(145c) Bimetallic Nanoporous Pd Alloys as CO Tolerant Electrocatalysts for the Electrohydrogenation of CO2 to Formate

Electrochemical CO₂ reduction reaction (CO₂RR) has been widely investigated for conversion of water dissolved CO₂ into useful carbon based products e.g CO, formate/formic acid, alcohols etc. Depending upon the binding affinities of the different reaction intermediates, metals available for the CO₂RR show variability in product selectivity, overpotential requirements and sustained activity. Thus development of a suitable electrocatalyst with a high activity/faradaic efficiency (FE) for a single product is a challenge¹. Among the simple 2 electron reduced products, formate/formic acid 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, Pd, originally known to produce CO at overpotentials above 0.6 V RHE², has recently demonstrated formate FE’s near unity at overpotentials below 0.2 V³. The gradual poisoning of Pd-based CO₂RR electrocatalysts through the slow evolution of CO, even at near unity formate FEs⁴, results in catalyst deactivation and limits the viability of Pd in commercial applications.

Here, we report fabrication of free standing, core-shell, nanoporous bimetallic Pd (np-PdX, where X = Ag, Cu, Ni, Co) alloys that display a compositional dependent CO deactivation rate. The np-PdX electrocatalysts are made 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 deactivation. The np-PdX electrodes have a residual composition of ~ 30 atomic % of the less noble metal and pore sizes ranging from 5 – 10 nm. 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 and Pd/C show above 90% formate FE within 200 mV overpotential, there is a significant variation 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 avoid 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 weighted average 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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