Abstract
Microbial fuel cells (MFCs) are biochemical systems having the benefit of producing green energy through the microbial degradation of organic contaminants in wastewater. The efficiency of MFCs largely depends on the cathode oxygen reduction reaction (ORR). A preferable ORR catalyst must have good oxygen reduction kinetics, high conductivity and durability, together with cost-effectiveness. Platinum-based electrodes are considered a state-of-the-art ORR catalyst. However, the scarcity and higher cost of Pt are the main challenges for the commercialization of MFCs; therefore, in search of alternative, cost-effective catalysts, those such as doped carbons and transition-metal-based electrocatalysts have been researched for more than a decade. Recently, perovskite-oxide-based nanocomposites have emerged as a potential ORR catalyst due to their versatile elemental composition, molecular mechanism and the scope of nanoengineering for further developments. In this article, we discuss various studies conducted and opportunities associated with perovskite-based catalysts for ORR in MFCs. Special focus is given to a basic understanding of the ORR reaction mechanism through oxygen vacancy, modification of its microstructure by introducing alkaline earth metals, electron transfer pathways and the synergistic effect of perovskite and carbon. At the end, we also propose various challenges and prospects to further improve the ORR activity of perovskite-based catalysts.
Highlights
The literature and the vast array of information we have on their molecular mechanisms, characteristics and behavior offer a basic understanding of perovskite cathodes
The modification of the perovskite crystal structure through doping of the A and B site has proven to be a sensible technique in improving the oxygen reduction reaction (ORR) performance
The increase in ORR kinetics is associated with an optimal balance of ionic mobility, surface electron transfer and oxygen vacancies as aided by the synergistic effects of B sites
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. As a result of microbial respiration, electrons are generated and they are transferred to the anode electrode, which further performs the electrical work by passing through the circuit, and protons are transferred to the cathode via a proton exchange membrane. The protons and electrodes are combined in the presence of oxidants such as oxygen to generate water as the final product [6] By this combined bio–electrocatalytic reaction, the wastewater is treated at the anode, together with the useful energy output (Figure 1). Proton exchange membranes such as Nafion 117, which is expensive, have been replaced by sulfonated polyether ether ketone (SPEEK) [7], sulfonated SiO2 and sulfonated polystyrene ethylene butylene polystyrene (SSEBS), etc. This study reviews the perovskite electrocatalysts that have been used in the MFC system, their unique reaction pathway and the feasibility of maximizing the energy output through changes in perovskite’s elemental type and composition
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