Design And Catalytic Properties Of Noble-Metal-Free Electrocatalysts For Water Splitting

With the increasing environmental pollution and the concerns of the nonrenewable energy resources, human beings needs to explore a clean, cheap, more convenient and effective way for future energy supply to reduce the current dependence on the fossil fuels. Hydrogen (H2), as an ideal clean energy carrier, gradually attracts much attention all over the world. This thesis describes the design and applications of earth-abundant, robust and highly efficient electrocatalysts for water splitting. The contents of this thesis are as follows:1. Cobalt porphyrin complexs as molecular catalysts for water oxidationTwo cobalt porphyrin complexes (CoP-1 and CoP-2) were synthesized and used as electrocatlaysts for water oxidation. CoP-1 showed a better performance for water oxidation and the onsetpotential for water oxidation was only 560 mV. The TOF was as high as 0.5 s-1 under a potential of 1.3 V vs. Ag/AgCl. Physical characterizations and electrochemical methods demonstrated that these two cobalt porphyrins complexes were not decomposed to cobalt oxide (CoOx) to catalyze water oxidation. These catalysts may be used to study the photo-electrochemical catalysis for water oxidation.2. Cobaloximes as precursors of heterogeneous catalysts for water oxidationThe CoOx nanoribbon catalysts were obtained by an electrodeposition method in aqueous solution containing cobaloximes. Compared with molecular catalysts, the heterogeneous catalysts have the advantages with good robustness and high efficiency for water oxidation. When the potential was applied at 1.5 V vs. Ag/AgCl potential, the current density was > 10 mA/cm2. The results showed that the morphologies of catalysts, the electrolytes, pH of electrolytes and concentrations of the electrolyte have great effects on the catalytic performances.3. Ni2P nanowires for electrocatalytic water oxidationThis work focused on the Ni2P nanowires for water oxidation performance in alkaline media. After the activation process by continuous cyclic voltammetry, the catalytic property of Ni2P nanowires became better and better. Compared with Ni2P nanoparticles, Ni2P nanbwire showed a better catalytic performance due to the large specific area. The XPS results demonstrated that NiOx was formed on the surface of Ni2P nanowires, which was the real catalyst on Ni2P nanowires for OER catalysis.4. Design a self-supported Ni2P-G@NF electrode for hydrogen evolution reaction by heat-treated methodThe Ni2P-G@NF electrode was synthesized by one-step method and was used for hydrogen production in different electrolyte. This binder-free electrode showed good combination of catalysts and electrodes. A few layers of graphene were initially grown on the nickel foam substrate by CVD method to increase the conductivity of the electrode and accelerate electron transfer between the electrode and catalyst. The experimental results showed that the onset potential of Ni2P-G@NF electrode was only 7.0 mV. To reach a current density of 10 mA/cm2, the applied overpotential was as low as 50 mV.5. Design a self-supported CoNiP@NF electrode for hydrogen evolution reactionThe bimetallic phosphide CoNiP nanosheets was grown on nickel foam substrate (CoNiP@NF) by a hydrothermal method and subsequent low-temperature reduction process. The CoNiP@NF electrode was then used to study the catalytic performance for hydrogen evolution in different electrolytes. Compared to other high-temperature calcination method to form metal phosphides, this synthesis method showed many advantages including low-cost and facile operation. It provided an effective way for synthesis of other kinds of metal phosphides. Experimental results showed that the Tafel slope of CoNiP@NF electrode was only 39 mV/dec and when the current density was 10 mA/cm2, the overpotential was 60 mV under acidic conditions.6. Design a self-supported Cu3P@NF electrode for overall water spitting The copper phosphide CU3P nanosheets was grown on nickel foam substrate(Cu3P@NF) by a hydrothermal method and subsequent low-temperature reduction process. The CU3P@NF electrode was then used for overall water splitting catalysis in alkaline solutions. The mechanism of Cu3P@NF for water oxidation in alkaline solution was deeply studied by further physical and electrochemical characterizations. When the applied voltage was 1.67 V, the overall current density was as high as 10 mA/cm2, which showed the best performance for water splitting among the reported copper-based bifunctional catalysts.