Revealing the structure of the active sites for the electrocatalytic CO2 reduction to CO over Co single atom catalysts using operando XANES and machine learning.

Transition-metal nitrogen-doped carbons (TM-N-C) are emerging as a highly promising catalyst class for several important electrocatalytic processes, including the electrocatalytic CO2 reduction reaction (CO2RR). The unique local environment around the singly dispersed metal site in TM-N-C catalysts is likely to be responsible for their catalytic properties, which differ significantly from those of bulk or nanostructured catalysts. However, the identification of the actual working structure of the main active units in TM-N-C remains a challenging task due to the fluctional, dynamic nature of these catalysts, and scarcity of experimental techniques that could probe the structure of these materials under realistic working conditions. This issue is addressed in this work and the local atomistic and electronic structure of the metal site in a Co-N-C catalyst for CO2RR is investigated by employing time-resolved operando X-ray absorption spectroscopy (XAS) combined with advanced data analysis techniques. This multi-step approach, based on principal component analysis, spectral decomposition and supervised machine learning methods, allows the contributions of several co-existing species in the working Co-N-C catalysts to be decoupled, and their XAS spectra deciphered, paving the way for understanding the CO2RR mechanisms in the Co-N-C catalysts, and further optimization of this class of electrocatalytic systems.

Interfacial pH measurements during CO2 reduction on gold using a rotating ring-disk electrode.

Insights into how to control the activity and selectivity of the electrochemical CO2 reduction reaction are still limited because of insufficient knowledge of the reaction mechanism and kinetics, which is partially due to the lack of information on the interfacial pH, an important parameter for proton-coupled reactions like CO2 reduction. Here, we used a reliable and sensitive pH sensor combined with the rotating ring-disk electrode technique, in which a functionalized Au ring electrode works as a real-time detector of the OH- generated during the CO2 reduction reaction at a gold disk electrode. Variations of the interfacial pH due to both electrochemical and homogeneous reactions are mapped and the correlation of the interfacial pH with these reactions is inferred. The interfacial pH near the disk electrode increases from 7 to 12 with increasing current density, with a sharp increase at around -0.5 V vs. RHE, which indicates a change of the dominant buffering species. Through scan rate-dependent voltammetry and chronopotentiometry experiments, the homogenous reactions are shown to reach equilibrium within the time scale of the pH measurements, so that the interfacial concentrations of different carbonaceous species can be calculated using equilibrium constants. Furthermore, pH measurements were also performed under different conditions to disentangle the relationship between the interfacial pH and other electrolyte effects. The buffer effect of alkali metal cations is confirmed, showing that weakly hydrated cations lead to less pronounced pH gradients. Finally, we probe to which extent increasing mass transport and the electrolyte buffer capacity can aid in suppressing the increase of the interfacial pH, showing that the buffer capacity is the dominant factor in suppressing interfacial pH variations.

Electrolyte Effects on CO2 Electrochemical Reduction to CO.

ConspectusThe electrochemical reduction of CO2 (CO2RR) constitutes an alternative to fossil fuel-based technologies for the production of fuels and commodity chemicals. Yet the application of CO2RR electrolyzers is hampered by low energy and Faradaic efficiencies. Concomitant electrochemical reactions, like hydrogen evolution (HER), lower the selectivity, while the conversion of CO2 into (bi)carbonate through solution acid-base reactions induces an additional concentration overpotential. During CO2RR in aqueous media, the local pH becomes more alkaline than the bulk causing an additional consumption of CO2 by the homogeneous reactions. The latter effect, in combination with the low solubility of CO2 in aqueous electrolytes (33 mM), leads to a significant depletion in CO2 concentration at the electrode surface.The nature of the electrolyte, in terms of pH and cation identity, has recently emerged as an important factor to tune both the energy and Faradaic efficiency. In this Account, we summarize the recent advances in understanding electrolyte effects on CO2RR to CO in aqueous solutions, which is the first, and crucial, step to further reduced products. To compare literature findings in a meaningful way, we focus on results reported under well-defined mass transport conditions and using online analytical techniques. The discussion covers the molecular-level understanding of the effects of the proton donor, in terms of the suppression of the CO2 gradient vs enhancement of HER at a given mass transport rate and of the cation, which is crucial in enabling both CO2RR and HER. These mechanistic insights are then translated into possible implications for industrially relevant cell geometries and current densities.

The Role of Cation Acidity on the Competition between Hydrogen Evolution and CO2 Reduction on Gold Electrodes.

CO2 electroreduction (CO2RR) is a sustainable alternative for producing fuels and chemicals. Metal cations in the electrolyte have a strong impact on the reaction, but mainly alkali species have been studied in detail. In this work, we elucidate how multivalent cations (Li+, Cs+, Be2+, Mg2+, Ca2+, Ba2+, Al3+, Nd3+, and Ce3+) affect CO2RR and the competing hydrogen evolution by studying these reactions on polycrystalline gold at pH = 3. We observe that cations have no effect on proton reduction at low overpotentials, but at alkaline surface pH acidic cations undergo hydrolysis, generating a second proton reduction regime. The activity and onset for the water reduction reaction correlate with cation acidity, with weakly hydrated trivalent species leading to the highest activity. Acidic cations only favor CO2RR at low overpotentials and in acidic media. At high overpotentials, the activity for CO increases in the order Ca2+ < Li+ < Ba2+ < Cs+. To favor this reaction there must be an interplay between cation stabilization of the *CO2- intermediate, cation accumulation at the outer Helmholtz plane (OHP), and activity for water reduction. Ab initio molecular dynamics simulations with explicit electric field show that nonacidic cations show lower repulsion at the interface, accumulating more at the OHP, thus triggering local promoting effects. Water dissociation kinetics is increasingly promoted by strongly acidic cations (Nd3+, Al3+), in agreement with experimental evidence. Cs+, Ba2+, and Nd3+ coordinate to adsorbed CO2 steadily; thus they enable *CO2- stabilization and barrierless protonation to COOH and further reduction products.

Electrolyte buffering species as oxygen donor shuttles in CO electrooxidation.

Electrolyte buffering species have been shown to act as proton donors in the hydrogen evolution reaction (HER). Analogously, we study here whether these electrolyte species may participate in other reactions by investigating CO electrooxidation (COOR) on a gold rotating disk electrode. This model system, characterized by fast kinetics, exhibits a diffusion-limited regime, which helps in the identification of the species dictating the diffusion-limited current. Through a systematic concentration dependence study in a variety of buffers, we show that electrolyte buffering species act as oxygen donor shuttles in COOR, lowering the reaction overpotential. A similar correlation between electrolyte and electrocatalytic activity was observed for COOR on a different electrode material (Pt). Probing the electrode-electrolyte interface by attenuated total reflection infrared spectroscopy (ATR-FTIR) and modelling the surface speciation to include the effect of the solution reactions, we propose that the buffer conjugated base generates the oxygen donor (i.e. OH-) through its acid-base reaction with water.

Understanding Cation Trends for Hydrogen Evolution on Platinum and Gold Electrodes in Alkaline Media.

In this work, we study how the cation identity and concentration alter the kinetics of the hydrogen evolution reaction (HER) on platinum and gold electrodes. A previous work suggested an inverted activity trend as a function of alkali metal cation when comparing the performance of platinum and gold catalysts in alkaline media. We show that weakly hydrated cations (K+) favor HER on gold only at low overpotentials (or lower alkalinity), whereas in more alkaline pH (or high overpotentials), a higher activity is observed using electrolytes containing strongly hydrated cations (Li+). We find a similar trend for platinum; however, the inhibition of HER by weakly hydrated cations on platinum is observed already at lower alkalinity and lower cation concentrations, suggesting that platinum interacts more strongly with metal cations than gold. We propose that weakly hydrated cations stabilize the transition state of the water dissociation step more favorably due to their higher near-surface concentration in comparison to a strongly hydrated cation such as Li+. However, at high pH and consequently higher near-surface cation concentrations, the accumulation of these species at the outer Helmholtz plane inhibits HER. This is especially pronounced on platinum, where a change in the rate-determining step is observed at pH 13 when using a Li+- or K+-containing electrolyte.

Time-Resolved Local pH Measurements during CO2 Reduction Using Scanning Electrochemical Microscopy: Buffering and Tip Effects.

The electrochemical reduction of CO2 is widely studied as a sustainable alternative for the production of fuels and chemicals. The electrolyte's bulk pH and composition play an important role in the reaction activity and selectivity and can affect the extent of the buildup of pH gradients between the electrode surface and the bulk of the electrolyte. Quantifying the local pH and how it is affected by the solution species is desirable to gain a better understanding of the CO2 reduction reaction. Local pH measurements can be realized using Scanning Electrochemical Microscopy (SECM); however, finding a pH probe that is stable and selective under CO2 reduction reaction conditions is challenging. Here, we have used our recently developed voltammetric pH sensor to perform pH measurements in the diffusion layer during CO2 reduction using SECM, with high time resolution. Using a 4-hydroxylaminothiophenol (4-HATP)/4-nitrosothiophenol (4-NSTP) functionalized gold ultramicroelectrode, we compare the local pH developed above a gold substrate in an argon atmosphere, when only hydrogen evolution is taking place, to the pH developed in a CO2 atmosphere. The pH is monitored at a fixed distance from the surface, and the sample potential is varied in time. In argon, we observe a gradual increase of pH, while a plateau region is present in CO2 atmosphere due to the formation of HCO3 - buffering the reaction interface. By analyzing the diffusion layer dynamics once the sample reaction is turned "off", we gain insightful information on the time scale of the homogeneous reactions happening in solution and on the time required for the diffusion layer to fully recover to the initial bulk concentration of species. In order to account for the effect of the presence of the SECM tip on the measured pH, we performed finite element method simulations of the fluid and reaction dynamics. The results show the significant localized diffusion hindrance caused by the tip, so that in its absence, the pH values are more acidic than when the tip is present. Nonetheless, through the simulation, we can account for this effect and estimate the real local pH values across the diffusion layer.

Efficiency and selectivity of CO2 reduction to CO on gold gas diffusion electrodes in acidic media.

The electrochemical reduction of CO2 to CO is a promising technology for replacing production processes employing fossil fuels. Still, low energy efficiencies hinder the production of CO at commercial scale. CO2 electrolysis has mainly been performed in neutral or alkaline media, but recent fundamental work shows that high selectivities for CO can also be achieved in acidic media. Therefore, we investigate the feasibility of CO2 electrolysis at pH 2-4 at indrustrially relevant conditions, using 10 cm2 gold gas diffusion electrodes. Operating at current densities up to 200 mA cm-2, we obtain CO faradaic efficiencies between 80-90% in sulfate electrolyte, with a 30% improvement of the overall process energy efficiency, in comparison with neutral media. Additionally, we find that weakly hydrated cations are crucial for accomplishing high reaction rates and enabling CO2 electrolysis in acidic media. This study represents a step towards the application of acidic electrolyzers for CO2 electroreduction.

Probing the local activity of CO2 reduction on gold gas diffusion electrodes: effect of the catalyst loading and CO2 pressure.

Large scale CO2 electrolysis can be achieved using gas diffusion electrodes (GDEs), and is an essential step towards broader implementation of carbon capture and utilization strategies. Different variables are known to affect the performance of GDEs. Especially regarding the catalyst loading, there are diverging trends reported in terms of activity and selectivity, e.g. for CO2 reduction to CO. We have used shear-force based Au nanoelectrode positioning and scanning electrochemical microscopy (SECM) in the surface-generation tip collection mode to evaluate the activity of Au GDEs for CO2 reduction as a function of catalyst loading and CO2 back pressure. Using a Au nanoelectrode, we have locally measured the amount of CO produced along a catalyst loading gradient under operando conditions. We observed that an optimum local loading of catalyst is necessary to achieve high activities. However, this optimum is directly dependent on the CO2 back pressure. Our work does not only present a tool to evaluate the activity of GDEs locally, it also allows drawing a more precise picture regarding the effect of catalyst loading and CO2 back pressure on their performance.

Understanding the Voltammetry of Bulk CO Electrooxidation in Neutral Media through Combined SECM Measurements.

Recently, the bulk electrooxidation of CO on gold or platinum has been used to detect CO produced during CO2 reduction in neutral media. The CO bulk oxidation voltammetry may show two distinct peaks depending on the reaction conditions, which up to now have not been understood. We have used scanning electrochemical microscopy (SECM) to probe CO oxidation and pH in the diffusion layer during CO2 reduction. Our results show that the two different peaks are due to diffusion limitation by two different species, namely, CO and OH-. We find that between pH 7 and 11, CO oxidation by water and OH- gives rise to the first and second peak observed in the voltammetry, respectively. Additional rotating disc experiments showed that specifically in this pH range the current of the second peak is diffusion limited by the OH- concentration, since it is lower than the CO concentration.

Mediator-Free SECM for Probing the Diffusion Layer pH with Functionalized Gold Ultramicroelectrodes.

Probing pH gradients during electrochemical reactions is important to better understand reaction mechanisms and to separate the influence of pH and pH gradients from intrinsic electrolyte effects. Here, we develop a pH sensor to measure pH changes in the diffusion layer during hydrogen evolution. The probe was synthesized by functionalizing a gold ultramicroelectrode with a self-assembled monolayer of 4-nitrothiophenol (4-NTP) and further converting it to form a hydroxylaminothiophenol (4-HATP)/4-nitrosothiophenol (4-NSTP) redox couple. The pH sensing is realized by recording the tip cyclic voltammetry and monitoring the Nernstian shift of the midpeak potential. We employ a capacitive approach technique in our home-built Scanning Electrochemical Microscope (SECM) setup in which an AC potential is applied to the sample and the capacitive current generated at the tip is recorded as a function of distance. This method allows for an approach of the tip to the electrode that is electrolyte-free and consequently also mediator-free. Hydrogen evolution on gold in a neutral electrolyte was studied as a model system. The pH was measured with the probe at a constant distance from the electrode (ca. 75 µm), while the electrode potential was varied in time. In the nonbuffered electrolyte used (0.1 M Li2SO4), even at relatively low current densities, a pH difference of three units is measured between the location of the probe and the bulk electrolyte. The time scale of the diffusion layer transient is captured, due to the high time resolution that can be achieved with this probe. The sensor has high sensitivity, measuring differences of more than 8 pH units with a resolution better than 0.1 pH unit.

Metal-Phosphate Bilayers for Anatase Surface Modification.

Compared to many other metal oxides, anatase TiO2 shows relatively lower reactivity toward carboxylic acid anchor groups. The latter is crucial for applications, for example, in dye-sensitized solar cells (DSSCs), where the most used dyes bind to the metal oxide surface through carboxylic acid terminations. To improve the surface reactivity, metal-phosphate bilayers of Ni or Co were synthesized on anatase TiO2 compact oxide and nanotubes. In both cases, time-of-flight secondary ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS) results showed that the bilayers were successfully formed and that the phosphate layer works as an intermediate between TiO2 and the other species. ToF-SIMS depth profiles of modified nanotubes showed that Ni and Co are present through the whole tube length and reduce in content after heat treatment, in agreement with XPS results. Phosphate groups, on the other hand, are more present in the tubes' depth, and their content on the surface is reduced upon exposure to temperature. The reactivity of the modified surfaces toward carboxylic acid-terminated molecules, as stearic acid and Ru-based N719 dye, was evaluated. Contact angle measurements together with dye desorption experiments demonstrated that the Co-phosphate bilayers heat-treated at 300 °C resulted in the largest enhancement compared to the reference. Bilayer-modified compact anatase TiO2 and anatase TiO2 nanotubes were utilized as photoanodes in DSSCs. An increase in efficiency was observed for all modified electrodes with phosphate-Co treatment, leading to the highest JSC values and an efficiency improvement of 48%.

Tuning Anatase Surface Reactivity toward Carboxylic Acid Anchor Groups.

The effect of different post-treatments on TiO2 anatase surface reactivity was investigated in order to obtain the best techniques for enhancing anatase performance in diverse applications, e.g., in photocatalysis and especially as photoelectrodes for dye-sensitized solar cells (DSSCs). Different post-treatments of compact anodic anatase TiO2 were compared, including O2 plasma, UV irradiation, immersion in H2O2, vapor thermal treatment, and post-anodization, evaluating the increase of the amount of OH reactive groups on the surface and removal of surface contamination. In XPS spectra, the increase of OH groups is evident by the O 1s peak at higher binding energy. ToF-SIMS principal component analysis demonstrated that treatments performed in aqueous media led to a cleaner surface, with substantial removal of electrolyte residues. Stearic acid and the organic dye N719 were adsorbed to the differently post-treated anatase, and adsorption was evaluated by contact angle and dye desorption measurements. A higher loading with molecules containing carboxylic acid functionalities was confirmed by both techniques on the treated samples. The post-treatments that presented the highest amounts of dye were used to prepare photoelectrodes, and these were tested in DSSCs where the efficiency values doubled in comparison with the non-post-treated electrode.