Assessment of coal fly ash char as a substituting material of graphite with electrocatalytic activity for the oxygen reduction reaction

Abstract

The search for functional and economical electrocatalysts derived from abundant materials is essential to enable the mass implementation of renewable energy conversion systems, such as regenerative fuel cells.Herein, char recovering from coal combustion fly ashes using simple concentration methods, such as sieving, gravimetry, impaction, elutriation, magnetic separation and demineralization, is proposed as electrocatalyst for the oxygen reduction reaction (ORR), allowing to maximize the coal value chain. The ORR electrocatalytic activity of the different materials obtained after each concentration step was tested and compared with that of demineralized char concentrates and natural graphite; their potentiality to be used as substitute of natural graphite was evaluated. All the materials showed ORR electrocatalytic activity, with selectivity for the indirect two-electron pathway. The char concentrated after elutriation and magnetic separation and the demineralized sample exhibited the most promising performances (onset potential (Eonset) and diffusion-limiting current density (jL) values between 0.76 and 0.80 V vs. RHE and −2.04 and −2.54 mA cm−2, respectively), with good methanol tolerance and long-term electrochemical stability (current retentions of 75–80% after ≈17 h of continuous activity). Very importantly, the results of this work showed that the prepared coal-derived materials present similar or even better ORR electrocatalytic activity than natural graphite (Eonset = 0.67 V vs. RHE, jL = −1.10 mA cm−2, current retention of 65% after ≈17 h) and, thus, can be able to replace it in similar energy-related applications, which can be of a great importance in terms of circular economy and environmental sustainability in a near future.