Polymer Electrolyte Membrane Fuel Cell Cathode Research Articles

Ceria tubular nanoarchitecture antioxidants achieve sustainable fuel cell devices via tuning the oxophilicity of Pt catalytic surfaces and radical scavenging

The durability of polymer electrolyte membrane fuel cells (PEMFCs) crucially depends on the use of antioxidants to prevent electrocatalyst degradation. Here, we report for the first time, by in situ X-ray absorption and H2O2 electrochemistry, that ceria significantly weakens the Pt-surface oxophilicity, which determines oxygen reduction (ORR) activity and durability, within the PEMFC cathode. Ceria mitigates catalyst disintegration and improves ORR durability by Pt oxophilicity reduction and its inherent radical scavenging behavior. We also found that the antioxidation efficacy of ceria could be finely tuned through nanostructuring. Among various ceria nanostructures, tubular ceria nanoarchitectures (CeOx NT), designed to have the largest surface area and abundant oxygen vacancies, enable the most potent interaction between Pt and Ce without direct chemical contact with Pt. The nanotubular structure confers superior multifunctional antioxidant therapeutic efficacy to Pt/C catalyst in PEMFCs, resulting in outstanding durability that retains 94% of initial performance after 100-hour tests.

Effect of the high oxygen excess ratio design on the performance of air-cooling polymer electrolyte membrane fuel cells for unmanned aerial vehicles

Oxygen excess ratio (OER) is a key parameter that affects output power and operating current density of air-cooling polymer electrolyte membrane fuel cells (PEMFCs). In this work, the design method and performance model of air-cooling PEMFCs for UAVs with high OER are proposed. The high OER is realized by using high-speed airflow behind propeller to feed PEMFC cathode. The effect of high OER on the PEMFC performance is investigated by measuring OER and the output power of PEMFC. The test results indicate that the high OER design can further release the performance potential of the PEMFC constrained by heat dissipation while discarding the parasitic power caused by cooling fans. Compared with current PEMFC with low OER by using cooling fans, the temperature distribution variation of PEMFC stack is reduced by 29.8% and the voltage consistency of the single cell at high current density is doubled. The PEMFC with the high OER design exhibits the “current jump” effect, the continuous operating current of PEMFC is increased by 25.0%. The PEMFC net power is increased from 1070 W to 1320 W, and the power density is also increased by 67.8%. Such results provide good reference values for the current PEMFC-powered propulsion system.

Heterogeneous pore-scale model analysis of micro-patterned PEMFC cathodes

Polymer electrolyte membrane fuel cells (PEMFCs) are a key feature of future hydrogen-based societies. The surface-patterning method for electrolyte membranes has been proposed as a performance improvement technique for PEMFCs. However, its underlying mechanism remains unclear because of complicated mass transport and chemical reaction phenomena. In this study, the surface patterning of a PEMFC cathode was simulated using a unique simulation method of a heterogeneous pore-scale model. The simulation was validated by a comparison of the calculated results with experimental measurements. The model was subsequently applied to investigate variations in the pattern aspect, Pt loading, and ionomer amount and to conduct an advanced pillar structural analysis. A series of numerical simulations suggested that the performance improvement from surface patterning is based on improved proton conduction through the catalyst layer resulting from shortened paths through a tortuous ionomer. Additionally, surface patterning potentially promotes oxygen diffusion by reducing the amount of ionomer and taking advantage of improved proton conduction.

A kW-level integrated propulsion system for UAV powered by PEMFC with inclined cathode flow structure design

Open-cathode air-cooling polymer electrolyte membrane fuel cells (PEMFCs) with the lightweight and simple structure are widely used in state-of-the-art UAVs. In this work, a 1 kW-level PEMFC powered propulsion system with an inclined cathode flow structure is developed. The inclined cathode flow structure uses the airflow behind the blade root of the propeller to meet the reaction and cooling requirements of the PEMFC cathode. Such design discards the cathode axial fan in the current commercial PEMFC system and reduces the parasitic power, weight and volume caused by the axial fan. Compared with the current commercial PEMFC powered propulsion system, the specific power of the 1 kW PEMFC system, power-to-weight ratio and thrust-to-weight ratio of the 1 kW integrated propulsion system are increased by 33.0 %, 23.8 %, and 18.3 %, respectively. Besides, the space in the fuselage for the fuel cells and the weight of distribution wiring, as well as the wire loss, is also significantly saved by replacing the axial fan with proposed inclined cathode flow structure.

(Invited) Investigation of Pt-Alloy and Strained Pt As Electrocatalysts

The need to replace conventional sources of energy has driven the development of fuel cell technology. The polymer electrolyte membrane fuel cells (PEMFCs) show high performance and are important power sources for hydrogen fuel cell vehicles due to the fact that the cells can operate at relatively low temperatures (50 - 90 oC) , has high energy efficiency and only emits water as a byproduct. Furthermore, the direct methanol fuel cell emerge as an important technology for the green energy transition with the use methanol fuel. The most important electrocatalyst for the fuel cells is platinum and efforts are made to enhance its performance and reduce its requirement by alloying with other metals. Therefore, novel approaches are required to develop electrocatalysts for the oxygen reduction reaction (ORR) and methanol oxidation using reduced amounts of Pt for use at PEMFC cathodes with high electrocatalytic activity.Investigation of alloying Pt with transition metals (Ni, Co, Fe) has demonstrated increased electrocatalytic activity and is attributed to changes in the Pt-Pt atomic interaction distances. The electrocatalytic activity is linked to the adsorption of molecular oxygen to the electrocatalytic sites, which is a process enhanced by creating favorable Pt-Pt distances at the electrocatalyst. It is also understood that Pt-alloys have an increased density of d-band vacancies, resulting in stronger binding of OH species to catalyst surfaces. Furthermore, the use of alloying metals causes distortions in the atomic lattice, resulting in compressive strain in the lattice, which increases orbital overlap, widens the d-band, lowers the d-band centre and affects electrocatalytic performance. In addition, enhanced methanol oxidation on strained pure Pt platinum was recently demonstrated and is associated with the preferential absorption of reactant at optimum Pt-Pt interatomic distances.Here, we show results of electrocatalytic activity for oxygen reduction reaction of Fe-Pt alloy with a range of iron content (0 to 80%), the electrocatalytic activity of highly ordered array of Pt nanotubes for methanol oxidation and the effect of strained Pt on methanol oxidation. Structural and electrochemical data of the catalysts will be presented including rotating disk electrode (RDE) voltammetry and SEM images.

(Digital Presentation) Impact of the Cathode Catalyst Ink Solvent Composition on the Polymer Electrolyte Membrane Fuel Cell (PEMFC) Performance at High Current Densities

Polymer electrolyte membrane fuel cells (PEMFC) are promising candidates for the replacement of internal combustion engines. Still, alongside the necessary enhancement of the mass activity of cathode catalyst material, the performances of the PEMFC cathode must be improved in the high current densities range where the O2 reactant mass transport towards the catalytic site is limiting for the reaction rate [1]. High current density performance of the PEMFC strongly depends on the ionomer distribution in the catalytic layer constituting the electrode, which depends on the solvent used for the catalyst ink preparation [2]. From different studies, it has been observed that the nature of the solvent composition modifies the resulting structure and size of the dispersed ionomer agglomerates in the catalyst ink [3–5]. The main objective of our investigation is to resolve in more detail the influence of the ink solvent composition on the PEMFC performances. For this purpose, the agglomerate size distribution of the 1100 EW Nafion® ionomer dispersed into various solvents was determined at first by dynamic light scattering (DLS) in a systematic fashion and found to be strongly influenced by the kind of alcohol (isopropanol, ethanol and methanol) and its concentration in water. Then, in order to attempt a correlation between the agglomerate size of the ionomer and the MEA performances, cathode catalyst layers were prepared from inks with these alcohols at different concentrations, while keeping all the other parameters in the CCM preparation unchanged. Electrochemical impedance spectroscopy (EIS) and O2 mass transport resistance measurements (O2-MRT) were performed in addition to the recording of single cell polarization curves. The latter were modeled following the equation suggested by Larminie et al. [6] to discriminate the kinetic, ohmic and mass transport overvoltages (losses).The ink-solvent compositional characteristics significantly influence the O2 reactant mass transport overvoltages. An analog equivalent circuit including a Nernst impedance element to account for the finite O2 diffusional limitations at the cathode catalyst layer was fitted to the experimental EIS data. The Warburg parameter scales very well with the O2 transport resistance through the catalytic layer derived from O2-MRT measurements, this strengthening the robustness of the overall set of experimental data. As the fraction of ionomer agglomerates in the range 50-500nm progressively increases in the catalyst ink up to ca. 37-45%, the losses attributed to the limited O2 mass transport to the catalytic sites are lowered by down to ca. 70% at 1.76 A∙cm-2 current load under the given cell testing conditions, independently of the nature of the alcohol and its relative content. For the higher fraction of ionomer agglomerates in this size range, no further change is observed in the O2 transport loss parameter. These results highlight quantitatively the significant influence of the size distribution of ionomer agglomerates on the cell performances under high current loads and suggest a convolution with other parameters, such as, e.g., the porous structure of the catalyst layer, that has yet to be quantitatively asserted.

3D Computational Model for an Electrochemical Gas Separation and Inerting System

Aircraft fuel tank inerting is employed to reduce the flammability of the fuel vapor in the ullage (air volume above the fuel) by restricting its oxygen concentration to a safe value—12% for commercial aircraft and 9% for military aircraft. Inerting is typically accomplished by displacing oxygen in the ullage with an inert gas like nitrogen. Electrochemical gas separation and inerting system (EGSIS) is an on-board method to generate and supply nitrogen-enriched air (NEA) to the fuel tank. EGSIS combines a polymer electrolyte membrane (PEM) electrolyzer anode which dissociates water to evolve oxygen, and a PEM fuel cell cathode which reduces oxygen from atmospheric air to produce NEA at its outlet. This paper represents the first attempt to model and simulate EGSIS using a three-dimensional, steady state, isothermal model. Various EGSIS performance indicators such as current density, reactant concentration distribution, and polarization curves are studied as a function of operating conditions and design parameters. The results from the computational model are validated against our previous experimental results for various operating conditions. The simulation results reveal the effects of temperature, reactant flowrates, and material property optimization on EGSIS performance. Different operating strategies are explored with the goal of improving system performance.

Computational Model for Electrochemical Gas Separation and Inerting System

In July 1996, TWA flight 800 crashed into the Atlantic Ocean only 12 minutes after takeoff killing all 230 people on board. After a 4-year investigation into this catastrophe, the National Transportation Safety Board concluded that the accident was probably caused by the explosion of flammable fuel vapor in the Boeing 747’s center fuel tank. Following this landmark incident new regulations for aircraft fuel tank inerting were developed to prevent such explosions. Fuel tank inerting is the process of reducing the flammability of the fuel vapor by restricting the oxygen concentration in the ullage (air volume above the fuel) to a safe value – 12% for commercial aircraft and 9% for military aircraft. Generally, inerting is carried out by driving oxygen out the ullage using an inert gas like nitrogen.Existing inerting technologies include systems like liquid nitrogen, explosion suppressant foam, halon extinguishment, and on-board inert gas generation systems (OBIGGS). Compared to other techniques, OBIGGS is seen as viable option since nitrogen enriched air (NEA) can be produced during flight by supplying pressurized air into separation modules that preferentially retain nitrogen while allowing oxygen to escape. However, such systems are expensive and consume substantial amounts of energy, while also undergoing premature failure. Additionally, the NEA generation rate is fixed in such systems, whereas the actual NEA requirement depends on the aircraft’s flight stage. Thus, incumbent technologies are designed based on the highest demand making them overweight and expensive.Here, we propose a novel on-board electrochemical gas separation and inerting system (EGSIS) to overcome such problems. EGSIS is an electrically powered system that can be used to produce and supply NEA to the fuel tank. It combines a polymer electrolyte membrane (PEM) electrolyzer anode wherein water is dissociated to generate oxygen, and a PEM fuel cell cathode where oxygen from the inlet air inlet is reduced to produce humidified NEA as the output gas. After proper dehumidification, the NEA can be supplied directly to the ullage to reduce its oxygen concentration. Unlike current technologies, the NEA production rate in EGSIS can be easily varied with demand just by controlling the voltage applied to the system. Experimental investigations of EGSIS have been previously conducted by our group. This paper represents the first attempt to model and simulate EGSIS.We present a two-dimensional mathematical model of a single cell EGSIS that provides a foundation for the simulation and analysis of a full-scale EGSIS module. A single phase, two-dimensional, steady state, isothermal model is implemented in an EGSIS cell using COMSOL. Various EGSIS performance parameters such as current densities, reactant concentration distributions, and polarization curves are investigated as a function of operating voltage and temperature. The model involves the material properties of the various components including the polymer electrolyte membrane, the catalyst later and gas diffusion media; the transport of water vapor and air in the anode and cathode flow field channels, respectively; the transport of protons and water in the catalyst layer and the membrane; and the transport of electrical charge in the solid phase. The governing equations include continuity and steady-state incompressible Navier-Stokes equations. Additionally, species transport is modeled using the generalized Stefan-Maxwell equations whereas the Butler-Volmer equation is employed for charge transport. All of these coupled model equations are solved by the finite element method using the COMSOL Multi physics software package. The results from the computational model are then compared and validated with our experimental results for various operating conditions. Different operating strategies are explored with the goal of improving system performance. The simulated results reveal the influence of temperature, reaction flowrate, and optimized material properties on EGSIS performance. This model also provides a base for future research into the effect of alternate catalysts and gas diffusion media. It also serves as a foundation for the analysis of a practical EGSIS stack.

Amino Functionalization Approach to Synthesis of Carbon Supported Intermetallic Platinum Based Alloy Catalysts for Fuel Cell Application

Platinum (Pt) is a critical element in the electrocatalysts for oxygen reduction reaction (ORR) occurring in the cathode of polymer electrolyte membrane fuel cells (PEMFCs). To address the Pt abundance issue and to enhance Pt ORR activity, Pt is often alloyed with another transition metal M (i.e., M=Fe, Co, and Ni, etc). (1-3) Generally, the atoms in PtM alloys are randomly distributed in the Pt fcc lattice. In the corrosive working environment of a PEMFC cathode, these transition metals are rapidly etched, thus leading to the reduced catalytic activity and stability. In these regards, ordered intermetallic PtM alloys are considered as one of the most promising candidates to achieve both high activity and stability in practical fuel cell applications. (4) The transition metals in ordered intermetallic PtM alloys occupy specific sites, and are stabilized by both metallic and ionic bonding. Ordered intermetallic structures are formed via high temperature (>600 °C) annealing of disordered PtM alloys, as the atomic ordering is a thermodynamically driven process. However, the high temperature annealing inevitably leads to the migration and agglomeration of the catalyst nanoparticles forming randomly alloyed particles with poor dispersion and broad size distributions, due to weak adhesions to the carbon support under typical synthesis conditions. To prevent this coalescence during annealing, protective coating of the nanoparticles with inorganic shells or physical barriers has been suggested. (5-10) However, these studies were limited to the synthesis of intermetallic nanoparticles on carbon supports at low metal loadings, or require an additional step of removing the protective coating layer from the surface of the nanoparticles to expose the active sites. Thus, it is essential to develop a general approach that can produce highly dispersed, structurally ordered nanoparticles while achieving controls over the size and size distribution.We propose to use the functionalized carbon supports to control PtM alloy nanoparticle size and prevent nanoparticles from aggregating during the high temperature annealing through improving the metal-support interactions. A strong electrostatic attraction between the negative charge from Pt precursor (PtCl6 2 -) and positive charge from the amino groups (C-NH2+) on the surface of the functionalized carbon will be established during the wet impregnation synthesis. Such bonds will help to make the PtM nanoparticle size smaller and more uniformly distributed over the surface of support. The ordered intermetallic 30 wt.% PtCo/KB-NH2 catalyst demonstrated an average size of 2.7 nm and uniform size distribution. In addition, 30 wt.% PtCo/KB-NH2 catalyst exhibited a mass activity of 535 A/gPt (H2/O2) and a rated power density of 1.05 W/cm2 at 0.67 V (H2/air), meeting/exceeding the DOE targets.(1) V. Stamenkovic, B.S. Mun, K.J.J. Mayrhofer, P.N. Ross, N.M. Markovic, J. Rossmeisl, J. Greeley, J.K. Nørskov, Angew. Chem. Int. Ed. 2006, 118, 2963–2967.(2) P. Strasser, S. Koh, T. Anniyev, J. Greeley, K. More, C. Yu, Z. Liu, S. Kaya, D. Nordlund, H. Ogasawara, M.F. Toney, A. Nilsson, Nat. Chem. 2010, 2, 454–460.(3) D. Wu, X. Shen, Y. Pan, L. Yao, Z. Peng, ChemNanoMat 2019, 6, 32–41.(4) X. Wang, M. T. Swihart, G. Wu, Nat. Catal. 2019, 2, 578–589.(5) D. C. Lee, F. V. Mikulec, J. M. Pelaez, B. Koo, B. Korgel, J. Phys. Chem. B 2006, 110, 11160−11166.(6) H. Chen, D. Wang, Y. Yu, K. A. Newton, D. A. Muller, H. Abruna, F. J. Disalvo, J. Am. Chem. Soc. 2012, 134, 18453−18459.(7) E. F. Holby, W. Sheng, Y. Shao-Horn, D. Morgan, Energy Environ. Sci. 2009, 2, 865−871. (8) L. Guo, W. J. Jiang, Y. Zhang, J. S. Hu, Z. D. Wei, L Wan, J. ACS Catal. 2015, 5, 2903−2909. (9) N. Cheng, Banis, N. M. J. Liu, A. Riese, X. Li, R. Li, S. Ye, S. Knights, X. Sun, Adv. Mater. 2015, 27, 277−281.(10) J. Li, S. Sharma, X. Liu, Yung-Tin Pan, J. S. Spendelow, M. Chi, Y. Jia, P. Zhang, D. A. Cullen, Z. Xi, Joule 2019, 3, 124-135.

Temperature dependences of the double layer capacitance of some solid/liquid and solid/solid electrified interfaces. An experimental study

This study investigates the influence of the temperature on the electrical double layer capacitance (CDL) of various materials, which are essential for fuel cells and solid-state Li-ion batteries. Electrochemical impedance spectroscopy is utilized to measure the CDL of polycrystalline Pt/aqueous electrolytes interfaces, cathode catalyst layers of polymer electrolyte membrane fuel cells (PEMFC), and Au or Li electrodes in contact with a solid-state electrolyte (SSE), a prime example for solid-state ionics. Our results show that within the investigated temperature ranges, the CDL decreases with an increase in the temperature for Pt electrodes in an aqueous acidic electrolyte. However, for SSE and PEMFC cathode catalyst layers, the CDL increases with temperature. The CDL behavior with the temperature of herein presented systems is important for understanding and modeling of the interface processes for renewable energy conversion systems such as fuel cells, water electrolyzers, and batteries.

Open-circuit dissolution of platinum from the cathode in polymer electrolyte membrane water electrolysers

Platinum is the state-of-the-art catalyst for hydrogen evolution in polymer electrolyte membrane (PEM) water electrolysers; however, its stability has only been characterized to a limited extent in situ. This study measures platinum dissolving from the cathode during intermittent operation of a 3-electrode PEM electrolyser cell, using a differential pulse voltammetry technique that provided detection limits for platinum of less than 2 ng L−1. Water samples were periodically taken during on-off current cycling, and during periods of open-circuit voltage (OCV) platinum dissolution was detected when the cathode potential rose above 0.85 V NHE due to diffusion of oxygen from the anode. This reached a maximum dissolution rate at the highest cathode potential of 1.02 V NHE, and gradually decayed over a 90 h period. The average total amount of platinum dissolved per 90 h OCV period was estimated to be 152 ng cm−2 or 0.005% of the initial electrode catalyst mass. The dissolution mechanism was predicted to be the same as that occurring in PEM fuel cell cathodes, although being kinetically hindered in PEM electrolysers by the slow diffusion of oxygen from the anode to the cathode.

Circular use of Pt/C through Pt dissolution from spent PEMFC cathode and direct reproduction of new catalyst with microwave synthesis

Nanoparticulate platinum supported on high surface area carbon (Pt/C) remains the electrocatalyst of choice at the cathode of the low-temperature polymer electrolyte membrane fuel cell (PEMFCs). Recycling of the Pt/C from the spent PEMFC electrodes through environment-friendly and economic processes is of high industrial significance. Here, we report such a Pt/C recycling process involving recovery of Pt from spent PEMFC electrodes through dissolution under mild conditions followed by synthesis of Pt/C using the Pt containing dissolution bath as the Pt source. Prior to recycling, the spent PEMFC electrodes were characterized for the physical and chemical states of Pt nanoparticles (size, agglomeration, surface adsorbed species, electrochemical surface area, etc.), owing to their influence on the Pt dissolution under mild conditions. Pt from the spent electrode was recovered through dissolution in 1 M HCl and the Pt/C catalyst (20 wt% Pt) was synthesized by reduction of the dissolved Pt species using microwave assisted polyol route and ethylene glycol as the reducing agent. The process eliminates the steps necessary to recover the dissolved Pt in form of a Pt compound suitable for conventional Pt/C synthesis (e.g. H2PtCl6). The synthesized Pt/C was characterized for its structure and electrochemical performance. The study may be of interest for the PEMFC industry, which relies on the primary sources of Pt for their Pt/C electrocatalyst demand.

Operando Analysis of Cation Migration in PEMFC Electrodes Using Nanoscale X-Ray Computed Tomography

Polymer electrolyte membrane fuel cells (PEMFCs) are beginning to be accepted as the electrification technology of choice for long-range, high-power vehicles due to their clean operation, high efficiency, and the ability to independently scale their range and power. However, PEMFC vehicles are still not cost-competitive with internal combustion engines, largely because of the high cost of the platinum (Pt) used as the electrocatalyst in the cathodes of PEMFCs (1). Recent efforts have succeeded in reducing the Pt content of PEMFC cathodes by using high-performance Pt alloys like PtCo or PtNi, which have improved catalytic activities over pure Pt (2, 3). While these alloys achieve higher power densities with smaller amounts of Pt in beginning-of-life performance, they experience significant degradation over time, including loss of Pt to particle coarsening and a Pt band and ionization and dissolution of the less-stable alloying element (i.e. Co or Ni). These contaminant cations are absorbed by the ionomer, replacing protons on the sulfonate groups and inhibiting proton conduction and affecting oxygen transport, leading to reduced performance at high current densities (4). While the overall loadings of these cations with respect to the total number of sulfonate groups in the membrane is low, these cations are suspected to migrate during operation, causing high local loadings of contaminant cations in the cathode catalyst layers.The focus of this work was the operando analysis of contaminant cation migration and accumulation using the nanoscale X-ray computed tomography (nano-CT) imaging technique. We designed a miniature nano-CT conducive fuel cell hardware with a Kapton-sealed X-ray viewing port, allowing us to monitor the temporal concentrations of contaminant cations (i.e. Co2+ and Cs+) at the catalyst layer/membrane interface using 2D absorption contrast imaging during cell polarization. The preliminary results from our method have demonstrated back-and-forth cation migration and accumulation under hydrogen pumping conditions, confirming the mobility of these cations under constant current application and demonstrating promise in the operando evaluation of their impact on proton and oxygen transport.This work was partially supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy under grant DE-EE0007271.

Spacers to Improve Performance and Porosity of Graphene Based Polymer Electrolyte Fuel Cells

Fuel Cell are energy generation alternatives to the traditional fossil fuel dependant transport and energy sectors. A typical polymer electrolyte membrane fuel cell (PEMFC) generates power from the electrochemical reaction between hydrogen and oxygen which forms water. While PEMFC have become increasingly commercialised in the last decade there needs to be a significant cost reduction before the technology sees a much wider uptake. One of the key methods to reduce costs of PEMFCs is to reduce the platinum loading. Significant research has investigated non-platinum groups metals or ultra-low Pt loadings, however these have other technological barriers that need to be surpassed before they are widely commercialised. Increasing the lifetimes of fuel cell stacks is one other method to make costs competitive with traditional power generation. Carbon support corrosion is one of the key decomposition pathways of a fuel cell, this entails degradation and loss of the carbon support with use leading to loss of Pt catalyst, ionomer and reduction of proton and electron conduction.Highly graphitised carbon supports have been reported to be have superior carbon corrosion resistance compared to commercial carbon black. This has often been attributed to the sp2 carbon framework being more resilient to oxidation. Graphene and few layer graphite (FLG) are composed of sheets of sp2 carbon, high surface areas graphene has been developed. However when deposited as a catalyst layer the sheets restack during drying, forming a dense, low porosity electrode with low current densities. The potential carbon corrosion benefits of graphene cannot be observed reliably when operated at low current densities, it is therefore imperative to develop methods to control the morphology and layering of graphene based electrodes to mitigate this performance loss. In this work we report a series of method that utilises sacrificial additives and spacers to scalably control the morphology of graphene based electrodes. We show significant increases in current density, porosity and Pt utilisation via this method. In addition we discuss the effect of graphene size, source and treatment on performance of PEMFC cathodes. Figure 1

Controlling Electrochemical Reactivity of Mesoporous Carbons By Surface Modification

Carbon nanomaterials are used in a wide array of electrochemical applications, such as in fuel cells, batteries, capacitors, and sensors. These materials typically have the high surface area and conductivity required for good electrochemical performance and are low in cost. Many different types of carbon nanomaterials are studied, including microporous Vulcan carbon and Ketjenblack, carbon nanotubes, templated carbons, graphene and its derivatives. While microporous carbons are often used as cathode supports in polymer electrolyte membrane fuel cells (PEMFCs), most carbons are susceptible to corrosion, with pores too small to fit catalytic Pt nanoparticles or contain Nafion ionomer, and have varying and uncontrolled pathways between carbon particles, yielding transport limitations that influence electrochemical performance. Thus, interest is increasingly being focused on mesoporous carbons as catalyst support materials. Colloid imprinted carbons (CICs) are one example of promising mesoporous carbon materials and are the focus of this presentation. CICs are prepared from abundant and inexpensive precursors that are templated with silica colloids of a precise size to achieve ordered mesoporosity. Previously, CIC powders with pore sizes of 12, 22, and 50 nm (with still smaller internal necks) were studied, focusing on their corrosion resistance and wettability before and after functionalization with a hydrophobic surface group (-PhF5), achieved through diazonium chemistry.1–3 In the present work, we investigated CICs with larger pore sizes in the range of 85 nm to better simulate the typical size of the pathways and pores present in common, ink-derived microporous carbons, attaching both –PhF5 and a hydrophilic surface functional group, -PhSO3H, to the CIC-85 surface. The larger CIC pore size should be beneficial for use in PEMFC cathode supports, as they can accommodate both Pt nanoparticles and ionomer without hindering mass transport. The CIC-85 material was prepared by mixing a suspension of 85 nm silica particles and ground mesophase pitch precursor in ethanol and then imprinting at 400 °C for 2 hrs under nitrogen before being carbonized at 900 °C for 2 hrs in nitrogen. The silica was removed by 24 hr NaOH reflux, then washed with water until neutralized. Surface functionalization was achieved by refluxing the CIC-85 powder for 24 hr under nitrogen with amyl nitrate and either 2,3,4,5,6-pentafluoroaniline in acetonitrile, or sulfanilic acid in water, to form hydrophobic or hydrophilic CIC-85 surface properties, respectively. The product was then treated to a 24 hr acetonitrile Soxhlet extraction and air-dried. Characterization techniques, including Raman spectroscopy, XRD, SEM, and gas sorption (BET analysis), showed that the ordered porosity of the CIC-85 was fully preserved after functionalization and that average pore sizes decreased by a few nm, consistent with the presence of surface functional groups. Neutron activation analysis and CHNS elemental analysis confirmed the successful functionalization of the CIC-85 powder with hydrophobic -PhF5 and hydrophilic -PhSO3H groups, respectively. The electrochemistry of the CICs, before and after surface functionalization, were studied by using a CIC-85, 12% sulfuric acid, and 1 wt% Nafion in ethanol ink mixture, with 0.2 mg of the CIC-85 loaded onto a glassy carbon rod electrode. The CIC material was tested in 0.5 M sulfuric acid for its CV response in the normal potential range for carbon and also exposed to an electrochemical accelerated stress test (AST) protocol, designed to reduce testing time and to account for real time surface area changes as a result of surface roughening and mass loss. The AST involved holding at 1.0 V vs RHE for 10 s and then stepping to and holding at 1.4 V for 50 s, repeatedly, and then reporting the difference in the cumulative oxidation vs reduction charges, equivalent to the amount of total oxidized carbon. CVs collected before and after the AST to determine surface area changes from the double layer capacitance and changes in the density of surface oxide groups from the pseudocapacitance. The results showed that the presence of the hydrophobic –PhF5 group protects the CIC-85 material from corrosion, while the hydrophilic –PhSO3H group also decreases the extent of CIC-85 oxidation, but by a lesser extent. A new model is suggested, wherein steric hindrance introduced by the functional groups, in addition to surface wettability alteration, prevents access of solution to the surface and protects carbon from oxidation. XPS analysis, before and after the corrosion experiments, showed that the surface functionalities are not lost after repeated exposure of the CIC-85 to the more aggressive anodic potentials. 1. D. Bélanger and J. Pinson, Chem. Soc. Rev., 40, 3995–4048 (2011).2. X. Li et al., ACS Appl. Mater. Interfaces, 10, 2130–2142 (2018).3. F. Forouzandeh et al., J. Electrochem. Soc., 165, F3230–F3240 (2018). Figure 1

Applicability of double layer capacitance measurements to monitor local temperature changes at polymer electrolyte membrane fuel cell cathodes

One of the most sensitive parameters to control the performance of automotive-size polymer electrolyte membrane fuel cells (PEMFCs) is the cell temperature. For instance, when starting up the fuel cell, it is critical to know whether the cell temperature is below ~273 K or, approaches the operating conditions. The easiest way to determine this is to perform measurements using some commercially available temperature sensors at the active electrode surface. However, this only applies to single-cells and not for automotive fuel cell stacks with ~400 cells. In this case, the measurement with a standard resistive temperature sensor is practically problematic. In this paper, we demonstrate that the electric double layer capacitance of the PEMFC cathodes (geometric active area size of 43.6 cm2) increases linearly from 268 K up to 333 K; and using this information, it is possible to determine the catalyst temperature accurately within this temperature range. These results should be of particular significance for both modelling the double layer in the PEMFCs as well as for the practical use in automotive applications.

How the colloid chemistry of precursor electrocatalyst dispersions is related to the polymer electrolyte membrane fuel cell performance

Polymer electrolyte membrane fuel cells (PEMFCs) operating at low temperature (60–80 °C, up to 110 °C) are mostly limited in their performance by the kinetics of the oxygen reduction reaction (ORR), leading to high loadings of platinum (Pt) in the cathode. Pt catalysts are without alternative in numerous industrial applications, and since Pt resources are limited, the associated high costs for low temperature fuel cells are hindering among other factors their commercialization. In order to increase the fraction of electrocatalytically available Pt towards ORR, this work is devoted to the factors responsible for the microstructure of the PEMFC cathodes. Typically, the active layers are coated by processes like spraying, doctor blading, printing etc. Therefore, the final structure actually is strongly dependent on the coating process and the physicochemical properties of the catalyst dispersions used. Selecting commercially available electrocatalysts from Johnson-Matthey and Tanaka as active material and ultrasonically assisted spraying as the coating method, systematic variations of the surface chemistry of the catalyst particles and their influence on catalyst layer morphology and therefore electrical and electrochemical properties of resulting membrane electrode assemblies (MEA) have been investigated. It could be shown, that the colloid–chemical properties of the catalyst dispersions have a profound influence not only on the microstructure of the MEAs but also on the performance under operating conditions.

Enhanced Performance and Durability of Polymer Electrolyte Membrane Fuel Cell By Infiltration of Cerium Oxide

Polymer Electrolyte Membrane Fuel Cells (PEMFCs) has been drown much attentions due to its high energy density, pollution-free electricity generation. Because of its versatile advantages, PEMFCs are considered as next-generation transportation system, which is used for power system of fuel cell electronic vehicle (FCEV). However, the usage of Pt catalyst hinders the widespread application of PEMFC because of high price of Pt. In order to utilize Pt in cost-effective way, various researches have been conducted to maximize the performance and long-term stability of Pt catalyst. Recently, many researches of Pt-metal oxide/C catalyst in PEMFCs have been investigated due to improvement of adsorption characteristics for oxygen at the surface of Pt by adding metal oxide. Especially, Cerium oxide has superior oxygen storage capability originated from easy redox between Ce3+ and Ce4+. Also, Pt-CeOx interact regions increase active sites where oxygen and Pt catalyst could meet together. In this study, we treated PEMFC cathode by cerium oxide with infiltration process, which enables uniform distribution of added materials with cost-effective way. We investigated the performance of PEMFC membrane-electrode assembly (MEA) with cerium oxide treated cathode. PEMFC performance was evaluated by current-voltage measurement and cyclic voltammetry. Also, the long term stability of cerium oxide treated PEMFC MEA has been analyzed with electrocatalyst accelerated degradation protocol proposed by Department of Energy (DOE). In brief, performance and long term stability enhancement of PEMFC MEA has been observed in cerium oxide treated samples. In our presentation, detail process of experiment and PEMFC performance analysis will be introduced.

Platinum Nanorod Arrays As ORR Electrocatalyst for Polymer Electrolyte Membrane Fuel Cells

Polymer electrolyte membrane fuel cell (PEMFC) is one of the most promising energy devices for stationary, portable, and transportation power-supply applications. PEMFCs have high energy conversion efficiency, low operating temperature, and are environmentally friendly compared to the other types of fuel cells. The dissolution of platinum (Pt) during the oxygen reduction reaction (ORR) on the cathode causes voltage losses and requires a high loading of Pt compared to the anode, and thus increases the cost of the fuel cell. In addition, carbon support used in conventional Pt/C catalyst present problems including oxidation of carbon, the formation of peroxide species causing degradation of the polymeric membrane, and separation from the ionomer over time leading to loss of catalyst effectiveness. While extensive efforts and significant progress have been made, there are still some challenges related to the fuel cell performance, durability, and cost that need to be addressed in order to have successful commercialization. In order to overcome these challenges, columnar Pt nanorods stand out as a good candidate for non-conventional catalysts due to their high surface area, high stability, and no need for a support material. For this purpose, vertical arrays of Pt nanorods (NRs) were grown onto microporous layer (MPL) side of a gas diffusion layer and glassy carbon (GC) substrates by glancing angle deposition (GLAD) magnetron sputtering to investigate their ORR properties for PEMFC cathode applications. The weight loading of Pt NRs were calculated using quartz crystal microbalance (QCM). Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to determine the microstructure of NRs. Magnetron sputtering provides high-purity catalyst material, controlled and ultra-low loadings. It is also a scalable fabrication method with no toxic by-products. GLAD involves a modified substrate alignment geometry where incident flux of atoms approaches the substrate at oblique angles. ORR activity of Pt NRs was evaluated in aqueous acidic electrolyte using the rotating disk electrode technique. The electrochemically active surface area (ECSA) was measured for Pt NRs deposited on the flat GC and on the rough surface of MPL. The results indicate about an order of magnitude increase in ECSA for Pt NRs grown on GC compared to conventional high density Pt thin film on GC. The enhancement agrees well with the higher surface-to-volume ratio of NRs compared to a planar film. Isolated columnar structure and ECSA of NRs are further enhanced when it is deposited on MPL due to the “shadowing effect” involved in our growth process. We are currently developing a new methodology for performing more detailed electrochemical tests on samples of Pt NRs on MPL and will report ECSA enhancement and other relevant analysis during the presentation. Electrochemical stability of the Pt NRs against dissolution will be investigated by using inductively-coupled plasma-mass spectrometer (ICP-MS) capable of detecting trace concentrations (<ppb) of dissolved elements in solution. This work is supported by the U. S. Department of Energy (DOE) under contract number DE-EE0007652.

A rejuvenation process to enhance the durability of low Pt loaded polymer electrolyte membrane fuel cells

An effective method to enhance the durability of polymer electrolyte membrane fuel cells (PEMFCs) is reported. PEMFC performance loss is mitigated by exposing the electrodes of fuel cells to dry nitrogen gas periodically at high temperature. This method extends the lifetime of fuel cells significantly compared to their non-treated counterparts. The impact of treatment temperature and exposure time on PEMFC durability is reported, using potential cycling accelerated stress tests. The enhanced durability is attributed to the suppression of “ionomer relaxation” that occurs under the nearly water saturated operating conditions of a PEMFC cathode.