(471d) Online Quantification of the Electrochemical CO2 Reduction Reaction Via a Novel Differential Electrochemical Mass Spectrometer Cell Design

The promise of utilizing solar energy to promote the electrochemical or photoelectrochemical reduction of CO₂ to transportation fuels has motivated extensive research efforts aimed at identifying highly active and selective  CO₂ reduction (CO₂R) electrocatalysts.^1–4  These efforts have revealed that copper is the only metallic electrocatalyst capable of reducing CO₂ to hydrocarbons and alcohols.^5–7  Unfortunately, the reaction requires an overpotential of approximately -1 V, resulting in a cathodic CO₂R energy efficiency of roughly 25% (see supplementary information S1).^8–11  Furthermore, CO₂R over metallic copper can produce up to 16 different products depending on the surface morphology and the applied potential.^10,11  As a consequence, there is considerable motivation to discover novel electrocatalysts that can reduce CO₂to fuels with higher efficiency and selectivity than metallic copper.

A combination of analytical techniques must be employed to fully characterize the products of CO₂R because the reaction produces both gaseous and liquid-phase products.^8,10  Gas chromatography is used to quantify the gaseous products by periodically sampling the headspace of the electrochemical cell over the course of electrolysis.  The liquid phase products are analyzed after electrolysis using either liquid chromatography (LC) or nuclear magnetic resonance (NMR).^8,10  While gas chromatography is sufficiently sensitive to quantify gaseous products from the effluent of an electrochemical cell, constant potential electrolysis must be performed for roughly one hour in order to reach the detection limits of LC or NMR because the Faradaic efficiencies of most liquid phase products are less than 1%.¹⁰  Therefore, there is considerable interest in developing an analytical technique capable of continuously quantifying the generation rates of the major reaction products in both phases in real time. 

Differential electrochemical mass spectrometry (DEMS) is an analytical technique that utilizes pervaporation to continuously separate and collect electrochemical reaction products.¹²  Because the analysis time of mass spectrometry is on the order of a second the generation rates of gaseous or volatile reaction products can be quantified in real time by recording the relevant mass ion currents and relating them to the partial current densities of the corresponding reaction products.¹²  This ultimately enables the potential dependence and transient nature of the reaction selectivity to be rapidly screened.  However, the efficacy of DEMS is heavily reliant on the design of the electrochemical cell, which must be capable of achieving both a rapid response time and a high product collection efficiency¹³.  As a result, electrochemical cells are specially designed for this application.  Current DEMS cell designs suffer from a variety of drawbacks including excessive delay between product generation and detection, high impedances, non-parallel electrode configurations, and non-spatially separated electrodes. 

This study outlines the rational design, modeling, and experimental validation of an electrochemical cell designed to study the electrochemical CO₂R via the DEMS technique.  The proposed cell is able to track the formation of gaseous hydrocarbon products as well as liquid phase alcohols, which is unprecedented in this field.  The proposed cell is also capable of recording product onsets and partial current trends that accurately reflect those observed by other research groups using the conventional combination of analytical techniques.      

References

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