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