We are currently engaged in a project [1] aimed at demonstrating the fixation of dinitrogen (N2, a very stable molecule) to ammonium (NH4+, an essential fertilizer) by a cathode colonized by a mixed biofilm accepting electrons directly from the electrode or in the form of dihydrogen (H2) electrogenerated at the electrode. In this context, our primary objective is to elaborate electrodes that are biocompatible and can produce H2 under biological conditions, i.e. in aqueous electrolyte at neutral pH. Here we report preliminary results on electrodes modified by [Cp*RhCl(phendiamine)]+ or [Cp*Co(CH3CN)(phendiamine)]2+ (scheme 1). The diamine moiety allows the grafting of these organometallic complexes on oxidized carbon surfaces through covalent and conjugatedbonds [2]. The rhodium complex grafted on oxidized carbon black powder (Vulcan) has already shown its efficiency and stability under strongly acidic conditions [3]. Herein, we scrutinize its electrochemical behaviour in less acidic electrolytes and compare them to those of the cobalt related complex, which could represent an abundant transition metal alternative. [1] CATHOMIX, Projet n° ANR-19-CE43-0013-02. [2] M.N. Jackson, C.J. Kaminsky, S. Oh, J.F. Melville, Y. Surendranath, J. Am. Chem. Soc., 2019, 141, 14160. [3] G. Pérez, G. Diaz-Sainz, L. Gómez-Coma, L. Álvarez-Miguel, A. Garnier, N. Cabon, A. Ortiz, F. Gloaguen, I. Ortiz, J. Environ. Chem. Eng., 2022, 10, 107682.