The electrocatalytic reduction of CO2 on gold cathodes was found to differ significantly between a standard batch electrochemical cell and a flow cell incorporating a porous gold cathode. While the ...well-known influence of KHCO3 concentration on product selectivity was observed in the batch cell, the selectivity of the CO2 reduction reaction was shown to be independent of KHCO3 concentration in the flow cell. The Faradaic efficiency for CO production was found to be 80–90% regardless of the KHCO3 concentration whereas in the batch cell it decreased from 75% to 35% as the KHCO3 concentration is increased from 0.05 to 0.5 mol L−1. The current density was found to be independent of the KHCO3 concentration and similar in both cells (−4 to −10 mA cm−2 at −1.3 V vs Ag|AgCl). As the KHCO3 concentration effect is normally attributed to changes in the local pH at the cathode–electrolyte interface brought about by the buffering action of the electrolyte, the results found in this work suggest that pH buffering can be suppressed or manipulated in some cell/electrode configurations. In the flow cell used in this work, it is suggested that poor transport of the KHCO3 through the porous cathode support to the active surface results in higher local pH than that found at the surface of a cathode immersed in a traditional batch cell.
•The direct comparison between a batch (H-cell) and a flow-cell has been made.•The influence of KHCO3 concentration on Faradaic efficiency is significantly different in the two cells.•Up to 90% Faradaic efficiency towards CO production can be obtained in the flow-cell.
The electrochemical reduction of CO
2
on gold cathodes was investigated, and the major products were found to be CO, H
2
and formate, which is consistent with existing literature. The Faradaic ...efficiency for CO production decreased from around 60 to 10% over the course of 4 h when the electrolysis was performed at – 5 mA cm
–2
in 0.2 M KHCO
3
saturated with CO
2
. This deactivation was accompanied by an increase in the selectivity of the cathode towards H
2
and formate production, which is normally attributed to the deposition of metals from trace impurities in the electrolyte or surface-bound species formed during the reaction. In this case, the deactivation was found to be due to the deposition of Cu, Zn and possibly Fe from the electrolyte, with the presence of Fe strongly enhancing H
2
production, the Cu deposition increasing the formate production rate and Zn enhancing both H
2
and formate production. While the accumulation of these poisons can be prevented with periodic anodic treatments (using methods previously described in the literature), these treatments lead to significant gold dissolution, with up to 450 ppb of gold found in the electrolyte after 4 h of electrolysis, and thus is unsuitable for use in long-term CO
2
reduction systems. This dissolution is expected to alter the surface structure and thus selectivity of the cathode. Therefore, alternative electrochemical cleaning protocols (periodic cyclic voltammetry, open-circuit and low anodic current treatments) were investigated as methods to remove these poisons without significant gold corrosion occurring. The best approach to prevent the deactivation of gold cathodes during CO
2
reduction is to cycle the potential between − 0.5 and 0.5 V vs Ag|AgCl every 15 min during long-term electrolysis. It is also shown that simply interrupting the CO
2
reduction process every 15 min with 4 min at open circuit can also partially prevent the deactivation of the CO
2
reduction reaction as will short anodic current pulses.
Graphical abstract
Metal clusters often exhibit superior chemical, electronic, and geometrical properties and can show exciting catalytic performance. The catalytic behaviour of the clusters is strongly affected by ...their size and composition, offering unique opportunities to fine-tune such materials for a specific application. In this study, atomically precise Au
6
(dppp)
4
(NO
3
)
2
, Au
9
(PPh
3
)
8
(NO
3
)
3
, Au
13
(dppe)
5
Cl
2
Cl
3
and Au
101
(PPPh
3
)
21
Cl
5
clusters were synthesised, characterised and their activity for electrocatalytic CO
2
reduction is compared. These Au clusters were deposited onto carbon paper to serve as the cathode for the electrochemical reduction of CO
2
. The experimental studies suggest that the clusters remain intact upon deposition on the carbon paper but undergo agglomeration during CO
2
electrolysis. The cluster-based catalysts demonstrated high selectivity (75%—90%) for CO production over hydrogen evolution reaction. Upon calcination, the activity of the cluster-based electrodes decreases, which can be attributed to the agglomeration of small clusters into larger bulk-like nanoparticles, as suggested by XPS, XAS and SEM.
Graphical Abstract
A series of phosphine-protected gold clusters supported on carbon paper were studied as CO
2
RR electrocatalysts. The CO
2
RR activity was dependent on their size and ligand density. Thermal annealing of the catalysts invariably lowered CO selectivity due to agglomeration of the clusters into larger nanoparticles.
Electrocatalytic reduction of CO
2
has the potential to convert CO
2
into carbon-based fuels by using renewable energy. While a wide range of electrocatalytic materials have been investigated, the ...process is still limited by poor reaction selectivity or large overpotentials. Hence many have studied the effects of surface oxides, crystal facets, and nanoparticles on the activity and selectivity of CO
2
reduction. Others have also shown that the reaction is sensitive to conditions such as temperature, CO
2
pressure, electrolyte buffer concentration and electrode porosity. These effects mean that it can be challenging to determine the underlying cause for differences in intrinsic catalytic behaviour 1,2, and thus it is important to understand the effect of experimental conditions on CO
2
reduction.
Firstly, we will review the literature and our own experimental work which clearly highlight the importance of interfacial pH on the reaction 2-4. It is well known that in weak buffers such as KHCO
3
, the pH at the surface of the cathode is significantly higher than the bulk due to the hydrogen evolution and CO
2
reduction reactions. By conducting experiments over a range of KHCO
3
concentrations (and thus different interfacial pH values), it is clear that this alters the selectivity and activity of CO
2
reduction. As the hydrodynamics at the cathode surface will also alter interfacial pH, we have examined the effect of hydrodynamics on the CO
2
reduction reaction using a Cu rotating cylinder electrode 5. Given that the enhanced mass transport will also increase the CO
2
concentration at the electrode surface 6, it seems clear that mass transfer effects should influence the reaction selectivity. We confirm that this is indeed an important factor and suggest that increasing mass transport not only decreases the interfacial pH (closer to bulk values) but also decreases the surface coverage of CO on the cathode (a key immediate during CO
2
reduction), which ultimately lowers the current going to the CO
2
reduction reaction.
As these experiments revealed the importance of both KHCO
3
concentration and mass transport, we developed a numerical model to predict how the bulk electrolyte composition changes during long term electrolysis experiments. This model was validated against experiment data and shows that changes in the bulk electrolyte (ionic resistance, pH, CO
2
-electrolyte equilibria) can occur over the course of long-term electrolysis experiments. These changes can complicate the interpretation of long-term electrode behaviour as well as the control of the electrochemical process. From these findings, we suggest a range of strategies to improve the experimental aspects of electrochemical CO
2
reduction investigations.
1 A.S. Hall, Y. Yoon, A. Wuttig, Y. Surendranath, Mesostructure-Induced Selectivity in CO
2
Reduction Catalysis, Journal of the American Chemical Society 137 (2015) 14834-14837.
2 R. Kas, R. Kortlever, H. Yılmaz, M.T.M. Koper, G. Mul, Manipulating the Hydrocarbon Selectivity of Copper Nanoparticles in CO
2
Electroreduction by Process Conditions, ChemElectroChem 2 (2015) 354-358.
3 K.J.P. Schouten, E. Pérez Gallent, M.T.M. Koper, The influence of pH on the reduction of CO and to hydrocarbons on copper electrodes, Journal of Electroanalytical Chemistry 716 (2014) 53-57.
4 A.S. Varela, M. Kroschel, T. Reier, P. Strasser, Controlling the selectivity of CO
2
electroreduction on copper: The effect of the electrolyte concentration and the importance of the local pH, Catalysis Today 260 (2016) 8-13.
5 C.F.C. Lim, D.A. Harrington, A.T. Marshall, Effects of mass transfer on the electrocatalytic CO
2
reduction on Cu, Electrochimica Acta (2017)
in press
.
6 N. Gupta, M. Gattrell, B. MacDougall, Calculation for the cathode surface concentrations in the electrochemical reduction of CO
2
in KHCO
3
solutions, Journal of Applied Electrochemistry 36 (2006) 161-172.
7 K. Hara, A. Tsuneto, A. Kudo, T. Sakata, Electrochemical Reduction of CO
2
on a Cu Electrode under High Pressure: Factors that Determine the Product Selectivity, Journal of The Electrochemical Society 141 (1994) 2097-2103.
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