Performance Of Gas Diffusion Electrodes Research Articles

Inhibiting salt precipitation on the gas diffusion electrode surface in gas-phase CO2 electroreduction to formate by using an acidic anolyte

The gas-phase CO2 electroreduction to formate represents one of the most promising CO2 conversion processes due to its scalability, as the product concentration surpasses 30 % wt. However, the use of alkaline media anolytes, intended to improve the efficiency and selectivity of formate production, causes the carbonate and bicarbonate salts to precipitate over the Gas Diffusion Electrode (GDE). This precipitation clogs the porous structure, leading to a rapid loss of electrode stability. In this work, we address this issue by proposing the use of acid anolytes, based on K2SO4, to mitigate the precipitation of insoluble salt on the GDE structure, thereby achieving longer and more stable GDE operation times. Various anolyte concentrations and pHs are evaluated, with 0.3 M K2SO4 at pH 1, adjusted using H2SO4, providing the best compromise. This condition inhibited potassium carbonate and bicarbonate precipitation, as observed through XRD, SEM, and EDS analysis, while maintaining high CO2 electroreduction to formate performance, with a concentration of 69 g L−1, and a Faradaic Efficiency of 33 %. Furthermore, the anolyte flowrate per geometric area is optimized to maximize the system performance. At a flowrate of 0.85 mL min−1 cm−2, enhanced concentration of 88 g L−1 and a Faradaic Efficiency of 42 % are reached. Besides, long-term experiments demonstrated that GDEs used with alkaline conditions exhibit a larger deactivation constant (0.7652) compared to the GDEs used with acid anolytes (0.3891). This indicates that salt precipitation more rapidly reduces GDE performance under alkaline conditions. These results represent a promising advance in obtaining longer-lasting GDEs, which are crucial to successfully scaling up the CO2 electroreduction to formate.

Improving Gas Diffusion Electrodes for Electrochemical CO2 Reduction: Influence of the Ionomer

Electrochemical CO2 reduction is a promising way of harnessing greenhouse gas as an abundant resource for the production of value-added hydrocarbons. By employing copper as the catalyst, CO2 can be converted to multi-carbon compounds such as ethylene. Highly porous gas diffusion electrodes (GDEs) enable efficient contacting of gaseous CO2 and the electrolyte. Copper-coated GDEs are commonly manufactured by depositing a mixture of copper nanoparticles and ionomer on a gas diffusion layer. The ionomer not only acts as a binder, but also influences the GDE’s wetting behavior and can improve mass transport within the catalyst layer due to the combination of a hydrophobic PTFE-backbone and hydrophilic ion groups. Thus, optimizing the type and content of ionomer in the catalyst layer is of utmost importance.In this study, we investigate how the composition of catalyst layers affects the faradaic efficiency (FE) toward ethylene during electrochemical CO2 reduction. For this purpose, we fabricate different GDEs with constant copper loading but varied ionomer content and type (Nafion, Sustainion). To assess the performance of the GDEs, we conduct electrolysis experiments in a flow cell (Flex-E-Cell) setup with KHCO3 as the catholyte. Preliminary screening experiments at up to 50 mA cm-2 reveal a strong effect of the Nafion/copper mass ratio (NCR) on the FE toward ethylene. Within the investigated range (NCRs of 0.1, 0.2, 0.3, 0.4 and 0.5), we identify a sweet spot at an NCR of approximately 0.4. We hypothesize that at higher NCRs negative effects, such as decreasing porosity and an increasingly acidic pH, outweigh the improved mass transport. To assess the performance of the different GDEs at industrially relevant conditions, we perform constant current experiments at ≥ 100 mA cm-2. Here, we corroborate the previously observed trend and find that increasing the NCR from 0.2 to 0.4 doubles the maximum observed FE toward ethylene. When replacing Nafion with an anion exchange ionomer such as Sustainion, the total FE toward carbon products remains nearly unchanged. However, the selectivity shifts from ethylene toward single-carbon species such as CO and formic acid. These results highlight that ionomers play a decisive role in hydrogen evolution mitigation and tuning of the product spectrum during electrochemical CO2 reduction. Figure 1

Importance of Substrate Pore Size and Wetting Behavior in Gas Diffusion Electrodes for CO2 Reduction

We study the effect of substrate pore size and wetting behavior on the performance of polymer-based Ag gas diffusion electrodes (GDEs) for electrochemical CO2 reduction. We find a strong correlation between the pore size of the substrate and the GDE product selectivity and performance stability during CO2 electrolysis. We attribute this correlation to the GDE’s wetting behavior, in particular, to its resistance to aqueous electrolyte penetration. We quantify this resistance by the water entry pressure, that is, the minimum pressure required to push water through a hydrophobic porous substrate, which depends on the pore size of the substrate and on the hydrophobicity of the substrate polymer. While substrates with low water entry pressure yield GDEs that exhibit poor CO selectivity and stability, high water entry pressure substrates lead to greatly improved Faradaic efficiency toward CO (up to 95% at 100 mA/cm2) and remarkably longer performance stability (97% of initial CO selectivity retained after >40 h). We also assess the sensitivity to surface Cu contaminants and find that substrates with high water entry pressures lead to GDEs that are more resilient to impurities, with an almost unvaried selectivity even when contaminated with >1 at % Cu vs Ag. On this basis, we propose the water entry pressure as a metric to assess GDE quality. These results highlight how acting on the substrate is a powerful and scalable handle for improving the performance of GDEs for the electrochemical CO2 reduction.

Optimization of Porosity Distribution in Gas Diffusion Electrodes for CO2 Electrolysis

The criteria dictating the performance of gas diffusion electrodes (GDEs) for CO2 electrolysis are not well understood, due to the complex and highly coupled relationships of the underlying physical and chemical phenomena. In addition, a number of key performance indicators (KPIs) have been identified (e.g., Faradaic efficiency, reaction selectivity, single-pass conversion, and productivity), and optimizing for any single metric often leads to inherent tradeoffs. Consequently, much recent work has focused on understanding how operational and architectural parameters control GDE performance. The porous catalyst supports and diffusion media in these devices are critical components as they mediate the transport and reactive processes. The microstructure of these components influences the balance between the electrochemical surface area, which dictates CO2 consumption, and mass transfer of the aqueous and gaseous species. Traditional porous media is often spatially homogeneous and provides limited opportunity to tailor this balance. In contrast, novel advanced manufacturing methods have now enabled researchers to create electrodes with variable porosity that can be tuned for optimal performance. To explore the impact of locally altering the porous media structure, we leverage previous modeling work by Weng et al. to allow for spatially varying porosity. Further, we couple the forward simulation to adjoint based optimization algorithms to determine optimal porosity distribution in the diffusion media and catalyst layer of a GDE. The cost functions for the optimization are derived from the previously mentioned KPIs. Finally, the performance of the resultant spatially varying porosity GDEs is compared to the performance of homogenous porosity GDEs, and we identify key features of the porosity distribution leading to improved performance. Weng, Lien-Chun, Alexis T. Bell, and Adam Z. Weber. "Modeling gas-diffusion electrodes for CO2 reduction." Physical Chemistry Chemical Physics 20.25 (2018): 16973-16984. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.LLNL-ABS-830082

Effect of Catalyst Double Layer on Performance without Micro Porous Layer in Anode for High Temperature Polymer Electrolyte Membrane Fuel Cell

Catalyst double layer (CDL) structure on gas diffusion electrode (GDE) without microporous layer (MPL) is developed for the anode of high temperature polymer electrolyte membrane fuel cell (HT–PEMFC). Polytetrafluoroethylene and polyvinylidene fluoride is applied for the outer and inner layer as the hydrophobic binder for fabricating 3D electrode structure. The effect of Pt ratio in anode CDLs on the GDE performance was investigated by single cell test and electrochemical analysis using poly(benzimidazole) (PBI)–based membrane, with Pt loading under 0.3 mg/cm2 at ambient pressure air on 150℃. The results show that optimal CDL anodes present performance of 0.65V at 0.2A/cm2 and peak power density is 0.41W/cm2 at mass transfer region which is higher than conventional single layer anode with MPL. Furthermore, the 3D structure of the outer catalyst layer in anode read high catalyst utilization and preventing leakage of electrolyte with a short–term durability test shows good stability with PBI membrane.

CVD-Deposited Oxygen-Selective Fluorinated Siloxane Copolymers as Gas Diffusion Layers

Copolymer thin films of 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane (V4D4), 2-(perfluorohexylethylacrylate) (PFHEA), and 2-(perfluoroalkylethylmethacrylate) (PFEMA) were synthesized via initiated chemical vapor deposition (iCVD) as potential candidates for gas diffusion layers (GDLs) in gas diffusion electrodes (GDEs) for aqueous metal–air batteries. Thin-film GDLs exhibited an average water vapor transmission rate of 7.5 g m–2 day–1 and enhanced oxygen diffusion with oxygen permeabilities as high as 3.53 × 10–15 mol m m–2 s–1 Pa–1 (10.5 Barrer). The electrochemical performance of GDEs fabricated using commercial catalysts, current collectors, and synthesized GDLs was investigated by cyclic voltammetry, electrochemical impedance spectroscopy, and potentiodynamic polarization measurements. The fabricated GDEs exhibited higher oxygen reduction current densities (228.2 mA cm–2) compared to commercial GDEs (132.7 mA cm–2). Copolymer GLDs exhibited an order of magnitude higher oxygen diffusion (39.5 × 10–8 cm2 s–1) in GDEs compared to commercial counterparts (1.84 × 10–8 cm2 s–1). Due to the high oxygen solubility of V4D4 and excellent hydrophobic behavior of PFHEA and PFEMA, their copolymers can effectively promote the diffusion of oxygen and restrict moisture intake, making them ideal materials for GDLs. Combining well-balanced properties of siloxane and fluorinated polymer chemistries, the iCVD process is an excellent low-cost method for the fabrication of GDLs for metal–air battery applications.

Probing the local activity of CO2 reduction on gold gas diffusion electrodes: effect of the catalyst loading and CO2 pressure.

Large scale CO2 electrolysis can be achieved using gas diffusion electrodes (GDEs), and is an essential step towards broader implementation of carbon capture and utilization strategies. Different variables are known to affect the performance of GDEs. Especially regarding the catalyst loading, there are diverging trends reported in terms of activity and selectivity, e.g. for CO2 reduction to CO. We have used shear–force based Au nanoelectrode positioning and scanning electrochemical microscopy (SECM) in the surface-generation tip collection mode to evaluate the activity of Au GDEs for CO2 reduction as a function of catalyst loading and CO2 back pressure. Using a Au nanoelectrode, we have locally measured the amount of CO produced along a catalyst loading gradient under operando conditions. We observed that an optimum local loading of catalyst is necessary to achieve high activities. However, this optimum is directly dependent on the CO2 back pressure. Our work does not only present a tool to evaluate the activity of GDEs locally, it also allows drawing a more precise picture regarding the effect of catalyst loading and CO2 back pressure on their performance.

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.

Modeling and Numerical Investigation of the Performance of Gas Diffusion Electrodes for the Electrochemical Reduction of Carbon Dioxide to Methanol

In ‎this study, a model was ‎built to investigate the role ‎of Cu2O-ZnO based gas ‎diffusion ‎electrodes in enhancing the‎ reduction ‎of carbon dioxide into ‎methanol inside an ‎‎‎electrochemical ‎...

Looking Deep inside the Cathode of Li-O2 Batteries: Unraveling the Local Distribution of Li2O2 with a Combined Experimental and Model-Based Approach

Carbon-based, high surface area gas diffusion electrodes (GDEs) are commonly used as cathodes in Li-O2 batteries. These GDEs provide an sufficient amount of active sites to deposit the main discharge product, Li2O2. Despite the fast electron transfer at the electrode surface to form Li2O2, experimental results show that the very high theoretical energy densities of Li-O2 batteries cannot be achieved at the moment. Instead of the desired formation of high amounts of soluble LiO2, predominantly insoluble Li2O2 precipitates at the active sites of the cathode during discharge. Unfortunately Li2O2 is poorly conductive for electrons, which leads to a low amount of deposited Li2O2 per active site and a fast passivation of the cathode. Thus, only the cathode surface area and not the pore volume is utilized by Li2O2, which explains the discrepancy between theory and experiments. Furthermore, the precipitation leads to continuous changes in porosity, available active surface area and predominant reaction pathway. For this reason a detailed analysis of the distribution of Li2O2 at all points in the GDE and of the factors that influence the Li2O2 morphology is needed to overcome performance limitations. [1]In this work, we elucidate how the local distribution of Li2O2 inside the GDE and its particle size evolve as a function of discharge current density. We apply a powerful combination of experimental and model-based analysis. To the best of our knowledge, this is the first study on the cathode surface utilization by Li2O2 and its particle sizes performed for different locations and states of discharge (SOD). The impact of the distribution and the resulting changes in transport resistance and active surface area on the battery performance are derived thereof.In the experimental part, decreasing capacities and Li2O2 particle sizes are observed for increasing discharge current densities (see SEM analysis of discharged battery cathodes in fig. 1 a)). Furthermore, the experimental results show that particle precipitation starts mainly at the side of the cathode that faces the oxygen reservoir of the battery (see fig. 1 b)). Discharging a battery at low current densities leads to a uniform cathode surface coverage by Li2O2 even at low SOD whereas discharging at high current densities yields a gradient of blocked active sites through the cathode.Simulations based on a physical GDE model show that a two-step reaction mechanism with the soluble species LiO2 as reaction intermediate can quantitatively explain the experimental findings. Depending on the current density, either a chemical or an electrochemical particle growth process predominates, which leads to the distinct different particle size distributions (see fig. 1 c) and d)). The strong dependence of capacity on the current density is related to different capacities per carbon cathode surface area at the end of discharge. At high discharge rates and thereby lowered potential more nucleation takes place. As a result, the particle number is higher and the average size smaller. Due to the surface volume ratio, smaller particles entail lower capacities per cathode surface area. The results point out that even at moderate current densities the battery capacity is limited by the surface area of the GDE and not by the concentration of dissolved O2 in the liquid electrolyte. Therefore, the solubility and reaction kinetics of the reaction intermediate LiO2 play a crucial role to enhance Li2O2 particle size and with it the obtainable discharge capacity.The presented results provide detailed insight into the cathode surface utilization and the underlying processes that limit the performance of GDEs in Li-O2 batteries. In the end, the study will help to achieve higher discharge capacities for this type of battery and thus will propel the ongoing research and the efforts in commercialization of metal-oxygen batteries.Acknowledgement:The authors are thankful for financial support by the BMBF (Federal Ministry of Education and Research) within the project “MeLuBatt” (03XP0110A).

Pulsed Electrodeposition of Tin Gas-Diffusion Electrocatalysts for CO2 Reduction to Formate

The performance of electrocatalysts for the electrochemical carbon dioxide (CO2) reduction reaction (eCO2RR) is largely dependent on the ability to efficiently deliver CO2 to the active sites. To date, the best-identified reactor designs for CO2 conversion are based upon gas diffusion electrodes (GDEs) to supply CO2 to cathodic electrocatalytic sites, as gas-phase mass transport is significantly more rapid than that of CO2 dissolved in an aqueous electrolyte. As such, GDE-based electroreactors typically provide current densities that are significantly greater than those that convert solvated CO2, often by an order of magnitude or more [1]. However, the performance of GDEs for eCO2RR can be hampered by poor catalyst utilization and transport limitations within the catalyst layer. Higher catalyst loadings, generally achieved via application of thicker catalyst layers, can yield higher total reaction rates, but often at the cost of reduced conversion efficiency arising from accelerated formation of undesired side products (e.g., hydrogen evolution from water splitting or diversion of carbon to alternative reaction pathways) due to reactant starvation. Reducing catalyst particle size typically enhances both utilization and activity per unit mass. This in turn may allow use of thinner catalyst layers, mitigating or avoiding such decreases in product selectivity. While synthetic methods exist for generating smaller (< 10 nm) particles of many catalytic metals, these particles must still be applied to a gas-diffusion layer (GDL) substrate in a fashion that maintains ionic and electronic contact with the electrolyte and GDL, respectively, in order to obtain both high current density and high selectivity.Previous work directed towards platinum (Pt) catalyst utilization in polymer electrolyte fuel cell GDEs demonstrated an “electrocatalyzation” approach that used pulse and pulse-reverse electrodeposition to obtain highly dispersed and uniform Pt catalyst nanoparticles (~5 nm) [2-4]. Moreover, since the catalyst was electroplated through an ionomer layer onto the bare GDL, the formed nanoparticles were inherently in both electronic and ionic contact within the GDE and, consequently, utilization was enhanced. Specifically, for the oxygen reduction reaction, the electrodeposited catalyst exhibited equivalent performance at 0.05 mg/cm2 loading compared to a conventionally prepared GDE with a loading of 0.5 mg/cm2 [4].This talk will present recent and ongoing results from the application of this electrocatalyzation approach to fabrication of tin-loaded GDEs for reduction of CO2 to formic acid. The effect of GDL pre-treatment, pulse/pulse-reverse waveform parameters and duration of electrocatalyzation on the size distribution and uniformity of deposited tin catalyst particles will be discussed, and the performance of these catalysts in a flow electroreactor will be reported in terms of total current density and faradaic efficiency for formic acid. The electroreactor form factor includes an anion-exchange membrane at the GDE cathode and ion-exchange media within the single flow channel to facilitate ionic transport across the cell, similar to a form factor reported in literature to provide high performance [5]. This configuration provides at least two significant benefits: the anion-exchange membrane avoids the excessive wetting of and electrolyte permeation through the GDE in membraneless configurations; and, the ion-exchange media allows use of deionized or reverse-osmosis purity water as the liquid feed, potentially eliminating the need for a costly salt/formic acid separation step in a production context.

Polarization Analysis of Gas Diffusion Electrode with Different Fabrication Parameters in Metal–Air Batteries

The gas diffusion electrode (GDE) represents the most significant component in metal–air batteries (MABs). It is essential to understand the underlying behaviors of GDE and optimize its performance. Herein, a polarization separation method from total polarization curve to single activation, ohmic, and mass transport polarizations of GDE in MABs is proposed for the first time, which is then applied in the investigation of detailed polarization behaviors of the GDE by changing the fabrication parameters. As the polytetrafluoroethylene contents in the catalyst and gas diffusion layers and pressing pressure are adjusted, hydrophobicity, porosity, coverage, and thickness are changed to influence the performance of GDE. Through this analysis, a detailed understanding of the structure–performance relationship of GDE is achieved to improve the electrode design, architecture, and fabrication.

Influence of Temperature on the Performance of Gas Diffusion Electrodes in the CO2 Reduction Reaction

AbstractA detailed investigation of the influence of operating temperature on the electrochemical reduction of CO2 to formate at tin oxide loaded gas diffusion electrodes (GDEs). Ambient pressure electrolysis is performed between 20 and 70 °C with a focus on maximizing current density and energy efficiency while maintaining an average formate faradaic efficiency of at least 80 %. The best performance is achieved at a temperature of 50 °C, which allows a current density of 1000 mA cm−2. Lower or higher temperatures both show an increased hydrogen evolution at said current density. Further investigation of CO2 transport limitation revealed a minimum at 50 °C, which is explained by the opposing influence of temperature on CO2 diffusion coefficients and solubility. This explanation is supported by an estimate of the current density at which hydrogen evolution starts to increase based on the flooded agglomerate model. Long‐term operation for 24 h also revealed an optimum temperature of 50 °C, which helps to suppress the increasing rate of hydrogen evolution and with that a mechanical degradation of the GDE.

Pulsed Electrodeposition of Gas-Diffusion Electrocatalysts for CO2 Reduction

The performance of electrocatalysts for the electrochemical carbon dioxide (CO2) reduction reaction (eCO2RR) is largely dependent on the ability to efficiently deliver CO2 to the active sites. A variety of reactor configurations have been explored in the literature that can be broadly classified as based on either liquid- or gas-phase reactant delivery. These configurations utilize a range of electrode types including metal plates, meshes, packed granules, and gas diffusion electrodes (GDEs) [1]. Amongst these methods, the use of gas-phase reactor designs employing GDEs enables a dramatic increase in current density (typically an order of magnitude or larger) over liquid-phase reactor designs, where the low solubility and aqueous diffusivity of CO2 result in severe mass transport limitations. However, the performance of GDEs in various CO2 electroreduction processes can be hampered by poor catalyst utilization and transport limitations within the catalyst layer. At higher catalyst loadings (thicker catalyst layers), which are desirable for high production rates, conversion efficiencies drop and undesirable side product formation (both from hydrogen evolution and diversion of carbon to alternative reaction pathways) increases due to reactant starvation. Reducing particle size typically enhances both catalyst utilization and activity per unit mass. This, in turn, may enable thinner catalyst layers, mitigating or avoiding such decreases in product selectivity. While synthesis methods exist for generating smaller (< 10 nm) particles, these particles must still be deposited on a gas-diffusion layer (GDL) substrate such that ionic and electronic contact can be maintained with the electrolyte and GDL, respectively. Previous work directed towards platinum (Pt) catalyst utilization in polymer electrolyte fuel cell GDEs demonstrated an “electrocatalyzation” (EC) approach that used pulse and pulse-reverse electrodeposition to obtain highly dispersed and uniform Pt catalyst nanoparticles (~5 nm) [2-4]. Moreover, since the catalyst was electroplated through an ionomer layer onto the bare GDL, the formed nanoparticles were inherently in both electronic and ionic contact within the GDE and, consequently, utilization was enhanced (see Figure - Schematic comparison of catalyst particle size and location distributions in conventional spray-painting versus electrodeposition application methods). Specifically, for the oxygen reduction reaction, the electrodeposited catalyst exhibited equivalent performance at 0.05 mg/cm2 loading compared to a conventionally prepared GDE with a loading of 0.5 mg/cm2 [4]. This talk will discuss the electrodeposition of tin (Sn) and copper (Cu) onto both commercially-available and custom-fabricated GDLs through an EC process, and the electrocatalysis performance of these catalysts as compared to state-of-the-art Sn and Cu nanoparticle catalysts (75-150 nm) prepared by spray-coating. Testing in a custom flow-cell electroreactor has demonstrated that the EC GDEs exhibit electrocatalytic performance comparable or superior to both literature reports and the spray-painted catalysts. Further, clear effects of the pulsed-waveform EC parameters on product distribution and total current density will be highlighted. Preliminary work toward development of GDLs robust against electrolyte saturation/penetration over many hours of operation will also be discussed. In summary, the highly scalable EC approach appears promising for fabricating active catalytic layers directly onto GDL substrates for carbon dioxide reduction applications.

Highly Porous Nitrogen-Doped Carbon-Supported Pt Nanoparticles with Enhanced Activity for the Oxygen Reduction Reaction in Alkaline Media

Fuel cells and metal-air batteries have attracted the attention of many researchers because of their superior power storage capacity. Pt and Pt-based alloys supported on carbon black (CB) are widely used on air electrodes in the batteries. However, the electrocatalytic activity for oxygen reduction reaction (ORR) is still insufficient, which is a barrier to the further commercial deployment of the batteries. Pt supported on nitrogen-doped graphene has attracted attention because the N functionalities modified nucleation and growth kinetics during catalyst nanoparticle deposition, which results in smaller catalyst particle size and increased catalyst particle dispersion, and increased support/catalyst chemical binding, which results in enhanced durability.1 However, the stacking of graphene inflicted by van der Waals forces would significantly reduce the number of active sites. Furthermore, for optimal utilization of the available reaction sites, large pore volumes in catalyst supports play an essential role in gas mass-transport by acting as gas-supplying channels in the catalyst layer. Our group recently reported that highly porous nitrogen-doped carbon nanoparticles (NC) based on CB and cyanamide have a fine pore network that provides efficient access for mass transport.2In this study, for improving the oxygen reduction activity, we used NC as a Pt catalyst support and measured the electrocatalytic activity of synthesized Pt/NC for ORR. NC was synthesized by catalyst-free thermal treatments using CB and cyanamide, and have large pore volume (2.23 cm3g−1) and high surface area (1141 m2g−1).2 Pt nanoparticles supported on NC were prepared by a simple alcohol-reduction method using methanol and dinitro-diamine platinum (II) solution as a reducing agent and platinum source. The Pt content in the catalysts was fixed at 30 wt%. Compared to Pt/CB (mean diameter = 3.2 nm), the smaller Pt nanoparticle size (mean diameter = 2.2 nm) and improved nanoparticle dispersion on NC were observed in TEM images, because the incorporated nitrogen sites on the NC would provide a strong metal–support interaction to anchor efficiently Pt precursor on NC and to stabilize the Pt nanoclusters. Electrochemical measurements were executed using two types of electrodes, rotating disk electrode (RDE) and gas diffusion electrode (GDE). Linear sweep voltammograms at a scan rate of 20 mV s—1 for Pt/NC and Pt/CB are shown in Fig. 1. The kinetic current density value of 6.13 mA cm—2 at 0.80 V vs. RHE for Pt/NC calculated from a mass-transport correction of RDE is about twice as high as that for Pt/CB (3.33 mA cm—2). The steady-state current density—voltage curves of the three-electrode cells installed with the fabricated GDEs were measured in 8.0 M KOH under an O2 flow (100 mL min—1). The GDE performance for ORR of Pt/NC exhibited higher than that of Pt supported on non-doped CB at the measured current density (<300 mA cm—2). The current density value of 52 mA cm—2 at 50 mV vs. Hg/HgO (8.0 M KOH) for Pt/NC obtained from the iR-corrected polarization curve is approximately two times higher than that for Pt/CB (25 mA cm—2). No obvious potential drop at the measured current density for Pt/NC was observed, suggesting the use of NC as a catalyst support was effective in the GDE system because the hierarchical pore structure of the NC would provide both excellent electron transfer and reactant transport rate towards ORR. Figure caption Fig. 1. Linear sweep voltammograms of Pt/NC and Pt/CB at a rotation rate of 1600 rpm measured by RDE in O2-saturated 0.1 M KOH. Reference 1. 1. Y. Zhou et al., Energy Environ. Sci., 3, 1437 (2010). 2. N. Tachibana, et al., Carbon, 115, 515 (2017). Figure 1

Pulsed Electrodeposition of Tin Electrocatalysts Onto Gas Diffusion Layers for CO2 Reduction to Formate

The performance of electrocatalysts for the electrochemical carbon dioxide (CO2) reduction reaction (eCO2RR) is largely dependent on the ability to efficiently deliver CO2 to the active sites. A variety of reactor configurations have been explored in the literature that can be broadly classified as based on either liquid- or gas-phase reactant delivery. These configurations utilize a range of electrode types including metal plates, meshes, packed granules, and gas diffusion electrodes (GDEs).[1] Amongst these methods, the use of gas-phase reactor designs employing GDEs enables a dramatic increase in current density (typically an order of magnitude or larger) over liquid-phase reactor designs, where the low solubility and aqueous diffusivity of CO2 result in severe mass transport limitations. However, per existing literature, the performance of GDEs in various CO2 electroreduction processes can be hampered by poor catalyst utilization and transport limitations within the catalyst layer. Prior reports have demonstrated electroreduction of CO2 to formate (FA) on commercial tin nanoparticle (150 nm) loaded gas diffusion layers (GDLs) at current densities of 200 mA/cm2, 80% selectivity to FA, and cathodic potentials of -0.8 V vs. RHE.[2, 3] However, these and other studies have also reported that at higher catalyst loadings (thicker catalyst layers), which are desirable for high production rates, conversion efficiencies drop and undesirable side product formation increases due to reactant starvation. Reducing particle size typically enhances both catalyst utilization and activity per unit mass. This, in turn, may enable thinner catalyst layers. While synthesis methods exist for generating smaller (< 10 nm) particles, these particles must still be deposited on a GDL such that ionic and electronic contact can be maintained with the electrolyte and GDL, respectively. These critical interfaces are key to maximizing electrode performance in terms of product generation rate, selectivity, and catalyst utilization. Previous work directed towards platinum (Pt) catalyst utilization in polymer electrolyte fuel cell GDEs demonstrated an “electrocatalyzation” (EC) approach that used pulse and pulse-reverse electrodeposition to obtain highly dispersed and uniform Pt catalyst nanoparticles (~5 nm).[4-6] Moreover, since the catalyst was electroplated through an ionomer layer onto the bare GDL, the formed nanoparticles were inherently in both electronic and ionic contact within the GDE and, consequently, utilization was enhanced. Specifically, for the oxygen reduction reaction, the electrodeposited catalyst exhibited equivalent performance at 0.05 mg/cm2 loading compared to a conventionally prepared GDE with a loading of 0.5 mg/cm2.[6] Here we investigate the electrodeposition of tin (Sn) onto commercially available GDLs through an EC process and benchmark our results against a state-of-the-art Sn nanoparticle catalysts (150 nm) spray-coated on a GDL. Electrolysis experiments are conducted in a three compartment W-Cell setup using the Sn-coated GDEs as cathodes and Pt/H2 counter electrode. We demonstrate that the EC GDE samples can exhibit up to 388 mA/cm2 total current density and 76% selectivity to formate at cathodic potentials of -0.8 V vs. RHE, representing a two-fold improvement in current density over both our benchmark electrode and existing reports using Sn-loaded GDEs prepared by conventional methods.[2, 3] We hypothesize that this enhancement arises from improved catalyst utilization, leading to high electrode activity. Surprisingly, SEM imaging of the EC GDE reveals Sn particles no smaller than the micron scale (~10 μm). Thus, we anticipate further improvement in electrode activity may be realized through suitable tuning of the EC waveform to yield nanoscale Sn particles (< 10 nm). In summary, the EC approach appears promising for fabricating active catalytic layers directly onto GDL substrates.

Mixed Reactant Low Temperature Fuel Cells: The Swiss-Roll Cell Design and Selective Electrocatalysts

Eliminating the proton exchange membrane (PEM) together with the platinum-based anode and cathode catalysts and replacing the bulky bipolar plates with lighter and flexible alternatives, would significantly improve the economic outlook of low temperature fuel cells operated with liquid fuels. To address the afore-mentioned issues of PEM fuel cells, at the University of British Columbia we have developed a novel mixed reactant fuel cell design referred to as the Swiss-roll cell, fed with a two-phase mixture of liquid fuel and air (or oxygen) [1-5]. Here, a research progress review is presented related to both selective electrocatalysis and Swiss-roll cell engineering applied to two alkaline fuel cell systems: sodium borohydride – oxygen and sodium borohydride – nitrous oxide, respectively. On the anode, the borohydride electro-oxidation reaction is a complex, up to eight electron transfer process, with possibly competing hydrogen evolution generated by either faradaic (i.e., electrocatalytic) or non-faradaic (i.e., thermocatalytic) pathways. Experimental results correlated with density functional theory modelling predictions are discussed focused on the borohdyride electro-oxidation mechanism on catalysts such as: Pt, Au and Os. On the cathode side, in case of the oxygen reduction reaction (ORR) in alkaline media the comparative performance of gas diffusion electrodes with electrocatalysts such as MnO2, Ag and Fe-aminoantypyrine (Fe-AAPyr) is presented. For nitrous oxide reduction, the electrode kinetics on Pd and Pt electrocatalysts are compared. It is shown that the selective electrocatalyst discovery combined with cell electrochemical engineering innovations enables significant improvements in the power output, reaching 250 mW/cm2 for the membrane-free Swiss-roll mixed-reactant direct borohdyride fuel cell operated at 50 0C and near atmospheric pressure using non-platinum anode and cathode catalysts.

Gas Diffusion Electrodes for Li/O2 Batteries: Impact of Porosity, Wettabiliy and Electrode Design

Beyond lithium ion batteries the metal air technology is considered to be very promising for various energy storage applications in a long-term view. Due to its very high theoretical energy density the aprotic Li/O2 system in particular has attracted broad attention during the recent years. Here molecular oxygen O2 is reduced at a gas diffusion electrode (GDE) upon discharge forming Li2O2 as a solid discharge product which precipitates within the electrode to be reoxidized under the release of oxygen upon recharge. The performance of a Li/O2cell is therefore critically influenced by the structural properties of the used GDE. Based on carbon, especially the porosity, the wetting behavior and the general design of the GDE has a huge impact on the capacity and efficiency of the cell. Regarding the wettability of a GDE, a thin electrolyte film within the inner pore structure is considered to be ideal to provide efficient oxygen transport through open gas channels as well as improved diffusion properties towards the active surface. Using mesoporous carbon xerogels which exhibit defined open pore structures, large inner surfaces and pore volumes we were able to investigate the pore wettability with different solvents by using 1H relaxation MRT techniques. To investigate them as active material in the Li/O2 system they were synthesized directly into a woven carbon paper to create ready-to-use electrodes without any binder. Here, the discharge reaction was studied by electrical impedance spectroscopy (EIS) using 1M LiTFSI/DMSO as electrolyte and Li metal as reference electrode in a three electrode setup. A model for the interpretation of the limiting factors regarding the GDE performance due to the formation and deposition of the discharge product could be developed. For further investigation of these factors the chemical and morphological characterization of the discharge products in dependence on the GDE design was addressed by XPS analyses. Besides looking at the electrolyte facing side the deposition of discharge product in the bulk of the active layer in the GDE and at the oxygen facing side were investigated. For information about the bulk of the GDE cross-section measurements as well as depth profiling by argon ion sputtering were conducted.

Enhanced performance of gas diffusion electrode for electrochemical reduction of carbon dioxide to formate by adding polytetrafluoroethylene into catalyst layer

Gas diffusion electrode (GDE) with Nafion bonded catalyst layer (CL) for electrochemical reduction of CO2 to formate (ERCF) suffers from CO2 mass transfer limitation. In this work, polytetrafluoroethylene (PTFE) with contents of 5.9 wt%, 7.7 wt%, 11.1 wt% and 20 wt% are added into the CL of the GDE with Sn catalyst (P-SGDE) for ERCF. The morphologies and porous structures of the P-SGDEs are examined by scanning electron microscope and mercury intrusion measurement, respectively. The electrochemical performances of the P-SGDEs are investigated by linear sweep voltammetry, electrochemical impedance spectroscopy and constant potential electrolysis. The results show that the Faraday efficiency (86.75 ± 2.89%) and current density (21.67 ± 1.29 mA cm−2) for ERCF were improved by 25.4% and 25.8% respectively when the content of PTFE is 11.1 wt%, probably owing to the enhancement in the catalyst active surface area and CO2 diffusion. This Faraday efficiency is the highest one found for ERCF with Sn GDE under similar conductions.

Determination of Charge and Mass Transfer for Molten Carbonate Fuel Cell Cathode Using NiO Coated Ring Electrode

Introduction Molten carbonate fuel cell (MCFC) systems have very high energy conversion for distributed generation systems, and the largest electric power generation capacity in stationary fuel cell systems. In order to improve energy conversion efficiency and durability, cathode and electrolyte materials are key factors. We found that the solubility of LaNiO3 in the La2O3 saturated molten alkaline metal carbonates is significantly smaller than that of NiO in the La2O3 free molten carbonates1), and should know the kinetics on the LaNiO3 in the carbonates for oxygen reduction reaction (ORR). However, even the kinetics of the ORR on the NiO is not clarified enough2). Furthermore, the performance of gas diffusion electrode for practical system should be affected by both charge and mass transfer.In this study, charge transfer on a sintered NiO electrode and mass transfer through a meniscus on the electrode have been investigated with chronoamperometry using a meniscus ring electrode to establish electrochemical method for the evaluation of charge and mass transfer separately. Experimental Li/Na (=52/48 mol %) or Li/K (=62/38 mol %) molten carbonate were used as an electrolyte. The working electrode was a sintered NiO on a gold ring electrode. The height of the working electrode from surface of molten carbonate was controlled by a micrometer. Reference electrode was a reversible oxygen electrode (ROE), and counter electrode was a gold coil electrode. The surface area of the sintered NiO was BET method.The oxygen reduction reaction current was determined with chronoamperometry (CA) from 10 to -10 mV vs. ROE. The initial current density was evaluated by extrapolation to t = 0 with the linear relation between current and the square root of time to determine mass transfer free polarization at 2mm of the height. The steady state current density was evaluated for 100 s after 500 s of CA at 4mm of the height with mass transfer resistance of the meniscus.Symmetry factor: a was determined with Allen-Hickling equation for the mass transfer free polarization curve, and the exchange current density: i 0 was determined with mass transfer free Butler-Volmer equation, the a and polarization curve. The limiting current density: i l was determined with Butler-Volmer equation with the i l, i 0, a, the steady state polarization curve. Results and discussion Figure 1 shows the i 0 (Fig. 1-a)) and the i lc (Fig. 1-b)) as a function of CO2 or O2 partial pressure in CO2-O2-Ar. The CO2 or O2 partial pressure was fixed at 0.1 atm for p O2 or p O2 function, respectively. The i 0 the i lc were determined at 2 mm height to minimize the influence of ohmic drop of the electrolyte and at 4 mm height to minimize mass transfer resistance of reaction gas through the meniscus. Here, the i 0 and the i lc are normalized the BET surface area and geometrical surface area, respectively. The BET surface area of the sintered NiO was 2.5 m2g-1. Therefore, the apparent exchange current density per geometrical surface area was 1000 times larger than the limiting current density. The dependence of the i 0s on the partial pressures were smaller than that of the i lcs. All slopes could not explained by a simple reaction mechanism model of peroxide, superoxide, or parcarbonate path2). The polarization of practical electrode depends on liquid – gas interface surface area rather than real surface area of the electrode3). Therefore, performance of the practical electrode is controlled by the combination of the charge and mass transfer, and the i 0 and the i lc, which is a function of gas solubility, diffusion coefficient, and morphology of molten carbonate film, should be an effective index to determine activity of electrode and property of electrolyte. References 1) K. Matsuzawa, Y. Akinaga, S. Mitsushima, and K. Ota, J. Power Sources, 196, 5007 (2011).2) T. Nishina, Y. Masuda and I. Uchida, Molten Salt Chem., 93, 424 (1993).S. Kuroe, M. Takeuchi, S. Nishimura, K. Ohtsuka, DENKIKAGAKU, 58, 928 (1990).