Electrochemically decoupled reduction of CO2 to formate over a dispersed heterogeneous bismuth catalyst enabled via redox mediators

Electrochemical CO2 reduction is a topic of major interest in contemporary research as an approach to use renewably-derived electricity to synthesise useful hydrocarbons from waste CO2. Various strategies have been developed to optimise this challenging reaction at electrode interfaces, but to-date, decoupled electrolysis has not been demonstrated for the reduction of CO2. Decoupled electrolysis aims to use electrochemically-derived charged redox mediators – electrical charge and potential vectors – to separate catalytic product formation from the electrode surface. Utilising an electrochemically generated highly reducing redox mediator; chromium propanediamine tetraacetate, we report the first successful application of decoupled electrolysis to electrochemical CO2 reduction. A study of metals and metal composites found formate to be the most accessible product, with bismuth metal giving the highest selectivity. Copper, tin, gold, nickel and molybdenum carbide heterogeneous catalysts were also investigated, in which cases H2 was found to be the major product, with minor yields of two-electron CO2 reduction products. Subsequent optimisation of the bismuth catalyst achieved a high formate selectivity of 85%. This method represents a radical new approach to CO2 electrolysis, which may be coupled directly with renewable energy storage technology and green electricity.


Voltammetry
Unless otherwise stated, cyclic voltammetry was performed in a three-electrode cell with a glassy carbon working electrode (area of 0.0707 cm 2 ), Ag/AgCl reference electrode (1 M KCl inner solution), and a platinum wire counter electrode at a scan rate of 100 mV s −1 .

UV-Vis Spectroscopy
Fig. S5 UV-Vis spectra of Cr PDTA with KHCO3 in its ground and charged states.The samples consisted of 10 mM Cr PDTA with 1 M KHCO3 in water.Spectra were collected from a 10 mm path length quartz cuvette.In its fresh Cr(III) ground state, the complex is a bright red (blue line) with two distinct absorbance peaks at 506 and 383 nm respectively, and an exponential increase in absorbance below 280 nm.When reduced to Cr(II) under an N2 atmosphere, the complex changes colour to a pale green (green line) which displays no distinct absorbance peaks, with the exponential increase in current now onset around 400 nm.When the atmosphere is replaced with CO2, the complex visibly changes to a blue-green colour (pink line), however the UV-Vis spectra show minimal change.
Fig. S6 UV-Vis spectra of Cr PDTA with KCl in its ground and charged state.The samples consisted of 10 mM Cr PDTA with 1 M KCl in water.Spectra were collected from a 10 mm path length quartz cuvette.In its Cr(III) ground state, the complex appears identical to when KHCO3 is present, with the same red colour and absorbance peaks.When reduced to Cr (III), the complex changes to a pale sky-blue colour.Again, the complex displays no distinct absorbance peaks with the spectrum again appearing very similar to when KHCO3 is present.The complex appears identical when either an N2 or CO2 atmosphere is used.

Fig. S7 UV-Vis spectra of used
Cr PDTA with KCl.The samples consisted of 10 mM Cr PDTA with 1 M KCl in water.Spectra were collected from a 10 mm path length quartz cuvette.When KCl is used as the supporting electrolyte, mediator solution that has been used in batch decoupled electrolysis does not return to the red ground state as initially expected, instead taking on a purple hue.As purple is also the colour resulting from a mixture of the red Cr(III) and blue Cr(II) states when KCl is present, colour can no longer be used to determine when the reaction is finished.The UV-Vis spectrum of the used purple sample has two peaks in broadly the same positions as the fresh solution, however the higher wavelength peak is broader with lower max absorbance.The solution was found to have a pH of 11, which is much higher than its initial pH of 7.8.Dropwise addition of acid to return the pH to 7.8 restored the red colouration and UV-Vis spectrum of the fresh solution.Gaseous Products.To determine the faradaic efficiency for the gaseous products H2 and CO, headspace gas was taken from the flask and analysed by gas chromatography.An aliquot of gas was collected in a gas tight syringe and diluted with additional background gas to ensure the products were within the calibration range (5 or 10 mL of sample diluted into 50 mL for five-and ten-fold dilutions respectively).The sample was then directly injected into the GC inlet for separation.Using the above calibration plots, the concentration of each product within the diluted sample was determined in ppm (molar ratio).This can then be multiplied by the dilution factor to estimate the concentration of each product within the vessel headspace.It is assumed that 100% of all gaseous products were in the headspace, as both H2 and CO are sparing soluble in water and the electrolyte was degassed via sonication immediately before sampling.
The amount of each gaseous product in moles within the headspace was estimated; × Moles of gas in headspace [mol] where the ideal gas law was used to determine the total number of moles within the headspace;

Fig. S1
Fig. S1 Cyclic voltammograms of Cr PDTA with KHCO3 supporting electrolyte in water saturated with N2 at sequentially more negative potential vertices.The electrolyte consisted of 1 M KHCO3 in water with (solid blue) and without (dashed green) 10 mM Cr PDTA target analyte.The negative potential vertex was shifted more negative for each subsequent voltammogram, a, −1.7 V, b, −1.8 V, c, −1.9 V, d, −2.0 V.

XRDFig.
Fig. S8 Powder XRD spectra of the bismuth catalyst materials.Largest peak (012) set to the same intensity for each sample.Blue, 100-mesh bismuth powder, Pink, solvothermally reduced bismuth nanospheres, Green, chemically reduced bismuth rhombic dodecahedra.All three samples contain the expected peaks for the (012), (104) and (110) facets.As the lowest energy facet, (012) is the dominant facet in each sample.The Bi RD are theorised to selectively expose only 104 and 110 facets, while the powder and NS are not expected to selectively expose any particular facets.The key difference between the spectra is the ratio of the (104) and (110) peaks, where the Bi RD favour the (104) facet while the Bi NS does not favour either.This lines up reasonably with the previously reported characterisation of these material.1

Fig. S10 SEM
Fig. S10 SEM image of spent Bi rhombic dodecahedra used in mediated CO2 reduction.Sample collected post discharge with 200 nm syringe filter and washed with water and ethanol, some salt remained.Nanoparticles have retained their size and shape post catalysis.

Fig. S12 SEM
Fig. S12 SEM image of spent Bi nanospheres used in mediated CO2 reduction.Scale bar 1 µm.Sample collected post discharge with 200 nm syringe filter and washed with water, some salt remained.Particle shape and size unaltered by use as catalyst.

Fig. S14
Fig. S14 Gas chromatography calibration for carbon monoxide concentration with high and low concentration fittings.Samples were prepared from a calibration standard containing 976 ppm H2, 1009 ppm CH4, 1040 CO, and 1018 ppm C2H4, with He balance.Dilutions were performed in a gas tight pressure vessel with He to give concentrations of approximately

Fig. S15
Fig. S15 Ion chromatography calibration for HCOO − concentration in ppm.Samples were prepared from a calibration standard containing 1000 ppm HCOO − by dilution with ultrapure water to concentrations of 100, 50, 20, 10, 5, 2, and 1 ppm.The calibration is strongly correlated, allowing for a high degree of certainty in the accuracy of determined concentrations.An equation in the form y = mx + c was used to determine the concentration of HCOO − in a real sample from the IC peak area; Peak area = (0.038070 × ppm) + 0.018176 Fig. S16 10 mM Cr PDTA with 1 M KHCO3 in water.a, red ground state.b, green reduced state under N2.c, blue-green reduced state under CO2.

Fig. S17 10 mM Cr PDTA with 1 M
Fig. S17 10 mM Cr PDTA with 1 M KCl in water.a, red ground state.b, blue reduced state.c, high pH purple discharged state.

Fig. S18
Fig. S18 Cr PDTA stability testing.Samples of 10 mM Cr PDTA with 1 M KHCO3 in water with (left to right) Cu, Sn, Bi and Mo2C powder immediately after preparation.

Fig. S19 Cr
Fig. S19 Cr PDTA stability testing.Samples of 10 mM Cr PDTA with 1 M KHCO3 in water with (left to right) Cu, Sn, Bi and Mo2C powder after three months.The copper powder has displaced the chromium from the complex, resulting in a deep blue Cu(II) solution and plating the copper particles light grey with chromium.The Sn powder has plated the glass vial however the complex is unaffected.Both bismuth and molybdenum carbide underwent no visible reaction with the complex.