Effect of pore diameter and length on electrochemical CO2 reduction reaction at nanoporous gold catalysts

Cation Effects on the Adsorbed Intermediates of CO2 Electroreduction Are Systematic and Predictable

The electrode-electrolyte interface, and in particular the nature of the cation, has considerable effects on the activity and product selectivity of the electrochemical reduction of CO2. Therefore, to improve the electrocatalysis of this challenging reaction, it is paramount to ascertain whether cation effects on adsorbed intermediates are systematic. Here, DFT calculations are used to show that the effects of K+, Na+, and Mg2+, on single carbon CO2 reduction intermediates can either be stabilizing or destabilizing depending on the metal and the adsorbate. Because systematic trends are observed, cation effects can be accurately predicted in simple terms for a wide variety of metals, cations and adsorbed species. These results are then applied to the reduction of CO2 to CO on four different catalytic surfaces (Au, Ag, Cu, Pd) and activation of weak-binding metals is consistently observed by virtue of the stabilization of the key intermediate *COOH.

Regulating Cu Oxidation State for Electrocatalytic CO2 Conversion into CO with Near-Unity Selectivity via Oxygen Spillover

Cu-based catalysts are the most intensively studied in the field of electrocatalytic CO2 reduction reaction (CO2RR), demonstrating the capacity to yield diverse C1 and C2+ products albeit with unsatisfactory selectivity. Manipulation of the oxidation state of Cu sites during CO2RR process proves advantageous in modulating the selectivity of productions, but poses a formidable challenge. Here, an oxygen spillover strategy is proposed to enhance the oxidation state of Cu during CO2RR by incorporating the oxygen donor Sb2O4. The Cu-Sb bimetallic oxide catalyst attains a remarkable CO2-to-CO selectivity approaching unity, in stark contrast to the diverse product distribution observed with bare CuO. The exceptional Faradaic efficiency of CO can be maintained across a wide range of potential windows of ≈700 mV in 1 m KOH, and remains independent of the Cu/Sb ratio (ranging from 0.1:1 to 10:1). Correlative calculations and experimental results reveal that oxygen spillover from Sb2O4 to Cu sites maintains the relatively high valence state of Cu during CO2RR, which diminishes the binding strength of *CO, thereby achieving heightened selectivity in CO production. These findings propose the role of oxygen spillover in CO2RR over Cu-based catalysts, and shed light on the rational design of highly selective CO2 reduction catalysts.

Selective Reduction of Aqueous Nitrate to Ammonium with an Electropolymerized Chromium Molecular Catalyst

Nitrate (NO3-) is a common nitrogen-containing contaminant in agricultural, industrial, and low-level nuclear wastewater that causes significant environmental damage. In this work, we report a bioinspired Cr-based molecular catalyst incorporated into a redox polymer that selectively and efficiently reduces aqueous NO3- to ammonium (NH4+), a desirable value-added fertilizer component and industrial precursor, at rates of ∼0.36 mmol NH4+ mgcat-1 h-1 with >90% Faradaic efficiency for NH4+. The NO3- reduction reaction occurs through a cascade catalysis mechanism involving the stepwise reduction of NO3- to NH4+ via observed NO2- and NH2OH intermediates. To our knowledge, this is one of the first examples of a molecular catalyst, homogeneous or heterogenized, that is reported to reduce aqueous NO3- to NH4+ with rates and Faradaic efficiencies comparable to those of state-of-the-art solid-state electrocatalysts. This work highlights a promising and previously unexplored area of electrocatalyst research using polymer-catalyst composites containing complexes with oxophilic transition metal active sites for electrochemical nitrate remediation with nutrient recovery.

Facet-Dependent Formation and Adhesion of Au Oxide and Nanoporous Au on Poly-Oriented Au Single Crystals

Nanoporous Au (NPG) has different properties compared to bulk Au, making it an interesting material for numerous applications. To modify the structure of NPG films for specific applications, e. g., the porosity, thickness, and homogeneity of the films, a fundamental understanding of the structure formation is essential. Here, we focus on NPG prepared via electrochemical reduction from Au oxide formed during high voltage (HV) electrolysis on poly-oriented Au single crystal (Au POSC) electrodes. These POSCs consist of a metal bead, with faces with different crystallographic orientations and allow screening of the influence of crystallographic orientation on the structure formation for different facets in one experiment. The HV electrolysis is performed between 100 ms and 30 s at 300 V and 540 V. The amount of Au oxide formed is determined by electrochemical measurements and the structural properties are investigated by scanning electron and optical microscopy. We show that the formation of Au oxide is mostly independent of the crystallographic orientation, except for thick layers, while the macroscopic structure of the NPG films depends on experimental parameters such as the Au oxide precursor thickness and the crystallographic orientation of the substrate. Possible reasons for the frequently observed exfoliation of the NPG films are discussed.

Ternary Ionic-Liquid-Based Electrolyte Enables Efficient Electro-reduction of CO2 over Bulk Metal Electrodes

Using bulk metals as catalysts to get high efficiency in electro-reduction of CO₂ is ideal but challenging. Here, we report the coupling of bulk metal electrodes and a ternary ionic-liquid-based electrolyte, 1-butyl-3-methylimidazolium tetrafluoroborate/1-dodecyl-3-methylimidazolium tetrafluoroborate/MeCN to realize highly efficient electro-reduction of CO₂ to CO. Over various bulk metal electrodes, the ternary electrolyte not only increases the current density but also suppresses the hydrogen evolution reaction to obtain a high Faradaic efficiency (FE) toward CO. FE_CO could maintain ∼100% over a wide potential range, and metal electrodes showed very high stability in the ternary electrolyte. It is demonstrated that the aggregation behavior of the ternary electrolyte and the arrangement of two kinds of IL cations with different chain lengths in the electrochemical double layer not only increase the wettability to electrode and CO₂ adsorption but also extend the diffusion channel of H⁺, rendering the high current density and FE_CO.

Operando Forming of Lattice Vacancy Defect in Ultrathin Crumpled NiVW-Layered Metal Hydroxides Nanosheets for Valorization of Biomass

The 2D layered metal hydroxides (LMHs) have been developed for electrooxidation of 5-hydroxymethylfurfural (HMF). In this work, an effective strategy is proposed to tailor the electronic structure of active sites at the atomic level, which is by introducing defects into the lattice structure. As an example, a series of ultrathin crumpled ternary NiVW-LMH electrocatalysts with abundant lattice vacancies (denoted as NiVW_v -LMH) are prepared in this way. The introduction of tungsten (W) endows the catalyst with a special crumpled structure, which promotes the generation of lattice vacancies and thus exposes more unsaturated Ni activity sites. The NiVW_v -LMH displays superb performance in the electrooxidation of HMF. The Tafel slope for electrodehydrogenation of Ni²⁺ OH bond to Ni(OH)O species is 12.04 mV dec^-1 . The current density at 1.43 V versus reversible hydrogen electrode (RHE) toward the oxidation reaction of HMF reaches about 193 mA cm^-2 , which is better than most of the common electrocatalysts, with an 5.37-fold improvement compared with Ni(OH)₂ electrode. The preparation strategy demonstrates in this work can be useful for developing highly efficient electrocatalysts.

Nanoporous Au Formation on Au Substrates via High Voltage Electrolysis

Nanoporous Au (NPG) films have promising properties, making them suitable for various applications in (electro)catalysis or (bio)sensing. Tuning the structural properties, such as the pore size or the surface-to-volume ratio, often requires complex starting materials such as alloys, multiple synthesis steps, lengthy preparation procedures or a combination of these factors. Here we present an approach that circumvents these difficulties, enabling for a rapid and controlled preparation of NPG films starting from a bare Au electrode. In a first approach a Au oxide film is prepared by high voltage (HV) electrolysis in a KOH solution, which is then reduced either electrochemically or in the presence of H₂ O₂ . The resulting NPG structures and their electrochemically active surface areas strongly depend on the reduction procedure, the concentration and temperature of the H₂ O₂ -containing KOH solution, as well as the applied voltage and temperature during HV electrolysis. Secondly, the NPG film can be prepared directly by applying voltages that result in anodic contact glow discharge electrolysis (aCGDE). By carefully adjusting the corresponding parameters, the surface area of the final NPG film can be specifically controlled. The structural properties of the electrodes are investigated by means of XPS, SEM and electrochemical methods.

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.

CO2 electroreduction to multicarbon products in strongly acidic electrolyte via synergistically modulating the local microenvironment

Electrochemical CO₂ reduction to multicarbon products faces challenges of unsatisfactory selectivity, productivity, and long-term stability. Herein, we demonstrate CO₂ electroreduction in strongly acidic electrolyte (pH ≤ 1) on electrochemically reduced porous Cu nanosheets by combining the confinement effect and cation effect to synergistically modulate the local microenvironment. A Faradaic efficiency of 83.7 ± 1.4% and partial current density of 0.56 ± 0.02 A cm^-2, single-pass carbon efficiency of 54.4%, and stable electrolysis of 30 h in a flow cell are demonstrated for multicarbon products in a strongly acidic aqueous electrolyte consisting of sulfuric acid and KCl with pH ≤ 1. Mechanistically, the accumulated species (e.g., K⁺ and OH^-) on the Helmholtz plane account for the selectivity and activity toward multicarbon products by kinetically reducing the proton coverage and thermodynamically favoring the CO₂ conversion. We find that the K⁺ cations facilitate C-C coupling through local interaction between K⁺ and the key intermediate *OCCO.

Boosting the Electrocatalytic CO2 Reduction Reaction by Nanostructured Metal Materials via Defects Engineering

Electrocatalytic CO₂ reduction reaction (CO₂RR) is one of the most effective methods to convert CO₂ into useful fuels. Introducing defects into metal nanostructures can effectively improve the catalytic activity and selectivity towards CO₂RR. This review provides the recent progress on the use of metal nanomaterials with defects towards electrochemical CO₂RR and defects engineering methods. Accompanying these ideas, we introduce the structure of defects characterized by electron microscopy techniques as the characterization and analysis of defects are relatively difficult. Subsequently, we present the intrinsic mechanism of how the defects affect CO₂RR performance. Finally, to promote a wide and deep study in this field, the perspectives and challenges concerning defects engineering in metal nanomaterials towards CO₂RR are put forward.

Electrolyte Effects on CO2 Electrochemical Reduction to CO

The 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.