Pulsed voltammetry has been used to study formic acid oxidation on platinum stepped surfaces to determine the kinetics of the reaction and the role of the surface structure in the reactivity. From the current transients at different potentials, the intrinsic activity of the electrode through the active intermediate reaction path (j(theta = 0)), as well as the rate constant for the CO formation (k(ads)) have been calculated. The kinetics for formic acid oxidation through the active intermediate reaction path is strongly dependent on the surface structure of the electrode, with the highest activity found for the Pt(100) surface. The presence of steps, both on (100) and (111) terraces, does not increase the activity of these surfaces. CO formation only takes place in a narrow potential window very close to the local potential of zero total charge. The extrapolation of the results obtained with stepped surfaces with (111) terraces to zero step density indicates that CO formation should not occur on an ideal Pt(111) electrode. Additionally, the analysis of the Tafel slopes obtained for the different electrodes suggests that the oxidation of formic acid is strongly affected by the presence of adsorbed anions, hydrogen and water.

Pulsed voltammetry was used to study formic acid electrooxidation on Pt nanoparticles with well-characterized surfaces. Polyoriented and preferential (100), (100−111), and (111) Pt nanoparticles were characterized and employed to evaluate the influence of the surface structure and shape of the Pt nanoparticles on this model electrochemical reaction. The results pointed out that, in agreement with fundamental studies with Pt single crystal electrodes, the surface structure of the electrodes plays an important role on the reactivity and kinetics of formic acid oxidation. Thus the electrocatalytic properties for this reaction strongly depend on the dominant structure on the surface of the nanoparticles, in particular on the presence of domains with (100) and (111) symmetry. Among the Pt nanoparticles studied, those containing (100) domains are clearly the most active toward formic acid electrooxidation via the active intermediate path, but also exhibit the highest poisoning rate. (111) Pt nanoparticles show the lowest CO formation constant in the series and a moderate reaction rate via the active intermediate reaction path.

Pulsed voltammetry is used to study formic acid oxidation on Pt(2n-1,1,1) surfaces and determine the effects of the size of the (100) terrace and the (111) step density on the reaction mechanism. The intrinsic activity of the electrode through the active intermediate reaction path (j(theta=) (0)), as well as the rate constant for the CO formation (k(ads)), are calculated from the current transients obtained at different potentials. For surfaces with wide terraces, j(theta=) (0) and k(ads) are almost insensitive to the step density, which suggests that step and terrace sites have a similar activity for this reaction. For narrow terraces (n<6), the intrinsic activity diminishes. The dependence of the reaction rates on the electrode potential is also elucidated. The CO formation only takes place in a narrow potential window, very close to the potential of zero total charge, while the direct oxidation takes place even when the surface is covered by anions. The different behavior for both reactions suggests that the adsorption mode of formic acid is different for each path.

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

Among the electrolyzers under development for CO2 electroreduction at practical reaction rates, gas-fed approaches that use gas diffusion electrodes (GDEs) as cathodes are the most promising. However, the insufficient long-term stability of these technologies precludes their commercial deployment. The structural deterioration of the catalyst material is one possible source of device durability issues. Unfortunately, this issue has been insufficiently studied in systems using actual technical electrodes. Herein, we make use of a morphologically tailored Ag-based model nanocatalyst [Ag nanocubes (NCs)] assembled on a zero-gap GDE electrolyzer to establish correlations between catalyst structures, experimental environments, electrocatalytic performances, and morphological degradation mechanisms in highly alkaline media. The morphological evolution of the Ag–NCs on the GDEs induced by the CO2 electrochemical reduction reaction (CO2RR), as well as the direct mechanical contact between the catalyst layer and anion-exchange membrane, is analyzed by identical location and post-electrolysis scanning electron microscopy investigations. We find that at low and mild potentials positive of −1.8 V versus Ag/AgCl, the Ag–NCs undergo no apparent morphological alteration induced by the CO2RR, and the device performance remains stable. At more stringent cathodic conditions, device failure commences within minutes, and catalyst corrosion leads to slightly truncated cube morphologies and the appearance of smaller Ag nanoparticles. However, comparison with complementary CO2RR experiments performed in H-cell configurations in a neutral environment clearly proves that the system failure typically encountered in the gas-fed approaches does not stem solely from the catalyst morphological degradation. Instead, the observed CO2RR performance deterioration is mainly due to the local high alkalinity that inevitably develops at high current densities in the zero-gap approach and leads to the massive precipitation of carbonates which is not observed in the aqueous environment (H-cell configuration).