Oxygen reduction reaction and X-ray photoelectron spectroscopy characterisation of carbon nitride-supported bimetallic electrocatalysts

Five bimetallic electrocatalysts (ECs) including a carbon nitride (CN) support are synthesised through the pyrolysis of a solid precursor obtained through sol-gel and gel-plastic processes. The resulting ECs are characterised through ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectroscopy) and XPS (X-ray Photoelectron spectroscopy); their performance and reaction mechanism in the oxygen reduction reaction (ORR) are evaluated with the CV-TF-RRDE method (Cyclic Voltammetry Thin-Film Rotating Ring-Disk Electrode). Special attention is given to XPS results with the aim to carry out a thorough investigation of the surface concentration and the chemical environments of the different elements, as well as providing information on the structure of the metal components of the ECs and their interactions with the carbon nitride support. The correlation of the results obtained from the chemical analyses, XPS and the electrochemical studies allows to improve the fundamental understanding of: (i) the interplay between the preparation parameters and the surface and bulk chemical composition of the CN-supported ECs; and (ii) the factors controlling the ORR kinetics and reaction pathway in bimetallic CN-supported ECs, identifying the roles played by the various metal species. In this regard, the VI-period metal included in each EC has the highest impact on the ORR performance. Indeed, the ORR onset potential rises from ca. 300 to ca. 780 to ca. 900 mV vs. RHE for ECs based on Au, Ir and Pt, respectively. On the other hand, the second metal (i.e., Ni, Rh, and also Ir in the case of an EC including both Pt and Ir) plays a secondary role. With respect to the Pt/C ref., the CN-supported EC including Pt and Ni exhibits a higher onset potential (900 and 911 mV vs. RHE, respectively); finally, it is shown that Ir and Rh promote the selectivity of the ORR in the 4-electron mechanism.