Exploiting the Multifunctionality of M²⁺/Imidazole–Etidronates for Proton Conductivity (Zn²⁺) and Electrocatalysis (Co²⁺, Ni²⁺) toward the HER, OER, and ORR

This work deals with the synthesis and characterization of one-dimensional (1D) imidazole-containing etidronates, [M₂(ETID)(Im)₃]·nH₂O (M = Co²⁺ and Ni²⁺; n = 0, 1, 3) and [Zn₂(ETID)₂(H₂O)₂](Im)₂, as well as the corresponding Co²⁺/Ni²⁺ solid solutions, to evaluate their properties as multipurpose materials for energy conversion processes. Depending on the water content, metal ions in the isostructural Co²⁺ and Ni²⁺ derivatives are octahedrally coordinated (n = 3) or consist of octahedral together with dimeric trigonal bipyramidal (n = 1) or square pyramidal (n = 0) environments. The imidazole molecule acts as a ligand (Co²⁺, Ni²⁺ derivatives) or charge-compensating protonated species (Zn²⁺ derivative). For the latter, the proton conductivity is determined to be ∼6 × 10^-4 S·cm^-1 at 80 °C and 95% relative humidity (RH). By pyrolyzing in 5%H₂-Ar at 700-850 °C, core-shell electrocatalysts consisting of Co²⁺-, Ni²⁺-phosphides or Co²⁺/Ni²⁺-phosphide solid solution particles embedded in a N-doped carbon graphitic matrix are obtained, which exhibit improved catalytic performances compared to the non-N-doped carbon materials. Co²⁺ phosphides consist of CoP and Co₂P in variable proportions according to the used precursor and pyrolytic conditions. However, the Ni²⁺ phosphide is composed of Ni₂P exclusively at high temperatures. Exploration of the electrochemical activity of these metal phosphides toward the oxygen evolution reaction (OER), oxygen reduction reaction (ORR), and hydrogen evolution reaction (HER) reveals that the anhydrous Co₂(ETID)(Im)₃ pyrolyzed at 800 °C (CoP/Co₂P = 80/20 wt %) is the most active trifunctional electrocatalyst, with good integrated capabilities as an anode for overall water splitting (cell voltage of 1.61 V) and potential application in Zn-air batteries. This solid also displays a moderate activity for the HER with an overpotential of 156 mV and a Tafel slope of 79.7 mV·dec^-1 in 0.5 M H₂SO₄. Ni²⁺- and Co²⁺/Ni²⁺-phosphide solid solutions show lower electrochemical performances, which are correlated with the formation of less active crystalline phases.