Transition metal-based electrocatalysts for efficient water splitting and its self-reconstruction mechanism

Abstract

In the past several decades, resource crisis of non-renewable fossil fuels is raising as a major concern over the security of our energy future. Developing sustainable and eco-friendly fuels could play a major role in reducing greenhouse effect while providing sufficient feedstocks to power industrial production and to supply households use. Among various energy sources, hydrogen is widely accepted as a promising candidate with high power density and zero-carbon emission. From this prospective, hydrogen evolution from water electrolysis plays a key role in this energy conversion process and numerous electrocatalysts have been developed to improve the efficiency of the total reaction. By now, series of transition metal-based electrocatalysts, including transition metal dichalcogenides, phosphides and carbides have been reported to be effective for water electrolysis. However, the electrocatalytic performance of bifunctional catalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is not satisfying. Besides, the reaction mechanism and the active sites are still unclear. Additionally, water splitting in neutral media is rarely discussed, which composes the basis of seawater splitting. In this thesis, NiSe2 catalyst was prepared through a thermochemical method followed by a chemical vapor deposition step. The as-prepared NiSe2 nanoparticle is applicable in both acidic and alkaline solutions with excellent stability. The self-assembled NiSe2/carbon fiber paper electrode exhibits high conductivity and catalytic activity which can deliver a high current density of 100 mA cm-2 at a low overpotential of 138 mV in acidic HER, superior to many transition metal-based selenides. Moreover, it can work as both cathode and anode with a full cell voltage of 1.66 V to power the electrolysis at a current density of 10 mA cm-2. Additionally, a systematic characterization of the NiSe2 prior to and post electrolysis was conducted, an oxidation process during OER was proposed as well. To further explore the phase evolution of the NiSe2 during HER in base medium, an operando investigation was conducted, two cells were specially designed for the experiments. By applying operando synchrotron X-ray powder diffraction (SXRD) measurement, a series of time-resolved patterns were obtained, a phase transformation from cubic NiSe2 to hexagonal NiSe was revealed. By applying operando Raman spectroscopy measurement, the decomposition of NiSe2 was further confirmed. Additionally, ex situ measurements were also combined to detect the structural, morphological and compositional changes. Based on these results, a phase evolution mechanism was proposed. To further investigate the phase transformation from atomic level, density functional theory (DFT) study was performed. The self-reorganization was found to improve the conductivity and raise the d-band center, enhancing the catalytic activity. To develop the hydrogen evolution under neutral conditions, pure Co3O4 and Cu-doped Co3O4 nanowire arrays were synthesized through a hydrothermal method. The effect of substrates and varied electrolytes on the HER performance of pure Co3O4 was examined. The copper foam substrate and metal salt were confirmed to increase the catalytic activity. Based on these results, a series of Cu-doped Co3O4 nanowire arrays were prepared with different Cu concentrations. The Cu-doped Co3O4 catalyst exhibits much superior activity to the pristine Co3O4. The optimization ratio of Cu/Co was verified to be 1:4 (CuCo-4). Furthermore, the CuCo-4 was employed in simulated seawater with different salinity. With a NaCl concentration of 0.15 M, the CuCo-4 exhibits a highest current density of 50 mA cm-2 at an overpotential of 300 mV, demonstrating its huge potential in practical application.