Precise Regulation of Transition Metal Oxides Cathode for Efficient Electrochemical Energy Storage and Conversion


Student thesis: Doctoral Thesis

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Award date4 May 2023


With the increasing crisis of traditional energy depletion and environmental deterioration, it has become hot issues all around the world to develop advanced energy storage and conversion technologies based on electrochemistry. It is crucial to exploit high-performance electrode materials for the application of energy storage and conversion devices. Transition metal oxides (TMOs) have been extensively studied in the energy storage and conversion area due to their abundant distribution, various structural properties, diversified oxidation states, high theoretical activity and superior stability. However, they are also subjected in the practical application by their limited specific surface area, poor electrical conductivity, ionic diffusion coefficients and weak adsorption at the electrochemical interface. Therefore, it is necessary to regulate the fine structure of TMOs for their further practice in the large-scale applications. Meanwhile, cathode materials usually determine the reactivities and energy density of the whole electrochemical devices, thereby innovative regulation strategies towards high-performance TMOs cathodes materials are proposed in this thesis.

For energy storage system, it is significant to develop high-voltage TMOs cathodes to achieve high energy density of commercial batteries. Herein, we propose a hierarchical doping strategy with active/inert dual elements to stabilize commercial LiCoO2 under 4.6 V. The ingenious outside-in structure design enables Ni2+ occupying at Li layer in the bulk layered phase and P gradient doping at superficial lattice. Comparing with the conventional inert element substitution strategy, the doped active Ni2+ can not only serve as “pillar” to restrain the formation of metastable H1-3 phase, but also regulate the electronic structure of LCO and trigger the superexchange interaction of Ni2+-O-Co4+, altogether with the strong P-O coordination to substantially suppress the lattice oxygen escape from the surface. Therefore, it considerably reduces the risk of layer structure collapse and consequently achieves stable and high-capacity operation over 4.6V.

For energy conversion system, developing advanced TMOs electrocatalysts for oxygen reduction reaction (ORR) is critically important for the cathodes reactions in fuel cells and metal-air batteries. It is urgently necessary to develop appropriate descriptors to assist unravel structure–performance relationships of TMOs electrocatalysts, which plays important roles in understanding the trends in electrocatalytic performance and predicting promising electrocatalysts with rational structure construction. Herein, we propose a low-temperature electrochemical anodic oxidation method to definitely tailor the valence of silver oxides from zero valence to tri-valence. Taking their valence to normalize the valence–activity relationship, we demonstrate that the oxidation state can also serve as an effective descriptor for designing ORR electrocatalyst. Our results unravel, for the first time, the electrocatalytic activities of silver species can be improved through raising their valence, conforming the order of Ag < Ag2O < Ag2O2 < Ag3O4 < Ag2O3. The computational studies reveal that higher valence Ag species possess higher proportion of d band holes and more electrons closer to the Fermi level. Therefore, the oxygen adsorption and activation energy on the Ag sites can be regulated to a near-optimal level and the ORR catalytic efficiency get the raise. This work clearly presents that oxidation state is another significant freedom to design efficient ORR electrocatalysts.

Besides, developing low-cost and high-performance TMOs electrocatalysts for ORR is significant to proceed the efficient applications of metal-air batteries. Catalysts containing isolated single atoms have attracted much interest due to their good catalytic behavior, bridging the gap between homogeneous and heterogeneous catalysts. Here, we proposed a precise regulation strategy for TMOs with metal single atoms modification through pyrolyzing metal organic framework (MOF) precursors of NH2–MIL-125(Ti) with Cu(acac)2 coordination. By constructing synergistic TiO2-x electrocatalyst with atomically dispersed Cu sites confined by defective mixed-phased TiO2–x on nitrogen-doped carbon substrates, significantly enhanced ORR activity and stability were achieved on this special Cu single atoms modified TiO2-x. The promising application of this novel electrocatalyst was demonstrated in a prototype Zn–air battery. This precise regulation strategy for TMOs with single atoms modification sheds light on the development of highly efficient electrocatalysts.

In general, this thesis provides useful insights into the precisely regulation strategies and mechanism study of TMOs for energy storage and conversion application, which could serve as a guide for designing advanced electrodes materials in electrochemical storage and conversion devices.