Project description:Electrocatalytic reduction of CO2 to valuable fuels and chemicals can not only alleviate the energy crisis but also improve the atmospheric environment. The key is to develop electrocatalysts that are extremely stable, efficient, selective, and reasonably priced. In this study, spin-polarized density function theory (DFT) calculations were used to comprehensively examine the catalytic efficacy of transition metal-hexaaminobenzene (TM-HAB) monolayers as single-atom catalysts for the electroreduction of CO2. In the modified two-dimensional TM-HAB monolayer, our findings demonstrate that the binding of individual metal atoms to HAB can be strong enough for the atoms to be evenly disseminated and immobilized. In light of the conflicting hydrogen evolution processes, TM-HAB effectively inhibits hydrogen evolution. CH4 dominates the reduction byproducts of Sc, Ti, V, Cr, and Cu. HCOOH makes up the majority of Zn's reduction products. Co's primary reduction products are CH3OH and CH4, whereas Mn and Fe's primary reduction products are HCHO, CH3OH, and CH4. Among these, the Ti-HAB reduction products have a 1.14 eV limiting potential and a 1.31 V overpotential. The other monolayers have relatively low overpotentials between 0.01 V and 0.7 V; therefore, we predict that TM-HAB monolayers will exhibit strong catalytic activity in the electrocatalytic reduction of CO2, making them promising electrocatalysts for CO2 reduction.
Project description:Single-atom catalysts (SACs) with metal-nitrogen (M-N) sites are one of the most promising electrocatalysts for electrochemical carbon dioxide reduction (ECO2R). However, challenges in simultaneously enhancing the activity and selectivity greatly limit the efficiency of ECO2R due to the improper interaction of reactants/intermediates on these catalytic sites. Herein, we report a carbon-based nickel (Ni) cluster catalyst containing both single-atom and cluster sites (NiNx-T, T = 500-800) through a ligand-mediated method and realize a highly active and selective electrocatalytic CO2R process. The catalytic performance can be regulated by the dispersion of Ni-N species via controlling the pyrolysis condition. Benefitting from the synergistic effect of pyrrolic-nitrogen coordinated Ni single-atom and cluster sites, NiNx-600 exhibits a satisfying catalytic performance, including a high partial current density of 61.85 mA cm-2 and a high turnover frequency (TOF) of 7,291 h-1 at -1.2 V vs. RHE, and almost 100% selectivity toward carbon monoxide (CO) production, as well as good stability under 10 h of continuous electrolysis. This work discloses the significant role of regulating the coordination environment of the transition metal sites and the synergistic effect between the isolated single-site and cluster site in enhancing the ECO2R performance.
Project description:A seemingly catalytically inactive electrode, boron-doped diamond (BDD), is found to be active for CO2 and CO reduction to formaldehyde and even methane. At very cathodic potentials, formic acid and methanol are formed as well. However, these products are the result of base-catalyzed Cannizzaro-type disproportionation reactions. A local alkaline environment near the electrode surface, caused by the hydrogen evolution reaction, initiates aldehyde disproportionation promoted by hydroxide ions, which leads to the formation of the corresponding carboxylic acid and alcohol. This phenomenon is strongly influenced by the electrolyte pH and buffer capacity and not limited to BDD or formaldehyde, but can be generalized to different electrode materials and to C2 and C3 aldehydes as well. The importance of these reactions is emphasized as the formation of acids and alcohols is often ascribed to direct CO2 reduction products. The results obtained here may explain the concomitant formation of acids and alcohols often observed during CO2 reduction.
Project description:Innovative and robust photosensitisation materials play a cardinal role in advancing the combined effort towards efficient solar energy harvesting. Here, we demonstrate the photocathode functionality of a Metal-Organic Framework (MOF) featuring cofacial pairs of photo- and electro-active 1,4,5,8-naphthalenediimide (NDI) ligands, which was successfully applied to markedly reduce the overpotential required for CO2 reduction to CO by a well-known rhenium molecular electrocatalyst. Reduction of [Cd(DPNDI)(TDC)] n (DPNDI = N,N'-di(4-pyridyl)-1,4,5,8-naphthalenediimide, H2TDC = thiophene-2,5-dicarboxylic acid) to its mixed-valence state induces through-space Intervalence Charge Transfer (IVCT) within cofacial DPNDI units. Irradiation of the mixed-valence MOF in the visible region generates a DPNDI photoexcited radical monoanion state, which is stabilised as a persistent species by the inherent IVCT interactions and has been rationalised using Density Functional Theory (DFT). This photoexcited radical monoanion state was able to undergo charge transfer (CT) reduction of the rhenium molecular electrocatalyst to effect CO generation at a lower overpotential than that required by the discrete electrocatalyst itself. The exploitation of cofacial MOFs opens new directions for the design philosophy behind light harvesting materials.
Project description:A rational design of an electrocatalyst presents a promising avenue for solar fuels synthesis from carbon dioxide (CO2) fixation but is extremely challenging. Herein, we use density functional theory calculations to study an inexpensive binary copper-iron catalyst for photoelectrochemical CO2 reduction toward methane. The calculations of reaction energetics suggest that Cu and Fe in the binary system can work in synergy to significantly deform the linear configuration of CO2 and reduce the high energy barrier by stabilizing the reaction intermediates, thus spontaneously favoring CO2 activation and conversion for methane synthesis. Experimentally, the designed CuFe catalyst exhibits a high current density of -38.3 mA⋅cm-2 using industry-ready silicon photoelectrodes with an impressive methane Faradaic efficiency of up to 51%, leading to a distinct turnover frequency of 2,176 h-1 under air mass 1.5 global (AM 1.5G) one-sun illumination.
Project description:Pulsed electrolysis can significantly improve carbon dioxide reduction on metal electrodes, but the effect of short (millisecond to seconds) voltage steps on molecular electrocatalysts is largely unstudied. In this work, we investigate the effect pulse electrolysis has on the selectivity and stability of the homogeneous electrocatalyst [Ni(cyclam)]2+ at a carbon electrode. By tuning the potential and pulse duration, we achieve a significant improvement in CO Faradaic efficiencies (85%) after 3 h, double that of the system under potentiostatic conditions. The improved activity is due to in situ catalyst regeneration from an intermediate that occurs as part of the catalyst's degradation pathway. This study demonstrates the wider opportunity to apply pulsed electrolysis to molecular electrocatalysts to control activity and improve selectivity.
Project description:Electroreduction of carbon dioxide into higher-energy liquid fuels and chemicals is a promising but challenging renewable energy conversion technology. Among the electrocatalysts screened so far for carbon dioxide reduction, which includes metals, alloys, organometallics, layered materials and carbon nanostructures, only copper exhibits selectivity towards formation of hydrocarbons and multi-carbon oxygenates at fairly high efficiencies, whereas most others favour production of carbon monoxide or formate. Here we report that nanometre-size N-doped graphene quantum dots (NGQDs) catalyse the electrochemical reduction of carbon dioxide into multi-carbon hydrocarbons and oxygenates at high Faradaic efficiencies, high current densities and low overpotentials. The NGQDs show a high total Faradaic efficiency of carbon dioxide reduction of up to 90%, with selectivity for ethylene and ethanol conversions reaching 45%. The C2 and C3 product distribution and production rate for NGQD-catalysed carbon dioxide reduction is comparable to those obtained with copper nanoparticle-based electrocatalysts.
Project description:Artificial photosynthesis is a vibrant field of research aiming at converting abundant, low energy molecules such as water, nitrogen or carbon dioxide into fuels or useful chemicals by means of solar energy input. Photo-electrochemical reduction of carbon dioxide is an appealing strategy, aiming at reducing the greenhouse gas into valuable products such as carbon monoxide at low or without bias voltage. Yet, in such configuration, there is no catalytic system able to produce carbon monoxide selectively in aqueous media with high activity, and using earth-abundant molecular catalyst. Upon associating a p-type Cu(In,Ga)Se2 semi-conductor with cobalt quaterpyridine complex, we herein report a photocathode complying with the aforementioned requirements. Pure carbon dioxide dissolved in aqueous solution (pH 6.8) is converted to carbon monoxide under visible light illumination with partial current density above 3 mA cm-2 and 97% selectivity, showing good stability over time.
Project description:A cobalt(i) complex is shown to be capable of both electrocatalytic reduction and hydrogenation of CO2 to formate. Several proposed intermediates are characterized and thus form the basis for a proposed mechanism that allows for the dual reactivity: reduction of CO2via H2 addition, and H+/e- equivalents. The work makes use of a novel tris(phosphino) ligand. When a pendent amine is attached to the ligand, no change in catalytic reactivity is observed.
Project description:A selective noble-metal-free molecular catalyst has emerged as a fruitful approach in the quest for designing efficient and stable catalytic materials for CO2 reduction. In this work, we report that a sodium pectate complex of copper (PG-NaCu) proved to be highly active in the electrocatalytic conversion of CO2 to CH4 in water. Stability and selectivity of conversion of CO2 to CH4 as a product at a glassy carbon electrode were discovered. The copper complex PG-NaCu was synthesized and characterized by physicochemical methods. The electrochemical CO2 reduction reaction (CO2RR) proceeds at -1.5 V vs. Ag/AgCl at ~10 mA/cm2 current densities in the presence of the catalyst. The current density decreases by less than 20% within 12 h of electrolysis (the main decrease occurs in the first 3 h of electrolysis in the presence of CO2). This copper pectate complex (PG-NaCu) combines the advantages of heterogeneous and homogeneous catalysts, the stability of heterogeneous solid materials and the performance (high activity and selectivity) of molecular catalysts.