Use Of Gas Diffusion Electrodes Research Articles

Investigation of Biofilm Formation on Air Cathodes with Quaternary Ammonium Compounds in Microbial Fuel Cells

The use of gas diffusion electrodes (GDEs) in microbial fuel cells (MFCs) can improve their cell performance, but tends to cause fouling. In order to allow long-term stable operation, the search for antifouling methods is necessary. Therefore, an antibacterial coating with ammonium compounds is investigated. Within the first 30 days of operation, the maximum measured power density of a GDE with antibacterial ionomer was 606 mW m−2. The GDE without an antifouling treatment could only reach a maximum of 284 mW m−2. Furthermore, there was an optimum in the loading amount with ionomer below 2.6 mg cm−2. Further investigations showed that additional aeration of the GDEs by a fan had a negative effect on their performance. Despite the higher performance, the antibacterial coating could not prevent biofilm growth at the surface of the GDE. The thickness of the biofilm was only reduced by 14–16%. However, the weight of the biofilm on the treated GDEs was 62–80% less than on a GDE without an antifouling treatment. Consequently, the coating cannot completely prevent fouling, but possibly leads to a lower density of the biofilm or prevents clogging of the pores inside the electrodes and improves their long-term stability.

Local reaction microenvironment impacts on H2O2 electrosynthesis in a dual membrane electrode assembly solid electrolyte electrolyzer

Hydrogen peroxide (H2O2) synthesis via the electrocatalytic reduction of oxygen is a sustainable alternative to the energy-intensive anthraquinone oxidation process. The use of gas diffusion electrodes in dual membrane electrode assembly (MEA) solid electrolyte (SE) electrolyzers has substantially improved H2O2 production, but the influence of mass transport and local reaction environment on H2O2 performance in these cell architectures is still unclear and unoptimized. Herein, we investigate the impacts of electrode components and reactor operating conditions on the H2O2 performance and cell potential required to reach current densities up to 400 mA cm−2. Results show an intermediate catalyst loading of 2 mg cm−2 improves H2O2 production through balancing O2 diffusion and active site exposure. Hydrophobic treatment via fluoropolymer improves electrode stability, but addition of >15 wt% fluoropolymer worsens performance, likely by limiting active site accessibility at the catalyst-membrane interface. Moreover, decreasing O2 concentration from typical pure streams to match the composition of air has a negligible effect at moderate current densities (∼50 mA cm−2), but significantly impacts overall performance at higher current densities (∼200 mA cm−2). This work also highlights benefits of operating the reactor with a recycled product stream rather than tuning the flow rate of water over the SE to extremely low values to obtain high H2O2 concentrations, as the latter likely contributes to exacerbated H2O2 degradation in the electrolyzer. This work provides insights into how macroscale system properties in flow cell electrolyzers impact the local reaction environment and mass transport, which in turn dictate overall catalytic performance.

Spatio-Temporal Electrowetting and Reaction Monitoring in Microfluidic Gas Diffusion Electrode Elucidates Mass Transport Limitations.

The use of gas diffusion electrodes (GDEs) enables efficient electrochemical CO2 reduction and may be a viable technology in CO2 utilization after carbon capture. Understanding the spatio-temporal phenomena at the triple-phase boundary formed inside GDEs remains a challenge; yet it is critical to design and optimize industrial electrodes for gas-fed electrolyzers. Thus far, transport and reaction phenomena are not yet fully understood at the microscale, among other factors, due to a lack of experimental analysis methods for porous electrodes under operating conditions. In this work, a realistic microfluidic GDE surrogate is presented. Combined with fluorescence lifetime imaging microscopy (FLIM), the methodology allows monitoring of wetting and local pH, representing the dynamic (in)stability of the triple phase boundary in operando. Upon charging the electrode, immediate wetting leads to an initial flooding of the catalyst layer, followed by spatially oscillating pH changes. The micromodel presented gives an experimental insight into transport phenomena within porous electrodes, which is so far difficult to achieve. The methodology and proof of the spatio-temporal pH and wetting oscillations open new opportunities to further comprehend the relationship between gas diffusion electrode properties and electrical currents originating at a given surfacepotential.

An integrated carbonized wood-based gas-diffusion electrode for high-current-density CO electrosynthesis in flow cells

An integrated carbonized wood-based gas-diffusion electrode embedded with reconstructed Ag nanoparticles is developed, exhibiting activity and selectivity towards the CO2RR.

Ultra-High-Rate CO2 Reduction Reactions to Multicarbon Products with a Current Density of 1.7 a cm-2 in Neutral Electrolyte

The electrochemical carbon dioxide reduction reaction (CO2RR) has attracted considerable attention as a promising strategy for the conversion of anthropogenic CO2 into value-added products.1 An advantage of these methods is that this greenhouse gas can be reduced under mild conditions (close to ambient temperature and pressure). However, the operative efficiency, product selectivity and rate must be improved for practical implementation. Among them, it has been reported that the current density for the production of high value-added products is correlated with the capital cost of the electrolyzers employed. Therefore, improving the current density would significantly affect the feasibility of this technology. The use of gas diffusion electrodes (GDEs), which allow the CO2RR to occur at the three-phase interfaces (solid-catalyst/liquid-electrolyte/gaseous-CO2), effectively accelerates the CO2RR by solving the problem of the mass transport limitation due to the inherently low diffusion of CO2 in water. A catalyst layer (CL) composed of metal Cu particles and ionomers on carbon-based GDEs has been widely studied as the standard cathode for gaseous CO2RR to produce C2+ organics, such as C2H4, C2H5OH, n-C3H7OH and CH3COOH (Figure (a)). Recently, a few studies have achieved an ultra-high-rate (UHR-) CO2RR with the partial current density for C2+ organics (j C2+) of over 1 A cm-2 using special catalysts or electrodes in high alkaline solution.2, 3, 4 However, in addition to the toxicity and corrosivity, alkaline solutions quickly absorb CO2 and convert it to electrochemically inert (bi)carbonate. Based on these drawbacks, UHR-CO2RR in neutral electrolytes is thus required from a practical viewpoint. Furthermore, the obtainable maximum j C2+ value is still unknown when optimizing the standard components; CuO nanoparticle catalysts (CuONPs), which are known to serve as oxide-derived Cu catalysts for effective C2+ production, and conventional carbon-based GDEs.In this work, we attempt to pursue a high partial current density for C2+ (j C2+) using neutral electrolytes by optimizing the standard components and properly assembling them as the cathode. In particular, we achieved a record j C2+ of 1.7 A cm-2 in neutral electrolytes using CuONPs and carbon-based GDEs (Figure (b)).5 Moreover, j C2+ reached 1.8 A cm-2 when we used 1 M KOH as catholytes, which is also the record j C2+ value in alkaline electrolytes. When the total current density (J total) was increased, the Faradaic efficiencies for C2+ organics increased monotonically below J total = 2000 mA cm-2. Therefore, the trade-on relationship between the reaction rate and selectivity was shown in the UHR-CO2RR under those J total regions. Although the cathode of this electrochemical system was composed of the conventional materials, we successfully maximized the potential of the catalytic activity by constructing a CL with optimal porosity and thickness. Especially, we revealed that the thickness of CL was directly correlated to j C2+. With a thinner CL (the amount of catalyst loadings is 0.34 mg cm-2), the maximum j C2+ didn’t reach 1 A cm-2. This is because the constructed three-phase interface was too thin. With a thicker CL (the amount of catalyst loadings is 3.1 mg cm-2), the faradaic efficiency for H2 production was higher than optimized CL (the amount of catalyst loadings is 1.7 mg cm-2). This is because the Cu sites fully immersed in the electrolyte cannot serve as CO2RR site but only for H2 evolution site. Therefore, the optimal thickness of CL was required for UHR-CO2RR. The present system is expected to become one of the standards when pursuing UHR-CO2RR because only conventional materials were used in this work.

(Invited) High-Rate CO2 Reduction Reactions: From Electrocatalysts to Gas-Diffusion Electrodes

Excessive emissions of carbon dioxide (CO2) from the use of fossil fuels are becoming a serious obstacle to the sustainable development of society. Electrochemical CO2 reduction (CO2RR) into value-added products using solar electricity is a promising technology to close the carbon cycle and sequester anthropogenic CO2 into chemical feedstocks.[1] The practical implementation of CO2RR requires a high current density, as the current density for CO2RR is directly correlated to the capital cost of the electrodes and electrochemical cells. The use of gas diffusion electrodes (GDEs) effectively accelerates CO2RR by overcoming the mass transport limitation due to the inherently low diffusion and solubility of CO2 in water. This presentation summarises our recent studies on high-rate CO2RR from the point of view of both novel electrocatalyst designs and appropriate electrode assembly. Single-atom electrocatalysts (SAECs), consisting of single isolated metal sites dispersed on heterogeneous supports, are one of the promising electrocatalysts for high-rate gaseous CO2RR. We have employed various single metal-doped covalent triazine frameworks (M-CTFs) as a platform for CO2RR electrocatalysts on GDEs and systematically investigated them to derive sophisticated design principles using a combined computational and experimental approach.[2-4] The Ni-CTF exhibited both high selectivity and high reaction rate for CO production. In contrast, the Sn-CTF exhibited selective formic acid production, and the faradaic efficiencies (FEs) and partial current density reached 85% and 150 mA/cm2, respectively.[2] These results were in close agreement with the intermediate CO2RR adsorption strength obtained by DFT calculations. In addition to the synthesis of efficient electrocatalysts, the triple phase boundary (TPB) at the GDE, where the catalyst material, electrolyte, and gas pores intersect, needs to be enlarged for high-rate gaseous CO2RR.[5,6] We successfully increased the partial current density for multicarbon products (C2+) over cupric oxide (CuO) nanoparticles on gas diffusion electrodes in neutral electrolytes to a record value of 1.7 A/cm2 by maximizing the area of the CO2RR active interface.[5] In particular, we demonstrated that the thickness of catalyst layers was one highly sensitive factor in determining the maximum current density for C2+. Although the GDE and electrocatalyst used in this case are not unique, the optimized assembly elicits their undermined potential.[1] K. Kamiya, Nakanishi* et al. Chem. Lett. 2021, 50, 166-179. [2] S. Kato, K. Kamiya* et al. Chem. Sci., 2023, 14, 613–620. [3] P. Su, K. Kamiya*et al. , Chem. Sci., 2018, 9, 3941-3947. [4] K. Kamiya* Chem. Sci. 2020, 11, 8339–8349. [5] A. Inoue, K. Kamiya* et al. EES Catal. 2023, 1, 9-16. [6] T. Liu*, K. Kamiya* et al. Small 2022, 18, 2205323.

How the Choice of Electrolyte Affects the Carbon Efficiency in Electrochemical CO2 Reduction at Silver Gas Diffusion Electrodes

Reducing CO2 emissions by replacing fossil fuels with renewable energy sources is an important step in the context of climate change. Electrochemical CO2 reduction is a promising technology for supporting this shift, since it provides the possibility to store electrical energy in form of high value chemicals while consuming CO2. It is now generally accepted that the use of gas diffusion electrodes (GDEs) is beneficial due to the limited solubility of CO2 in water [1].To achieve a better understanding of the complex processes taking place in the electrode, the description by a mathematical model is desirable. We present a TFFA (thin-film flooded agglomerate) model for silver GDEs based on the approach presented by Franzen et al. [2] for the oxygen reduction reaction. The model describes the processes in the GDE, including reaction, diffusion, and migration. Therefore, the model provides an in-depth view on the local reaction environment in the electrode, which becomes highly alkaline at elevated current densities due to the formation of hydroxide ions by the electrochemical reactions [3]. This causes the rate of bicarbonate/carbonate formation to exceed the rate of electrochemical CO2 reduction, resulting in a limited carbon efficiency [4]. Using acidic electrolytes might significantly reduce this limitation by decreasing the local alkalinity. This has already been experimentally demonstrated for gold electrodes using electrolytes with pH values in the range of 2-4 [5]. In this study, we present experimental results for silver GDEs using acidic electrolytes. These results show that the carbon efficiency can be increased while maintaining a similar Faradaic efficiency as obtained for KHCO3 electrolytes at industrially relevant current densities. Our model calculations support these findings and give insights about the development of the local pH in dependence of the applied current density for different pH values of the bulk electrolyte.[1] T. Burdyny, W. A. Smith, CO2 reduction on gas-diffusion electrodes and why catalytic performance must be assessed at commercially-relevant conditions, Energy & Environmental Science 12 (5) (2019), 1442-1453.[2] D. Franzen, M. C. Paulisch, B. Ellendorff, I. Manke, T. Turek, Spatially resolved model of oxygen reduction reaction in silver-based porous gas-diffusion electrodes based on operando measurements, Electrochimica Acta 375 (2021), 137976.[3] M. Löffelholz, J. Osiewacz, A. Lüken, K. Perrey, A. Bulan, T. Turek, Modeling electrochemical CO2 reduction at silver gas diffusion electrodes using a TFFA approach. Chemical Engineering Journal 435(2) (2022), 134920.[4] J. A. Rabinowitz, M.W. Kanan, The future of low-temperature carbon dioxide electrolysis depends on solving one basic problem. Nature Communications 11 (2020), 5231.[5] M.C.O. Monteiro, M.F. Philips, K.J.P. Schouten et al., Efficiency and selectivity of CO2 reduction to CO on gold gas diffusion electrodes in acidic media. Nature Communication 12 (2021), 4943. Figure 1

Inhomogeneities in the Catholyte Channel Limit the Upscaling of CO2 Flow Electrolysers.

The use of gas diffusion electrodes that supply gaseous CO2 directly to the catalyst layer has greatly improved the performance of electrochemical CO2 conversion. However, reports of high current densities and Faradaic efficiencies primarily come from small lab scale electrolysers. Such electrolysers typically have a geometric area of 5 cm2, while an industrial electrolyser would require an area closer to 1 m2. The difference in scales means that many limitations that manifest only for larger electrolysers are not captured in lab scale setups. We develop a 2D computational model of both a lab scale and upscaled CO2 electrolyser to determine performance limitations at larger scales and how they compare to the performance limitations observed at the lab scale. We find that for the same current density larger electrolysers exhibit much greater reaction and local environment inhomogeneity. Increasing catalyst layer pH and widening concentration boundary layers of the KHCO3 buffer in the electrolyte channel lead to higher activation overpotential and increased parasitic loss of reactant CO2 to the electrolyte solution. We show that a variable catalyst loading along the direction of the flow channel may improve the economics of a large scale CO2 electrolyser.

Bridging knowledge gaps in liquid- and vapor-fed CO2 electrolysis through active electrode area

Increased use of gas diffusion electrodes for CO 2 electroreduction widens the experimental phase space that was previously inaccessible using foil electrodes, raising fundamental questions over the impacts of key variables that translate between liquid- and vapor-fed CO 2 electrolysis systems. This work focuses on studying the interplay of current-potential profiles and electrochemically active surface area (ECSA) by implementing a Cu nanoflower catalyst morphology. The results show decreased overpotentials as much as 460 and 174 mV for foil and gas diffusion electrodes, respectively, while maintaining or improving multi-carbon product current density. These overpotential shifts and product activities normalized by ECSA lead to current-potential relationships akin to those of the Tafel description, which are found through a continuum model to be useful for describing the roughness dependence for both liquid- and vapor-fed systems. This analysis establishes a holistic approach for establishing catalyst design criteria to improve materials development for CO 2 electrolysis technologies. • Distinct catalyst morphologies are translated between electrode types • Reaction rate toward CO 2 R and C 2+ products is enhanced at lower overpotentials • Increase in J geo up to ∼735× and ∼10× are observed with foil and GDEs, respectively • Experiments and model show Tafel-like description of roughness and potential shift CO 2 electrolysis coupled with renewable energy sources is an attractive method for producing industrially relevant chemicals while reducing emissions. Recent advancements have focused on evolving the electrode architecture from metal foils to porous gas diffusion electrodes for improving the reaction rate. However, the contributions from surface kinetics and local reaction environment are convoluted and vary when translating from liquid- to vapor-fed systems. Herein, we draw connections between these systems by evaluating identical catalyst structures of varying roughness for both electrode types. Our experimental and modeling approaches provide better understanding of the interplay between electrode structure and reaction environment on the activity and selectivity. We observe nominal changes in the intrinsic activity of Cu with significant overpotential savings corresponding to the roughness factors across electrode types. We achieve a ∼700- and ∼10-fold increase in multi-carbon product formation by designing roughened catalysts for both foil and gas diffusion-type electrodes, respectively. We showcase catalyst surface area as a key descriptor for understanding the interplay between the electrode structure and local reaction environment on activity and selectivity. Both experimental and computational modeling determine heightened reaction rates toward CO 2 R at lower overpotentials, which uncover a clear dependence on electrode roughness.

Effect of Substrate Microstructure and Hydrophobicity on Ag Gas Diffusion Electrodes for Electrochemical CO2 Reduction

Electrochemical CO2 reduction (CO2RR) is a very promising way to convert detrimental CO2 emissions into sustainable fuels and chemicals, and move us closer to a circular economy.1 To compete with traditional fuel/chemical production routes, we need to develop electrocatalysts that are highly active, selectivity towards a desired product, and stable. Thanks to the use of gas diffusion electrodes (GDEs) that provide gaseous CO2 close to the catalyst, high activities can be achieved.2,3 However, product selectivity and stability must still be improved before practical application. An emerging strategy in this regard is to control the local environment surrounding a catalyst, i.e. the concentration of the various products and reagents therein.4 In this work, we achieve such control by varying the microstructure of substrates later used to fabricate GDEs by Ag deposition. By systematically studying the CO2RR performance, we observe a clear correlation between the pore size of the GDEs and their product selectivity, stability, and even resilience to Cu impurities.5 We rationalize it with the different effective hydrophobicity of the GDEs, which control the extent of electrolyte penetration within the electrode and depends on both substrate chemistry and pore size. We show that a higher effective hydrophobicity correlates with higher Faradaic efficiency towards carbon monoxide production (FECO up to 95 % at 100 mA/cm2) and longer stability (~100 % retention after a 1 hour electrolysis at 100 mA/cm2). These results show the importance of the substrate's microstructural properties in GDE for CO2RR, and highlight a new strategy to achieve control over selectivity and stability of these electrodes.

Synthesis of Nanostructures Using Gas-Diffusion Electrodes

Gas diffusion electrodes (GDEs) intertwine an ionically conducting liquid and a gas with an electrically conducting solid, supporting electrochemical reactions involving constituents linked to the three phases (i.e., chemical species, electrons). GDEs are broadly used in electrochemical energy-conversion devices, such as fuel cells and metal-air batteries, as well as in electrolyzers aiming at chemical synthesis, like in the chlor-alkali industry, hydrogen peroxide production, CO2 conversions to fuels and fine chemicals, or N2 reduction to ammonia. Recently, the use of GDEs was pioneered for metal recovery and the synthesis of nanostructures, in a process named gas-diffusion electrocrystallization (GDEx).[1–4] A liquid solution containing dissolved metal or metalloid ions (e.g., Cu2+, Fe3+, As3+, PtCl2 −6) flows through an electrochemical cell equipped with a GDE, filling in its porosity. The gas (e.g., O2, O2 in air, CO2, etc.) percolates through a hydrophobic backing (e.g., PTFE) on the GDE. After the gas diffuses to the electrically conducting layer acting as an electrocatalyst (e.g., hydrophilic porous activated carbon), the gas is electrochemically reduced. For instance, by imposing specific cathodic polarization conditions (e.g., at −0.145 VSHE O2 is reduced producing H2O2, H2O and OH–). As the highly abundant hydroxyl ions accompanied by redox reactive species spread to the bulk electrolyte, a reaction front develops throughout the hydrodynamic boundary layer. This creates local saturation conditions at the electrochemical interface, where metal ions precipitate in metastable or stable phases, depending on the operational variables. When O2 is the oxidizing gas, GDEx has been explained with an oxidation-assisted alkaline precipitation mechanism.[4] Conversely, when CO2 is used, the reaction front, rich in reducing species, yields elemental nanoparticles.This centennial celebration talk will explain the underlying principles of GDEx, portray reflections on its design and scale-up, and substantiate some of the experimental merits achieved. It will include the GDEx: (a) synthesis of iron oxide nanoparticles with high control of their magnetic susceptibility[1]; (b) recovery and immobilization of arsenic into crystalline scorodite[2]; (c) synthesis of nanoparticles with novel magnetic ground states (e.g., spin liquids and spin glasses)[3]; (d) synthesis of libraries of electrochemically-active materials[5] and (e) formation of elemental nanoparticles of platinum group metals (PGMs).

Visualisation and quantification of flooding phenomena in gas diffusion electrodes used for electrochemical [formula omitted] reduction: A combined EDX/ICP–MS approach

The most promising strategy to scale up the electrochemical CO2 reduction reaction (ec-CO2RR) is based on the use of gas diffusion electrodes (GDEs) that allow current densities close to the range of 1 A/cm2 to be reached. At such high current densities, however, the flooding of the GDE cathode is often observed in CO2 electrolysers. Flooding hinders the access of CO2 to the catalyst, and by thus leaving space for (unwanted) hydrogen evolution, it usually leads to a decrease of the observable Faradaic efficiency of CO2 reduction products. To avoid flooding as much as possible has thus become one of the most important aims of to-date ec-CO2RR engineering, and robust analytical methods that can quantitatively assess flooding are now in demand. As flooding is very closely related to the formation of carbonate salts within the GDE structure, in this paper we use alkali (in particular, potassium) carbonates as a tracer of flooding. We present a novel analytical approach —based on the combination of cross-sectional energy-dispersive X-ray (EDX) mapping and inductively coupled plasma mass spectrometry (ICP–MS) analysis— that can not only visualise, but can also quantitatively describe the electrolysis time dependent flooding in GDEs, leading to a better understanding of electrolyser malfunctions.

Magnetic fields enhance mass transport during electrocatalytic reduction of CO2

The selectivity of electrocatalytic reduction of CO 2 (CDR) is dictated not only by the intrinsic reactivity of the catalyst but also by the transport of reactants to the catalyst (i.e., mass transport). Current methods for increasing mass transport in CDR rely upon either (1) mechanical agitation or (2) use of gas-diffusion electrodes and are unable to eliminate concentration polarization completely. This work demonstrates that magnetic fields orthogonal to the ionic current (i.e., the Lorentz force) can be used to increase mass transport during CDR by generating convective flow in the fluid, thus modifying the observed selectivity of CDR. This increase in mass transport leads to a corresponding increase in current densities (up to 1.3× higher than the analogous system with no B → -field or agitation) and increased selectivity of CDR relative to the hydrogen evolution reaction (up to 2.5× higher than the system with no B → -field or agitation). • During CDR, magnetic fields (B-fields) generate convective currents in the fluid • The convection generated by B-fields decreases pH gradients during electrolysis • The magnitude of the B-field effect is dictated by the structure of the cathode • B-fields decrease the cost of operating a CO 2 electrolyzer by ∼10% At high current densities (>100 mA/cm 2 ), the electrocatalytic reduction of CO 2 (CDR) can be limited by mass transport, resulting in decreased selectivity for CDR relative to the reduction of protons to hydrogen. Common approaches to increase mass transport for CDR rely upon either (1) gas-diffusion electrodes or (2) mechanical agitation. This work demonstrates that magnetic fields acting through the Lorentz force can increase mass transport during CDR. Magnetic fields generate convective currents by the Lorentz force acting on ions moving during electrolysis and decreases the cost of operating a CO 2 electrolyzer (by ∼10% in this work). Magnetic fields parallel to the surface of an electrode can be used to increase mass transport during electrocatalytic reduction of CO 2 .

The Role of Carbonate Formation on the Long-Term Performance of Gas Diffusion Electrodes for Carbon Dioxide Electrolysis in Alkaline Media

Anthropogenic CO2 emissions present an immediate and unprecedented threat to our planet. Electroreduction of CO2 (CO2RR) offers the dual benefits of reducing CO2 emissions from point sources and producing valuable chemicals such as CO, formic acid, and ethanol in a carbon-neutral manner.1,2 Over the past decade, a significant body of research has focused on improving the product selectivity and activity of CO2RR catalysts, but studies regarding long-term stability and durability of these electrocatalytic systems have been rare.3 To date, only a few studies have reported performance durations exceeding 100 hours, and even fewer have reported on possible mechanisms underlying decrease in performance and system failure.4 Techno-economic analyses suggest a lifetime of 3000 hours or greater is needed for economically feasible CO2RR systems5. Progress in this area will be crucial for further development of CO2RR technology.The use of gas diffusion electrodes (GDEs) in flow electrolyzer and membrane electrode assembly-based systems has enabled the conversion of a gaseous stream of CO2 into products at a high rate5. However, the layers of the GDE are plagued by a variety of degradation modes, which have not been extensively studied to date. Our study investigates the detrimental role of carbonate deposits on the catalyst layer and within the GDE; by understanding the conditions that promote or inhibit their formation, we can better understand electrode performance loss and how to design long-lasting electrodes.In this talk, we will discuss the role of electrolyte concentration and composition in carbonate deposit formation, investigated via a series of 6-hour tests in our GDE-based alkaline flow electrolyzer. The insights gained from these tests using a broad range of characterization techniques (SEM, EDS, Micro-CT, XRD) provide a deeper mechanistic understanding of long-term GDE performance for CO2 electroreduction in alkaline media. This will be followed by a discussion of strategies for overcoming the challenges currently facing GDEs in our system, including the use of durable and hydrophobic electrode substrate materials, and electrolyte engineering-based solutions. While pursuing these studies, we also developed protocols for durability and stability testing based on those used in the fuel cell, water electrolysis and solid oxide electrolysis communities. This allows tests to be completed in an accelerated fashion while simulating a variety of operating conditions. Acknowledgement We gratefully acknowledge financial support from Shell’s New Energies Research & Technology - Dense Energy Carriers Program and I2CNER.

Revealing Mechanistic Processes in Gas-Diffusion Electrodes During CO2 Reduction via Impedance Spectroscopy

The use of gas-diffusion electrodes (GDEs) as CO2 converting electrodes increases the achieved current densities for the CO2 reduction reaction (CO2RR) by a multiple (> 100 mA cm-2) compared to pla...

Deconvolution of Degradation Mechanisms Towards Durable Gas Diffusion Electrodes for the Efficient Electrochemical Reduction of CO2

Anthropogenic CO2 emissions present an immediate and unprecedented threat to our planet. Electroreduction of CO2 (CO2RR) offers the dual benefits of reducing CO2 emissions from point sources and producing valuable chemicals such as CO, formic acid, and ethanol in a carbon-neutral manner1,2. Over the past decade, a significant body of research has focused on improving the product selectivity and activity of CO2RR catalysts, but studies regarding long-term stability and durability of these electrocatalytic systems have been rare3. To date, only a few studies have reported performance durations exceeding 100 hours, and even fewer have reported on possible mechanisms underlying decrease in performance and system failure. Techno-economic analyses suggest a lifetime of 3000 hours or greater is needed for economically feasible CO2RR systems4. Progress in this area will be crucial for further development of CO2RR technology.The use of gas diffusion electrodes (GDEs) in flow electrolyzer and membrane electrode assembly-based systems has enabled the conversion of a gaseous stream of CO2 into products at a high rate5. However, the layers of the GDE are plagued by a variety of degradation modes, the mechanisms of which are still unclear. By determining the physical and chemical degradation mechanisms present in the catalyst layer, as well as the detrimental role of carbonate deposits within the GDE, we can better understand electrode performance loss and how to design long-lasting electrodes.In this talk, we will discuss the various mechanisms that cause performance degradation in GDEs over time, investigated via a series of 6-hour tests in our GDE-based alkaline flow electrolyzer. The insights gained from these tests using a broad range of characterization techniques (i.e. SEM, EDS, Micro-CT, XRD, XPS) provide a deeper mechanistic understanding of long-term GDE performance for CO2 electroreduction. This will be followed by a discussion of strategies for overcoming the challenges currently facing GDEs in our system, including the use of durable electrode substrate materials and catalyst layer coatings for improved hydrophobicity and stability for long-term operation. While pursuing these studies, we also developed protocols for durability and stability testing based on those used in the fuel cell, water electrolysis and solid oxide electrolysis communities. This allows tests to be completed in an accelerated fashion while simulating a variety of operating conditions. Acknowledgement We gratefully acknowledge financial support from Royal Dutch Shell and I2CNER.

Electrocatalysis of Hydrogen Peroxide Generation Using Oxygen-Fed Gas Diffusion Electrodes Made of Carbon Black Modified with Quinone Compounds

Hydrogen peroxide (H2O2) is one of the most popular and widely used oxidants. Among the wide range of synthesis techniques used for the production of H2O2; the electrochemical method allows the use of gas diffusion electrodes to generate H2O2 without limiting the low solubility of O2 in water. The present work reports the modification of carbon black with quinone, where the generation of H2O2 occurs in an electrochemical/chemical mechanism. The results obtained by this technique were found to be highly promising. The use of the organic compound 1,2-dihydroxyanthraquinone to modify carbon black electrode resulted in greater production of H2O2. Carbon black electrode modified with 1% of 1,2-dihydroxyanthraquinone yielded 298 mg L−1 of H2O2 at the end of 90 min of experiment, reaching an electrical efficiency of approximately 25.5%. Based on the findings of this study, H2O2 generation is found to be directly associated with the chemical structure of the carbon modifier and not solely related to the presence of quinone groups.

On the Conversion Efficiency of CO2 Electroreduction on Gold

Benjamin A. Zhang is a PhD student at Harvard University. His research focus is on the control of mass transport and concentration gradients in electrochemical reactions, in particular CO2 reduction, through nano- and mesostructuring of electrodes. Cyrille Costentin is Professor at Universite Paris Diderot. He received his undergraduate education at Ecole Normale Superieure de Cachan and his PhD at Universite Paris Diderot. He is currently a Visiting Scientist in the group of D.G. Nocera at Harvard University. His interests include mechanisms and reactivity in electron transfer chemistry with particular emphasis on proton-coupled electron transfer and catalysis of electrochemical reactions. Daniel G. Nocera is the Patterson Rockwood Professor of Energy at Harvard University. His group studies mechanisms of biological and chemical energy conversion.

What would it take for renewably powered electrosynthesis to displace petrochemical processes?

Electrocatalytic transformation of carbon dioxide (CO2) and water into chemical feedstocks offers the potential to reduce carbon emissions by shifting the chemical industry away from fossil fuel dependence. We provide a technoeconomic and carbon emission analysis of possible products, offering targets that would need to be met for economically compelling industrial implementation to be achieved. We also provide a comparison of the projected costs and CO2 emissions across electrocatalytic, biocatalytic, and fossil fuel-derived production of chemical feedstocks. We find that for electrosynthesis to become competitive with fossil fuel-derived feedstocks, electrical-to-chemical conversion efficiencies need to reach at least 60%, and renewable electricity prices need to fall below 4 cents per kilowatt-hour. We discuss the possibility of combining electro- and biocatalytic processes, using sequential upgrading of CO2 as a representative case. We describe the technical challenges and economic barriers to marketable electrosynthesized chemicals.

Electrosynthesis of hydrogen peroxide using modified gas diffusion electrodes (MGDE) for environmental applications: Quinones and azo compounds employed as redox modifiers

Although the electrosynthesis of hydrogen peroxide (H2O2) using gas diffusion electrodes (GDE) is a viable option for the production of this oxidizing agent in advanced oxidation processes (AOP) for wastewater treatment, the quest for more efficient electrodes is still regarded a matter of great importance in this area. The present study sought to investigate different redox organic compounds employed as modifiers of carbon black Printex L6 (CP) with the aim of increasing H2O2 production using carbon-based electrodes. Varying amounts of the modifiers, including Sudan Red 7B (SR7B), methyl-p-benzoquinone (MPB), anthraflavic acid (AA) and anthraquinone-2-carboxylic acid (A2CA), were added to carbon black, where the electrochemical activity was studied by applying a microporous catalyst layer on a rotating ring-disk electrode (RRDE). The materials containing 0.5% of SR7B and 5.0% of MPB increased the current efficiency for the electrogeneration of hydrogen peroxide to 86.2% and 85.5%, respectively, compared to 82.8% obtained for unmodified carbon. Carbon Printex L6 gas diffusion electrodes modified with 0.5% of SR7B were studied and the following results were obtained: the application of current density of 75 mA cm−2 led to the production of 1020.1 mg L-1 of H2O2, with an energy consumption of 118.0 kW h kg-1, apparent kinetic constant of 37.34 mg L-1 min-1 and current efficiency of 17.87%. Conversely, the use of GDE with unmodified carbon resulted in the production of relatively less quantity of H2O2 which amounted to 717.3 mg L-1, with more energy consumption of 168.5 kW h kg-1, lower apparent kinetic constant of 21.41 mg L-1 min-1 and lower current efficiency of 12.57%. Based on these results, carbon Printex L6 GDE modified with 0.5% of Sudan Red 7B is seen as a suitable alternative for the production of high amounts of H2O2 which can be applied in advanced oxidation processes in acidic medium.