Role of ion-selective membranes in the carbon balance for CO<sub>2</sub> electroreduction <i>via</i> gas diffusion electrode reactor designs

Ma, Ming; Kim, Sang-Kuk; Chorkendorff, Ib; Seger, Brian

doi:10.1039/d0sc03047c

Cited by 118 publications

(126 citation statements)

References 47 publications

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“…1d). While this design avoids releasing CO 2 at the anode 17 , it has a lower energy efficiency than an AEM cell because the BPM imposes an additional overpotential in order to drive water dissociation (Fig. 1a).…”

Section: Consequences Of Operating In Carbonated Electrolytementioning

confidence: 99%

The future of low-temperature carbon dioxide electrolysis depends on solving one basic problem

2020

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Carbonate formation is the primary source of energy and carbon losses in lowtemperature carbon dioxide electrolysis. Realigning research priorities to address the carbonate problem is essential if this technology is to become a viable option for renewable chemical and fuel production. The plummeting cost and daily curtailment of renewable electricity have spurred growing interest in using CO 2 electrolysis to produce chemicals and fuels. High-temperature solid oxide cells that convert CO 2 to CO and O 2 have reached nascent commercialization. Low-temperature CO 2 electrolysis is an attractive alternative that offers more convenient and flexible operation and the ability to generate multicarbon products such as ethylene, ethanol, and propanol. Over the past 10 years, a dramatic expansion of research in this area has yielded substantial progress in fundamental understanding and prototype devices. Leveraging insights from fuel cells and membrane water electrolyzers, researchers have developed gas diffusion electrode (GDE) cells demonstrating synthetically relevant CO 2 electrolysis current densities (>100 mA cm −2) and promising stability. Despite these advances, the energy efficiency (power-to-product) and carbon efficiency (CO 2-toproduct) of low-temperature CO 2 electrolysis remain too low to support large-scale applications 1,2. While much current research is focused on CO 2 reduction catalyst design, the biggest obstacle to improving performance is an often overlooked basic chemistry problem: the rapid and thermodynamically favorable reaction of CO 2 with hydroxide (OH-) to form carbonate (CO 3 2-) imposes steady state electrolysis conditions that result in large voltage and CO 2 losses. Although recent work has brought attention to some aspects of the CO 3 2problem 2-4 , it is far more pernicious than what is widely appreciated. Here we explain how CO 3 2formation compromises efficiency to highlight the need for new research directions that address this problem. Hydroxide consumption makes alkaline CO 2 electrolyzers fuel-wasting devices The state-of-the-art for low-temperature CO 2 electrolysis has been obscured by studies that utilize a reservoir of flowing alkaline electrolyte to maintain a high pH in the cell 5-8. High pH minimizes the cell voltage, which makes these systems appear to have high energy efficiency, but consumption of OHin the reservoir by CO 3 2formation results in a net negative energy balance. Understanding why the cell voltage is minimized at high pH helps to clarify the CO 3 2problem (Fig. 1a). For most known catalyst materials, including Au and Cu, the CO 2 reduction rate depends on the electron transfer driving force but not explicitly on pH 9-13. As a result, synthetically relevant current densities require rather negative potentials versus an absolute reference such as the standard hydrogen electrode (SHE). Even with high surface area electrodes, CO 2 reduction at a geometric current density of >200 mA cm-2 has generally required potentials

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Section: Consequences Of Operating In Carbonated Electrolytementioning

confidence: 99%

The future of low-temperature carbon dioxide electrolysis depends on solving one basic problem

2020

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“…BPMs also present an additional benefit in terms of carbon utilisation. Although carbonate crossover is not expected to be significant with either membrane, [19] with the Nafion system, the generated carbonate stays in solution whereas with the BPM, carbonate is converted to CO 2 gas in the vicinity of the CEL, which can then be re‐circulated [63] . With such a system, overall CO 2 utilisation would be enhanced, avoiding significant costs associated with electrolyte regeneration [14] …”

Section: Discussionmentioning

confidence: 99%

“…[16][17][18] Under these specific conditions, minimisation of cell potentials requires optimisation of anode conditions as well as membrane choice. [19] The OER suffers from extremely sluggish kinetics, therefore finding the optimal operating conditions is key to the minimisation of E cell . Electrolyte properties (such as pH, buffer capacity, nature of the anion and cation) must be tuned to ensure low contribution to the cell potential and high stability.…”

Section: Introductionmentioning

confidence: 99%

“…As a consequence, cathodes are preferably operated in neutral pH using buffers such as phosphate which can limit bulk pH changes – this enables protonation of (bi)carbonate species to form CO 2 gas that can then be re‐circulated to limit regeneration costs [16–18] . Under these specific conditions, minimisation of cell potentials requires optimisation of anode conditions as well as membrane choice [19] …”

Section: Introductionmentioning

confidence: 99%

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Advancing the Anode Compartment for Energy Efficient CO₂ Reduction at Neutral pH

et al. 2021

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Energy efficient CO 2 reduction in neutral pH requires the development of electrochemical devices operating with minimal energy input. To reach this objective, we propose an original approach focused on the anode compartment where the oxygen evolution reaction (OER) takes place. Different anode catalysts, anolytes and membranes were tested, and the components resulting in the most energy efficient and stable CO 2 reduction reaction device were selected. A stainless-steelbased anode with significant OER activity and stability in a wide pH range was used to compare symmetric (neutral catholyte and anolyte) and asymmetric (neutral catholyte and alkaline anolyte) configurations. The gain in energy efficiency for carbon products of 6-8 % in the asymmetric configuration highlighted the advantages of a pH gradient. Although a Nafion membrane gave the highest energy efficiencies, the pH was unstable. Therefore, bipolar membranes that can maintain a pH gradient emerge as the best option for long-term electrolysis. This work paves the way for the development of highly energy efficient CO 2 -reduction devices with cathodes operating in low pH conditions.

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“…In CO 2 electrolyzers constructed around a BPM, on the other hand, parasitic CO 2 transport can be completely eliminated, since the current is supported by the water dissociation and ionic separation, as shown experimentally in a liquid–liquid environment. 1 The use of a (bicarbonate) buffer in the catholyte improves the CO 2 reduction efficiency by suppressing the HER, but increases the thickness of the reactor. Recent work shows that, by reducing the acidity of the CEL, the buffer can be omitted as the HER is suppressed (up to 40% increase in efficiency).…”

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confidence: 99%

Insights and Challenges for Applying Bipolar Membranes in Advanced Electrochemical Energy Systems

et al. 2021

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Bipolar membranes (BPMs) are gaining interest in energy conversion technologies. These membranes are composed of cation- and anion-exchange layers, with an interfacial layer in between. This gives the freedom to operate in different conditions (pH, concentration, composition) at both sides. Such membranes are used in two operational modes, forward and reverse bias. BPMs have been implemented in various electrochemical applications, like water and CO 2 electrolyzers, fuel cells, and flow batteries, while BPMs are historically designed for acid/base production. Therefore, current commercial BPMs are not optimized, as the conditions change per application. Although the ideal BPM has highly conductive layers, high water dissociation kinetics, long lifetime, and low ion crossover, each application has its own priorities to be competitive in its field. We describe the challenges and requirements for future BPMs, and identify existing developments that can be leveraged to develop BPMs toward the scale of practical applications.

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Role of ion-selective membranes in the carbon balance for CO₂ electroreduction via gas diffusion electrode reactor designs

Abstract: In this work, the effect of ion-selective membranes on the detailed carbon balance was systematically analyzed for high-rate CO2 reduction in GDE-type flow electrolyzers. By using different ion-selective membranes, we...

Cited by 118 publications

References 47 publications

The future of low-temperature carbon dioxide electrolysis depends on solving one basic problem

The future of low-temperature carbon dioxide electrolysis depends on solving one basic problem

Advancing the Anode Compartment for Energy Efficient CO₂ Reduction at Neutral pH

Insights and Challenges for Applying Bipolar Membranes in Advanced Electrochemical Energy Systems

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Role of ion-selective membranes in the carbon balance for CO2 electroreduction via gas diffusion electrode reactor designs

Abstract: In this work, the effect of ion-selective membranes on the detailed carbon balance was systematically analyzed for high-rate CO2 reduction in GDE-type flow electrolyzers. By using different ion-selective membranes, we...

Cited by 118 publications

References 47 publications

The future of low-temperature carbon dioxide electrolysis depends on solving one basic problem

The future of low-temperature carbon dioxide electrolysis depends on solving one basic problem

Advancing the Anode Compartment for Energy Efficient CO2 Reduction at Neutral pH

Insights and Challenges for Applying Bipolar Membranes in Advanced Electrochemical Energy Systems

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Resources

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Role of ion-selective membranes in the carbon balance for CO₂ electroreduction via gas diffusion electrode reactor designs

Advancing the Anode Compartment for Energy Efficient CO₂ Reduction at Neutral pH