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Journal of The Electrochemical Society, 2020 167 133504 Review—CO2 Separation and Transport via Electrochemical Methods Alexander P. Muroyama,z Alexandra Pătru, and Lorenz Gubler* Electrochemistry Laboratory, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland This review focuses on research advancements in electrochemical methods of CO2 separation as part of the broader field of CO2 capture. Such methods are a potentially effective way of separating CO2 from dilute gas mixtures (e.g., flue gas, air) such that it can be sequestered or recycled for other purposes. Electrodialysis using a liquid electrolyte capture solution is the most thoroughly explored electrochemical approach for CO2 capture. The purpose of this review is to provide a broad overview of developments in the field, highlighting and harmonizing relevant figures of merit such as specific energy consumption and faradaic efficiency. In addition, the use of alkaline membranes is separately surveyed as a promising means of electrochemical CO2 separation, as their CO2 transport phenomena are well understood within the context of alkaline fuel cells or electrochemical CO2 reduction. Recent materials advancements enable the use and modification of these membranes to promote electromigration of (bi)-carbonate ions, the result being CO2 concentration on the anode side of an electrochemical cell. © 2020 The Author(s). Published on behalf of The Electrochemical Society by IOP Publishing Limited. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/ by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. [DOI: 10.1149/ 1945-7111/abbbb9] Manuscript submitted June 15, 2020; revised manuscript received August 28, 2020. Published October 5, 2020. Persistent anthropogenic greenhouse gas emissions, particularly emissions of CO2, necessitate the development of novel mitigation solutions. Global anthropogenic fossil CO2 emissions exceeded a record estimated 37 Gt in 2017.1 To limit excessive CO2 emissions and to minimize impacts on the climate, a range of technologies including a transition from fossil fuel-based to renewable energy, improvement of vehicle and building energetic efficiencies, and CO2 capture and sequestration must be implemented together.2 CO2 can be captured from point sources (e.g., coal-fired power plants) or atmospheric air, the latter theoretically allowing for negative CO2 emissions.3 Although various demonstration projects have come online in recent years, the only CO2 capturing technique that has been utilized at an industrial scale is amine scrubbing.4 This is a post-combustion process that uses amine-based solvents (e.g., monoethanolamine) that absorb the carbon dioxide contained in the flue gas. The CO2 is then removed from the CO2-containing solvent by means of a regeneration process, driven by a temperature or pressure swing. The solvent is recycled and the captured CO2 is treated for transport and subsequent storage. The main drawbacks of this technology are related to the corrosive nature of the solvent and to the high cost (60–107 USD/ton of CO2 captured).5–7 In this context, considerable research efforts are needed to develop more efficient and, at the same time, less costly CO capture technologies. Alternative to amine scrubbing are the capture of CO2 using solid sorbents such as alkaline earth metal oxides,8 layered double oxides,9 carbon,10 or metal organic frameworks11 but also electrochemical methods. Using solid sorbent-like metal oxides (e.g., CaO) or carbon present the advantage of using cheap abundant materials, less corrosive and potentially more selective for CO capture. Electrochemical gas separation was successfully used for H2 separation from gas mixture containing CH4,12 reformate gas,13 and N2.14 The method consists of selectively oxidizing (or reducing) a gas at one electrode to an ion which is further transported through a membrane or liquid electrolyte to the other electrode where is reduced (or oxidized) back to the gas. The advantages of this technology is that the separation occurs at low temperature and pressure and that, in principle, the energy requirements are low.15 Moreover, electrochemical methods for CO2 capture can be per- formed at atmospheric concentration levels.16 While electrochemical CO2 separation methods have been investigated for almost half a century, recent years have brought a surge of interest and encoura- ging developments. *Electrochemical Society Member. zE-mail: alexander.muroyama@psi.ch This review is divided into two primary parts; the first focuses on electrochemical processes for CO2 separation. Although this review is agnostic in the source or purpose of captured CO2, a comparison of direct-air capture (DAC) and point-source capture is provided. The second part deals with CO2 transport in anion exchange membranes (AEM) and proton exchange membranes (PEM) as an ancillary process in fuel cell or co-electrolyzer operation. Electrochemical CO2 Separation Electrochemical CO2 separation is herein defined as any process by which CO2 is selectively removed from a gas mixture using electrochemical reactions as the driving mechanism. This is most commonly performed using a liquid electrolyte as the medium for absorbing and releasing CO2 in a two-step process, as shown in Fig. 1a, with ion-conducting membranes used to regenerate the electrolyte.17–25 Alternatively, the capture and transport of CO2 can occur in a static environment (e.g., a polymer electrolyte membrane electrochemical cell) in which the CO2 is delivered as a gas to the feed side and evolved from the permeate side,15,16,26–28 as shown in Fig. 1b. Depending on the configuration of gases, the cell operates either in a fuel cell mode, which generates electricity, or a driven mode, which consumes electricity. A summary of notable experimental studies from the past 20 years is presented in Table I. The key performance indicators, faradaic efficiency and molar specific energy consumption, are given for the selected current density value, although they are not necessarily the only values reported for a given study. Faradic efficiency describes the efficiency with which electrons release CO2 molecules, and is typically defined as the total number of CO2 molecules transferred over the total number of electrons transferred. In selecting values for the table, higher values of current density were favored, as current density is a critical factor in system scalability. Increasing current density tends to have a negative effect on specific energy consumption and a mixed effect on faradaic efficiency. Table I shows that a wide range of current densities have −2 2 2 been reported, varying between 0.5–139 mA cm , and that the corresponding performance indicators have varied widely as well. As the energy consumption calculation generally only includes the electrical input to the cell or stack, it can be assumed that the values given are conservative for a scaled up system. In future electro- chemical CO2 separation studies, efforts should be made to align the reporting of performance metrics, with both faradaic efficiency and specific energy consumption clearly stated or shown at a relevant testing conditions.PDF Image | CO2 Separation and Transport via Electrochemical Methods
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