Silver-M-Phathalocyanine (M= Co,Fe,Cu) Electrocatalysts for Oxygen Reduction Reaction in H2/O2 Anion Exchange Membrane Fuel Cells

Abstract

In the transition from fossil fuels to clean energy sources, H2 plays a key role due to its high energy density, and direct conversion to electricity in fuel cells (FCs). FCs are devices that allow the clean conversion of chemical energy into electrical energy. In H2/O2 fuel cell the hydrogen oxidation reaction (HOR) occurs at the anodic electrode, while at the cathodic side oxygen is reduced into water (ORR). The electrodes are separated by a conductive membrane, that could exchange protons (Proton Exchange Membrane, PEM) or anions (Anion Exchange Membrane, AEM) to maintain the electro-neutrality of the system. AEMFCs are an attractive alternative to PEMFCs, thanks to the alkaline noncorrosive enviroment that allows the use of cheaper structural materials such as non-noble metal electrocatalysts. This work is centred on the study of silver (Ag) nanostructured electrocatalysts in ORR for the realization of a platinum-free AEMFCs. Ag nanoparticles promote, in alkaline media, a 4e- pathway of the ORR, optimizing the conversion of oxygen into water at the expense of hydrogen peroxide.1 However, the activity of Ag is limited by the strong hydroxide anion (OH-) binding energy, which reduces the free active sites destined to oxygen adsorption.1 To overcome this, Ag electrocatalysts were modified with metal-phthalocyanines (M-Pc M=Co, Fe, Cu) and supported on Ketjen-black carbon and on Ceria/Ketjen black (CeO2/C) supports. The synergistic effect between M-Pc and Ag NPs optimizes the OH- binding energy enhancing the ORR performances.2,3 The employment of ceria, thanks to its high oxophylicity, improves the transfer of OH- to the metal surface.4,5 The dispersion and size of nanoparticles (5-15 nm) was controlled through the Turkevich synthesis method, in order to maximize the interaction between Ag and Me-Pc. Physical characterization has been realized by High Resolution Trasmission Electron Microscopy (HR-TEM), X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) while electrochemical behaviour has been explored in half cells and in complete AEMFCs. References Erikson, H., Sarapuu, A., & Tammeveski, K. (2019). Oxygen reduction reaction on silver catalysts in alkaline media: a minireview. ChemElectroChem, 6(1), 73-86.. Miller, H. A., Bevilacqua, M., Filippi, J., Lavacchi, A., Marchionni, A., Marelli, M., ... & Vizza, F. (2013). Nanostructured Fe–Ag electrocatalysts for the oxygen reduction reaction in alkaline media. Journal of Materials Chemistry A, 1(42), 13337-13347. Miller, H. A., Bellini, M., Oberhauser, W., Deng, X., Chen, H., He, Q., ... & Vizza, F. (2016). Heat treated carbon supported iron (ii) phthalocyanine oxygen reduction catalysts: elucidation of the structure–activity relationship using X-ray absorption spectroscopy. Physical Chemistry Chemical Physics, 18(48), 33142-33151. Miller, H. A., Bellini, M., Dekel, D. R., & Vizza, F. (2022). Recent developments in Pd-CeO2 nano-composite electrocatalysts for anodic reactions in anion exchange membrane fuel cells. Electrochemistry Communications, 135, 107219. Bellini, M., Pagliaro, M. V., Marchionni, A., Filippi, J., Miller, H. A., Bevilacqua, M., ... & Vizza, F. (2021). Hydrogen and chemicals from alcohols through electrochemical reforming by Pd-CeO2/C electrocatalyst. Inorganica Chimica Acta, 518, 120245. Figure 1