Insights from Operando and Identical Location (IL) Techniques on the Activation of Electrocatalysts for the Conversion of CO2: A Mini-Review

Abstract

In this mini-review we compare two prototypical metal foam electrocatalysts applied to the transformation of CO2 into value-added products (e.g. alcohols on Cu foams and formate on Bi foams). A substantial improvement in the catalyst performance is typically achieved through thermal annealing of the as-deposited foam materials, followed by the electro-reduction of the pre-formed oxidic precursors prior or during the actual CO2 electrolysis. Utilizing highly insightful and sensitive complementary operando analytical techniques (XAS, XRD, and Raman spectroscopy) we demonstrate that this catalyst pre-activation process is entirely accomplished in case of the oxidized Cu foams prior to the formation of hydrocarbons and alcohols from the CO2. The actually active catalyst is therefore the metallic Cu derived from the precursor by means of oxide electro-reduction. Conversely, in their oxidic form, the Cu-based foam catalysts are inactive towards the CO2 reduction reaction (denoted ec-CO2 RR). Oxidized Bi foams can be regarded as an excellent counter example to the above-mentioned Cu case as both metallic and the thermally derived oxidic Bi foams are highly active towards ec-CO2 RR (formate production). Indeed, operando Raman spectroscopy reveals that CO2 electrolysis occurs upon its embedment into the oxidic Bi2O3 foam precursor, which itself undergoes partial transformation into an active sub-carbonate phase. The potential-dependent transition of sub-carbonates/oxides into the corresponding metallic Bi foam dictates the characteristic changes of the ec-CO2 RR pathway. Identical location (IL) microscopic inspection of the catalyst materials, e.g. by means of scanning electron microscopy, demonstrates substantial morphological alterations on the nm length scale on the material surface as consequence of the sub-carbonate formation and the potential-driven oxide reduction into the metallic Bi foam. The foam morphology on a mesoscopic length scale (macroporosity) remains, by contrast, fully unaffected by these phase transitions.

Novel Ni foam catalysts for sustainable nitrate to ammonia electroreduction

“…S10), the crystallinity of the obtained Ni foams was largely independent on the foam layer thickness (deposition time) and resembled the characteristics of polycrystalline Ni. Similar to other foam materials reported in the literature (Dutta et al, 2021a(Dutta et al, , 2016, a gradient in pore diameters formed along the surface normal. Structural data on the formed Ni foams are summarized in Fig.…”

In situ electrochemical characterization of CuxO-based gas-diffusion electrodes (GDEs) for CO2 electrocatalytic reduction in presence and absence of liquid electrolyte and relationship with C2+ products formation

“…Recently, although still to a limited extent, activities about charac-terization in operando of electrocatalysts for CO2 reduction have been growing [29][30][31][32][33][34][35][36]. However, these studies mainly focused on under-standing factors related to the electrocatalyst itself, rather than on the aspects mentioned before.…”

CO2 Conversion at High Current Densities: Stabilization of Bi(III)-Containing Electrocatalysts under CO2 Gas Flow Conditions

“…Since the pioneering work by Hori et al, it has been known that particular oxophilic materials, e.g., Sn, − Pb, Cd, Tl, and In, favor CO 2 RR pathways toward formate production. However, debate continues regarding the mechanistic role of the respective metal oxides, which are either formed in situ during electrolysis or applied directly as the catalyst (precursor). − Advanced operando techniques, e.g., vibrational ,− and X-ray absorption spectroscopy, − have been successfully employed to gain deeper mechanistic insight into the electrolysis time, current density, and potential-dependent activation of CO 2 RR catalysts under operating conditions. , A recent example is based on the use of oxidic bismuth electrocatalysts, ,, which had already demonstrated superior selectivities toward formate production in classical H-type cell testing environments, with Faradaic efficiencies (FEs) exceeding 95% within an extraordinarily (approximately 1.1 V) , wide potential window. For a CO 2 -saturated 0.5 mol dm –3 KHCO 3 electrolyte solution, a combination of electrochemical analysis and operando Raman spectroscopy during CO 2 RR revealed the coupling of two potential-dependent CO 2 RR pathways as the origin of this superior catalytic performance .…”