CO2 is the most important greenhouse gas responsible for global warming. It is released to the atmosphere mainly due to the combustion of fossil fuels used for mobility and industrial processes. In recent years, electrochemical reduction of CO2 has raised as a promising route to convert CO2 in value added chemicals such as carbon monoxide, formic acid, and alcohols among others.
The electrochemical reduction of CO2 can be achieved in both homogeneous and heterogenous phase. In homogeneous catalysis, the catalyst is usually a metal complex serving a redox mediator between the electrode and the carbon dioxide molecule. In heterogeneous catalysis, the catalyst can consist of an immobilized metal complex or a highly porous heterogenous catalyst. There is a wide range of catalysts used for this purpose from metallic (i.e. Au, Ag, Cu, Pt), non-metallic (i.e. N-doped carbon, N-doped diamond, carbon nanofibers), and molecular catalysts (i.e. Fe(0) porphyrins, Pd phospine).
The molecules produced during carbon dioxide reduction can vary depending on the catalyst and the conditions at which the transformation in carried out. Usually a combination of analytical techniques is used to characterize the products, this is due to the difference in physical and chemical properties of such molecules. The typical techniques involved are Gas chromatography (GC) to quantify gaseous products such as CH4, C2H4 and other hydrocarbons, while liquid products like HCOOH, CH3COOH and alcohols are normally analyzed by liquid chromatography (LC) or nuclear magnetic resonance (NMR). The major drawback of these techniques is the long periods of time required, making difficult to analyze time sensitive products or catalyst degradation.
Conversely to conventional methods, electroanalytical techniques can offer product analysis time in the range of seconds. However, some of the electrochemical techniques require complex and expensive setups like in the case of Differential electrochemical mass spectrometry (DEMS) and Scanning electrochemical Microscopy (SECM). A convenient approach to avoid complex setups is the implementation of the rotating ring-disc electrode (RRDE), which place a detector probe in the vicinity of the electrocatalyst surface allowing the detection of products in situ under the disc-generation/ring-collection mode.
An example of the implementation of the RRDE was recently reported by Zhang and Co from the Ohio State University. They performed a systematic study of four model electrocatalyst such as Pt, Au, Sn, [NiII(cyclam)]2+ for CO2 reduction and the quantification of its products (Zhang & Co, 2020). Almost all the CO2 reduction products have characteristic electrochemical oxidation features, this fact allows the use of the RRDE as a quantification method. In this approach the concentration of a molecule can be related to the magnitude of a peak current, peak potential or the integrated charge in simple electrochemical techniques such cyclic voltammetry.
In their work, Zhang and Co proved the feasibility of the implementation of the RRDE as a reliable and fast quantification method during CO2 electrochemical reduction including both heterogeneous and homogeneous catalysts. The results obtained using this electroanalytical technique showed to be comparable to those obtained with conventional methods such as GC, NMR, and LC. In this case, the detected molecules included small molecules such as H2, CO, HCOO− and their mixtures.
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For more details of this application please refer to the full article, which is available at https://iopscience.iop.org/article/10.1149/1945-7111/ab7a80.
Zhang, F., & Co, A. C. (2020). Rapid Product Analysis for the Electroreduction of CO 2 on Heterogeneous and Homogeneous Catalysts Using a Rotating Ring Detector . Journal of The Electrochemical Society, 167(4), 046517. https://doi.org/10.1149/1945-7111/ab7a80