Sucrose-derived activated carbons: electron transfer properties and application as oxygen reduction electrocatalysts

Abstract

The development of carbon-based metal-free electrocatalysts for the oxygen reduction reaction (ORR) is one of the most attractive topics in fuel cell field. Herein, we report the application of two sustainable sucrose-based activated carbons (ACs), denominated SC800 and SH800, as ORR electrocatalysts. In alkaline medium the ACs showed similar onset potentials at E_onset ≈ −0.20 V vs. Ag/AgCl (0.76 V vs. E_RHE), which are 0.06 V more negative than that observed for 20 wt% Pt/C used as a reference. Higher diffusion-limiting current densities (j_{L(−1.0 V, 1600 rpm)} = −3.44 mA cm⁻²) were obtained for the SH800 electrocatalyst, in contrast to SC800 (j_{L(−1.0 V, 1600 rpm)} = −3.04 mA cm⁻²). These differences can be related with their different textural properties. The SH800 electrocatalyst revealed a higher specific surface area (A_BET ≈ 2500 m² g⁻¹), larger micropores (widths between 0.7 and 2 nm) and sponge-like morphology. Conversely, SC800 showed a spherical shape, A_BET ≈ 1400 m² g⁻¹ and narrow micropores with pore width <0.7 nm. Both ACs were neither selective to 2- or 4-electron ORR processes, opposing Pt/C which showed selectivity towards direct O₂ reduction to water. SH800 and SC800 showed very similar Tafel plots, but with SH800 showing in both low and high current density regions, the lowest slopes values 53/171 mV dec⁻¹ vs. 68/217 mV dec⁻¹. Furthermore, the ACs presented excellent tolerance to methanol, with the SH800 electrocatalyst also showing greater long-term electrochemical stability than the Pt/C electrocatalyst which are very important advantages. The ACs-based electrocatalysts also showed ORR catalytic activity in acidic media, which makes them promising candidates for applications with acidic electrolytes (e.g. proton exchange fuel cells). In this case, E_onset = 0.06 V vs. Ag/AgCl (0.41 V vs. E_RHE) for SC800 and E_onset = −0.01 V vs. Ag/AgCl (0.34 V vs. E_RHE) for SH800, and the diffusion-limiting current densities are very similar for both ACs (j_L = −2.59/−2.76 mA cm⁻² at −1.3 V vs. Ag/AgCl, at 1600 rpm). SH800 and SC800 Tafel plots also showed two different slopes, but with higher values in both low and high current density regions, when compared with those obtained in an alkaline medium; still SH800 continues to show the lowest slopes.