Preparation and mechanism study of non-precious metal catalysts for electrochemical oxygen reduction

Abstract

Fuel cell is an electrochemical device which can convert chemical energy into to electricity. Compared with steam engine and internal combustion engine, the fuel cell has higher conversion efficiency and lower pollution. The energy conversion in fuel cells is based on the oxidation of fuel and reduction of oxygen, both of which need electrocatalyts to smooth the reactions. However, the oxygen reduction reaction (ORR) is sluggish. Therefore, high-performance ORR catalyst is crucial to the overall performance of the fuel cell. State-to-art ORR catalyst are those based on Pt and its alloys. Therefore, developing new inexpensive eletrocatalysts for ORR is very demanding. This thesis is committed to develop carbon based ORR catalysts. The research comprises synthesis of material, characterization and electrochemical test. The research specially emphasizes on facile synthesis of nitrogen and non-precious metal co-doped graphitic carbon, characterization of metal-nitrogen interaction and methodology in electrochemical analysis. The first three parts of the thesis is about the synthesis of non-precious metal/ nitrogen-doped reduced graphene oxide (N-rGO) composite and study of their ORR catalytic activity. The first part of the thesis is focused on synthesis of N-rGO, and for the first time, a nanocomposite of CuO/N-rGO with a very high ORR performance in alkaline electrolyte was synthesized. It is confirmed that CuO, with negligible ORR reactivity by itself, introduces a synergistic effect by combining with N-rGO through the interactions with nitrogen, resulting in a significant ORR activity enhancement. The experiment revealed that rapid reduction of HOO- intermediate by the CuO/N-rGO leads to more positive onset potential, higher current density and higher four-electron selectivity catalysed than both the CuO/rGO and N-rGO. Meanwhile, the CuO hindered restacking of N-rGO, providing the resultant nanocomposite much higher specific surface area and larger pore size to facilitate oxygen transfer, resulting in higher ORR current than that of Pt/C at low potential. In the second part of the thesis, three types of silver/ reduced graphene oxide (Ag/rGO) nanocomposites (one doped with nitrogen and another two without) are synthesized to investigate their atomic structures and the oxygen reduction reaction (ORR) performance with them as the electrocatalysts. For the first time, the bonding interaction between Ag and N in N-rGO is confirmed by both high resolution X-ray photoelectron spectroscopy (XPS) and surface enhanced Raman spectroscopy (SERS). The Ag/N-rGO shows excellent ORR performance, including very high onset potential and current density, which outperforms those Ag/rGOs without N doping. Detailed electrochemical analysis shows that the ORR mechanism on Ag/N-rGO is different from both Ag and N-rGO, and its excellent performance is caused by the Ag-N bonding which alters the electronic structure of N-rGO. In the third part of the thesis, a novel Fe/N co-doped graphitic carbon bulb (Fe/N-gCB) is synthesized as a high-performance ORR catalyst. The key findings include: 1) highly graphitic and lowly defective carbon with extraordinarily high N content can be synthesized at low temperature of 550 °C. 2) High Fe content facilitates the formation of high specific surface area and graphitic structure at low temperature. 3) The low temperature process retains high level of N. 4) Fe/N-gCB shows comparable performance with Pt/C in alkaline electrolyte and adequate performance in acidic electrolyte. 5) Only the Fe coordinated to N in the shell can contribute ORR activity. The forth part of the thesis focus on the study methodology of ORR. The forced convection methods on rotation electrode are carefully investigated in theory and verified by experiment to study the electron transfer number (n) of ORR on non-Pt catalysts in aqueous electrolyte. It is found that the widely used Koutechy-Levich (KL) plot is not suitable in determining n neither theoretically nor experimentally. The oxygen reduction reaction is neither a single-step nor irreversible so it does not meet the precondition of KL theory. By construction of a simple mathematical model, it is prove that the ORR is even not a first-order reaction in some cases, which is also essential to KL theory. Practically, the n of oxygen reduction is significantly dependent on the angular velocity of the electrode, which makes the linear fitting of KL plot meaningless. The KL plot is always not linear, even though the measurement error is excluded. Therefore, forced linear fitting results in incorrect n. The fifth part of the thesis is the extension of the forth part. A new mathematical method is developed to determine the rate constants and the orders of reaction of the sub-reactions in ORR. The method can extract the oxygen and peroxide concentration on the electrode surface and the current destiny from different sub-reactions of ORR. So it can be used to determine order of reaction. This method is applied to various catalysts and the orders of reactions are determined not to be 1 for most cases.