Theoretical and Experimental Investigations in Metal−N−C Composites for Oxygen Reduction Reaction and Hydrogen Evolution Reaction at Universal Phs

Abstract

During the development of platinum group metal (PGM)-free electrocatalysts for both polymer electrolyte membrane fuel cells and electrolyzers, non-precious metal and nitrogen co-doped carbon (denoted as Me-N-C) has been widely investigated as a promising catalyst for oxygen reduction reactions (ORR)1-3 and hydrogen evolution reactions (HER)4. However, these studies were conducted by different groups under various experimental conditions and most of the experiments were focused on a certain kind of reaction (ORR or HER) in either acid or alkaline media. In addition, current understanding can not explain well the activity discrepancy between Fe-N-C and Co-N-C. For instance, the former is much active than the latter for ORR, while the trend is reversed for HER4. This study aims to solve this dilemma by a first-principles density functional theory (DFT) approach. In the theoretical simulations, the catalytic activities on Fe-N-C and Co-N-C were studied for ORR and HER in both acid and alkaline solutions by constructing the free energy diagrams. Meanwhile, the effect of applied potential on the activity was also included. Figure 1 shows the free energy diagrams of ORR for Fe-N-C and Co-N-C at low pH at 0 V (Fig. 1a) and 0.80 V (Fig. 1b), respectively. At 0 V, all elementary reaction steps are downhill for Fe-N-C (blue line) and Co-N-C (red line), indicating that both catalysts have good ORR activity in acid media; however, when the potential increases to 0.80 V, the OOH* and O* formation steps become uphill for Co-N-C while the Fe-N-C still possesses the downhill trend, illustrating the latter one has a better activity at low overpotentials. Similarly, the free energy diagrams based on the elementary reaction steps were constructed for ORR in alkaline media as well as the HER, from which the comparison between the Fe-N-C and Co-N-C regarding the ORR and HER activities at universal pHs could be obtained. Our DFT calculation results demonstrateed that Fe-N-C had the better ORR activity than Co-N-C in both acid and alkaline media but a completely different trend was found for HER. These theoretical findings were also verified by our experimental results. References (1) Wu, G.; More, K. L.; Johnston, C. M.; Zelenay, P. Science 2011, 332, 443. (2) Shao, M.; Chang, Q.; Dodelet, J.-P.; Chenitz, R. Chemical reviews 2016, 116, 3594. (3) Lefèvre, M.; Proietti, E.; Jaouen, F.; Dodelet, J.-P. science 2009, 324, 71. (4) Zhang, L.; Liu, W.; Dou, Y.; Du, Z.; Shao, M. The Journal of Physical Chemistry C 2016, 120, 29047. Figure 1. Gibbs free energy diagrams for ORR in acid media for Fe-N-C (blue) and Co-N-C (red) at 0 V (a) and 0.80 V (b). Figure 1