Electrochemical reduction of CO2 in aqueous media is a strategy for sustainable production of fuels and commodity chemicals. Cu is the only catalyst which converts CO2 to significant quantities of ...hydrocarbons and oxygenates. Here we demonstrate that oxygenate products can be favored over hydrocarbons by positioning a local source of CO generated by a CO producing catalyst (Au or Ag) in close proximity to a Cu catalyst. Use of a bimetallic device comprising interdigitated and independently controllable lines of Au and Cu allows the local CO concentration to be modulated. Notably, diffusional simulations show that the saturation concentration of CO can be exceeded locally. Actuating both the Au and Cu lines increases the oxygenate to ethylene ratio compared to actuating Cu only. Increasing the relative area of CO-producing Au relative to Cu also increases this ratio. These insights are translated into a second bimetallic system comprising Cu dots/lines patterned directly onto a Ag substrate, allowing for the distance between Cu and the CO generating metal to be precisely controlled. Controlling the relative areas of Ag and Cu allows for tuning of the oxygenate to ethylene ratio from 0.59 to 2.39 and an increase in oxygenate faradaic efficiency from 21.4% to 41.4%, while maintaining the selectivity to C2/C3 products in the 50–65% range. We attribute this change in selectivity to be due to an increased *CO coverage on Cu. By utilizing diffusional transport of CO to the Cu, a sequential catalysis pathway is created which allows for the control of oxygenate selectivity in aqueous CO2 reduction.
Formic acid has been proposed as a hydrogen energy carrier because of its many desirable properties, such as low toxicity and flammability, and a high volumetric hydrogen storage capacity of 53 g H
2
...L
−1
under ambient conditions. Compared to liquid hydrogen, formic acid is thus more convenient and safer to store and transport. Converting formic acid to power has been demonstrated in direct formic acid fuel cells and in dehydrogenation reactions to supply hydrogen for polymer electrolyte membrane fuel cells. However, to enable a complete cycle for the storage and utilization of low-carbon or carbon-free electricity, processes for the hydrogenation and electrochemical reduction of carbon dioxide (CO
2
) to formic acid, namely power to formic acid, are needed. In this review, representative homogenous and heterogeneous catalysts for CO
2
hydrogenation will be summarized. Apart from catalytic systems for CO
2
hydrogenation, a wide range of catalysts, electrodes, and reactor systems for the electrochemical CO
2
reduction reaction (eCO
2
RR) will be discussed. An analysis for practical applications from the engineering viewpoint will be provided with concluding remarks and an outlook for future challenges and R&D directions.
Power to formic acid
via
CO
2
hydrogenation or electrochemical CO
2
reduction has great potential to enable a complete cycle with formic acid to power for the storage and utilization of low-carbon electricity at a scale of multi-gigatonnes per year.
Electrolyte cation size is known to influence the electrochemical reduction of CO2 over metals; however, a satisfactory explanation for this phenomenon has not been developed. We report here that ...these effects can be attributed to a previously unrecognized consequence of cation hydrolysis occurring in the vicinity of the cathode. With increasing cation size, the pK a for cation hydrolysis decreases and is sufficiently low for hydrated K+, Rb+, and Cs+ to serve as buffering agents. Buffering lowers the pH near the cathode, leading to an increase in the local concentration of dissolved CO2. The consequences of these changes are an increase in cathode activity, a decrease in Faradaic efficiencies for H2 and CH4, and an increase in Faradaic efficiencies for CO, C2H4, and C2H5OH, in full agreement with experimental observations for CO2 reduction over Ag and Cu.
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IJS, KILJ, NUK, PNG, UL, UM
Abstract
Electroreduction uses renewable energy to upgrade carbon dioxide to value-added chemicals and fuels. Renewable methane synthesized using such a route stands to be readily deployed using ...existing infrastructure for the distribution and utilization of natural gas. Here we design a suite of ligand-stabilized metal oxide clusters and find that these modulate carbon dioxide reduction pathways on a copper catalyst, enabling thereby a record activity for methane electroproduction. Density functional theory calculations show adsorbed hydrogen donation from clusters to copper active sites for the *CO hydrogenation pathway towards *CHO. We promote this effect via control over cluster size and composition and demonstrate the effect on metal oxides including cobalt(II), molybdenum(VI), tungsten(VI), nickel(II) and palladium(II) oxides. We report a carbon dioxide-to-methane faradaic efficiency of 60% at a partial current density to methane of 135 milliampere per square centimetre. We showcase operation over 18 h that retains a faradaic efficiency exceeding 55%.
Copper electrodes, prepared by reduction of oxidized metallic copper, have been reported to exhibit higher activity for the electrochemical reduction of CO2 and better selectivity toward C2 and C3 ...(C2+) products than metallic copper that has not been preoxidized. We report here an investigation of the effects of four different preparations of oxide-derived electrocatalysts on their activity and selectivity for CO2 reduction, with particular attention given to the selectivity to C2+ products. All catalysts were tested for CO2 reduction in 0.1 M KHCO3 and 0.1 M CsHCO3 at applied voltages in the range from −0.7 to −1.0 V vs RHE. The best performing oxide-derived catalysts show up to ∼70% selectivity to C2+ products and only ∼3% selectivity to C1 products at −1.0 V vs RHE when CsHCO3 is used as the electrolyte. In contrast, the selectivity to C2+ products decreases to ∼56% for the same catalysts tested in KHCO3. By studying all catalysts under identical conditions, the key factors affecting product selectivity could be discerned. These efforts reveal that the surface area of the oxide-derived layer is a critical parameter affecting selectivity. A high selectivity to C2+ products is attained at an overpotential of −1 V vs RHE by operating at a current density sufficiently high to achieve a moderately high pH near the catalyst surface but not so high as to cause a significant reduction in the local concentration of CO2. On the basis of recent theoretical studies, a high pH suppresses the formation of C1 relative to C2+ products. At the same time, however, a high local CO2 concentration is necessary for the formation of C2+ products.
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IJS, KILJ, NUK, PNG, UL, UM
Electrochemical conversion of nitrate (NO3 –) into ammonia (NH3) recycles nitrogen and offers a route to the production of NH3, which is more valuable than dinitrogen gas. However, today’s ...development of NO3 – electroreduction remains hindered by the lack of a mechanistic picture of how catalyst structure may be tuned to enhance catalytic activity. Here we demonstrate enhanced NO3 – reduction reaction (NO3 –RR) performance on Cu50Ni50 alloy catalysts, including a 0.12 V upshift in the half-wave potential and a 6-fold increase in activity compared to those obtained with pure Cu at 0 V vs reversible hydrogen electrode (RHE). Ni alloying enables tuning of the Cu d-band center and modulates the adsorption energies of intermediates such as *NO3 –, *NO2, and *NH2. Using density functional theory calculations, we identify a NO3 –RR-to-NH3 pathway and offer an adsorption energy–activity relationship for the CuNi alloy system. This correlation between catalyst electronic structure and NO3 –RR activity offers a design platform for further development of NO3 –RR catalysts.
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IJS, KILJ, NUK, PNG, UL, UM
Multi-carbon alcohols such as ethanol are valued as fuels in view of their high energy density and ready transport. Unfortunately, the selectivity toward alcohols in CO
/CO electroreduction is ...diminished by ethylene production, especially when operating at high current densities (>100 mA cm
). Here we report a metal doping approach to tune the adsorption of hydrogen at the copper surface and thereby promote alcohol production. Using density functional theory calculations, we screen a suite of transition metal dopants and find that incorporating Pd in Cu moderates hydrogen adsorption and assists the hydrogenation of C
intermediates, providing a means to favour alcohol production and suppress ethylene. We synthesize a Pd-doped Cu catalyst that achieves a Faradaic efficiency of 40% toward alcohols and a partial current density of 277 mA cm
from CO electroreduction. The activity exceeds that of prior reports by a factor of 2.
Producing liquid fuels such as ethanol from CO
, H
O, and renewable electricity offers a route to store sustainable energy. The search for efficient electrocatalysts for the CO
reduction reaction ...relies on tuning the adsorption strength of carbonaceous intermediates. Here, we report a complementary approach in which we utilize hydroxide and oxide doping of a catalyst surface to tune the adsorbed hydrogen on Cu. Density functional theory studies indicate that this doping accelerates water dissociation and changes the hydrogen adsorption energy on Cu. We synthesize and investigate a suite of metal-hydroxide-interface-doped-Cu catalysts, and find that the most efficient, Ce(OH)
-doped-Cu, exhibits an ethanol Faradaic efficiency of 43% and a partial current density of 128 mA cm
. Mechanistic studies, wherein we combine investigation of hydrogen evolution performance with the results of operando Raman spectroscopy, show that adsorbed hydrogen hydrogenates surface *HCCOH, a key intermediate whose fate determines branching to ethanol versus ethylene.
Solar to chemical energy conversion could provide an alternative to mankind's unsustainable use of fossil fuels. One promising approach is the electrochemical reduction of CO2 into chemical products, ...in particular hydrocarbons and oxygenates which are formed by multi-electron transfer reactions. Here, a nanostructured Cu-Ag bimetallic cathode is utilized to selectively and efficiently facilitate these reactions. When operated in an electrolysis cell, the cathode provides a constant energetic efficiency for hydrocarbon and oxygenate production. As a result, when coupled to Si photovoltaic cells, solar conversion efficiencies of 3-4% to the target products are achieved for 0.35 to 1 Sun illumination. Use of a four-terminal III-V/Si tandem solar cell configuration yields a conversion efficiency to hydrocarbons and oxygenates exceeding 5% at 1 Sun illumination. Here, this study provides a clear framework for the future advancement of efficient solar-driven CO2 reduction devices.In a process analogous to natural photosynthesis, solar-driven reduction of carbon dioxide to hydrocarbon and oxygenate products is demonstrated with an overall efficiency exceeding 5%.
Electrochemical CO2 reduction reaction (CO2RR) is a promising technology for mitigating global warming and storing renewable energy. Designing low-cost and efficient electrocatalysts with high ...selectivity is a priority to facilitate CO2 conversion. Halide ion (F–, Cl–, Br–, I–) modified electrocatalysts is a potential strategy to promote CO2 reduction and suppress the competitive hydrogen evolution reaction (HER). Therefore, a comprehensive review of the role and mechanism of halide ions in the CO2RR process can help better guide the future design of efficient electrocatalysts. In this review, we first discuss the role of halide ions on the structure and morphology of electrocatalysts. Secondly, the relationship between the halide ions and the valence states of the active sites on the catalyst surface is further elaborated on. Thirdly, the mechanisms of halide in enhancing CO2 conversion efficiency are also summarized, including the involvement of halide ions in electron transfer and their influence on the reaction pathway. Finally, we conclude with a summary and future outlook.
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