatalysts. In this dissertation, we focus on the synthesis of nonprecious, cheap and highly efficient transition-metal oxalates, study their performances for water splitting in different media, and discuss the related mechansim on electrocatalytic process. The main contents are as follows:1. The cobalt oxalate electrocatalysts have been successfully synthesized by a simple solvothermal process. Their morphologies could be easily controlled by reasonable adjusting the solution composition. Water solution is favorable to generate cobalt oxalate micropolyhedrons, while cobalt oxalate microrods are easily obtained by using ethylene glycol as solvent. The two cobalt oxalate electrocatalysts exhibit different OER activity, which is closely related to specific surface area and intrinsic electrochemical active sites of cobalt oxalate. The OER mechanism of cobalt oxalates was revealed by Raman spectra and X-ray photoelectron spectroscopy, which indicates that the enhanced activity is atrributed to the cobalt species with higher oxidation states produced during OER process.2. We use a similar method to prepare the Ni-Co oxalate solid solution electrocatalysts. These Ni-Co oxalates retain the origin morphology of cobalt oxalate, which is mainly determined by the solvent. X-ray diffraction, scanning electron microscope and Fourier transform infrared(FTIR) spectroscopy are used to character the structure and morphology of Ni-Co oxalate solid solution electrocatalysts. Among these electrocatalysts, nickel oxalate shows no OER activity but the activity can be improved by increasing Co amounts. It is noticeable that the current density of Ni-Co oxalate solid solution electrocatalysts is greatly improved and the overpotencial is lower compared to those of the pure cobalt oxalate and nickel oxalate. Chronoamperometry test also confirms the good stability of the Ni-Co oxalate solid solution electrocatalysts in alkaline solution.3. A series of Co2 P electrocatalytic materials were prepared by a low temperature phosphate treatment using cobalt oxalate micropolyhedrons as the precursor. Unlike the precursor, cobalt phosphide presents the morphology of nanosticks with uniform size. Experimental results have shown that the amount of sodium hypophosphite and phosphate temperature can affect the electrochemical performance of Co2 P. Co2 P electrocatalyst shows excellent activity for electrocatalytic hydrogen evolution in an acidic medium. In alkaline medium, Co2 P electrocatalysts present good activity for overall water splitting. Chronoamperometric and chronopotentiometric tests exhibit the excellent OER and HER stability of the electrocatalytic Co2 P materials.