Tuning Binding Energy in Nanoporous Core Shell Alloys of Palladium to Suppress CO Poisoning in Electrochemical Reduction of CO2

Abstract

Electrochemical CO2 reduction reaction (CO2RR) has been extensively explored for conversion of dissolved CO2 into useful carbon based products e.g CO, formate/formic acid, alcohols etc. Formate/formic acid as a CO2RR product is drawing focus because of its high energy density and utility as a chemical precursor. Traditional formate producing metals e.g. Co, In, Bi, Sn etc. require high overpotentials, limiting their energy effectiveness. Pd on the other hand, originally known to produce CO at overpotentials above 600 mV RHE, has recently shown substantial formate FE’s at overpotentials below 200 mV. The gradual poisoning of Pd-based CO2RR electrocatalysts through the slow production of CO, even at near unity formate FEs, results in deactivation of the catalyst surface and limits the viability of Pd for sustained electrolysis. Here, we report synthesis of core-shell nanoporous multimetallic Pd (np-PdX, where X = Co, Ni, Cu, Ag, Cu-Sn, Cu-Ti) alloys that show suppressed CO deactivation based on the subsurface composition of the alloy electrocatalysts. The Pd skinned nanoporous alloys have been obtained by electrochemical dealloying where the presence of less noble metal/alloy under the Pd shell changes the electronic structure of the Pd and consequently alters the CO adsorption strengths and subsequent catalyst deactivation. The tortuous pores (pore size: 5 – 10 nm) give rise to roughness factors above 200 which generates high geometric formate partial current densities (> 30 mA/cm2). Furthermore, the np-Pd electrodes are free standing and obviate the use of any binder and/or support which otherwise causes extra overpotential losses and morphological instability. The suppression of surface poisoning is explained on the basis of weakening of CO binding strengths due to electronic interactions of the alloying component that shifts the d-band center of Pd, as well as changes to the near surface hydrogen solubility which can also affect the adsorption strength of active/inactive intermediates and reaction selectivity.