(544gm) Nanoporous Palladium Alloys As CO Poisoning Suppressing Electrocatalysts for Electrochemical Conversion of CO2 to Formate

Electrochemical carbon dioxide reduction reaction (CO₂RR) has been widely studied for conversion of CO₂ into useful carbon based products e.g CO, formate/formic acid, alcohols etc. As the binding affinities of the different reaction intermediates differ, metals available for the CO₂RR show variability in product selectivity, overpotential requirements and sustained activity. Thus development of an appropriate electrocatalyst with a high activity/faradaic efficiency (FE) for a single product is a challenge¹. Among the simple 2 electron reduced products, formate is gaining focus because of its high energy density and utility as a chemical precursor. Traditional formate producing metals e.g. Sn, In, Bi, Co etc. require high overpotentials (>700 mV), limiting their effectiveness. Recently Palladium, originally known to produce CO at overpotentials above 600 mV RHE², has demonstrated formate FE’s close to 100% at overpotentials below 200 mV³. The gradual poisoning of Pd-based CO₂RR electrocatalysts through the slow generation of CO, even at near unity formate FEs⁴, results in catalyst deactivation and limits the viability of Pd in commercial applications.

Herein, we present making of free standing, core-shell, nanoporous Pd (np-PdX, where X = Ag, Cu, Ni, Co) alloys that display a compositional dependent CO deactivation rate. The np-PdX electrocatalysts are synthesized through electrochemical dealloying⁵ of 4 binary Pd₁₅X₈₅ (X = Co, Ni, Cu, Ag) alloys. Electrochemical dealloying drives the formation of Pd skinned nanoporous architectures where the presence of less noble metal under the Pd shell alters the electronic structure of the Pd skin and consequently affects the CO adsorption strengths thereby changing the rate of catalyst poisoning. The np-PdX electrodes have pore sizes ranging from 5 – 10 nm and a residual composition of ~ 30 atomic % of the less noble metal. The tortuous pores give rise to roughness factors above 500 which generates high geometric formate partial current densities (> 40 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.

Upon performing extended electrolysis, the CO₂RR activity, formate FE and degree of deactivation of the np-PdX elecrodes are compared with a traditional Pd/C electrocatalyst having particle sizes of 5-10 nm. While all the np-PdX electrodes show formate FE above 90% within 200 mV overpotential as already known for Pd/C, there is a significant dissimilarity in tolerance to CO poisoning as evidenced by the compositionally dependent time of deactivation. While np-PdAg and np-PdCu show faster deactivation compared to Pd/C, np-PdCo and np-PdNi suppress poisoning for much longer times. The results are 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 Pd⁶. This is also shown by the trend of CO stripping peak potentials on Pd-skinned polycrystalline PdX (X = Co, Ni, Cu and Ag) alloys. Thus np-PdX electrocatalysts based on a suitable choice of alloying component, offer substantial suppression of CO poisoning, much higher formate geometric current densities and enhanced operational stability compared to traditional Pd based CO₂RR electrocatalysts.

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