Lin, Runjia; (2022) Design of metal-carbon hybrids for electrosynthesis. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Electrocatalytic devices are of great importance for promoting global electrification and solving renewable energy intermittency problem by enabling efficient and flexible conversion between electrical and chemical energy. For instance, green hydrogen, which refers to hydrogen gas produced by renewable energy, is considered an ideal candidate to replace fossil fuels as energy carriers and raw materials for domestic and industrial applications. Green hydrogen can be generated by water electrolyser through catalytic hydrogen evolution reaction and then be utilized as the fuel of fuel cells to power electronics. Hydrogen economy is currently hindered by the high production (due to the large overpotential and low economic value of the anodic product of the water splitting process) and utilization (due to the expensive fuel cell cathode material) cost, thus requires the electrocatalytic systems/catalysts to be further studied and designed. Here, this thesis firstly aims at minimising the operation potential of a H2 generator by replacing the sluggish anodic oxygen evolution reaction of conventional water electrolysers with advanced anodic reactions (i.e. organic compound oxidation reactions) which either requires lower overpotential (i.e. urea oxidation reaction) or generates high value products (i.e. methanol upgrading reaction). Step-by-step research is carried out by firstly investigating the products, selectivity and active site of organic compound oxidation reactions governed by nickel-carbon catalyst, then evaluating the activity limitations, finally optimising the electrocatalyst design. The second objective of this thesis is to design oxygen reduction reaction electrocatalysts made of earth-abundant elements such as carbon and transition metals. Attention is laid on correlating oxygen reduction reaction selectivity with catalytic active moiety at atomic level to accommodate the demand for fuel cell application or green hydrogen peroxide synthesis. The details of these two main work in this PhD thesis are as follows: (1) Electrocatalytic organic compound oxidation reactions have been intensively studied for energy and environmentally benign applications. However, relatively little effort has been devoted to developing a fundamental understanding of the electrooxidation of organic compounds, including the detailed competition with side reactions and activity limitations, thus inhibiting the rational design of high-performance electrocatalysts. Herein, by taking NiWO4-catalysed urea oxidation reaction in aqueous media as an example, the competition between the organic compound oxidation reaction and the oxygen evolution reaction within a wide potential range was examined. It is shown that the root of the competition can be ascribed to insufficient surface concentration of dynamic Ni3+, an active site shared by both UOR and OER. Similar problems are observed in other OCOR electrocatalysts and systems. To address the issue, a “controllable reconstruction of pseudo-crystalline bimetal oxides” design strategy is proposed to maximise the dynamic Ni3+ population and manipulate the competition between urea oxidation and oxygen evolution reactions. The optimised electrocatalyst delivers best-in-class performance and a ~10-fold increase in current density at 1.6 V versus the reversible hydrogen electrode for alkaline urea electrolysis compared to that of the pristine materials. (2) Electrochemical oxygen reduction reaction (ORR), in which O2 is either reduced to H2O for fuel cell/metal air battery applications; or undergone a 2e- pathway to produce green H2O2, is critical for global electrification and decarbonisation thanks to its multi-pathway character. Nevertheless, holding multiple reaction channels can be a double-edge sword which raises difficulty in selectivity control. This work firstly identified the limitations of the popular Sabatier principle-driven ORR catalyst design, owing to which current ORR electrocatalysts are restricted by a activity-selectivity dilemma. A “O2 hydrogenation kinetics modification” strategy was then predicted for synergising the activity and selectivity. By designing and examining a series of Co-Nx-C samples, ORR selectivity was correlated with Co-N coordination at atomic level. The most optimised sample exhibits a ~100% H2O2 selectivity and a high onset which is comparable to the thermodynamic limit. Through operando spectroscopic studies, the excellent 2e- ORR activity and selectivity can be attributed to the seemly O2 hydrogenation kinetics, validating the as-proposed hypothesis. Finally, a redox-induced electron delocalisation mechanism was identified via analysing the pseudocapacitive and ORR behaviours of the Co-Nx-C samples. Such mechanism was then applied for regulating the H2O2 formation of ORR 4e- catalysts. The above discoveries provide fresh insights for electrocatalyst design in both fuel cell and H2O2 generation applications.

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