Studies On Structure Design And Electrochemical Performance Of Catalytic Materials For Gas-evolving Reactions

Electrochemical technology plays an important role in national economy, such as chemical industry, machinery, electronics and other applications. The chlor-alkali industry is one of the most important practical applications of electrochemical reactions carried out on an industrial scale. In this reaction, sodium chloride is electrolyzed producing chlorine gas at the anode as well as sodium hydroxide and hydrogen at the cathode. However, its electricity cost makes up more than 50% of alkali production cost, seriously restricting the development of the chlor-alkali industry due to the large amount of consumed electrical energy in electrolysis reaction. The main problems are the electrodes with high overpotentials. For dimensionally stable anodes(DSA), the coating was deposited on the Ti substrate with an obvious mud-crack island-gap surface microstructure due to the thermally induced internal stresses. Although the cracks can increase the specific surface area of the electrodes, it can also cause the electrolyte into the Ti substrate. During the reactions, the byproducts of oxygen and the adsoprted intermediates gradually oxidize the surface of the Ti substrate, forming a Ti O2 passivation film between the Ti substrate and the coatings. In that case, the over-potential of the electrode for the chlorine evolution reaction(CER) gradually increases. And the catalysts coated on the Ti substrate flake off and active species dissolve at high potentials, resulting in the deactivation of the electrodes. For the cathodes, the main catalysts are non-precious metal materials in practical applications. Their catalytic performances for the hydrogen evolution reaction(HER) are by far inferior to Pt-group catalysts due to the high overpotentials and stability. Based on the above consideration, a series of high performance catalysts with low overpotentials and high durability are designed from the aspects of the modified substrate, the dispersion of the active species, and the optimization of the catalyst structure. In this thesis, we have carried out the following researches:Sn and Sb co-doped Ru Ti oxides supported on Ti O2 nanotubes anode for selectivity towards electrocatalytic chlorine evolution. The results indicate that highly ordered Ti O2 nanotubes(TNTs) with large specific surface area could be implanted with active metal oxides. And the(Ru0.3Ti0.34Sn0.3Sb0.06)O2-TNTs anode exhibits a uniform and compact morphology without crack, which is attributed to the uninterrupted mouth of the TNTs for relieving the accumulated stress of the coating at the process of annealing. The addition of an appropriate amount of tin and antimony, not only improved the electrochemically selectivity towards CER, but also enhanced the adhesion strength between the coating and the TNTs. These improvements were attributed to the Sn4+ and Sb3+ ions having ionic radii similar to those of Ru4+ and Ti4+, which means they satisfy the Hume-Rothery conditions for the formation of a substitution solid solution. The catalyst firmly binds with the TNTs and enhances the electrochemical stability of the electrode. It displays high over-potential for oxygen evolution reaction(OER). Accordingly, the constructed(Ru0.3Ti0.34Sn0.3Sb0.06)O2-TNTs anode exhibits a greater potential difference(△E) between the evolutions of oxygen and chlorine than that exhibited by the traditional dimensionally stable anode(DSA), which is beneficial for improving the selectivity towards chlorine evolution reaction(CER). This superior performance is explained in terms of the surface properties and geometric structure of coated catalyst, as well as the electrochemical selectivit y ascribed by the addition of tin and antimony species.Nanostructured Ru O2-Ti O2 catalysts with different morphologies are grown in situ onto the Ti substrates for anodic chlorine evolution. Solvothermal crystallization in the presence of hydrochloric acid plays a critical role in regulating the catalyst size and morphology during the nucleation and growth process of Ru O2-Ti O2. The role of hydrochloric acid can be two-fold: one is etching the Ti substrate to form titanium ions, which are easy to hydrolyze with water at the water/substrate interface, resulting in the slow oxidation process consuming dissolved oxygen to form a crystal nucleus on the substrate. After the formation of the first nanocrystalline layer, the deposition film is gradually formed with continuous hydrolysis and subsequent growth-crystallization. The acid condition could slow down the hydrolysis reaction of titanium ions by providing free H+, which is necessary for the growth of depositions. On the other hand, since the rutile(110) surface possesses the lowest energy and has abundant five-fold coordinated titanium atoms, two-fold coordinated oxygen atoms and oxygen vacancies, it may be the favorite source of the selective adsorption of Cl- on(110) plane and retard the growth rate of(110) surface. On the contrary, the other higher surface energy crystal surface can absorb more the Ti(Ⅳ) oxo species to decrease the surface energy. Thus, the crystal grows anisotropically along the(110) surface. With the extension of the hydrothermal reaction time, the crystal growth rate starts to decrease, and part of the crystals may begin to dissolve to form the Ti(Ⅳ) oxo species again. These species would diffuse to the solution and provide conditions for the random regrowth on the surface of the formed crystals. Accordingly, chlorine adsorption plays a critical role in affecting the preferred crystal planes. The designed Ru O2-Ti O2/Ti anode with a nano-flowerlike structure displays significantly enhanced activity toward anodic chlorine evolution reaction(CER) compared to the other two morphology anodes. Such excellent performance of Ru O2-Ti O2/Ti is explained in terms of the small charge transfer resistance and the unique surface structure with more active sites to be utilized during CER.Ru O2 loaded into porous Ni as a synergistic catalyst for hydrogen production. In this work, a porous nano/microarchitectured Ru O2/Ni composite catalyst has been elaborately designed via a facile and controllable route. The porous nano/microarchitectured interconnected Ni network has been grown in site on the Ni substrate by a facile and controllable process accompanying hydrogen production where hydrogen bubbles play a template role in the formation of the unique porous architecture. The designed Ru O2/p-Ni catalyst significantly displays enhanced catalytic activity and long-term durability toward hydrogen production compared with Pt catalyst. The excellent performance of the composite catalyst could be ascribed to the fact that Ru O2 can be well inosculated into the constructed porous Ni network with large specific surface area, in which the presence of Ru O2 and Ni network in pairs on the surface of the composite catalyst may not only result in a synergistically enhanced catalytic effect between Ru O2 and porous Ni network by hydrogen spillover, but also Ru O2 firmly bind with the porous Ni network, consequently ensuring the long-term durability of the catalyst during the whole reaction.One-dimensional Ni-doped Mo2 C nanowires supported on Ni foam as a binder-free Electrode for enhancing the hydrogen evolution performance. We developed a facile and controllable strategy combined with the post carburization treatment to directly construct 1D Ni Mo2 C nanowire arrays onto conductive 3D Ni foam(Ni Mo2C/NF) as a binder-free integrated catalyst for the HER. Such catalyst exhibits excellent catalytic HER activity and long-term stability in terms of inosculating the merits of high intrinsic activityfrom the combination of Ni and Mo2 C, as well as the exposure of more active sites provided by the high aspect ratio of 1D structure and rich surface area. We also employed density functional theory(DFT) calculations to elucidate the underlying reasons behind the distinct performance by comparing the electronic structure, hydrogen chemisorptions process and Gibbs free energy of Mo2 C and Ni Mo2 C. The DFT calculations clearly demonstrate that the incorporation of Ni into the Mo2 C brings about the changes of the charge distribution on the catalyst, which results in a synergistic effect of Ni and Mo2 C toward decreasing the hydrogen binding energy. Accordingly, the Ni Mo2C/NF catalyst is able to enhance the catalytic performance of the HER.