Effect of Chloride Anions on the Synthesis and Enhanced Catalytic Activity of Silver Nanocoral Electrodes for CO₂ Electroreduction

Abstract

Metallic silver (Ag) is known as an efficient electrocatalyst for the conversion of carbon dioxide (CO 2 ) to carbon monoxide (CO) in aqueous or non-aqueous electrolytes. However, polycrystalline silver electrocatalysts require significant overpotentials in order to achieve high selectivity toward CO 2 reduction, as compared to the side reaction of hydrogen evolution. Here we report a high-surface-area Ag nano-coral catalyst, fabricated by an oxidation-reduction method in the presence of chloride anions in an aqueous medium, for the electro-reduction of CO 2 to CO with a current efficiency of 95% at the low overpotential of 0.37 V and the current density of 2 mA cm −2 . A lower limit of TOF of 0.4 s −1 and TON > 8.8 × 10 4 (over 72 h) was estimated for the Ag nano-coral catalyst at an overpotential of 0.49 V. The Ag nano-coral catalyst demonstrated a 32-fold enhancement in surface-area-normalized activity, at an overpotential of 0.49 V, as compared to Ag foil. We found that, in addition to the effect on nanomorphology, the adsorbed chloride anions play a critical role in the observed enhanced activity and selectivity of the Ag nano-coral electrocatalyst toward CO 2 reduction. Synchrotron X-ray photoelectron spectroscopy (XPS) studies along with a series of control experiments suggest that the chloride anions, remaining adsorbed on the catalyst surface under electrocatalytic conditions, can effectively inhibit the side reaction of hydrogen evolution and enhance the catalytic performance for CO 2 reduction.

Enhanced CO selectivity and stability for electrocatalytic reduction of CO 2 on electrodeposited nanostructured porous Ag electrode

“…4. In contrast, The slops for two electrodes are both consistent with the reported value of the rate determining step of one-electron reduction of CO 2 to form a surface adsorbed CO 2 À intermediate [6,16,17,21,30]. The noteworthy difference in CO faradaic efficiency between the two electrode suggests that the nanostructured surface of porous Ag foams lower the barrier for the formation of CO 2 À intermediate while suppressing HER [32].…”

“…Faradaic efficiency for CO remained above 90% throughout the electrocatalytic process. Similar characteristics in activity and stability have been found in other nanostructured electrocatalysts [16,17,21,30]. Much more accessible surface active sites provided by porous surface than the flat one result in the improvement of activity.…”

“…Compared with Ag foams deposited on Pt/Ti/Si substrate [23], as-prepared porous Ag foams on Ag foil have the smaller average pore size and are composed by smaller Ag grains, due to the different property of hydrogen evolution on these two substrates. Moreover, the morphology and the size of Ag grains here are distinctly different from Ag nanocorals fabricated by oxidation-reduction of Ag foil as well [17].…”

“…Lu et al [16] prepared a nanoporous Ag catalyst by a dealloy process, which achieve a high activity of approximately 20 mA cm À2 with 92% selectivity at a potential of 0.6 V (vs. RHE) in aqueous KHCO 3 electrolyte. Polyansky et al [17] reported a nanocoral Ag catalyst fabricated by an oxidationreduction method, demonstrated a 32-fold enhancement in surface-area normalized activity at the low potential of 0.37 V, as compared to Ag foil.…”

Optimizing C–C Coupling on Oxide-Derived Copper Catalysts for Electrochemical CO2 Reduction

“…[12][13][14][15][16][17][18][19][20][21] Oxidation-reduction processing has also been applied to gold and silver, and decreases in the overpotential required for CO2 reduction to CO have been reported by a number of groups. [23][24][25][26][27] There are two important issues not addressed in previous studies of oxide-derived Cu catalysts.…”