Mesoporous Nanocarbon-Based PGM-Free Electrocatalysts Prepared with MgO-Template for Anion-Exchange Membrane Fuel Cell

Abstract

Anion-exchange membrane fuel cells (AEMFCs) are receiving more and more attention as a viable alternative to proton-exchange membrane fuel cells, because they permit the use of precious metal-free electrocatalysts for oxygen reduction reaction (ORR), such as transition metal and nitrogen-doped carbon nanomaterials, containing MNx ORR-active sites.1 However, in addition to active sites, the structure of the catalyst support material is also essential to ensure efficient mass transfer in the catalyst layer, especially in AEMFCs where mass transport issues are caused by the higher loading that PGM-free catalysts require.2 Easily removable inorganic particles like MgO offer a sustainable alternative to silica-based templates to achieve optimal porosity. The catalyst materials can be created via a template-directed synthesis, by pyrolyzing a carbon precursor with MgO or other thermally unstable magnesium compounds like acetate or citrate. The precursors break down by heat-treatment to create MgO nanoparticles, which are subsequently dissolved using dilute acids such as HCl.3 Unlike with silica-based templates, scaling up the method is significantly easier and more sustainable since no corrosive chemicals like hydrofluoric acid are used in the process.Using HoneyolTM (a mixture of alkylresorcinols) as the carbon source and magnesium acetate as the template precursor, we demonstrate a straightforward MgO template-assisted approach for obtaining mesoporous nitrogen and transition metal co-doped nanocarbon catalysts for AEMFCs.4 The effect of the mass ratio of Honeyol to magnesium acetate on the porosity and electrocatalytic activity of the catalysts was studied. The addition of Mg acetate increases the surface area of the materials, the prepared materials are mostly mesoporous with a broad pore size distribution. The catalyst with the highest electrocatalytic activity toward the ORR was synthesized using a 2:1 mass ratio of Honeyol to Mg acetate. The bimetallic FeCoNC-MgOAc catalyst displayed the highest electrocatalytic activity surpassing the benchmark Pt/C catalyst, showing a E 1/2 value of -0.14 V vs. SCE in the RDE test. The catalyst demonstrated excellent stability, only a minor decay was observed after 15,000 potential cycles (ΔE 1/2 = 10 mV). In AEMFC as the cathode catalyst, FeCoNC-MgOAc reached a peak power density of 0.92 W cm-2 (Figure 1), more than twice higher as the untemplated FeCoNC catalyst and slightly surpassing the benchmark Pt/C (20 wt.%). The catalyst also displayed promising durability in AEMFC with a minor voltage degradation rate of 0.0017 V h–1 after 24 h. References Osmieri, L.; Pezzolato, L.; Specchia, S. Recent Trends on the Application of PGM-Free Catalysts at the Cathode of Anion Exchange Membrane Fuel Cells. Current Opinion in Electrochemistry 2018, 9, 240–256.Dekel, D. R. Review of Cell Performance in Anion Exchange Membrane Fuel Cells. Journal of Power Sources 2018, 375, 158–169.Morishita, T.; Tsumura, T.; Toyoda, M.; Przepiórski, J.; Morawski, A. W.; Konno, H.; Inagaki, M. A Review of the Control of Pore Structure in MgO-Templated Nanoporous Carbons. Carbon 2010, 48 (10), 2690–2707.Kisand, K.; Sarapuu, A.; Douglin, J. C.; Kikas, A.; Treshchalov, A.; Käärik, M.; Piirsoo, H.-M.; Paiste, P.; Aruväli, J.; Leis, J.; Kisand, V.; Tamm, A.; Dekel, D. R.; Tammeveski, K. Templated Nitrogen-, Iron-, and Cobalt-Doped Mesoporous Nanocarbon Derived from an Alkylresorcinol Mixture for Anion-Exchange Membrane Fuel Cell Application. ACS Catalysis 2022, 12 (22), 14050–14061. Figure 1