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A novel bismuth oxide nanofoam, produced by means of the dynamic hydrogen bubble template (DHBT) electrodeposition approach followed by thermal annealing at 300 °C for 12 h, demonstrates excellent electrocatalytic selectivity toward formate production with faradaic efficiencies (FEs) never falling below 90% within an extended potential window of ∼1100 mV (max. FE formate = 100% at −0.6 V vs RHE). These promising electrocatalytic characteristics result from the coupling of two distinct reaction pathways of formate formation in the aqueous CO 2 -sat. 0.5 M KHCO 3 electrolyte, which are active on (i) the partly reduced Bi 2 O 3 foam at low overpotentials (sub-carbonate pathway) and (ii) on the corresponding metallic Bi met foam catalyst at medium and high overpotentials (Bi−O pathway). For the first time, operando Raman spectroscopy provides experimental evidence for the embedment of CO 2 into the oxidic Bi 2 O 3 matrix (sub-carbonate formation) at low overpotentials prior to and during the CO 2 reduction reaction (CO 2 RR). The gradual transition of the formed carbonate/oxide composite catalyst into its fully metallic state is monitored by operando Raman spectroscopy as a function of electrolysis time and applied potential. The observed structural and compositional alterations correlate with changes in the faradaic efficiency and partial current density of formate production (PCD formate ), which reaches a maximum value of PCD formate = −84.1 mA cm −2 at −1.5 V vs RHE. The so-called identical location scanning electron microscopy technique was applied to monitor morphological changes that take place on the nanometer length scale upon sub-carbonate formation and partial electro-reduction of the oxidic precursor during the CO 2 RR. However, the macroporous structure of the foam catalyst remains unaffected by the (oxide/ carbonate → metal) transition and the catalytic CO 2 RR. KEYWORDS: ec-CO 2 reduction, operando Raman spectroscopy, identical location (IL) SEM, formate production, carbon fiber cloth, (BiO) 2 CO 3 , Bi 2 O 3 nanofoam

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The creation of open porous structures with an extremely high surface area is of great technological relevance. The electrochemical deposition of metal foams around co‐generated hydrogen bubbles that act as templates for the deposition is a promising, cheap and simple approach to the fabrication of new electrocatalyst materials. Metal foams obtained by dynamic hydrogen bubble templating (DHBT) offer an intrinsically high electrical conductance with an open porous structure that enables the fast transport of gases and liquids. As an additional benefit, the confined space within the pores of DHBT metal foams may act as small reactors that can harbour reactions not possible at an open electrode interface. The number, distribution, and size of the pores can be fine‐tuned by an appropriate choice of the electrolysis parameters so that metal foam catalysts prepared by the DHBT technique meet certain requirements. In this paper, we review the preparation of certain metal foams, and their applications as catalysts for the electrochemical reduction of CO2.

We present a novel, foam-type, high surface area electrocatalyst for the CO2 reduction reaction (CO2RR) that is not only highly selective toward n-propanol (PrOH) formation (FEPrOH = 13.7%, jPrOH =...

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Direct electrosynthesis of formate through CO2 electroreduction (denoted CO2RR) is currently attracting great attention because formate is a highly valuable commodity chemical that is already used in a wide range of applications (e.g., formic acid fuel cells, tanning, rubber production, preservatives, and antibacterial agents). Herein, we demonstrate highly selective production of formate through CO2RR from a CO2-saturated aqueous bicarbonate solution using a porous In55Cu45 alloy as the electrocatalyst. This novel high-surface-area material was produced by means of an electrodeposition process utilizing the dynamic hydrogen bubble template approach. Faradaic efficiencies (FEs) of formate production (FEformate) never fell below 90% within a relatively broad potential window of approximately 400 mV, ranging from −0.8 to −1.2 V vs the reversible hydrogen electrode (RHE). A maximum FEformate of 96.8%, corresponding to a partial current density of j formate = −8.9 mA cm–2, was yielded at −1.0 V vs RHE. The experimental findings suggested a CO2RR mechanism involving stabilization of the HCOO* intermediate on the In55Cu45 alloy surface in combination with effective suppression of the parasitic hydrogen evolution reaction. What makes this CO2RR alloy catalyst particularly valuable is its stability against degradation and chemical poisoning. An almost constant formate efficiency of ∼94% was maintained in an extended 30 h electrolysis experiment, whereas pure In film catalysts (the reference benchmark system) showed a pronounced decrease in formate efficiency from 82% to 50% under similar experimental conditions. The identical location scanning electron microscopy approach was applied to demonstrate the structural stability of the applied In55Cu45 alloy foam catalysts at various length scales. We demonstrate that the proposed catalyst concept could be transferred to technically relevant support materials (e.g., carbon cloth gas diffusion electrode) without altering its excellent figures of merit.

Herein, we demonstrate the superior performance of bismuth subcarbonate ((BiO) 2 CO 3 ) layer catalysts for formate production using a fluidic CO 2 -fed electrolyzer device. The subcarbonate catalyst readily forms in situ from a CO 2 -absorbing Bi 2 O 3 precursor material during the CO 2 reduction reaction (CO 2 RR). In 1 mol dm −3 KOH electrolyte solution, a maximum Faradaic efficiency of FE formate = 97.4% (corresponding partial current density of formate formation: PCD formate = −111.6 mA cm −2 ) was achieved at a comparably low applied electrolysis potential of −0.8 V vs the reversible hydrogen electrode (RHE). Even higher values of PCD formate = −441.2 mA cm −2 (FE formate = 62%) were observed at a more cathodic potential, −2.5 V vs RHE. As the alkalinity of the liquid electrolyte is further increased (e.g., by using 5 mol dm −3 KOH solution), the performance of formate production is boosted beyond PCD formate values of −1 A cm −2 . Combined X-ray diffraction and Raman spectroscopic investigations demonstrate an extraordinarily high stability of Bi(III) cations in the catalytically active subcarbonate catalyst phase down to cathode potentials of −1.5 V vs RHE. This stabilization effect can clearly be attributed to the high abundance of gaseous CO 2 under the operating conditions of the gas-fed electrolyzer. In the absence of any CO 2 supply, the reductive Bi(III) → Bi(0) transition already occurs under much milder conditions of −0.3 V vs RHE, as evidenced by in situ Raman spectroscopy in CO 2 -free 1 mol dm −3 KOH electrolyte solution. An advanced X-ray diffraction computed tomography technique was applied to gain deeper insights into the spatial distribution of the metallic and subcarbonate phases comprising the active composite catalyst layer during the CO 2 RR.