CO-saturated Electrolyte Research Articles

Electrochemical syngas production over Au/SrTiO3 and Fischer–Tropsch synthesis chemistry for long-chain hydrocarbons

Syngas (a mixture of CO and H2) has been utilized in industrial settings for the production of liquid fuels through Fischer–Tropsch (F-T) synthesis. The conventional methods of producing syngas involve energy-intensive processes utilizing coal, natural gas, or biomass. However, in order to promote more environmentally friendly practices, electrochemistry has emerged as an alternative approach. In this study, we demonstrate that syngas can be efficiently produced through electrochemical (EC) CO2 conversion using Au-loaded perovskite strontium titanate (SrTiO3) as a catalyst. By manipulating various parameters such as overlayer Au thickness, applied potential, electrolyte type, and concentration, we can easily control the ratio of CO to H2. Moreover, our findings reveal that electrochemistry can directly initiate the minor channel of the F-T synthesis process. We also observe the occurrence of the EC F-T process in CO-saturated electrolyte, where CO molecules directly adsorb and interact with surface H to generate hydrocarbons. The analysis of the Anderson-Schulz-Flory equation demonstrates a strong linear relationship, confirming that the EC F-T synthesis follows the conventional F-T synthesis mechanism. This novel demonstration not only offers valuable insights into the development of EC syngas production but also enhances our understanding of the mechanism behind the formation of C2+ hydrocarbon products.

Compressive Stress and Charge Redistribution during CO Adsorption Onto Pt

Electroorganic reactions, where energy is either stored or released from chemical bonds within carbon-based molecules, are essential to help achieve a circular carbon-based economy. Central to electrochemical conversion of organic molecules is carbon monoxide (CO), which serves as both an intermediate in electrochemical CO2 reduction to hydrocarbons and electrochemical oxidation in fuel cells that operate via oxygenated hydrocarbons (e.g., ethanol, methanol). For the latter, Pt has been the predominate catalyst of choice, and substantial effort has been spent to identify the mechanistic details for complete oxidation of alcohol-based fuels. With CO conversion serving as an important if not rate-limiting step of the oxidative process, it is critical to understand the state of a catalytic surface when CO is present.The change in surface stress associated with the adsorption and oxidative stripping of CO on (111)-textured Pt is examined using the wafer curvature method in 0.1 mol/L KHCO3 electrolyte.1 The curvature of the Pt cantilever electrode was monitored as a function of potential in both CO free and CO-saturated electrolyte. Although CO adsorbs as a neutral molecule, significant compressive stress, up to -1.3 N/m, is induced in the Pt. The magnitude of the stress change correlates directly with the CO coverage, and within the detection limits of the stress measurement, is elastically reversible. Density functional theory calculations of a CO bound Pt surface indicate that charge redistribution from the first atomic layer of Pt to subsurface layers account for the observed compressive stress induced by the charge neutral adsorption of CO. A better understanding of adsorbate-induced surface stress is critical for the development of materials platforms for sensing and catalysis. References D. Raciti, K.A. Schwarz, J. Vinson, and G.R. Stafford, The Journal of Physical Chemistry C , 126, 4446-4457 (2022).

How Local Reaction and Process Conditions Influence CO2 Reduction to Multicarbon Products on Copper Gas-Diffusion Electrodes

Electrochemical CO2 reduction is one technology being investigated to convert waste CO2 into value-added products due to the ability for the process to be performed under room-temperature conditions using intermittent electricity. Within the last 2 years, a number of new reports have moved to both gas-diffusion electrode (GDE) architectures and membrane electrode assemblies has increased greatly.Through these new architectures, a substantial amount of CO2 reduction studies have been performed using copper catalysts. The wide range of CO2 reduction products formed has further been shown to vary with catalyst, electrolyte, current density and even CO concentration. The latter parameter has been shown to play a critical role in CO2 reduction selectivity/activity as shown through Au-Cu tandem catalysis (Lum and Ager, EES, 2018; Morales-Guio et al, N. Cat., 2018) and by varying CO2:CO feedstocks (Wang et al. N. Nano., 2019). What is often-overlooked, however, is that a copper GDE will, by itself, produce a substantial amount of CO during CO2 reduction at current densities >300 mA/cm2.Understanding the role of CO(aq) in CO2 reduction is slowly being understood through various mechanistic studies, as well as some modelling work (Wu et al, JECS, 2015; Weng et al, PCCP, 2018), but as of yet there does not exist a tool for researchers to quickly take experimental data and predict the local concentrations of species and products within a GDE configuration. Such a tool would be invaluable for providing an often-overlooked aspect for analyzing experimental data.Here in this work, we begin to distill many of the complexities of GDE operation down to a singular crucial factor, the local reaction environment. Specifically, we use advanced mass transport models to predict both CO2 and product concentrations throughout the gaseous GDE and liquid catalyst layer using data from experiments on controlled Cu nanostructures. Of primary novelty, using experimental data (Faradaic efficiency and gas channel composition) we can understand transport to and from the catalyst’s surface for a variety of current densities and electrolytes, giving us the steady-state concentration of important species (CO2, CO, OH-, C2H4) at the electrode’s surface.Here we experimentally show that despite large differences in product selectivity of CO2 reduction at lower current densities (<200 mA/cm2) in 1M KHCO3, KCl and KOH, once a current density of 300 mA/cm2 is applied, we find that the selectivity, activity, local environment and potential required to reduce CO2 on a copper electrode are identical. All electrolytes were found to produce C2H4 selectivities of 50-55%. We then show that the concentration of CO(aq) within the catalyst layer is 2-3 times that of a CO-saturated electrolyte. Given the recent demonstration that even small amounts of CO can influence ethylene formation (Wang et al. N. Nano, 2019), we conclude that the presence of by-product CO already plays a strong role in the high observed ethylene selectivity in all previous GDE studies on copper. Extending our results further, we discuss what these findings mean for future studies that aim to maximize either single-pass conversion efficiencies or move to larger cell sizes.In short, we have shown with experiments and modeling that process conditions and the local reaction environment for CO2 reduction on copper electrodes in a gas-diffusion layer configuration are complex, but can be predicted. Without the field taking mass transport of copper GDE’s into consideration when analyzing experimental results, we feel that a critical piece of information for describing selectivity and activity is missing.

In Situ Monitoring Using ATR-SEIRAS of the Electrocatalytic Reduction of CO2 on Au in an Ionic Liquid/Water Mixture

The electrochemical reduction of CO2 to CO in ionic liquids and ionic-liquid/water mixtures has received considerable attention due to recent claims of extraordinarily high energy efficiencies. We report here a study of CO2 electroreduction on Au in an [EMIM]BF4/H2O mixture (18% mol/mol) combining cyclic voltammetry and surface-enhanced infrared absorption spectroscopy in the attenuated total reflection mode (ATR-SEIRAS). The onset of the reduction current in the CV coincides with a decrease of the interfacial CO2 concentration, but the appearance of adsorbed CO (COad) is slightly delayed, as CO must probably first reach a minimum concentration at the interface. Comparisons with spectra collected in the absence of CO2 and in CO-saturated electrolyte reveal that the structure of the double layer at negative potentials is different when CO2 is present (probably due to the formation of COad) and allow us to assign the main band in the spectra to CO adsorbed linearly on Au (COL), with a smaller band correspon...

Pt Acts As a Catalyst to Activate RuO2: The Active Sites of RuO2 Co-Catalyst for the CO Oxidation

RuO2 has been suggested to act as stable co-catalysts for CO oxidation on Pt. However, little is known on how the oxide functions to oxidize nearby CO. In this work, we use model rotating disk electrodes composed of RuO2 nanosheets (RuO2(ns)) deposited on the surface of polycrystalline Pt, polycrystalline Au and glassy carbon. A new co-catalytic mechanism regarding the Pt/RuO2 interface towards the electro-oxidation of CO is proposed. Two distinct synergetic effects were identified, decreasing the bulk CO electro-oxidation onset potential compared to bare Pt by 50 mV (the ‘high potential’ synergetic effect at 0.85 V vs. RHE) and 250 mV (the ‘low potential’ synergetic effect at 0.65 V vs. RHE). We discuss the later with a new mechanism, involving an activated surface of RuO2(ns) generated by a partial reduction catalyzed by Pt. The peculiar ‘low potential’ synergetic effect decays upon potential cycling in CO-saturated electrolyte where Pt is poisoned with COad, but can be reactivated by COad-free Pt in HUPD potential region. These results were resolved considering the formation of ‘Ru metal-like’ sites on RuO2(ns) which are generated from a partial reduction of the RuO2(ns) surface catalyzed by a Pt surface that is not poisoned by CO (spill-over hydrogen). Therefore, this ‘low potential’ synergetic effect interestingly differs from the customary bifunctional mechanism and shows a highly interdependent relationship where Pt-catalyzed activated surface of RuO2(ns) provides oxygenated species for the removal of CO adsorbed on Pt. Acknowledgments This work was supported in part by the ‘Polymer Electrolyte Fuel Cell Program’ from the New Energy and Industrial Technology Development Organization (NEDO), Japan.

Determination of the Electrochemically Active Surface Area of Metal-Oxide Supported Platinum Catalyst

The electrochemically active surface area (ECSA) of metal-oxide supported platinum catalysts as obtained from hydrogen underpotential deposition (Hupd) and from carbon monoxide stripping experiments was investigated. It was demonstrated that both methods fail to give meaningful values of the ECSA if they are performed in the conventional way as known for pure Pt and carbon supported Pt catalysts, respectively. For both methods, the reason for this failure is the lack of a correct baseline for the integration of the associated charges. It was found that the cyclic voltammogram recorded in CO saturated electrolyte gives an improved baseline for the Hupd analysis. For CO stripping, a novel baseline method was developed by performing a “CO stripping simulation” (COSS) experiment in CO-free electrolyte. The first cycle of this COSS-experiment is an improved baseline for the integration of the CO stripping peak, since possible support reduction/oxidation currents can be accounted for. With these modifications, Hupd and CO stripping voltammetry can be used for metal-oxide supported platinum to yield true, reproducible and consistent values for the ECSA.

Electrochemical instability of Pt nanoparticles probed by formic acid oxidation

A detrimental effect of CO on the stability of Pt nanoparticles, which is important for fuel cells technology, is demonstrated by the change in HCOOH kinetics on Pt nanoparticles caused by the presence of CO in the electrolyte. Pt nanoparticles supported on high area carbon (Pt/C) were applied on gold and glassy carbon substrates (Pt/C@Au and Pt/C@GC) and three different kinds of Pt surface were prepared by potential cycling in the various potential limits in N 2 and CO saturated electrolyte: untreated, OH-annealed and CO-annealed. On untreated Pt/C@Au catalyst HCOOH is oxidized predominantly through indirect mechanism, after OH-annealing direct path is slightly accelerated, but on CO-annealed catalyst the direct path in HCOOH oxidation is predominant. Based on the fact that more contiguous Pt sites are necessary for indirect than for direct path, it was concluded that OH- and especially CO-annealing of Pt/C produces small Pt domains. Since the same pretreatment did not show any effect on HCOOH oxidation on Pt/C@GC electrode, it was postulated that Pt nanoparticles dissolve in the presence of CO on the surface and in the electrolyte, and successively deposit on Au substrate. The results points out pronounced instability of Pt nanoparticles in the presence of CO.

Electrochemistry at Ru(0001) in a flowing CO-saturated electrolyte—reactive and inert adlayer phases

We investigated the electrochemical oxidation and reduction processes on ultrahigh vacuum prepared, smooth and structurally well-characterized Ru(0001) electrodes in a CO-saturated and, for comparison, in a CO-free flowing perchloric acid electrolyte by electrochemical methods and by comparison with previous structural data. Structure and reactivity of the adsorbed layers are largely governed by a critical potential of E = 0.57 V, which determines the onset of O(ad) formation on the CO(ad) saturated surface in the positive-going scan and of O(ad) reduction in the negative-going scan. O(ad) formation proceeds via nucleation and 2D growth of high-coverage O(ad) islands in a surrounding CO(ad) phase, and it is connected with CO(ad) oxidation at the interface between the two phases. In the negative-going scan, mixed (CO(ad) + O(ad)) phases, most likely a (2 × 2)-(CO + 2O) and a (2×2)-(2CO + O), are proposed to form at E < 0.57 V by reduction of the O(ad)-rich islands and CO adsorption into the resulting lower-density O(ad) structures. CO bulk oxidation rates in the potential range E > 0.57 V are low, but significantly higher than those observed during oxidation of pre-adsorbed CO in the CO-free electrolyte. We relate this to high local CO(ad) coverages due to CO adsorption in the CO-saturated electrolyte, which lowers the CO adsorption energy and thus the barrier for CO(ad) oxidation during CO bulk oxidation.

Broad-band sum frequency generation study of formic acid chemisorption on a Pt (1 0 0) electrode

Broad-band sum frequency generation spectroscopy (BB-SFG) was used to obtain vibrational spectra of CO adsorbates produced from formic acid oxidation on a Pt (1 0 0) electrode in sulfuric acid media. The BB-SFG simultaneously monitored all forms of the CO intermediates, including steady-state, as the potential was scanned at 5 mV/s. Spectra were compared to those obtained from CO adsorbed from a CO-saturated electrolyte. While adsorbed from HCOOH, the CO had a sharp atop transition near 2050 cm −1 and a broader multiply-bonded transitions in the 1700–1900 cm −1 range, which appear to result from bridge-like and higher-coordinated (possibly fourfold) CO. As the potential scanned from −0.2 to 0.3 V (vs. Ag/AgCl), the bridge-like CO disappeared, and the amount of atop CO increased. At potentials above 0.5 V, the CO was in steady-state, being oxidized on the surface to CO 2 and replenished by CO from HCOOH. These measurements show that BB-SFG can observe potential-dependent interconversion of different CO forms on the electrode surface and can measure steady-state reaction intermediates on a surface in real time.

Methanol Oxidation on Carbon Supported Pt and Ru – Modified Pt Nanoparticles: a Comparison with Single Crystal and Polycrystalline Electrodes

AbstractThe rate of methanol adsorption and oxidation on carbon supported Pt nanoparticles and polycrystalline Pt has been studied by differential electrochemical mass spectrometry (DEMS) under continuous flow. At room temperature, the maximum adsorption rate at polycrystalline Pt is about 0.06 monolayer per second, whereas at Pt nanoparticles it is 0.04 monolayer per second. On polycrystalline Pt it amounts to 56% of a full CO monolayer, obtained by adsorbing CO from a CO saturated electrolyte. On the nanoparticles only 28% of a full CO monolayer can be obtained. The oxidation rate of the adsorbate is of zero order at polycrystalline Pt, whereas at nanoparticles it is of first order with respect to the coverage. Oxidation of methanol starts below the onset of CO2 formation. At polycrystalline Pt and Pt(111), the rate of CO2 formation is determined by the oxidation rate of the adsorbate. At Pt(332) and the nanoparticle electrodes, the rate of CO2 formation from bulk methanol is higher than that from methanol adsorbate, due to the heterogenity of these electrodes.Similarly to Ru modified Pt single crystal electrodes, Ru modified polycrystalline Pt and Pt nanoparticles offer different adsorption sites for CO, which can be selectively populated.

CO electrooxidation on a polycrystalline Pt electrode: A wall-jet EQCN study

Abstract Pre-adsorbed and bulk (continuous) CO oxidation on a polycrystalline Pt electrode were examined in a wall-jet electrochemical quartz crystal nanobalance (EQCN) setup, using both differential and integral evaluation of the EQCN data, to get further insights into the kinetics and mechanism of this important fuel-cell related electrocatalytic reaction. The hydrogen underpotential adsorption–desorption features in the base cyclic voltammogram of a Pt film are accompanied by significant changes in the electrode mass due H-upd induced desorption–adsorption of anion. In the double-layer region small capacitive currents are accompanied by comparatively large reversible mass changes indicating anion adsorption/desorption (96.5 g mol−1 assigned to bisulfate). OH and oxygen electrosorption from water at potentials more positive of 1.0 V result in relatively small variations in the electrode mass (16 g mol−1 for PtOH and ca. 9 g mol−1 for PtO formation, respectively). The CO-adlayer stripping first leads to the electrode mass decrease in the “pre-peak” region, followed by a fast mass increase within the main stripping peak due to re-adsorption of bisulfate anion (91 g mol−1). A mass-transport limited current for bulk CO oxidation under continuous flow of CO-saturated electrolyte leads to negligible mass changes (0–1 g mol−1) in the PtO region, suggesting that bulk CO oxidation is mediated by electroformed PtO.

Quantitative vibrational sum-frequency generation spectroscopy of thin layer electrochemistry: CO on a Pt electrode

Simultaneous electrochemical and vibrational sum-frequency generation (SFG) spectroscopy measurements are made of oxidation of CO on a polycrystalline Pt electrode with both CO-free and CO-saturated electrolytes. A femtosecond broad-band multiplex SFG apparatus permits rapid spectral acquisitions (5 s −1) synchronized with the potential sweep at a rate of 5 mV/s. The electrochemical cell has an ideal configuration, where the thickness of the supporting electrolyte (here 25 μm) was controlled by a spacer. The SFG lineshape is analyzed with a method that extracts the amplitude, peak frequency and Lorentzian linewidth of the CO stretch resonance as well as the nonresonant contribution from the Pt electrode. With a CO-free electrolyte, it is shown that the loss of SFG signal from CO quantitatively tracks the CO surface coverage, as determined by the integrated current or charge. The CO peak shift undergoes Stark tuning at a rate of 27.5 ± 1 cm −1 V −1 until an electrode potential slightly below the onset of CO oxidation. As CO is oxidized, the peak frequency dips and then recovers, while the peak linewidth broadens and then narrows. These effects are more pronounced with CO-saturated electrolyte. The relationships between the CO peak shift and linewidth and the microscopic dynamics of CO at the electrochemical surface are discussed briefly.

ATR-SEIRAS––an approach to probe the reactivity of Pd-modified quasi-single crystal gold film electrodes

Quasi-single crystalline gold films of 20 nm thickness and preferential (1 1 1) orientation on Si hemispheres were modified by controlled potentiostatic deposition of Pd (sub-ML, ML, multi-L) from sulphate and/or chloride-containing electrolyte. The electrochemical properties of these model electrodes were characterised for hydrogen and (hydrogen-) sulphate adsorption as well as for surface oxide formation by cyclic voltammetry. Conditions were developed to fabricate defined and stable Pd monolayers. In situ ATR-SEIRAS (Attenuated Total Reflection Surface Enhanced Infrared Reflection Absorption Spectroscopy) experiments were carried out to describe the electrochemical double layer of Pd modified gold film electrodes in contact with aqueous 0.1 M H 2SO 4 with focus on interfacial water and anion adsorption. Based on an analysis of the non-resonant IR background signal the potential of zero charge is estimated to 0.10–0.20 V (vs. RHE). CO was found to be weakly physisorbed in atop sites on Au(1 1 1-20 nm)/0.1 M H 2SO 4 only in CO saturated electrolyte. CO, deposited on a quasi-single crystal gold film modified with 1 ML Pd, is chemisorbed with preferential occupation of bridge sites and atop positions at step edges. Saturated CO adlayers, as obtained by deposition at 0.10 V, contain isolated water species and are covered by a second layer of hydrogen bonded water. Potentiodynamic SEIRAS experiments of CO electro-oxidation on Pd-modified gold film electrodes demonstrate clearly the existence of a “pre-oxidation” region. They also provide spectroscopic evidence that isolated water and weakly hydrogen bonded water are consumed during the reaction and that atop CO on defect sites is a preferential reactant. The simultaneous in situ monitoring of the potential- and time-dependent evolution of characteristic vibrational modes in the OH- and CO-stretching regions are in agreement with the Gilman (“reactant pair”) mechanism of CO oxidation.

Surface structure and relaxation at the Pt 3Sn(1 1 1)/electrolyte interface

In situ surface X-ray scattering (SXS) measurements have been performed to determine the surface structure of Pt 3Sn(1 1 1) in sulfuric acid electrolyte. Potentiodynamic measurements indicate that the ultra high vacuum (UHV) prepared p(2 × 2) alloy surface structure is stable upon transfer to electrolyte and remains stable during subsequent cycling of the applied potential. A detailed structural study by crystal truncation rod (CTR) analysis shows that the surface layer of Pt and Sn atoms undergoes an expansion of ∼2% of the (1 1 1) layer spacing at low potential (0.05 V vs. reversible hydrogen electrode) in CO-free electrolyte. At 0.55 V the expansion of the Pt atoms is reduced to ∼0.6%, whereas the Sn atoms are expanded by ∼6% of the layer spacing. The potential-induced buckling of the surface layer is also observed in CO-saturated electrolyte and is a precursor to Sn dissolution which occurs at ∼1.0 V, causing irreversible roughening of the surface.

Electrochemical evidence of intermediate formation of adsorbed CO in cathodic reduction of CO 2 at a nickel electrode

Electroreduction of CO 2 and CO was studied with voltammetric and coulometric measurements in aqueous electrolytes. The reaction products as well as their distribution are similar both for CO 2 and CO. CO is strongly adsorbed on Ni from a CO saturated electrolyte; adsorbed CO is not desorbed, even when dissolved CO is purged by sparging Ar in the solution. Adsorbed CO prevents significant hydrogen evolution in the cathodic region. The oxidative dissolution of the Ni electrode is markedly suppressed by adsorbed CO in the anodic region as well. Adsorbed CO is anodically oxidized at +0.12V vs nhe. On the basis of coulometric measurements, it was presumed in our previous reports that the electroreduction of CO 2 proceeds with intermediate formation of CO. The chemical species formed during the reduction of CO 2 on Ni electrode showed identical behavior with adsorbed CO.

Electrooxidation of adsorbed CO on polycrystalline platinum in alkaline solutions

The electrooxidation of adsorbed CO on polycristalline platinum in 1 M KOH was investigated under potentiodynamic conditions. A micro flux cell was used to avoid CO readsorption during the oxidation process. Three oxidation current peaks are observed at low sweep rates for the electrooxidation of (CO) ad obtained from CO-saturated electrolyte. The corresponding peak potentials shift towards more positive values, but the three peaks gradually overlap to give a single broad asymmetric current peak as v is increased. The multiplicity in the oxidation current peaks for (CO) ad under adsorption saturation is interpreted in terms of reactions involving different OH—substrate and CO—substrate interactions, instead of the existence of the “linear” or “bridge” adsorbed structures for (CO) ad. For (CO) ad below saturation values, a complex model involving different (CO) ad structures interacting with energetically different (OH) ad should be considered. From the dependence of peak potential on log v, a chemical reaction between adsorbed CO and platinum oxides can in principle be postulated as the rds. The splitting of the initial stages of oxide electroformation in the presence of adsorbed CO is evident from the triangular modulated linear potential sweep (TMLPS).