Article Cite This: Acc. Chem. Res. 2018, 51, 910−918
pubs.acs.org/accounts
Electrolytic CO2 Reduction in a Flow Cell David M. Weekes,† Danielle A. Salvatore,‡ Angelica Reyes,‡ Aoxue Huang,† and Curtis P. Berlinguette*,†,‡,§ †
Department of Chemistry, The University of British Columbia, 2036 Main Mall, Vancouver, British Columbia V6T 1Z3, Canada Department of Chemical & Biological Engineering, The University of British Columbia, 2360 East Mall, Vancouver, British Columbia V6H 1Z3, Canada § Stewart Blusson Quantum Matter Institute, The University of British Columbia, 2355 East Mall, Vancouver, British Columbia V6T 1Z4, Canada ‡
S Supporting Information *
CONSPECTUS: Electrocatalytic CO2 conversion at near ambient temperatures and pressures offers a potential means of converting waste greenhouse gases into fuels or commodity chemicals (e.g., CO, formic acid, methanol, ethylene, alkanes, and alcohols). This process is particularly compelling when driven by excess renewable electricity because the consequent production of solar fuels would lead to a closing of the carbon cycle. However, such a technology is not currently commercially available. While CO2 electrolysis in H-cells is widely used for screening electrocatalysts, these experiments generally do not effectively report on how CO2 electrocatalysts behave in flow reactors that are more relevant to a scalable CO2 electrolyzer system. Flow reactors also offer more control over reagent delivery, which includes enabling the use of a gaseous CO2 feed to the cathode of the cell. This setup provides a platform for generating much higher current densities (J) by reducing the mass transport issues inherent to the H-cells. In this Account, we examine some of the systems-level strategies that have been applied in an effort to tailor flow reactor components to improve electrocatalytic reduction. Flow reactors that have been utilized in CO2 electrolysis schemes can be categorized into two primary architectures: Membrane-based flow cells and microfluidic reactors. Each invoke different dynamic mechanisms for the delivery of gaseous CO2 to electrocatalytic sites, and both have been demonstrated to achieve high current densities (J > 200 mA cm−2) for CO2 reduction. One strategy common to both reactor architectures for improving J is the delivery of CO2 to the cathode in the gas phase rather than dissolved in a liquid electrolyte. This physical facet also presents a number of challenges that go beyond the nature of the electrocatalyst, and we scrutinize how the judicious selection and modification of certain components in microfluidic and/or membrane-based reactors can have a profound effect on electrocatalytic performance. In membrane-based flow cells, for example, the choice of either a cation exchange membrane (CEM), anion exchange membrane (AEM), or a bipolar membrane (BPM) affects the kinetics of ion transport pathways and the range of applicable electrolyte conditions. In microfluidic cells, extensive studies have been performed upon the properties of porous carbon gas diffusion layers, materials that are equally relevant to membrane reactors. A theme that is pervasive throughout our analyses is the challenges associated with precise and controlled water management in gas phase CO2 electrolyzers, and we highlight studies that demonstrate the importance of maintaining adequate flow cell hydration to achieve sustained electrolysis.
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INTRODUCTION Scalable energy storage technologies are needed in order to support the low renewable electricity prices that exist today.1 The ability to use renewable electricity to drive the formation of liquid fuels is a particularly appealing storage mechanism that addresses the geographic and seasonal mismatch of electricity supply and demand.2 These factors have motivated the design of electrolyzers capable of converting waste CO2 into useful fuel, chemical and polymer precursors or products (e.g., CO, © 2018 American Chemical Society
methanol, ethylene, propanol, and others; Figure 1) that would not rely on newly extracted fossil fuels.3,4 The electrolytic production of chemical fuels as a means of storing renewable electricity has long been championed by lowtemperature water electrolyzers in “power-to-gas” schemes.5 These highly-optimized systems are capable of operating with Received: January 5, 2018 Published: March 23, 2018 910
DOI: 10.1021/acs.accounts.8b00010 Acc. Chem. Res. 2018, 51, 910−918
Article
Accounts of Chemical Research
Figure 1. Overview of the approximate global market sizes (in United States dollars (USD)) and prices (in USD kg−1) for chemicals directly accessible via CO2 electrochemical reduction. (See the Supporting Information for details.)
(e.g., compression, storage, safety) are also likely to apply to electrolytically generated CO. Given the present lack of a commercial-scale system and the complexities of the supply chains, there is no one model that can define which product(s) should be targeted with absolute certainty. We therefore remain agnostic to the type of electrolytic reduction product, but the time is now to start developing the science and engineering technologies capable of achieving high product formation and stability for any type of CO2 electrolyzer, i.e., a “power-to-X” electrolyzer. The tendency for the CO2 reduction reaction (CO2RR) to selectively form a single conversion product(i.e., high FE) and at a low overpotential is affected by the cathodic electrocatalyst. While a rapidly growing body of research explores composition−structure−activity relationships for both metallic and nonmetallic catalysts toward CO2RR, the vast majority of these studies (including our own14,17,18) utilize a three-compartment “H-cell” geometry to quantify electrocatalyst CO2RR activity. However, the information from these experiments is not necessarily relevant to the dynamic environment of an electrolyzer.19−21 Moreover, mass transport in an H-cell limits testing to current densities of 200 mA cm−2) and long-term operation (>8000 h or 1 year), with high selectivity and low overpotential (i.e., high overall energy efficiency) figuring prominently in the analysis.12 The sizable global markets for methanol, methane, ethylene and ethanol are appealing, but it typically costs 6e− and 6H+) that inherently require large applied voltages with currently known catalysts.13−15 In contrast, formic acid formation, which invokes a 2e−/2H+ PCET reaction, can be produced with high Faradaic efficiencies (FEs), but the size of the market is likely too small to make a meaningful contribution to global CO2 emissions. A seemingly reasonable case can be made for producing CO from CO2:16 There is a sizable global demand for syngas (and downstream products thereof) along with a relatively higher market price in the 1−1.5 USD kg−1 range. Moreover, high FE values (up to 99%) for CO2 electroreduction are known. Notwithstanding, the drawbacks associated with the electrolytic formation of gaseous hydrogen 911
DOI: 10.1021/acs.accounts.8b00010 Acc. Chem. Res. 2018, 51, 910−918
Article
Accounts of Chemical Research
particularly on flow cells where CO2 is introduced in the gas phase rather than dissolved in aqueous media. We outline the opportunities and challenges of these promising “gas-phase flow cells” at this very early stage of development, highlighting the interfacial chemistry and components (membrane and gas diffusion layer) that need to be optimized to reach state-of-theart CO2 conversion performance metrics.
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FLOW-CELL ARCHITECTURES The most widely studied class of CO2 flow cells are membranecontaining reactors (Figure 3A). These systems are based on well-established low-temperature water electrolysis or fuel cell systems, and provide an effective translation from H-cell experiments involving liquid electrolytes. In a typical flow cell setup, CO2 is delivered to the cathode dissolved in mildly-basic solution (e.g., aqueous bicarbonate)23 or directly in the gas phase.20,24 The OER is driven at the anode and also affects the design of an efficient CO2 reactor.25,26 Cathodic and anodic chemistry is separated by a polymer electrolyte membrane (PEM) that facilitates the flow of ions while attenuating product crossover. Highly-porous gas diffusion layers (GDLs) situated between the electrode and the membrane support the catalyst and are used to promote prolonged contact between CO2 molecules and the electrocatalyst during electrolyte flow, serving to increase the upper limit of reaction rates (i.e.,
Figure 2. Number of papers published since 2007 on selected metalbased electrocatalysts for CO2 reduction in H-cell experiments (blue) and in continuous flow reactors (red). (Data obtained from Web of Science, collected December 29, 2017.)
electrocatalyst interface, compounded effects of a polymer electrolyte membrane in direct contact with the catalyst surface, and the available scope of suitable electrocatalyst subrates. This document will summarize the different types of flow cell reactors known today and their components therein, which serves to outline general strategies for maximizing CO2RR product formation and current density. We will focus
Figure 3. Exploded (left) and cross-sectional (right) diagrams of two common electrochemical reactors tested for CO2 electroreduction in flow. (A) Membrane-based reactor containing a membrane electrode assembly (MEA) consisting of the anode and cathode gas diffusion electrodes (GDEs) on either side of a polymer electrolyte membrane (PEM). The MEA is assembled in between the anode and cathode current collectors and flow field plates and separates the respective OER and CO2 reduction reactions. (B) Microfluidic reactor consisting a liquid electrolyte flow channel between the anode and cathode GDE materials. CO2(g) is supplied to the cathode side of the cell where it diffuses to the electrocatalyst supported on the cathode GDL. OER occurs at the anode and is vented to air. 912
DOI: 10.1021/acs.accounts.8b00010 Acc. Chem. Res. 2018, 51, 910−918
Article
Accounts of Chemical Research
Figure 4. Cartoon illustrating the relative saturated concentrations and diffusion coefficients (C and D, respectively) for (A) water, (B) CO2 in aqueous solution,37 and (C) humidified gaseous CO2.38 Values at room temperature and pressure are indicated.
continuously flowing catholyte solution over high surface area electrocatalyst materials (i.e., GDEs), but the concentration of CO2 in aqueous solution remains fundamentally restricted under ambient conditions. Alternatively, the delivery of gaseous CO2 to the cathode presents a means of further overcoming the mass transport limitations by simply increasing the concentration of available reactant molecules (Figure 4C), presenting the opportunity to drive up J values without elevated temperatures and pressures. The earliest examples of electrolytic reactions performed directly upon ambient temperature/pressure CO2 gas date back to the late 1970s with the invention of gas fixation solar cells for small molecule photoreduction,39 with the emergence of nonphoto-driven systems occurring approximately a decade later. These experiments typically applied gastight H-cell-type architectures, and in 1993 Sammells and co-workers reported a perovskite-catalyzed gas-phase CO2 reduction to alcohols with a cumulative FE ∼ 40% at J = 180 mA cm−2.40 Other examples of early gas-phase reactors include zero-gap solid polymer electrolyte (SPE) cells in which the electrocatalyst is deposited directly upon a PEM.41−43 In 2003, for example, Hori and coworkers reported a cell containing a silver-coated ion exchange membrane (Ag-SPE) capable of producing CO with >90% FE and J ∼ 60 mA cm−2 (i.e., roughly double the predicted upper limit in aqueous systems) at −3.0 V with gas-phase CO2.44 The performance ceiling in that case could be attributed to the absence of a GDL in the H-cell type setup, which precludes adequate CO2(g) supply to the cathode in a nonpressurized system. Certain groups have continued to investigate the use of SPE materials in gas-phase CO2 electrolysis flow systems, and indeed these more recent examples tend to include a GDL as a means of improving performance.45,46 These architectures therefore closely resemble present-day PEM-based CO2 flow reactors, and contain the same components surveyed in the remainder of this document. A recent collaborative study by Mallouk and our group sought to substantiate the advantages of working with gas-phase CO2 in flow reactors by benchmarking a PEM flow cell fed with a humidified gas phase CO2 cathode stream against a liquid electrolyte.20 Cyclic voltammograms (CVs) indicate J values approximately 2-fold higher for the gas-fed flow cell than the liquid-fed flow cell at −3.0 V (Figure 5). These results inspired the design of a gas-phase CO2-to-CO flow cell containing a GDL-supported silver catalyst, a bipolar membrane and a solidsupported water layer in which we could achieve J > 200 mA cm−2 with FE ∼ 50%, and prolonged (>24 h) stability at 100 mA cm−2 with stable FE for CO at 65%.20 There are currently very few reports that directly apply the strategy of using CO2(g) as a means of driving up J,20,47−49 a fact that speaks to the
increasing J) relative to H-cell systems. Each of these electrocatalyst, GDL, and PEM components affect CO2RR efficiency and selectivity. An alternative configuration based on a microfluidic cell was pioneered by Kenis in 2010 (Figure 3B).27,28 This architecture relies on a very thin (
