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Electrochemical CO2 Reduction (CO2R) can potentially allow for the sustainable production of valuable fuels and chemicals. Recently, single atom catalysts on a 2D support have been shown to be a promising catalyst candidate. Using state-of-the-art methods, we develop a model for Fe doped graphene which rationalises several critical experimental observations: the contentious origin of the pH dependence of reactivity and the dependence of current-potential relationships on active site. We show that single atom catalysts have the unique ability to stabilise different dipoles associated with critical reaction intermediates, which translates to significant shifts in activity. This provides a new rational design principle and paves the way for rigorous computation-guided catalyst design of new single atom catalysts for CO2R. File list (2) download file view on ChemRxiv SI_Dipole_field_interactions_determine_the_CO2_reducti... (3.76 MiB) download file view on ChemRxiv main.pdf (16.28 MiB)

involved, and affordable cost. [1][2][3] However, the commercial application of the sulfur cathode for the Li-S batteries is restrained by several technical barriers [4][5][6] compared with Li-ion battery. [7] First, the poor conductivity of sulfur and its reaction intermediates limit the sulfur utilization, [8] which leads to decreased energy density and power density. Second, during the charge/discharge process, there is a large volume change, resulting in rapid deterioration of the electrode structure. Various strategies have been investigated to increase electrode conductivity and to accommodate the volume expansion. [9][10][11] Last but not least, the dissolution and transport of lithium polysulfides (LiPSs) in the electrolyte result in the fatal "shuttle effect" that causes the deposition of Li 2 S on Li anode and then degrades the cycle performance. This shuttle effect could be mitigated by 1) trapping/confining the soluble LiPSs in the cathode by physical and chemical adsorption, [12][13][14][15] which prevents the transport of soluble LiPSs in the electrolyte, and 2) enhancing the kinetics of LiPS conversion reactions so that the soluble long-chain LiPS could transform to insoluble short-chain LiPS quickly, limiting the lifetime of soluble LiPS. [16,17] Therefore, a superior Li-S battery could be achieved by designing a cathode with high electricalThe lithium-sulfur (Li-S) battery is widely regarded as a promising energy storage device due to its low price and the high earth-abundance of the materials employed. However, the shuttle effect of lithium polysulfides (LiPSs) and sluggish redox conversion result in inefficient sulfur utilization, low power density, and rapid electrode deterioration. Herein, these challenges are addressed with two strategies 1) increasing LiPS conversion kinetics through catalysis, and 2) alleviating the shuttle effect by enhanced trapping and adsorption of LiPSs. These improvements are achieved by constructing double-shelled hollow nanocages decorated with a cobalt nitride catalyst. The N-doped hollow inner carbon shell not only serves as a physiochemical absorber for LiPSs, but also improves the electrical conductivity of the electrode; significantly suppressing shuttle effect. Cobalt nitride (Co 4 N) nanoparticles, embedded in nitrogen-doped carbon in the outer shell, catalyze the conversion of LiPSs, leading to decreased polarization and fast kinetics during cycling. Theoretical study of the Li intercalation energetics confirms the improved catalytic activity of the Co 4 N compared to metallic Co catalyst. Altogether, the electrode shows large reversible capacity (1242 mAh g −1 at 0.1 C), robust stability (capacity retention of 658 mAh g −1 at 5 C after 400 cycles), and superior cycling stability at high sulfur loading (4.5 mg cm −2 ).

Electrochemical CO2 reduction is a potential approach to convert CO2 into valuable chemicals using electricity as feedstock. Abundant and affordable catalyst materials are needed to upscale this process in a sustainable manner. Nickel‐nitrogen‐doped carbon (Ni‐N‐C) is an efficient catalyst for CO2 reduction to CO, and the single‐site Ni−Nx motif is believed to be the active site. However, critical metrics for its catalytic activity, such as active site density and intrinsic turnover frequency, so far lack systematic discussion. In this work, we prepared a set of covalent organic framework (COF)‐derived Ni‐N‐C catalysts, for which the Ni−Nx content could be adjusted by the pyrolysis temperature. The combination of high‐angle annular dark‐field scanning transmission electron microscopy and extended X‐ray absorption fine structure evidenced the presence of Ni single‐sites, and quantitative X‐ray photoemission addressed the relation between active site density and turnover frequency.

In situ/operando surface enhanced infrared and Raman spectroscopies are widely employed in electrocatalysis research to extract mechanistic information and establish structure-activity relations. However, these two spectroscopic techniques are more frequently employed in isolation than in combination, owing to the assumption that they provide largely overlapping information regarding reaction intermediates. Here we show that surface enhanced infrared and Raman spectroscopies tend to probe different subpopulations of adsorbates on weakly adsorbing surfaces while providing similar information on strongly binding surfaces by conducting both techniques on the same electrode surfaces, i.e., platinum, palladium, gold and oxide-derived copper, in tandem. Complementary density functional theory computations confirm that the infrared and Raman intensities do not necessarily track each other when carbon monoxide is adsorbed on different sites, given the lack of scaling between the derivatives of the dipole moment and the polarizability. Through a comparison of adsorbed carbon monoxide and water adsorption energies, we suggest that differences in the infrared vs. Raman responses amongst metal surfaces could stem from the competitive adsorption of water on weak binding metals. We further determined that only copper sites capable of adsorbing carbon monoxide in an atop configuration visible to the surface enhanced infrared spectroscopy are active in the electrochemical carbon monoxide reduction reaction.

Metrics & MoreArticle Recommendations CONSPECTUS: Electrochemical CO 2 reduction (eCO 2 R) enables the conversion of waste CO 2 to high-value fuels and commodity chemicals powered by renewable electricity, thereby offering a viable strategy for reaching the goal of net-zero carbon emissions. Research in the past few decades has focused both on the optimization of the catalyst (electrode) and the electrolyte environment. Surface-area normalized current densities show that the latter can affect the CO 2 reduction activity by up to a few orders of magnitude.In this Account, we review theories of the mechanisms behind the effects of the electrolyte (cations, anions, and the electrolyte pH) on eCO 2 R. As summarized in the conspectus graphic, the electrolyte influences eCO 2 R activity via a field (ε) effect on dipolar (μ) reaction intermediates, changing the proton donor for the multi-step proton-electron transfer reaction, specifically adsorbed anions on the catalyst surface to block active sites, and tuning the local environment by homogeneous reactions.To be specific, alkali metal cations (M + ) can stabilize reaction intermediates via electrostatic interactions with dipolar intermediates or buffer the interfacial pH via hydrolysis reactions, thereby promoting the eCO 2 R activity with the following trend in hydrated size (corresponding to the local field strength ε)/hydrolysis ability: Cs + > K + > Na + > Li + . The effect of the electrolyte pH can give a change in eCO 2 R activity of up to several orders of magnitude, arising from linearly shifting the absolute interfacial field via the relationship U SHE = U RHE − (2.3k B T)pH, homogeneous reactions between OH − and desorbed intermediates, or changing the proton donor from hydronium to water along with increasing pH. Anions have been suggested to affect the eCO 2 R reaction process by solution-phase reactions (e.g., buffer reactions to tune local pH), acting as proton donors or as a surface poison. So far, the existing models of electrolyte effects have been used to rationalize various experimentally observed trends, having yet to demonstrate general predictive capabilities. The major challenges in our understanding of the electrolyte effect in eCO 2 R are (i) the long time scale associated with a dynamic ab initio picture of the catalyst|electrolyte interface and (ii) the overall activity determined by the length-scale interplay of intrinsic microkinetics, homogeneous reactions, and mass transport limitations. New developments in ab initio dynamic models and coupling the effects of mass transport can provide a more accurate view of the structure and intrinsic functions of the electrode−electrolyte interface and the corresponding reaction energetics toward comprehensive and predictive models for electrolyte design.