Title: Direct Observation of the Local Reaction Environment during the Electrochemical Reduction of CO₂

Abstract

The electrochemical reduction of carbon dioxide is sensitive to electrolyte polarization, which causes gradients in pH and the concentration of carbon dioxide to form near the cathode surface. It is desirable to measure the concentration of reaction-relevant species in the immediate vicinity of the cathode because the intrinsic kinetics of carbon dioxide reduction depend on the composition of the local reaction environment. Meeting this objective has proven difficult because conventional analytical methods only sample products from the bulk electrolyte. In this study, we describe the use of differential electrochemical mass spectrometry to measure the concentration of carbon dioxide and reaction products in the immediate vicinity of the cathode surface. This capability is achieved by coating the electrocatalyst directly onto the pervaporation membrane used to transfer volatile species into the mass spectrometer, thereby enabling species to be sampled directly from the electrode–electrolyte interface. This approach has been used to investigate hydrogen evolution and carbon dioxide reduction over Ag and Cu. We find that the measured CO₂ reduction activity of Ag agrees well with what is measured by gas chromatography of the effluent from an H-cell operated with the same catalyst and electrolyte. A distinct advantage of our approach is that itmore » enables observation of the depletion of carbon dioxide near the cathode surface due to reaction with hydroxyl anions evolved at the cathode surface, something that cannot be done using conventional analytical techniques. We also demonstrate that the influence of this relatively slow chemical reaction can be minimized by evaluating electrocatalytic activity during a rapid potential sweep, thereby enabling measurement of the intrinsic kinetics. For CO₂ reduction over Cu, nine products can be observed simultaneously in real time. A notable finding is that the abundance of aldehydes relative to alcohols near the cathode surface is much higher than that observed in the bulk electrolyte. It is also observed that for increasingly cathodic potentials the relative abundance of ethanol increases at the expense of propionaldehyde. Finally, these findings suggest that acetaldehyde is a precursor to ethanol and propionaldehyde and that propionaldehyde is a precursor to n-propanol.« less

Authors:

Clark, Ezra L. ^[1]; Bell, Alexis T.  ^[1]

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1. Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States). Joint Center for Artificial Photosynthesis; Univ. of California, Berkeley, CA (United States)

Publication Date:

Mon May 14 00:00:00 EDT 2018

Research Org.:

Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States)

Sponsoring Org.:

USDOE Office of Science (SC), Basic Energy Sciences (BES). Chemical Sciences, Geosciences & Biosciences Division (CSGB)

OSTI Identifier:

1477378

Grant/Contract Number:  

AC02-05CH11231

Resource Type:

Accepted Manuscript

Journal Name:

Journal of the American Chemical Society

Additional Journal Information:

Journal Volume: 140; Journal Issue: 22; Journal ID: ISSN 0002-7863

Publisher:

American Chemical Society (ACS)

Country of Publication:

United States

Language:

English

Subject:

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Aldehydes; Electrical properties; Electrodes; Electrolytes; Oxides