Phosphorus and Iron Doped-Porous Carbon As an Efficient Electrocatalyst for Oxygen Reduction Reaction in Alkaline Medium

Abstract

The U.S. Energy Information Administration has projected a 27% growth in energy consumption by the year 2050.1 The continuous increase in the energy demands worldwide and the environmental concerns reaching critical levels, has required researchers to make the development of clean and sustainable energy conversion devices their paramount challenge. Fuel cells and metal-air batteries with high efficiency and low cost are the prime candidates. Platinum loaded carbon substrate is the catalyst of choice in fuel cells for anodic oxidation of H2 and cathodic reduction of O2 but large-scale commercial production is restricted by its prohibitive cost, limited supply, and weak durability.2 Platinum’s vulnerability to potential drift over time and easy loss of activity through methanol poisoning, questions its competitive catalytic activity. Electrocatalyst that are easily synthesized, have low cost and are earth-abundant is still highly desired. Heteroatom doped carbon electrocatalysts are highly considered candidates and are well researched because of their high abundance and low cost, with transition metal and nitrogen doped carbon material showing competitive result towards the oxygen reduction reaction (ORR).3 , 4 Among those heteroatoms phosphorus doped carbon is an emerging catalyst with high potential. The attraction towards phosphorus arises from the fact that it has a larger atomic radius- hence larger covalent radius- and higher electron-donating ability than nitrogen, making it a promising dopant with an ability to modify the electron transport properties. The Introduction of phosphorus, an atom with lower electronegativity, changes the charge density distribution and electronic properties of a high surface area carbon structure. Phosphorus doping increases the affinity towards an acceptor molecule such as oxygen5, hence a better interaction with the oxygen intermediate and enhances their electrocatalytic activities towards ORR. Such heteroatoms are typically covalently bonded within a carbon framework ensuring long-term stability. The advantages of a heteroatom doped porous carbon are twofold, stabilization of the non-precious metal in the alkaline medium and generation of active sites for facilitating the oxygen reduction reaction. Electrocatalytic activity of phosphorus doped and iron treated phosphorus doped carbon has only been covered by few groups and needs more improvements.6,7 The purpose of the study of electron transfer processes is to understand the reactivity of a relatively new, doped porous carbon complexes incorporating phosphorus as a heteroatom bonded to a metal center. According to DFT calculations, O2 and other ORR intermediates can be stably adsorbed on to Fe sites8, making phosphorus and iron doped carbon a prime candidate for ORR. Using a single precursor as a source of phosphorus, we propose a simple one-step chemical activation, with anhydrous ZnCl2 and anhydrous FeCl2 as the agents of carbonization, yielding P and Fe-doped porous carbon (PFeC). The electrocatalytic activity towards ORR was studied using a rotating ring-disk electrode (RRDE) technique and rotating disk electrode (RDE) connected to the CHI 600 potentiostat. The electrochemical activity of PFeC was compared with the commercially available 20 wt% Pt/C. PFeC showed high activity towards ORR, with comparable onset potential and a small negative shift in half-wave potential. PFeC electrocatalyst selectively catalyzes oxygen to water via a direct four-electron pathway. PFeC showed to be inert toward alcohol oxidation and had superior long-term stability. References (1) U.S. EIA. 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