Alkaline water-splitting reactions over Pd/Co-MOF-derived carbon obtained via microwave-assisted synthesis

Abstract

Cobalt-based metal–organic framework-derived carbon (MOFDC) has been studied as a new carbon-based support for a Pd catalyst for electrochemical water-splitting; i.e., the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline medium. The study shows a high increase in the HER activity, in terms of low onset overpotential (onset η = 35 mV vs. RHE), high exchange current density (j_o,s ≈ 0.22 mA cm⁻²), high mass activity (j_o,m ≈ 59 mA mg⁻¹), high kinetic current (j_K ≈ 5–8 mA cm⁻²) and heterogeneous rate constant (k⁰ ≈ 4 × 10⁻⁴ cm s⁻¹), which are attributed to the high porosity of MOFDC and contribution from residual Co, while the large Tafel slope (b_c = 261 mV dec⁻¹) is ascribed to the high degree of hydrogen adsorption onto polycrystalline Pd as a supplementary reaction step to the suggested Volmer–Heyrovsky mechanism. These values for the catalyst are comparable to or better than many recent reports that adopted nano-carbon materials and/or use bi- or ternary Pd-based electrocatalysts for the HER. The improved HER activity of Pd/MOFDC is associated with the positive impact of MOFDC and residual Co on the Pd catalyst (i.e., low activation energy, E_A ≈ 12 kJ mol⁻¹) which allows for easy desorption of the H_ads to generate hydrogen. Moreover, Pd/MOFDC displays better OER activity than its analogue, with lower onset η (1.29 V vs. RHE) and b_a (≈78 mV dec⁻¹), and higher current response (ca. 18 mA cm⁻²). Indeed, this study provides a new strategy of designing and synthesizing MOFDC with physico-chemical features for Pd-based electrocatalysts that will allow for efficient electrochemical water-splitting processes.