Review—Electrochemical CO2 Reduction for CO Production: Comparison of Low- and High-Temperature Electrolysis Technologies

Küngas, Rainer

doi:10.1149/1945-7111/ab7099

Cited by 292 publications

(266 citation statements)

References 56 publications

(166 reference statements)

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“…For the production of CO, the best reported performance is for lab-scale (5 cm 2 ) devices that have been operated at steady state for >4000 h at~200 mA cm -2 CO 2 -to-CO current density, nicely demonstrating the viability of durable CO electrosynthesis at a reasonable rate. However, the carbon efficiency is 50% and the cell voltage is 3.0 V, corresponding to an energy efficiency of only 43% 1,19 . Both of these values are consistent with the analysis in Fig.…”

Section: Current State-of-the-art and Outlookmentioning

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: Current State-of-the-art and Outlookmentioning

confidence: 99%

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

2020

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“…Solid-oxide electrolysers are important but outside the remit of this review, since they follow different operational conditions and principles. Readers interested in CO 2 solid-oxide electrolysers are directed to these reviews [114,115].…”

Section: Flow Cell Reactorsmentioning

confidence: 99%

“…Electrolyser and electrode design have a great influence on CO 2 R since productivity, selectivity, energy efficiency, and stability are not only dependent on the electrocatalyst. This is especially true for the industrialisation of CO 2 electrolysis, where current densities higher than 250 mA/cm 2 and cell voltages below 3 V are required [21,102,115]. In this review, the fundamentals of aqueous-fed and gas-fed CO 2 electrolytic cells have been presented and discussed.…”

Section: Conclusion and Future Perspectivesmentioning

confidence: 99%

Fundamentals of Gas Diffusion Electrodes and Electrolysers for Carbon Dioxide Utilisation: Challenges and Opportunities

2020

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Electrocatalysis plays a prominent role in the development of carbon dioxide utilisation technologies. Many new and improved CO2 conversion catalysts have been developed in recent years, progressively achieving better performance. However, within this flourishing field, a disconnect in catalyst performance evaluation has emerged as the Achilles heel of CO2 electrolysis. Too often, catalysts are assessed in electrochemical settings that are far removed from industrially relevant operational conditions, where CO2 mass transport limitations should be minimised. To overcome this issue, gas diffusion electrodes and gas-fed electrolysers need to be developed and applied, presenting new challenges and opportunities to the CO2 electrolysis community. In this review, we introduce the reader to the fundamentals of gas diffusion electrodes and gas-fed electrolysers, highlighting their advantages and disadvantages. We discuss in detail the design of gas diffusion electrodes and their operation within gas-fed electrolysers in both flow-through and flow-by configurations. Then, we correlate the structure and composition of gas diffusion electrodes to the operational performance of electrolysers, indicating options and prospects for improvement. Overall, this study will equip the reader with the fundamental understanding required to enhance and optimise CO2 catalysis beyond the laboratory scale.

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“…14,15 Today, CO2 electrolyzer to produce CO is commercially available at full-scale (from Haldor Topsoe 16 based on high temperature solid-oxide electrolysis cell (SOEC) technology) with a technology readiness level (TRL) of 8. 17 On the other hand, low temperature electrochemical CO2 conversion to multi-carbon products is relatively immature (TRL of 2-4) and often limited in labscale testing. 13,17 Interestingly, the electrochemical route offers a number of potential opportunities, including modular scaling for small to large-scale applications, lower capital investment, load balancing for electric grid, storage of intermittent renewable electricity (e.g., wind or solar energy) into energy-dense hydrocarbons that can be transported and traded globally etc.…”

Section: Introductionmentioning

confidence: 99%

“…17 On the other hand, low temperature electrochemical CO2 conversion to multi-carbon products is relatively immature (TRL of 2-4) and often limited in labscale testing. 13,17 Interestingly, the electrochemical route offers a number of potential opportunities, including modular scaling for small to large-scale applications, lower capital investment, load balancing for electric grid, storage of intermittent renewable electricity (e.g., wind or solar energy) into energy-dense hydrocarbons that can be transported and traded globally etc. 13 With continuous reduction in renewable electricity price, electrochemical CO2 conversion technologies could become economically competitive with thermochemical and conventional fossil-based synthesis pathways 12,18 .…”

Section: Introductionmentioning

confidence: 99%

Comparative Life Cycle Assessment of Electrochemical Upgrading of CO2 to Fuels and Feedstocks

Nabil¹,

McCoy²,

Kibria

2020

Preprint

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Development of electrochemical pathways to convert CO2 into fuels and feedstock is rapidly progressing over the past decade. Here we present a comparative cradle-to-gate life cycle assessment (LCA) of one and two-step electrochemical conversion of CO2 to eight major value-added products; wherein we consider CO2 capture, conversion and product separation in our process model. We measure the carbon intensity (i.e., global warming impact) of one and two-step electrochemical routes with its counterparts – thermochemical CO2 utilization and fossil-fuel based conventional synthesis routes for those same products. Despite inevitable carbonate formation in one-step CO2 electrolysis, this analysis reveals one-step electrosynthesis would be equally compelling (through the lens of climate benefits) as compared to two-step route. This analysis further reveals that the carbon intensity of electrosynthesis products is due to significant energy requirement for the conversion (70-80% for gas products) and product separation (40-85% for liquid products) phases. Electrochemical route is highly sensitive to the electricity emission factor and is compelling only when coupled with electricity with low emission intensity (<0.25 kg CO2e/kWh). As the technology advances, we identify the near-term products that would provide climate benefits over fossil-based routes, including syngas, ethylene and n-propanol. We further identify technological goals required for electrochemical route to be competitive, notably achieving liquid product concentration >20 wt%. It is our hope that this analysis will guide the CO2 electrosynthesis community to target achieving these technological goals, such that when coupled with low-carbon electricity, electrochemical route would bring climate benefits in near future.

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Review—Electrochemical CO₂ Reduction for CO Production: Comparison of Low- and High-Temperature Electrolysis Technologies

Cited by 292 publications

References 56 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

Fundamentals of Gas Diffusion Electrodes and Electrolysers for Carbon Dioxide Utilisation: Challenges and Opportunities

Comparative Life Cycle Assessment of Electrochemical Upgrading of CO2 to Fuels and Feedstocks

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