Investigation of Activation Protocols and Carbon Components for Core-Shell Mn@Mn3O4/Carbon Gas Diffusion Electrodes for Oxygen Reduction and Evolution Reactions

Abstract

For the wide application of reversible fuel cells and metal-air batteries, highly efficient and cost-effective oxygen electrodes for both the oxygen reduction and evolution reactions (ORR/OER) are highly desired. Manganese oxides are considered to be one of the most promising bifunctional electrocatalyst candidates to replace the precious Pt/C (for ORR) and IrO2 (for OER) catalysts. Although a tremendous effort has been made to develop advanced materials, the impact of experimental protocols on electrode performance has not been well studied. The objective of this work is to optimize the ORR/OER performance of core-shell Mn@Mn3O4/C gas diffusion electrodes (GDEs) with a focus on designing an effective electrode activation protocol and exploring suitable carbon components, including both carbon additives and wet proofed carbon paper gas diffusion layers (GDLs).To adjust the electrode wetting, two approaches were adopted: i) controlling the weight ratio of Teflon on carbon paper, and ii) pre-treatment with warm HNO3 acid. The HNO3 pre-treatment on Teflon-coated GDLs can slightly increase hydrophilicity (introduces C-O bonds) resulting in enhancement of the ORR/OER activities of Mn@Mn3O4/Vulcan carbon GDEs and the polarization curves are identical regardless of Teflon loading.In order to improve the catalytic activity and durability of Mn@Mn3O4/C GDEs, we explored a set of different carbon additive combinations. Among the different combinations, graphene/Vulcan (1:1) reached the longest lifespan and the ORR/OER overpotentials of Mn@Mn3O4/Vulcan/graphene (before degradation) were as low as that of the Pt/C-IrO2-Vulcan-graphene benchmark.Since MnOx can be irreversibly activated or passivated, it is necessary to exercise caution when polarizing a pristine Mn@Mn3O4/Vulcan/graphene GDE in the first few cycles. For the design of activation protocols, different polarization methods were used, i.e., fast/slow cyclic voltammetry (CV) in different potential ranges or cycling under constant currents (CC) or constant potentials (CP). The highest catalytic activity was achieved by a CV-activated GDE that was obtained by cycling between the ORR and OER-related potentials at a slow scan rate of 1 mV s-1. This better performance might be due to the potential-driven MnOx phase transitions that develop R/α/γ type structures. Figure 1 shows the effect of different electrochemical protocols on the GDE. In comparison to the CC and CP-activated GDEs, it has a lower Mn site average oxidation state (AOS) of 3.1 and a larger contact angle, which suggests a higher population of Mn(III) active sites and OER-improved electrode wetting properties.By systematically tuning the carbon components and activation protocols, we found the ORR/OER performance of Mn@Mn3O4/C GDEs can be much improved when using Vulcan/graphene (1:1) additives, HNO3-treated wetproofed GDL, and applying cyclic voltammetry between ORR and OER-related potentials at a slow scan rate. Although this work focuses on improving MnOx/C GDEs, the developed approaches can also help establish protocols for reaching the full scope of other transition-metal-based electrodes. Figure 1