Converting Captured CO₂ Into Valuable Chemicals

With the aim of advancing integrated CO₂ capture and electrochemical conversion, researchers from the Interface Science Department of the Fritz Haber Institute investigated how the concentration of KHCO3, a typical CO₂ capture medium, controls the electrochemical conversion of CO₂ on copper in a membrane-electrode-assembly (MEA) electrolyzer.

Dr. Hou and her colleagues, within Director Roldán’s department, found that, in bicarbonate electrolysis, more concentrated capture medium does not necessarily lead to better performance. Instead, the reaction follows a volcano-shaped dependence on KHCO₃ concentration. With a custom-designed operando Raman electrolyzer cell, they revealed that dilute electrolyte feeding limits the in-situ CO₂ supply, whereas concentrated bicarbonate blocks active sites through competitive anion adsorption. The work identifies the electrolyte concentration as a precise design parameter for coupling CO₂ capture streams directly to electrochemical conversion.

Bicarbonate as a feedstock for integrated CO₂ capture and conversion
Alkaline carbon-capture processes store CO₂ mainly as (bi)carbonate in aqueous solution. Direct bicarbonate electrolysis could convert this captured carbon without first releasing, compressing, and transporting gaseous CO₂. However, most bicarbonate electrolyzers have been studied with highly concentrated electrolytes, typically 2–3 M KHCO₃, whereas practical capture streams are generally more dilute. Understanding the role of the electrolyte concentration is therefore a critical system parameter, since it controls both, the supply of in situ generated CO₂ and the chemical environment at the catalyst surface.

A custom-designed operando platform
To investigate these coupled effects in a real device, the ISC-FHI researchers designed and developed a zero-gap MEA-type operando Raman cell. The Raman cell retained the essential membrane–electrode configuration of the working device while providing optical access to the Cu/electrolyte interface. This enabled the team to monitor (bi)carbonate species, *CO intermediates, and the apparent local pH during electrolysis, allowing them to gain mechanistic understanding on this important process by linking the device’s performance to the evolving interfacial chemistry directly.

Higher bicarbonate concentration is not always beneficial
At 400 mA cm−², the highest bicarbonate electroreduction performance was obtained with 0.5 M KHCO₃. The total Faradaic efficiency toward in situ CO_₂ reduction products reached approximately 40%, corresponding to a partial current density of ~160 mA cm−². This value was four times higher than that obtained with 3 M KHCO₃, while the cell voltage differed by less than 0.2 V.

The resulting volcano-shaped concentration dependence originates from two distinct limitations. At low concentration, bicarbonate mass transport is insufficient to sustain the required in situ CO₂ supply. At high concentration, operando Raman spectroscopy revealed increased interfacial (bi)carbonate signals together with suppressed ^*CO adsorption.

These results show that strongly adsorbed (bi)carbonate species compete with ^*CO for surface active sites. This competition limits the adsorption of a key intermediate, CO, and further C–C coupling, while the greater availability of bicarbonate-derived proton donors also promotes hydrogen evolution. Control experiments further showed that neither the K+ concentration nor the buffer-controlled local pH were the primary origins of the observed performance trend. The optimal concentration also shifted with current density, demonstrating that the bicarbonate supply must be matched to the proton flux and operating load of the electrolyzer.

Conclusion
The study establishes a direct design rule for integrated CO₂ capture and electrochemical conversion: bicarbonate concentration should be matched to the operating current rather than simply maximized.

A moderate concentration provides sufficient in situ CO₂ while preserving active surface sites for ^*CO adsorption and subsequent C–C coupling. By combining device-level performance measurements with a purpose-built MEA-type operando Raman platform, the researchers identified the interfacial mechanism governing this balance and provided a practical basis for designing CO₂ capture-to-chemicals electrolyzers.