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

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 (k ⁰ ≈ 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.