Fengwang Li

Hierarchical Mesoporous SnO2 Nanosheets on Carbon Cloth: A Robust and Flexible Electrocatalyst for CO2 Reduction with High Efficiency and Selectivity

Electrochemical reduction of CO2 into liquid fuels is a promising approach to achieve a carbon-neutral energy cycle. However, conventional electrocatalysts usually suffer from low energy efficiency and poor selectivity and stability. A 3D hierarchical structure composed of mesoporous SnO2 nanosheets on carbon cloth is proposed to efficiently and selectively electroreduce CO2 to formate in aqueous media. The electrode is fabricated by a facile combination of hydrothermal reaction and calcination. It exhibits an unprecedented partial current density of about 45 mA cm-2 at a moderate overpotential (0.88 V) with high faradaic efficiency (87±2 %), which is even larger than most gas diffusion electrodes. Additionally, the electrode also demonstrates flexibility and long-term stability. The superior performance is attributed to the robust and highly porous hierarchical structure, which provides a large surface area and facilitates charge and mass transfer.

Polyethylenimine promoted electrocatalytic reduction of CO2 to CO in aqueous medium by graphene-supported amorphous molybdenum sulphide

Efficiently and selectively converting CO2 to value-added carbon compounds remains a major challenge in sustainable energy research. In this paper, we report the synthesis of a cost-effective catalyst, i.e. amorphous molybdenum sulphide on a polyethylenimine modified reduced graphene oxide substrate, for electrocatalytically reducing CO2 into CO in CO2 saturated aqueous NaHCO3 medium with high efficiency and selectivity. The catalyst is capable of producing CO at overpotentials as low as 140 mV and reaches a maximum faradaic efficiency of 85.1% at an overpotential of 540 mV. At an overpotential of 290 mV with respect to the formation of CO, it catalyzes the formation of syngas with high stability. Detailed investigations reveal that PEI works as a co-catalyst by providing a synergetic effect with MoSx.

Electrochemical Reduction of CO2 at Metal Electrodes in a Distillable Ionic Liquid.

The electroreduction of CO2 in the distillable ionic liquid dimethylammonium dimethylcarbamate (dimcarb) has been investigated with 17 metal electrodes. Analysis of the electrolysis products reveals that aluminum, bismuth, lead, copper, nickel, palladium, platinum, iron, molybdenum, titanium and zirconium electroreduce the available protons in dimcarb to hydrogen rather than reducing CO2 . Conversely, indium, tin, zinc, silver and gold are able to catalyze the reduction of CO2 to predominantly carbon monoxide (CO) and to a lesser extent, formate ([HCOO](-) ). In all cases, the applied potential was found to have a minimal influence on the distribution of the reduction products. Overall, indium was found to be the best electrocatalyst for CO2 reduction in dimcarb, with faradaic efficiencies of approximately 45 % and 40 % for the generation of CO and [HCOO](-) , respectively, at a potential of -1.34 V versus Cc(+/0) (Cc(+) =cobaltocenium) employing a dimethylamine to CO2 ratio of less than 1.8:1.

Recent advances in the nanoengineering of electrocatalysts for CO2 reduction

Emissions of CO2 from fossil fuel combustion and industrial processes have been regarded as the dominant cause of global warming. Electrochemical CO2 reduction (ECR), ideally in aqueous media, could potentially solve this problem by the storage of energy from renewable sources in the form of chemical energy in fuels or value-added chemicals in a sustainable manner. However, because of the sluggish reaction kinetics of the ECR, efficient, selective, and durable electrocatalysts are required to increase the rate this reaction. Despite considerable progress in using bulk metallic electrodes for catalyzing the ECR, greater efforts are still needed to tackle this grand challenge. In this Review, we highlight recent progress in using nanoengineering strategies to promote the electrocatalysts for the ECR. Through these approaches, considerable improvements in catalytic performance have been achieved. An outlook of future developments in applying and optimizing these strategies is also proposed.

Electrochemical reduction of CO2 on defect-rich Bi derived from Bi2S3 with enhanced formate selectivity

A sulphide-derived bismuth catalyst, synthesised from a one-pot hydrothermal reaction followed by electrochemical reduction, exhibits excellent performance for converting CO2 into formate in an aqueous bicarbonate medium with high activity, selectivity and durability. The maximum faradaic efficiency of 84.0% for formate formation was achieved at an overpotential of 670 mV. A detailed study reveals that the lattice defects associated with the sulphide-derived Bi rather than residual sulphur are likely to engender a positive effect on the catalytic reduction of CO2.

Electrochemical reduction of carbon dioxide in a monoethanolamine capture medium

The electrocatalytic reduction of CO2 in a 30 % (w/w) monoethanolamine (MEA) aqueous solution was undertaken at In, Sn, Bi, Pb, Pd, Ag, Cu and Zn metal electrodes. Upon the dissolution of CO2 , the non-conducting MEA solution is transformed into a conducting one, as is required for the electrochemical reduction of CO2 . Both an increase in the electrode surface porosity and the addition of the surfactant cetyltrimethylammonium bromide (CTAB) suppress the competing hydrogen evolution reaction; the latter has a significantly stronger impact. The combination of a porous metal electrode and the addition of 0.1 % (w/w) CTAB results in the reduction of molecular CO2 to CO and formate ions, and the product distribution is highly dependent on the identity of the metal electrode used. At a potential of -0.8 V versus the reversible hydrogen electrode (RHE) with an indium electrode with a coralline-like structure, the faradaic efficiencies for the generation of CO and [HCOO]- ions are 22.8 and 54.5 %, respectively compared to efficiencies of 2.9 and 60.8 % with a porous lead electrode and 38.2 and 2.4 % with a porous silver electrode. Extensive data for the other five electrodes are also provided. The optimal conditions for CO2 reduction are identified, and mechanistic details for the reaction pathways are proposed in this proof-of-concept electrochemical study in a CO2 capture medium. The conditions and features needed to achieve industrially and commercially viable CO2 reduction in an amine-based capture medium are considered.

Stannate derived bimetallic nanoparticles for electrocatalytic CO2 reduction

The synthesis of 8 metal–Sn (metal = Mn, Co, Ni, Cu, Zn, Ag, Cd, and Pb) bimetallic materials by electrochemical reduction of their metal stannates is reported. When the metal–Sn bimetallic materials were used as electrocatalysts for electrochemical CO2 reduction, bulk electrolysis results revealed that the Ag–Sn and Cu–Sn bimetallic systems showed the highest activity and selectivity for formate. When incorporated with reduced graphene oxide (rGO), their electrocatalytic performance can be further improved, making them among the best performing tin based CO2 reduction electrocatalysts reported so far. The Ag–Sn/rGO catalyst reaches the highest faradaic efficiency for formate (FEformate) of 88.3% at −0.94 V vs. RHE with a current density of 21.3 mA cm−2 in a 0.5 M aqueous NaHCO3 solution. Comparable performance was observed from the Cu–Sn/rGO catalyst, where the highest FEformate of 87.4% and a current density of 23.6 mA cm−2 were obtained at −0.99 V vs. RHE. Both catalysts exhibited high stability over a 6 hour electrolysis period with the FEformate variation being less than 2%. The excellent performance of this class of bimetallic nanoparticle/rGO composite catalysts is attributed to the small size of the stannate derived bimetallic nanoparticles, the presence of a SnOx layer and the introduction of rGO which prevents the aggregation of the bimetallic nanoparticles and provides a 3D conductive network to facilitate fast charge transfer. This study demonstrates a facile yet general strategy that enables the synthesis of bimetallic systems for highly efficient electrocatalytic CO2 reduction.

Polyoxometalate-Promoted Electrocatalytic CO2 Reduction at Nanostructured Silver in Dimethylformamide

Electrochemical reduction of CO2 is a promising method to convert CO2 into fuels or useful chemicals, such as carbon monoxide (CO), hydrocarbons, and alcohols. In this study, nanostructured Ag was obtained by electrodeposition of Ag in the presence of a Keggin type polyoxometalate, [PMo12O40]3- (PMo). Metallic Ag is formed upon reduction of Ag+. Adsorption of PMo on the surface of the newly formed Ag lowers its surface energy thus stabilizes the nanostructure. The electrocatalytic performance of this Ag-PMo nanocomposite for CO2 reduction was evaluated in a CO2 saturated dimethylformamide medium containing 0.1 M [ n-Bu4N]PF6 and 0.5% (v/v) added H2O. The results show that this Ag-PMo nanocomposite can catalyze the reduction of CO2 to CO with an onset potential of -1.70 V versus Fc0/+, which is only 0.29 V more negative than the estimated reversible potential (-1.41 V) for this process and 0.70 V more positive than that on bulk Ag metal. High faradaic efficiencies of about 90% were obtained over a wide range of applied potentials. A Tafel slope of 60 mV dec-1 suggests that rapid formation of *CO2•- is followed by the rate-determining protonation step. This is consistent with the voltammetric data which suggest that the reduced PMo interacts strongly with CO2 (and presumably CO2•-) and hence promotes the formation of CO2•-.

Ultra-small Cu nanoparticles embedded in N-doped carbon arrays for electrocatalytic CO2 reduction reaction in dimethylformamide

The development of heterogeneous catalysts with a well-defined micro structure to promote their activity and stability for electrocatalytic CO2 reduction has been shown to be a promising strategy. In this work, Cu nanoparticles (∼ 4 nm in diameter) embedded in N-doped carbon (Cu@NC) arrays were fabricated by thermal decomposition of copper tetracyanoquinodimethane (CuTCNQ) under N2. Compared to polycrystalline copper electrodes, the Cu@NC arrays provide a significantly improved number of catalytically active sites. This resulted in a 0.7 V positive shift in onset potential, producing a catalytic current density an order magnitude larger at a potential of–2.7 V vs. Fc/Fc+ (Fc = ferrocene) in dimethylformamide (DMF). By controlling the water content in the DMF solvent, the CO2 reduction product distribution can be tuned. Under optimal conditions (0.5 vol% water), 64% HCOO–, 20% CO, and 13% H2 were obtained. The Cu@NC arrays exhibited excellent catalytic stability with only a 0.5% decrease in the steady-state catalytic current during 6 h of electrolysis. The three-dimensional (3D) array structure of the Cu@NC was demonstrated to be effective for improving the catalytic activity of copper based catalysts while maintaining long-term catalytic stability.