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

Abstract

This work deals with the synthesis and characterization of one-dimensional (1D) imidazole-containing etidronates, [M 2 (ETID)(Im) 3 ]· n H 2 O (M = Co 2+ and Ni 2+ ; n = 0, 1, 3) and [Zn 2 (ETID) 2 (H 2 O) 2 ](Im) 2 , as well as the corresponding Co 2+ /Ni 2+ solid solutions, to evaluate their properties as multipurpose materials for energy conversion processes. Depending on the water content, metal ions in the isostructural Co 2+ and Ni 2+ 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 (Co 2+ , Ni 2+ derivatives) or charge-compensating protonated species (Zn 2+ 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%H 2 –Ar at 700–850 °C, core–shell electrocatalysts consisting of Co 2+ -, Ni 2+ -phosphides or Co 2+ /Ni 2+ -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. Co 2+ phosphides consist of CoP and Co 2 P in variable proportions according to the used precursor and pyrolytic conditions. However, the Ni 2+ phosphide is composed of Ni 2 P 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 Co 2 (ETID)(Im) 3 pyrolyzed at 800 °C (CoP/Co 2 P = 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 H 2 SO 4 . Ni 2+ - and Co 2+ /Ni 2+ -phosphide solid solutions show lower electrochemical perform...

Intrinsic Specific Activity Enhancement for Bifunctional Electrocatalytic Activity toward Oxygen and Hydrogen Evolution Reactions via Structural Modification of Nickel Organophosphonates

“…15,17,32,52 Therefore, it can be clearly pointed out that the partial electron transfer can directly enhance the outstanding bifunctional electrocatalytic performances of the present materials, as evidenced from the pre-and postcatalytic XPS studies. 4,10,13,32,33,52,66,68 Crystallinity and morphology of compound II remain the same as measured by postcatalytic PXRD, SEM, and EDAX analyses (Figures S23 and S24, Supporting Information). Moreover, the present bifunctional electrocatalytic activities are superior compared to that of many hybrid frameworks and other classes of advanced wellexplored electrocatalysts, as evidenced by Tables 1 and S9 and S10 (Supporting Information).…”

“…In recent years, researchers are mainly focused to fabricate such electrocatalysts based on different classes of framework materials such as metal–organic frameworks (MOFs), hybrid open-frameworks, as well as different metal oxides, hydroxides, sulfides, nitrides, phosphides, and phosphonates. − Among them, transition-metal phosphonates can be considered as one of the most effective electrocatalysts, owing to their structural diversity, tunable morphology, long-term stability, and cost-effective generation. − …”

Labile Coordination Interphase for Regulating Lean Ion Dynamics in Reversible Zn Batteries

“…These results agree with the low thermodynamic stability of histidine‐metal complexes at applied potentials, as widely studied in imidazole‐based HER electrocatalysts. [ 33–37 ] Tafel plots in Figure 3d also confirm the role of carboxylates in stabilizing the Zn 2+ coordination and preventing HER, while introducing a large amount of histidine destabilizes the Zn 2+ coordination. The Tafel curves showed that ZnCND and 50%H‐ZnCND‐anchored anode had much lower current density over a wider voltage range from −1.5 V to −1.9 V (vs Ag/AgCl) than that of pure Zn, suggesting slower HER kinetics with the labile interphase.…”

NixCo1–x@NixCo1–xO/NCNT as Trifunctional ORR, OER, and HER Electrocatalysts and its Application in a Zn–Air Battery

“…The Tafel slope value of 68 mV dec −1 suggested the Volmer−Heyrovsky mechanism associated with the reaction, in which hydrogen adsorption and discharge steps are rate-determining steps (Figure 6b). 31,65 The HER catalytic stability was examined by chronoamperometric measurement at an applied potential of −0.15 V vs RHE for 24 h (Figure 6c). Ni x Co 1−x @Ni x Co 1−x O/NCNT revealed a steady current density of approximately 66 mA cm −2 with complete retention of activity.…”

“…From LSV, we found that Ni x Co 1 – x @Ni x Co 1 – x O/NCNT showed an overpotential of 74 mV at 10 mA cm –2 , however, the state-of-art Pt/C catalyst showed 33 mV overpotential for achieving the same current density (Figure a). The Tafel slope value of 68 mV dec –1 suggested the Volmer–Heyrovsky mechanism associated with the reaction, in which hydrogen adsorption and discharge steps are rate-determining steps (Figure b). , The HER catalytic stability was examined by chronoamperometric measurement at an applied potential of −0.15 V vs RHE for 24 h (Figure c). Ni x Co 1 – x @Ni x Co 1 – x O/NCNT revealed a steady current density of approximately 66 mA cm –2 with complete retention of activity.…”