Mass Transfer Overpotentials Research Articles

Energy consumption model of flow-through electrolyzer considering mass transfer overpotential for hydrogen production from alkaline water splitting

Flow-through electrolyzer can improve the efficiency of hydrogen production from alkaline water splitting and determining the optimal electrolyte velocity through electrode pores is crucial for reducing energy consumption. In this study, an energy consumption model of flow-through electrolyzer considering mass transfer overpotential (Emass) has been developed. The model of Emass due to gas hold-up (ϕ) in the electrolyzer accounts for the density, viscosity, surface tension, linear velocity, and current density. The model-predicted value is in good agreement with the experimental results, which indicated that the increase of velocity effectively reduces ϕ and Emass. Then, Voltage-Current curve models combining reversible voltage, activation overpotential, ohmic overpotential, and Emass are developed. Based on this, an energy consumption model, combining both consumption from electrolyzer and pumps, is established to evaluate the overall energy consumption of hydrogen production. It can be calculated the Emass at 400 mA cm−2, 6 M KOH, 353 K can be reduced from 0.363 V to 0.048 V if the circulation velocity rises from 0.001 m s−1 to 0.1 m s−1. The model predicts that total energy consumption is 4.36 kWh Nm−3 with energy consumption for pump being 0.02 kWh Nm−3 at 400 mA cm−2 and 0.063 m s−1.

Interleaved Electrode Structures for Enhanced Electrolyte Mass Transport in Li-Ion Batteries : A Numerical Study

The enhancement of energy and power density in Li-ion batteries remains a focal point in battery research. When Li-ion batteries are operated at high C-rates, mass transport limitations cause large concentration gradients of Lithium ions to form in the electrolyte. The resultant concentration overpotentials and underutilization of active material effectively imply a lower capacity for higher current rates, which creates a tradeoff between energy and power density. The integration of 3D interleaved electrode structures has been proposed to augment power density while preserving energy density. This work comprehensively analyzes the capacity improvement achieved by utilizing a wavy interleaved structure using electrochemical simulations. A pseudo-3D (P3D) model based on the Doyle-Fuller-Newman framework is employed to analyze the performance of various depths and frequencies of the wavy interleaved structure. Galvanostatic simulations are performed for NMC/Graphite-SiOx and LMO/Graphite cells at different C-rates, and the resultant voltage is systematically divided into component overpotentials to delineate the contribution of activation, ohmic, and mass-transfer overpotentials in both solid electrode and electrolyte. With the same amount of active material, simulations indicate a performance improvement of 7% in capacity at high C-rates. It is observed that using a wavy structure significantly reduces electrolyte concentration heterogeneities and resulting concentration gradients when compared to a conventional flat structure. The overpotential breakdown confirms that the improvement in voltage is indeed contributed precisely by the concentration overpotential. Concentration overpotential decreases with deeper interleaved structures as well as for more spatially frequent structures.Additionally, the impact of the interleaved electrode structure on cell degradation was also assessed. Lithium plating, which is the most prominent side reaction in the anode, was modeled using a Butler-Volmer kinetic equation, with deposition occurring locally when the solid phase potential drops below that of the electrolyte potential. The interleaved structure alleviates plating significantly due to higher uniformity in Li-ion concentration. With this work, we computationally demonstrate that interleaved structure works well for extracting higher capacity with the same active material, and also aids in suppressing Lithium plating.Figure Caption : Computed Li-ion concentration profile in the solid active material of an interleaved electrode Li-ion cell as a function of location (x, y) and distance from particle center (z) Figure 1

The Effect of PTFE Dip Coating on Enhancing the Durability and Performance of Anion Exchange Membrane Electrolyzers

Anion exchange membrane electrolyzers (AEMELs) are widely regarded as one of the most promising technologies to enable low-carbon hydrogen production. AEMELs can combine the advantages the traditional alkaline electrolyzer (AEL) and proton exchange membrane electrolyzer (PEMEL) – allowing for the use of PGM free electrodes, high operating current density, environmentally friendly materials, and compact design. Although significant progress has been made in the field of AEMELs, there is still a gap in performance compared to PEMELs that can be overcome through electrode design.One of the most critical aspects affecting the overall performance of the AEMEC is the water management. It has a major influence on cell behavior, and specifically mass transfer at the anode side of the cell. To improve AEMEL anode water dynamics, our team modified its porous transport layer (PTLs) through the addition of controlled amounts of hydrophobic polytetrafluoroethylene (PTFE). This study investigated the effect of PTFE loading and coating method on the performance of AEMELs. Ni-based PTLs were used. Experimental results revealed that a low amount of PTFE not only enhanced the performance, but it also enhanced the catalyst layer attachment. On the other hand, higher PTFE content led to a reduction in water availability in the electrode, leading to increased mass transfer overpotentials. In summary, increasing the hydrophobicity of Ni-PTLs appears to improve AEMEC performance, with the best performing electrodes having a PTFE loading of ~1-2% (equivalent to 5-10 wt.% on carbon GDLs).

Understanding the process of carbon corrosion and its impact on performance degradation during simulated start-stop operations for the proton exchange membrane fuel cell

There remains a need for a thorough understanding of the process of carbon corrosion in proton exchange membrane fuel cell and its impact on performance degradation so that appropriate preventive measures against carbon corrosion can be implemented. In this study, we examined the carbon corrosion in detail during simulated start-stop cycles. Using double-layer capacitance as an indicator, we tracked the transformations of electrode structure and surface properties. The impact of carbon corrosion on the performance degradation was analyzed via voltage loss breakdown based on the H2/air polarization curve. Ex-situ characterization techniques were conducted to monitor the change of the surface chemical properties and microstructures of electrode. The results show that carbon corrosion and cell performance degradation proceeded in three stages during simulated start-stop cycles. Within 150 cycles (Stage 1), the cell overpotential increased slightly, which was due to the increase in the ohmic and gas diffusion medium mass transfer overpotential rather than the electrochemical area loss. After 600 cycles (Stage 2), significant electrode degradation occurred and electrode thickness was reduced to less than its half. As a result, the cell overpotential increased dramatically (by 35.8% at 0.55 A cm−2), which is mainly attributed to the increase of mass transfer overpotential. Eventually (Stage 3), the electrode structure was stabilized after all the surface defects and amorphous carbon were removed. Among these three stages, Stage 2 is the critical stage that causes the major degradation of cell performance.

Understanding the Interaction between Flow Field Geometries and Porous Electrode Microstructures in Redox Flow Batteries

Redox flow batteries are a promising technological option to integrate the growing supply of renewable energies into the electricity grid, however their deployment is hampered by high costs. To increase cost competitiveness, research efforts have targeted design of new electrolytes, high performance materials, and alternative electrochemical reactor concepts [1]. One powerful strategy is to increase the overall efficiency of the electrochemical stack, which can be achieved by improving the electrochemical performance and reducing the pumping power requirements. Selecting and optimizing the flow field design and electrode microstructure is crucial to accomplish an optimum trade-off. Drawing inspiration from polymer electrolyte fuel cells, current flow battery technologies leverage flow-through, interdigitated and serpentine flow field designs [2]. However, while functional, these designs have not been tailored for the specific requirements of redox flow batteries where single-phase reactive flows are sustained. Recent studies have investigated the influence of the channels and ribs dimensions [3], branched channel geometries [4], as well as the electrode microstructure on the reactor performance [5]. However, the interaction between the flow field geometries and the electrode microstructure determines the accessible surface area, mass transfer phenomena, and pressure drop; but remains poorly understood. With this in mind, we are poised to answer the following scientific question: What is the optimal combination of flow field and electrodes in redox flow batteries? In this work, we evaluate the interaction between geometrically diverse flow field geometries and porous electrode microstructures. We study seven different flow field designs in combination with two commonly used fibrous electrode structures – a carbon paper and a cloth (Figure 1). Flow-through, serpentine, and multiple variations of interdigitated flow fields were designed and fabricated by graphite milling. We employ a suite of polarization, electrochemical impedance spectroscopy, capacitance, and pressure drop measurements to elucidate structure-property-performance relationships. We find that the interdigitated designs perform better with high density of channels (i.e. shorter rib-channel width), even though this leads to higher pressure losses. Interestingly, pressure drop measurements show a similar relative contribution of the flow field and the electrode to the pumping losses, which motivates engineering of flow field geometries and electrode structures in tandem. Mass transfer overpotentials and pressure losses in cloth electrodes are reduced when using flow field geometries that force electrolyte flow into an in-plane direction in the electrode (i.e. parallel to the membrane plane), such as flow-through and interdigitated designs with wider ribs between channels. On the contrary, carbon paper electrodes perform better with interdigitated designs that force electrolyte flow into the through-plane direction (i.e. perpendicular to the membrane plane). Based on these findings, we have undertaken the engineering of innovative flow fields designs by combining interdigitated and branched patterns using 3D-printing, obtaining promising results that will be discussed in the final part of my talk.Figure 1. Polarization results for the combination of cloth and paper electrodes with three different flow field designs, at 5 cm·s-1 in the electrode. References Sánchez-Díez, E. Ventosa, M. Guarnieri, A. Trovò, C. Flox, R. Marcilla, F. Soavi, P. Mazur, E. Aranzabe, R. Ferret, Journal of Power Sources, 481, 228804 (2021).D.Milshtein, K.M.Tenny, J.L.Barton, J.Drake, R.M.Darling and F.R.Brushett, J. Electrohcem. Soc., 164, E3265-E3275 (2017).R.Gerhardt, A.A. Wong, M.J. Aziz, J. Electrohcem. Soc., 165, A2625-A2643 (2018).Zeng, F. Li, F. Lu, X. Zhou, Y. Yuan, X. Cao, B. Xiang, Applied Energy, 238, 435-441 (2019).Forner-Cuenca, E.E. Penn, A.M. Oliveira, F.R.Brushett, J. Electrohcem. Soc., 166, A2230-A2241 (2019). Acknowledgments This work has been partially funded by the Agencia Estatal de Investigación (PID2019-106740RB-I00 and RTC-2017-5955-3/AEI/10.13039/501100011033). Figure 1

Electrografting As a Versatile Approach to Engineer Porous Electrode Interfaces for Redox Flow Batteries

The intermittent nature of renewable sources can be alleviated by grid-scale energy storage technologies, such as redox flow batteries (RFBs). However, these systems are currently not cost-competitive for widespread deployment. The system costs are linked to stack performance that is limited by kinetic, ohmic and mass-transfer overpotentials as well as materials stability. Carbon fiber-based porous electrodes, ubiquitous in electrochemical reactors, strongly influence these limiting phenomena as they facilitate redox reactions on their surface, conduct electrons, and provide flow paths for reactant transport. Unfortunately, the inherently hydrophobic and inert carbon surface is not optimal for aqueous electrolytes with kinetically sluggish redox couples such as vanadium and iron1. In these systems, controlling the surface chemical state of the electrode is vital to attain significant performance gains. Popular strategies to modify surface chemistry are thermal and acid treatments with a general aim to increase the heteroatom content and number of functional groups of the carbon surface2. These functional groups increase the surface energy of inherently hydrophobic carbon electrode and can serve as active sites for redox-reactions3. However, these treatment strategies can cause embrittlement of the electrode4, mass-loss5, and the formed functional groups, especially in the case of oxygen, can manifest in multiple chemical forms, hindering proper structure-property relationships to be established. Thus, to correlate the surface chemical state to the battery performance, there is a need to develop methodologies to synthesize homogenous, conformal, and stable interfaces onto three-dimensional porous electrodes6.Here we propose electrografting as a surface modification strategy for carbon fiber-based RFB electrodes with a model molecule, taurine. Electrografting is the electrochemical analogue of chemical grafting where covalent bonds are formed between species and the conductive substrate7, with an added advantage that the charge transfer reactions responsible for bond formation can be controlled with applied voltage. We selected taurine as a model molecule to graft on carbon cloth electrodes as its amine group can undergo oxidative electrografting and we hypothesize its sulfonic acid group to be beneficial for electrode wetting, allowing coverage of the electrode surface with a thin layer of desired functional groups. We performed diagnostic studies on glassy carbon electrodes by hydrodynamic voltammetry, where the kinetic rate for the reduction of Fe3+ on taurine treated electrodes is revealed to be an order of magnitude faster than untreated electrodes (1.23 x 10-4 vs 1.84 x 10-5 cm s-1). In-situ flow cell studies in single cell configuration revealed improved performance for mixed Fe2+/3+ electrolyte, especially at lower flow rates (Figure 1). Also, for the first time, we visualized wetting dynamics of porous RFB electrodes in flow cells using neutron radiography. We find that treated electrodes imbibe the acidic electrolyte instantaneously even at low flow rates, which indicates the electrode interfaces feature hydrophilic character. Finally, we performed extended in-situ stability tests under mixed Fe2+/3+ electrolyte flow at open-circuit and applied voltage conditions. Impedance spectroscopy revealed performance degradation with pristine electrodes but not with taurine treated electrodes, confirming the formation of a stable interface with electrografting of taurine. In summary, we show that electrografting of taurine is a facile and environmentally benign approach to functionalize porous electrodes for aqueous redox flow batteries. Beyond this specific application, the possibility to extend the molecular library makes electrografting a suitable approach to engineer interfaces for next-generation electrochemical devices. References P. Chen and R. L. McCreery, Anal. Chem., 68, 3958–3965 (1996).K. Jae Kim et al., Journal of Materials Chemistry A, 3, 16913–16933 (2015).R. H. Bradley and P. Pendleton, Adsorption Science & Technology, 31, 113–133 (2013).L. Yue, W. Li, F. Sun, L. Zhao, and L. Xing, Carbon, 48, 3079–3090 (2010).K. V. Greco, A. Forner-Cuenca, A. Mularczyk, J. Eller, and F. R. Brushett, ACS Appl. Mater. Interfaces, 10, 44430–44442 (2018).A. Forner-Cuenca and F. R. Brushett, Current Opinion in Electrochemistry, 18, 113–122 (2019).D. Bélanger and J. Pinson, Chem. Soc. Rev., 40, 3995–4048 (2011). Figure 1

Investigation of the correlation effects of catalyst loading and ionomer content in an anode electrode on the performance of polymer electrode membrane water electrolysis

In this study, the correlation effect of catalyst loading (Lc) and ionomer content (Ciono) on the performance of polymer electrolyte membrane water electrolysis (PEMWE) is investigated. Sixteen membrane electrode assemblies are constructed and electrochemically evaluated using a cross-combination of two factors. Although the coupling of the two factors does not significantly affect the performance at a low working voltage, the effect is dominant at a high working voltage. Based on the establishment of correlations between the changes in the contributing factors (including the ionomer fraction, thickness, and porosity) caused by variations in the two factors, the effects on the kinetic, ohmic, and mass transfer overpotential are estimated and analyzed. Since the Ciono and Lc simultaneously affect the porosity, the required Ciono for achieving the best performance changes according to the Lc and vice versa. Additionally, a rapid drop in performance when the Ciono exceeds 25 wt% is due to the inhibited electron transfer in electrode and interface between the electrode and porous transport layers, and a critical ionomer concentration is proposed. Finally, high-performance PEMWE (6.882 A cm−2 at 2.05 V) is achieved with a low Lc of 0.5 mg cm−2 and a Ciono of 10 wt%.

Non-Solvent Induced Phase Separation: A Versatile Synthetic Method for High Performance Redox Flow Battery Electrodes

Carbonaceous porous electrodes are ubiquitous to advanced electrochemical systems where they are responsible for multiple critical functions in the cell related to thermodynamics, kinetics, and transport, including providing surfaces for electrochemical reactions, conducting electrons and heat, and distributing fluids. Thus, their design governs the performance, durability, and consequently, the cost of these systems. However, there is limited knowledge about how to deterministically design these electrode materials which forces the repurposing of materials that have not been tailored for the specific application (e.g. the use of polymer electrolyte fuel cell gas diffusion layer as redox flow battery porous electrode1,2). Our current arsenal of materials is limited to fibrous electrodes which are manufactured using various mechanical methods (e.g. paper making, weaving, hydro-entangling) resulting in idiosyncratic structures such as papers, cloths, and felts. These fabrication methods involve multiple complex subprocessing steps impacting the final manufacturing cost and offering limited versatility to control the electrode microstructure and surface composition, which ultimately limits the performance of the electrochemical cell. Thus, there is a need to develop novel material sets with precise control over microstructure and composition while employing synthetic methods that are compatible with large scale manufacturing. In this work, we focus our efforts on designing tailored electrodes for redox flow batteries (RFBs), which are promising for grid-scale energy storage3 if their costs can be significantly reduced4.Here, we introduce the non-solvent induced phase separation (NIPS) as a simple and versatile fabrication method for carbonaceous porous electrodes for redox flow batteries5. Drawing inspiration from membrane technology, the NIPS method has been leveraged to synthesize morphologically-diverse microstructures (e.g., isoporous, macrovoids, porosity gradient) which are appealing to electrode manufacturing6. A polymer solution, containing polyacrylonitrile (PAN, carbon-containing) and polyvinylpyrrolidone (PVP, pore-forming agent) dissolved in N,N-dimethylformamide (solvent) was casted in a mold and subsequently immersed in water (non-solvent). Finally, the polymeric scaffold is carbonized under inert conditions to form a conductive network. Easily adjustable parameters, such as solvent type, polymer concentration and temperature enable control of the final electrode microstructure. To elucidate material synthesis-property-performance relationships, we vary the polymer content, polymer ratio, and solvent type and perform microscopic, spectroscopic, and electrochemical characterization techniques over the synthetized electrodes. Microstructural characterization revealed a multimodal pore size distribution composed of fine, interconnected microvoids (pore diameter 2-15μm) coupled with through plane, finger-like macrovoid channels (throat diameter > 50 μm) forming honeycomb networks. The unique microstructure, not attainable with traditional carbon-fiber manufacturing techniques, enables large surface area at the membrane-electrode interface and fast electrolyte replenishing which reduces mass transfer resistance within the electrode. We performed flow cell tests with an aqueous iron single electrolyte which revealed a considerable reduction of the charge transfer and mass transfer overpotentials of the novel electrodes compared to the commercial baseline at the expense of a slight increase in pressure drop. In the final part, we demonstrate the use of NIPS-electrodes in a full all-vanadium RFB which results in a significant improvement in power density compared to the baseline material, which can be attributed to reductions in the charge transfer and mass transport overpotentials. Although nascent, NIPS emerges as a promising platform to engineer porous electrodes for RFBs and other convection-enhanced electrochemical systems. References Mathias, M. F., Roth, J., Fleming, J. & Lehnert, W. Diffusion media materials and characterisation. Handb. Fuel Cells 3, 517–537 (2010).Forner-Cuenca, A. & Brushett, F. R. Engineering porous electrodes for next-generation redox flow batteries: recent progress and opportunities. Curr. Opin. Electrochem. 18, (2019).Weber, A. Z., Mench, M. M., Meyers, J. P., Ross, P. N., Gostick, J. T. & Liu, Q. Redox flow batteries: A review. J. Appl. Electrochem. 41, 1137–1164 (2011).Darling, R. M., Gallagher, K. G., Kowalski, J. A., Ha, S. & Brushett, F. R. Pathways to low-cost electrochemical energy storage: a comparison of aqueous and nonaqueous flow batteries. Energy Environ. Sci. 7, 3459–3477 (2014).Wan, C. T. C., Jacquemond, R. R., Chiang, Y. M., Nijmeijer, K., Brushett, F. R. & Forner-Cuenca, A. Non-Solvent Induced Phase Separation Enables Designer Redox Flow Battery Electrodes. Adv. Mater. 2006716, (2021).Young, T. H. & Chen, L. W. Pore formation mechanism of membranes from phase inversion process. Desalination 103, 233–247 (1995).

Contamination Mechanisms of Proton Exchange Membrane Fuel Cells - Mass Transfer Overpotential Origin

Proton exchange membrane fuel cells (PEMFCs), fed with oxygen from ambient air and hydrogen, spontaneously produce power. However, key materials, gas diffusion layers, catalysts, ionomers, and membranes, are sensitive to a multitude of other species that may be inadvertently introduced into PEMFCs even at parts per million (ppm) or less levels. For a commercially relevant catalyst loading of 0.1 mg Pt cm–2 and a severe case such as 100 ppm acetylene in air, the temporary cell voltage loss exceeds 80 % [1]. Exposure to species such as bromomethane is even more problematic because the cell voltage cannot be restored without a specific recovery procedure [2]. The development of preventive and recovery procedures to improve durability is facilitated by contamination mechanisms. Although mass transfer losses were often observed in the presence of organic species [1], the commonest contaminants either in air or released by system components [3], the origin of this effect is currently unclear. It may stem from adsorption on carbon [4] or platinum [5] respectively modifying surface hydrophobicity and liquid water management or reducing the catalyst active surface, which increases the real current density.Several diagnostic techniques able to either characterize water management or oxygen mass transfer were used to evaluate PEMFCs contaminated with propene or methyl methacrylate. Neutron radiographs were obtained for both in-plane and through-plane cell configurations to record changes in liquid water distribution [6,7]. Polarization curves were measured with O2, 21 % O2 in He, and air to derive concentration, molecular diffusion and ionomer permeability mass transfer overpotentials [8]. Overall oxygen mass transfer coefficients were acquired by first operating PEMFCs fed with 1 % to 7 % O2 in He, N2 or CO2 streams under limiting current conditions. Subsequently, a mathematical model was used to fit average limiting current data as a function of oxidant flow rate to extract the overall oxygen mass transfer coefficient. Separation of the overall mass transfer coefficient into molecular and Knudsen diffusion, and ionomer permeability contributions was conducted by plotting mass transfer resistance data (inverse of the mass transfer coefficient) either as a function of the O2 diluent molecular weight or O2 concentration and extrapolating linear correlations to the origin. The original separation method [9] was improved by taking advantage of the insensitivity of Knudsen diffusion to variations in O2 concentration. Propene and methyl methacrylate were selected because they have largely different solubility in water (surrogate hydrophobicity measure), minimize test durations and simplify test procedures (fast contamination and recovery kinetics), and facilitate data interpretation (minimal current distribution and effective concentration changes).Experimental data demonstrated that changes in liquid water distributions were subtle and cannot explain mass transfer losses induced by contaminants. In contrast, ionomer permeability overpotentials and mass transfer coefficients revealed major increases in the presence of both contaminants. These observations support the notion that propene and methyl methacrylate contaminants adsorb on the Pt catalyst, decrease the active area, and bring the real current density closer to the limiting value. Experimental data analyses based on a contamination model that include catalyst surface processes for both O2 and contaminant (adsorption, reaction, desorption) [10] confirmed this hypothesis. The proposed contamination mechanism is similar to the increase in oxygen mass transfer overpotential induced by a decrease in Pt catalyst loading [11].[1] J. St-Pierre, Y. Zhai, Molecules, 25 (2020) article 1060.[2] Y. Zhai, O. Baturina, D. Ramaker, E. Farquhar, J. St-Pierre, K. Swider-Lyons, Electrochim. Acta, 213 (2016) 482.[3] J. St-Pierre, M. Angelo, K. Bethune, J. Ge, S. Higgins, T. Reshetenko, M. Virji, Y. Zhai, Electrochem. Soc. Trans., 61(23) (2014) 1.[4] X. Zhang, B. Gao, A. E. Creamer, C. Cao, Y. Li, J. Hazard. Mater., 338 (2017) 102.[5] Y. Itagaki, M. Mori, Y. Sadaoka, Curr. Opin. Electrochem., 11 (2018) 72.[6] D. S. Hussey, D. L. Jacobson, M. Arif, K. J. Coakley, D. F. Vecchia, J. Fuel Cell Sci. Technol., 7 (2010) article 021024.[7] D. S. Hussey, D. Spernjak, A. Z. Weber, R. Mukundan, J. Fairweather, E. L. Brosha, J. Davey, J. S. Spendelow, D. L. Jacobson, R. L. Borup, J. Appl. Phys., 112 (2012) article 104906.[8] J. St-Pierre, M. Angelo, K. Bethune, J. Huizingh, T. Reshetenko, M. Virji, Y. Zhai, Modern Fuel Cell Testing Laboratory, in Springer Handbook of Electrochemical Energy, Part D, Chapter 19, Edited by C. Breitkopf, K. Swider-Lyons, Springer, 2017, p. 611.[9] T. Reshetenko, J. St-Pierre, J. Electrochem. Soc., 161 (2014) F1089.[10] J. St-Pierre, J. Electrochem. Soc., 156 (2009) B291.[11] A. Z. Weber, A. Kusoglu, J. Mater. Chem. A, 2 (2014) 17207.

Contamination Mechanisms of Proton Exchange Membrane Fuel Cells - Mass Transfer Overpotential Origin

Several experimental methods were used to identify the cause of the concurrent increase in kinetic and mass transfer overpotentials during the contamination of proton exchange membrane fuel cells o...

(Invited) Performance and Durability of Automotive Fuel Cell Stacks and Systems with Low-Loaded d-PtCo/C Cathode Catalyst in Membrane Electrode Assemblies

Previous studies have shown that meeting the 30 $/kWe cost and 8,000-h durability targets for automotive fuel cell systems (FCS) requires high-activity oxygen reduction reaction (ORR) cathode catalysts that are stable under cyclic potentials at low Pt loadings (<0.1 mg-Pt/cm2). We have evaluated the performance and durability of a state-of-the-art (SOA) fuel cell stack and system under automotive relevant conditions using data obtained for a membrane electrode assembly (MEA) operated under differential cell conditions. The MEA has de-alloyed Pt3Co catalyst supported on a high surface area carbon (HSAC) in cathode, Pt catalyst supported on Vulcan carbon in anode, thin 12-μm reinforced PFSA membrane, and 200-μm thick diffusion media with microporous layers. The Pt loadings are 0.1 mg/cm2 in the cathode catalyst and 0.025 mg/cm2 in the anode catalyst. We formulated a model for ORR kinetics on d-PtCo/C catalyst by using the measured polarization data at low current densities together with the ionic conductivity of the cathode catalyst layer derived from the Galvanostatic impedance data obtained in H2/N2, and a transient solid solution model for oxide coverage as a function of potential, relative humidity and temperature. The kinetic model indicates that d-PtCo/C catalyst has 650 A/gPt mass activity, which is double the mass activity of annealed Pt catalyst that has nearly the same particle size, and easily exceeds the target of 440 A/gPt. In accelerated stress tests (AST) with 0.6-0.95 V square wave potentials in cyclic voltammetry, there are negligible changes in kinetic parameters denoting the reaction order for O2 partial pressure, activation energy and relative humidity (RH) dependence. As indicated in Fig. 1, the specific activity (µA/cmPt 2) degraded by ~33% in AST during which time the electrochemically active surface area (ECSA) decreases by 7.5% and nearly stayed constant with further decrease in ECSA. We concluded that the mass activity, which is a product of specific activity and ECSA, degrades in AST initially because of decrease in specific activity due to cobalt dissolution and subsequently because of decrease in ECSA due to coarsening of catalyst particles. We determined the oxygen transport resistance of the d-PtCo/C electrode (Rcf) by (a) using the measured polarization data at high current densities; (b) estimating mass transfer overpotentials (ηm) from the ORR kinetic model; (c) determining the limiting current densities (iL) at which ηm equals 400 mV; (d) estimating the oxygen transport resistance (ηm) from iL; and (e) extracting the pressure-dependent and pressure-independent (Rcf) parts of Rm. We concluded that after 30,000 AST cycles, the pressure-dependent part of Rm representing O2 transport resistance in gas channel and gas diffusion layer (GDL) showed small changes; however Rcf has increased by >50% at 40% RH exposure and >125% at 100% RH exposure. Lower roughness (SPt, cmPt 2/cm2) because of catalyst coarsening accounts for 8% of the increase in Rcf at 40% exposure and for 45% increase at 100% exposure. Also, 48-60% of the increase in Rcf may be associated with the reduction in O2 permeability through the ionomer film on the catalyst particles supported on HSAC. We developed an integral cell model using the differential cell data to evaluate the performance and cost of an automotive FCS with the d-PtCo/C catalyst relative to the targets of 65% peak efficiency, 8.0 kWe/g stack Pt utilization, 1000 mW/cm2 stack power density and 40 $/kW cost, all subject to 1.45 kW/K (Q/ΔT) heat rejection constraint. The analysis indicates that the SOA catalyst can achieve 9.5 ± 0.5 kWe/g stack Pt utilization and 1180 ± 55 mW/cm2 gross stack power density at 656 mV cell voltage, 2.5 atm stack inlet pressure, 95oC stack coolant outlet temperature, and 1.5 cathode stoichiometry. The projected system cost is 46.0 ± 0.7 $/kWe at high manufacturing volume (500,000 units/year). At lower manufacturing volumes, the estimated costs are 51 $/kWe at 100,000 units/year and 88 $/kWe at 10,000 units/year. We projected FCS performance degradation in terms of ECSA loss by conducting simulations at constant air flow rate (variable cathode stoichiometry) and heat rejection (Q). The simulations indicate that meeting the target of 10% derating in FCS power over lifetime requires controlling the stack operating conditions to limit the ECSA loss to 35%. At end of life, we estimate that mass activity has degraded by ~50%, specific activity by 25%, and the cell voltage at rated power by 30 mV. Increase in kinetic overpotential due to Co dissolution and coarsening of catalyst particles accounts for the majority of the projected cell voltage loss at rated power. Figure 1

Experimental Measurement of Overpotential Sources during Anodic Gas Evolution in Aqueous and Molten Salt Systems

Current interrupt and galvanostatic EIS techniques were utilized in a complementary fashion to characterize the different sources of overpotential during anodic gas evolution. Room temperature anodic evolution of oxygen at a nickel working electrode in aqueous potassium hydroxide and the high temperature (348°C) anodic evolution of chlorine at a glassy carbon working electrode in molten (LiCl)57.5-(KCl)13.3-(CsCl)29.2 where investigatd. Combining of the two techniques enables to separate the total measured overpotential into its ohmic, charge transfer, and mass transfer components. Potential decay curves indicated that natural convection (due to both bubble evolution and density driven flow) was a major driving force in reestablishing equilibrium conditions at the working electrode surface. During oxygen evolution, charge transfer resistance dominated the total overpotential at low current densities, but as the current density approached ∼100mA/cm2, mass transfer overpotentials and ohmic overpotential became non-negligible. The mass transfer overpotential during chlorine evolution was found to be half that found during oxygen evolution.

Durability of Pt-Co Alloy Polymer Electrolyte Fuel Cell Cathode Catalysts under Accelerated Stress Tests

The durability of Pt-Co alloy cathode catalysts supported on high surface area carbon is investigated by subjecting them to accelerated stress tests (ASTs). The catalysts had different initial Co contents and nanoparticle morphologies: a “spongy” porous morphology for the high-Co (H) content catalyst, and a fully alloyed crystalline morphology for the medium-Co (M) and low-Co (L) content catalysts. The specific activity of the catalysts depends on their initial Co content, morphology and nanoparticle size, and remained higher than 1000 μA/cm2-Pt after 27–50% Co loss. The H-catalyst electrode showed the smallest kinetic overpotentials (ηcs) due to higher initial Pt loading than the other two electrodes, but it had the fastest increase in ηcs with AST cycling due to lower Co retention; the L-catalyst electrode showed higher ηcs due to a lower initial Pt loading, but had a smaller increase in ηcs with aging due to higher Co retention; the M-catalyst electrode showed a similar increase in ηcs with aging, but this increase was due to the combined effects of Co dissolution and electrochemically active surface area (ECSA) loss. The modeled increase in mass transfer overpotentials with aging correlates with the initial Pt loading, ECSA loss and the initial catalyst morphology.

Durability of De-Alloyed Platinum-Nickel Cathode Catalyst in Low Platinum Loading Membrane-Electrode Assemblies Subjected to Accelerated Stress Tests

The durability of de-alloyed platinum-nickel catalysts supported on high-surface area carbon (d-PtNi/C) in optimized electrodes and membrane electrode assemblies (MEAs) under an accelerated stress test (AST) protocol is investigated with the objective of developing a quantitative understanding of the degradation mechanisms and their relationship to the electrode structure, pre-conditioning, and operating conditions. It is found that the cell degradation can be mitigated by controlling the voltage cycle, acid washing the MEA to remove Ni contaminants that enter the electrode and membrane during fabrication, and monitoring the operating conditions. For example, the electrochemical surface area (ECSA) loss is <25% after 30,000 triangle cycles with 0.925 V upper potential limit if the MEA is acid washed and extensive diagnostics are avoided. The parameters that exacerbate the cell degradation also accelerate the rate at which Ni leaches out from the catalyst. A mechanistic model is presented for the degradation in performance of d-PtNi/C electrodes. The model correlates a) the degradation in ORR mass and specific activities with ECSA and Ni losses, b) the decrease in limiting current density (iL), which is inversely proportional to the O2 mass transport resistance, with the degradation in catalyst roughness factor, and c) the increase in mass transfer overpotentials with the reduced current density, i/iL.

Durability of Pt-Co Alloy Cathode Catalysts for PEM Fuel Cells Under Cyclic Potentials

Alloying Pt with transition metals, such as Co and Ni, enhances the catalytic activity for the oxygen reduction reaction (ORR) at cathodes of polymer electrolyte fuel cells (PEFCs). The alloying process necessarily involves a high-temperature thermal annealing step that grows the size of catalyst nanoparticles above 4-5 nm. Particles of this size are known to be more stable under cyclic potentials as they are less prone to Pt dissolution and coalescence than finer scale 2-3 nm Pt particles. However, transition metals are unstable at operating fuel cell potentials and can leach out of the catalyst in the cathode and into the ionomer, resulting in loss of ORR activity. Three formulations of Pt-Co alloy catalysts supported on high surface area carbon were subjected to accelerated stress tests (ASTs) in H2-N2 environment. The AST protocol consisted of repeated square waves with 0.6 V lower potential limit (LPL), 0.95 V upper potential limit (UPL), and 3-s holds at LPL and UPL. Changes in cell performance were monitored by measuring polarization in H2-air, high frequency resistance (HFR), electrochemical surface area (ECSA), impedance in air and helox (79% He, 21% O2), and mass activity. These measurements were made at beginning of life (BOL) and after 1000 (1k), 20,000 (20k) and 30,000 (30k) cycles. The three catalysts had a different initial Co content (34%, 20%, and 15%) and electrode microstructure. At BOL, transmission electron microscopy (TEM) analysis of the high-Co content (H) catalyst electrode, 0.2 mg/cm2 Pt loading, showed a “spongy” porous (hollow) catalyst nanoparticle morphology with ~6.7 nm median Pt-Co particle size after conditioning. The BOL medium-Co content (M) catalyst electrode, 0.1 mg/cm2 Pt loading, showed a crystalline fully alloyed catalyst nanparticle morphology with ~4.4 nm particle size after conditioning. The BOL low-Co content (L) catalyst electrode, 0.1 mg/cm2 Pt loading, also showed catalysts with a fully alloyed crystalline morphology with ~4.5 nm median particle size.Post-mortem TEM analysis of aged electrode specimens showed different amounts of Co dissolution from the nanoparticles. There is an apparent direct relationship between the initial alloy Co content the amount of Co dissolved after potential cycles. WAXS analysis confirms significant Co loss from the nanoparticles and also indicates that the Pt-Pt d-spacing in Pt-Co alloy approaches the values typical of pure Pt particles. The effect of Co dissolution on the loss in specific activity for ORR is summarized in Fig. 1a. The data suggest that the specific activity depends not only on the initial Co content but also on the catalyst nanoparticle morphology and size. Even with 27-50% Co loss, the specific activity of Pt-Co alloy remains higher than 1000 mA/cm2-Pt, and exceeds the activity (650 mA/cm2-Pt) of pure Pt of similar particle size. The measured specific activities are consistent with a kinetic model and catalyst-specific choice of kinetic parameters.The modeled ORR kinetic losses depend on specific activity, ECSA, catalyst loading, and kinetic parameters (electrode morphology). As shown in Fig. 1b, the H-catalyst electrode shows the smallest kinetic overpotentials (ηc s) due to high Pt loading, but has the fastest increase in ηc s with ageing because of low Co retention in the electrode. The L-catalyst electrode shows higher ηc s due to lower Pt loading but has a smaller increase in ηc s with ageing because of high Co retention. For this catalyst, increase in is mostly due to loss of ECSA with ageing. The M-catalyst electrode shows similar increase in ηc s with ageing as the L-catalyst electrode, but this increase is due to the combined effects of Co dissolution and ECSA loss. The modeled increase in mass transfer overpotentials (ηm) with ageing correlates with Pt loading and ECSA loss and depends on the initial electrode morphology. There is no clear evidence of a relationship between Co dissolution (i.e., effect of Co2+ on O2 permeability in ionomer film) and the resulting increase in ηm. AcknowledgmentsThe authors wish to acknowledge the financial support of the Fuel Cell Technologies Office and Fuel Cell Component R&D Team Lead, Dr. Dimitrios Papageorgopoulus, and Technology Development Manager, Dr. Nancy Garland. The authors also wish to acknowledge General Motors, IRD and Umicore for supplying materials used in this study. The submitted manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. Figure 1

Low grade thermal energy harvester using graphene-based thermocells

Graphene-based thermocells for the conversion of low grade waste heat into electrical energy are investigated. A maximum current and power density of 0.63 A m−2 and 0.19 W m−2 respectively for 1 mV s−1 at a temperature gradient of 50 °C is obtained. The maximum energy conversion efficiency relative to Carnot efficiency is 1.57% which is 1.1 times higher than the literature data. A constant ohmic overpotential of 0.07 V is calculated from equivalent circuit analysis. This low value of ohmic overpotential indicates higher ionic conductivity in the electrolyte medium. The mass transfer overpotential is low and is calculated as 0.2133 V for all load variations, indicating constant redox behaviour and an increased energy conversion efficiency of the device. The double layer capacitance is estimated as 3.72 F at a very low load (ca. 1 mV s−1) and 0.32 F at a very high load (ca. 100 mV s−1) thereby demonstrating the functioning ability of the device at higher loads. The Seebeck coefficient for a graphene electrode is evaluated to be 0.0117 V K−1 and is in satisfactory agreement with the literature value of 0.02 V K−1 for carbon-based devices.

Long-Term Stability of Nanostructured Thin Film Electrodes at Operating Potentials

Long-term stability of nanostructured thin film (NSTF) catalysts at operating potentials has been investigated. Compared to high surface area Pt/C catalysts, NSTF electrodes show 20–50x smaller F− emission rates (FER) because of their high specific activity for oxygen reduction reaction (ORR), but are susceptible to poisoning by the products of membrane degradation because of their low electrochemically active surface area (ECSA). The observed voltage degradation rates at potentials corresponding to 1–1.5 A/cm2 current density are much higher than the allowable 13–14 μV/h. Although F− is not itself responsible for performance decay, cumulative fluoride release (CFR) is a good marker for catalyst surface contamination. The observed performance decay is not only due to loss of active Pt sites but also adsorbed impurities impeding ORR kinetics. There is a strong correlation between measured CFR and observed decrease in specific ORR activity and limiting current density and increase in mass transfer overpotentials. The correlations indicate that the target of <10% lifetime performance degradation can be achieved by restricting CFR in NSTF electrodes to 0.7 μg/cm2, as may be possible with more stable membranes, higher surface area NSTF catalysts, and cell operation at lower temperatures and higher relative humidities.

PEMFC Reactant Mass Transfer Coefficient Measurement and Separation – Method Extension to the Mixed Kinetic and Mass Transfer Control Regime

Additional cost reductions are needed for proton exchange membrane fuel cells (PEMFC) to effectively compete with internal combustion engine powered vehicles. Recent trends have focused on the use of a high current density and a lower Pt catalyst loading to reduce the quantity of expensive materials. However, such strategies have led to higher mass transfer overpotentials (1). These performance losses are advantageously characterized by the mass transfer coefficient. Its separation into fundamental contributions will focus research efforts to improve cell performance. A separation method previously proposed (2) was extended, which provides information for a wider cell voltage range of relevance to duty cycling operation. The extended model yields a closed form and implicit current distribution expression (Eq. 1) where X represents the dimensionless Cartesian coordinate along the flow field channel, ie the inlet oxygen flow rate equivalent current density, ik the kinetic current density, iL ,0 the inlet limiting current density and i the current density. Experimental data obtained with a segmented single cell of 100 cm2 active area (Gore catalyst coated membrane, 18 μm thick membrane, 0.2 and 0.1 mg Pt cm- 2 catalyst loading, segmented and unsegmented Sigracet 25BC gas diffusion layer for the cathode and anode, respectively) validated Eq. 1 (Fig. 1a) and led to overall oxygen mass transfer coefficients k (derived from iL ,0 and operating conditions). A plot of 1/k versus M, the diluent molecular weight, yields a linear relation (Eq. 2) enabling the separation of the overall mass transfer coefficient into 2 contributions where ke +K represents the oxygen mass transfer coefficient in the electrolyte and the gas phase (Knudsen diffusion), km the oxygen mass transfer coefficient in the gas phase (molecular diffusion) for the specified diluent (subscript He for helium, N for N2, CF for C3F8) and b a parameter. The molecular diffusion mass transfer coefficient km is larger with a lighter diluent (Fig. 1b). Each mass transfer coefficient increases as the cell voltage is decreased. However, the increase in the mass transfer coefficient in the electrolyte and the gas phase (Knudsen diffusion) ke +K is more substantial than for the other contribution. As a result, the k/ke +K ratio decreases for lower cell voltages. Specifically for N2, the mass transfer rate limiting step is associated with the electrolyte and the gas phase (Knudsen diffusion), which suggests a focus on the micro-porous and catalyst layers to increase performance. Two reasons are proposed to explain the cell voltage dependency observed in Fig. 1b. First, the local change in gas phase composition promotes convection. This effect is particularly important before reaching the mass transfer control regime, in the mixed kinetic and mass transfer control region, as the interfacial oxygen concentration changes from its bulk flow field channel value to near 0 in the 0.75 to 0.6 V cell potential range creating a diluent enriched layer. Second, the local temperature in the gas diffusion electrode (through plane direction) is also higher at a lower cell voltage owing to additional heat production. The larger local temperatures in turn promote larger diffusion coefficients in the gas phase (molecular and Knudsen) and permeabilities in the ionomer phase. A comparison with other mass transfer coefficient values derived from impedance data (3) and the model limits of validity will also be discussed. Acknowledgments We gratefully acknowledge funding from the Army Research Office (W911NF-15-1-0188). The authors are grateful to the Hawaiian Electric Company for their ongoing support of the operations of the Hawaii Sustainable Energy Research Facility.

Mass Transfer Overpotentials in Dispersed Pt/C and De-Alloyed PtNi/C Polymer Electrolyte Fuel Cell Cathodes

While oxygen reduction reaction (ORR) mass activities, measured at high voltages, that exceed the DOE 2020 targets (>0.44 A/mg PGM @ 900 mV) have been demonstrated for carbon-supported Pt alloys and core-shell nanoparticle catalysts [1], the full performance of promising catalysts such as these have yet to be achieved in MEAs when operating at high current densities (> 1 A/cm2in air) [2]. To reach a high power output per gram PGM (8 kW/g, DOE 2017 Target), requires high utilization of the electrocatalytic sites as well as low oxygen transport resistance in the cathode catalyst layers. To determine the electrode properties limiting the performance of the de-alloyed PtNi/C based cathode, a standard Pt/C catalyst was annealed to grow the mean particle size to one that is comparable to that of d-PtNi/C catalysts (approximately 5.5 nm). Catalyst-coated membranes with cathodes comprised of a standard Pt/C, annealed Pt/C, and d-PtNi/C at loadings of 0.1 mg Pt/cm² with various ionomer to carbon ratios were fabricated, tested, and characterized under a variety of test conditions [3]. Modeling of these polarization curves to determine the sources of the observed voltage losses (i.e., purely resistive, kinetic, or mass transport) showed that mass transport losses are higher with d-PtNi/C and annealed Pt/C-based cathodes than standard non-annealed Pt/C, while the kinetic losses are lower for d-PtNi/C and standard non-annealed Pt/C as compared to the annealed Pt/C due to higher area-specific activity and higher enhanced surface area, respectively. Acknowledgements This work is part of a collaborative project with Johnson Matthey Fuel Cells, United Technologies Research Center, the University of Texas-Austin, and Indiana University-Purdue University of Indianapolis. The authors wish to thank the U.S. Department of Energy’s Fuel Cell Technologies Office for support. Argonne National Laboratory is managed for the U.S Department of Energy by the University of Chicago Argonne, LLC, under contract DE-AC-02-06CH11357. References Han, B.; Carlton, C.E.; Kongkanand, A.; Kukreja, R.S.; Theobald, B.R.; Gan, L.; O'Malley, R.; Strasser, P.; Wagner, F.T.; Shao-Horn, Y.; Energy Environ. Sci., DOI: 10.1039/c4ee02144d.Debe, M.K.; Steinbach, A.J.; Hendricks, S.M.; Kurkowski, M.J.; Vernstrom, G.D.; Atanasoski, R.T.; Hester, A.E.; Haug, A.T.; Lewinski, K.A.; Deppe, S.K.; “Advanced Cathode Catalysts and Supports for PEM Fuel Cells”, DOE Hydrogen Program Annual Report 2012;Myers, D.; Kariuki, N.; Ahluwalia R.; Wang X.; Peng J.-K.; “Rationally Designed Catalyst Layers for PEMFC Performance Optimization”, DOE Hydrogen Program Annual Report 2014;

Determining the Limiting Current Density of Vanadium Redox Flow Batteries

All-vanadium redox flow batteries (VRFBs) are used as energy storage systems for intermittent renewable power sources. The performance of VRFBs depends on materials of key components and operating conditions, such as current density, electrolyte flow rate and electrolyte composition. Mass transfer overpotential is affected by the electrolyte flow rate and electrolyte composition, which is related to the limiting current density. In order to investigate the effect of operating conditions on mass transport overpotential, this study established a relationship between the limiting current density and operating conditions. First, electrolyte solutions with different states of charge were prepared and used for a single cell to obtain discharging polarization curves under various operating conditions. The experimental results were then analyzed and are discussed in this paper. Finally, this paper proposes a limiting current density as a function of operating conditions. The result helps predict the effect of operating condition on the cell performance in a mathematical model.