Mn2O3, TiO2 and NiO based electrocatalysts with Ni and Pt for oxygen reduction and evolution reaction

Abstract

The constant increase in humanity's energy needs has led to increased consumption of fossil fuels, which according to some calculations will be completely consumed by the year 2050. On the other hand, the increased consumption of fossil fuels leads to the emission of a number of harmful compounds into the atmosphere, which in turn drastically impairs the quality of life of people and leaves enormous consequences for the living world. Therefore, scientists around the world are working rapidly to find a new source of energy that would be renewable and environmentally friendly. One of the alternatives to fossil fuels is the so-called "hydrogen economy" which is based on the use of hydrogen as a medium for storing and transporting energy. According to this model, energy would be stored in the form of hydrogen through the process of electrolysis when there is an inflow of energy from natural renewable sources such as solar energy, wind energy, water energy and nuclear energy, while in moments of lack of inflow of energy from the primary source, the stored hydrogen would be converted back into electricity in fuel cells. Fuel cells are systems that convert the chemical energy of reactants into electrical energy and water. If in the fuel cell it is possible for the reaction to take place in the opposite direction, i.e., to produce reactants (hydrogen and oxygen) from water and with the consumption of electricity, then such a cell is called a unitized regenerative fuel cell. The main problem during the design and construction of such a cell is the kinetically slow reactions of reduction and evolution of oxygen that take place on the physically identical electrode. Therefore, the design and testing of efficient bifunctional catalysts that would be used to catalyse these reactions is crucial for the wider implementation of these systems. Within this dissertation, a series of catalysts, based on transition metal oxides Mn, Ni and Ti, decorated with Pt and Ni nanoparticles, were synthesized which could potentially be used as bifunctional catalysts for the catalysis of oxygen reactions in regenerative fuel cells. At the beginning, three oxides were synthesized, Mn2O3, Mn2O3-TiO2 and Mn2O3-NiO, which were decorated with PtNi alloy nanoparticles by microwave irradiation and tested in 0.1 M KOH. Of the nine synthesized catalysts, the Mn2O3-NiO material decorated with PtNi nanoparticles showed the best catalytic performance. This catalyst showed the highest values of jd at 1800 rpm (-4.48 mA cm-2), the lowest values of the parameter b of 62 and 109 mV dec-1, and the Koutecky-Levich analysis concluded that the oxygen reduction reaction takes place on this catalyst by a kinetically more favourable four-electron mechanism. On the other hand, in the process of oxygen evolution, this catalyst showed one of the lowest values of the overpotential required to reach a current density of 10 mA cm-2, which apostrophizes its potential bifunctional application. In order to further improve the PtNi/Mn2O3-NiO catalyst, the oxide base was synthesized again by a partially modified method and doped with nitrogen in different ratios to obtain PtNi/Mn2O3-NiO-N catalysts with three different N:Mn2O3-NiO ratios. By testing the second set of synthesized catalysts, it was determined that the PtNi/Mn2O3-NiO-N (1:2) catalyst shows the best performance with a limiting diffusion current density at 1800 rpm during oxygen reduction of -2.94 mA cm-2 and a Tafel slope of 67 mV dec-1. The Koutecky-Levich analysis determined that on this catalyst the reduction of oxygen also predominantly takes place by a kinetically more favourable four-electron reduction mechanism with the number of exchanged electrons of 3.38. When examining the oxygen evolution reaction, it was found that PtNi/Mn2O3-NiO-N (1:2) reaches a current density of 10 mA cm-2 at an overvoltage of 0.50 V, while the reaction onset potential was 1.61 V vs. RHE. Catalysts synthesized and tested within this doctoral dissertation showed very good bifunctional performance in terms of the catalysis of the investigated reactions, comparable to the commercial catalysts Pt/C and IrO2, which are considered currently as the best catalysts for the oxygen reduction and evolution reactions, respectively. In addition to good catalytic performance, they also showed good stability during long-term exploitation in a real regenerative fuel cell (simulated by the so-called switch test), so in the opinion of the author, the potential for commercial application of the synthesized catalysts is very high. Further tests should be directed towards further optimization of the synthesis and preparation of catalytic inks as well as towards designing a realistic regenerative fuel cell and testing the performance of the catalyst in it.