Fuel cells utilize the chemical energy of liquid or gaseous fuels to generate electricity. As fuel cells extend their territory to include heavy-duty vehicles, new demands for proton conductors, a ...critical component of fuel cells, have emerged. A near-term need is ensuring the chemical and mechanical stability of proton exchange membranes to enable long lifetime vehicles. In the mid-term, achieving stable conductivity of proton conductors under hot (>100°C) and dynamic fuel cell operating conditions is desirable. In the long term, targeting high thermal stability and tolerance to water enables the utilization of high energy density liquid fuels that will increase pay-load space for heavy-duty vehicles. This article presents our perspective on these near-, mid-, and long-term targets for proton conductors of heavy-duty fuel cells.
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Fuel cells are an attractive technology to power zero-emission vehicles. Compared with battery-powered vehicles, fuel cells offer fast fueling and adequate fuel storage for long-range applications. Heavy-duty fuel cell vehicles have strenuous requirements with the most challenging target being the development of fuel cells with the durability to return capital investment over a longer lifetime. Fuel cell operation under hot and dry conditions enables simpler, low-cost fuel cell systems through better heat and water management. Utilizing high energy density liquid fuels can also increase pay-load space and eliminate the need for an expensive hydrogen infrastructure. Advanced proton conductors that can resolve these issues associated with heavy-duty fuel cell applications are needed. Here, we present the progress and promising options in meeting near-, mid-, and long-term targets with respect to performance, durability, and technical readiness to stimulate research on proton conductors for heavy-duty fuel cell vehicles.
Fuel cell technology is an attractive electrification platform for heavy-duty vehicles. As fuel cells expand their territory to include heavy-duty vehicles, new demands for proton conductors—a critical component of fuel cells—have emerged. This article summarizes the perspective of original equipment manufacturers on the research needs for heavy-duty fuel cell vehicle proton conductors in the near, mid, and long terms.
A highly accelerated stress test (HAST) has been developed to generate local stressful conditions that are representative of those in automotive fuel cell stacks. Using a 50-cm2 cell cycled between ...0.05 and 1.2 A/cm2 with a low inlet RH in the co-flow configuration, the HAST creates a distribution of combined mechanical/chemical stressors in the membrane with the maximum chemical stress occurring near the gas inlets and the maximum mechanical stress near the outlets. Conducting HASTs using a current distribution measurement tool and a shorting/crossover diagnostic method to track the state of health of a robust membrane containing both a mechanical support and a chemical stabilizing additive, the result shows that the membrane location with the most severe thinning coincides with that of the deepest membrane hydration cycling. Upon examination of the cerium redistribution patterns after the test, it was found that the severe humidity cycling generated by the HAST condition near the outlet region not only generated the highest membrane mechanical stress but also resulted in the strongest water flux, which may cause local depletion of the cerium added as chemical stabilizer. Further study is required to decouple the cerium migration effect from the possible mechanical/chemical synergistic degradation effect.
In this study, three commercially available proton exchange membranes (PEMs) are biaxially tested using pressure-loaded blisters to characterize their resistance to gas leakage under either static ...(creep) or cyclic fatigue loading. The pressurizing medium, air, is directly used for leak detection. These tests are believed to be more relevant to fuel cell applications than quasi-static uniaxial tensile-to-rupture tests because of the use of biaxial cyclic and sustained loading and the use of gas leakage as the failure criterion. They also have advantages over relative humidity cycling test, in which a bare PEM or catalyst coated membrane is clamped with gas diffusion media and flow field plates and subjected to cyclic changes in relative humidity, because of the flexibility in allowing controlled mechanical loading and accelerated testing. Nafion
® NRE-211 membranes are tested at three different temperatures and the time–temperature superposition principle is used to construct stress-lifetime master curve. Tested at 90
°C, 2%RH extruded Ion Power
® N111-IP membranes have a longer lifetime than Gore™-Select
® 57 and Nafion
® NRE-211 membranes.
Ion-conductive membranes used in energy devices are always under compression to minimize a device's contact resistance. In this paper, the conductivity and nanostructure of Nafion membrane, a ...commonly used ionomer in many electrochemical energy applications, are investigated under compression. Hydrophilic-domain spacing as a function pressure is determined both in the plane (face-on imaging) and thickness (edge-on imaging) directions using small-angle X-ray scattering (SAXS). SAXS results suggest a similar nanostructure in all directions indicating lack of a strong anisotropy when not under compression. However, compressing the membrane induces structural anisotropy where domain spacing gets smaller in the compression (thickness) direction while it elongates in the plane of the membrane. From the domain spacing and water content under compression, the change in conductivity is calculated as a function of pressure and compared with measured data. The findings of this work provide insight into the effect of compression on the three-dimensional morphology of a PFSA membrane and its conductivity, issues that have not been previously explored and are critical to the understanding of ion-conductive membranes.
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► This paper is one of the first to measure SAXS profiles of PFSA ionomers both edge-on and face-on. ► Shows how compression affects the SAXS profiles and domain spacing. ► Swelling studies and water content measurements are correlated with domain spacing. ► First to measure and correlate through-plane conductivity with water content under different applied compressive pressures.
Research and development of fuel cell materials often focuses on designing and discovering materials which will reduce the cost or improve the durability of an individual subcomponent. Examples of ...recent focus areas include non-Pt group metal catalysts, noncarbon catalyst supports, and nonfluorinated membranes. These studies rarely look at the entire system to comprehend the impact of these materials on the cost of ownership to the customer, including vehicle and fuel costs. This perspective takes a holistic look at the impact of functional materials on automotive fuel-cell systems and provides direction on which material properties will provide the greatest benefit. It also provides guidance on which material classes are the most likely to enable the achievement of systems which will result in the successful commercialization of light-duty fuel-cell vehicles.
Pinhole defects that form in proton exchange membranes (PEMs) due to the cyclic hygrothermal stresses induced during the operation of a fuel cell and cause gas crossover may be interpreted as a ...result of crack formation and propagation. The goal of this study is to employ a fracture test to approach the intrinsic fracture energy of a perflourosulfonic acid proton exchange membrane. The intrinsic fracture energy has been used to characterize the fracture resistance of polymeric materials with minimal plastic dissipation and the in absence of viscous dissipation, and has been associated with the long-term durability of polymeric materials where subcritical crack growth occurs under slow time-dependent or cyclic loading conditions. Insights into this limiting value of fracture resistance may offer insights into the durability of PEMs, including the formation of pinhole defects. In order to achieve this goal, a knife slit test which significantly reduces the plastic deformation during the test by limiting the plastic zone size with a sharp blade is conducted. Additionally, double edge notched tension tests and trouser tear tests are conducted to obtain the essential work of fracture and tear energy, respectively. It has been found that although the fracture energy obtained with the knife slit test is still several times larger than the intrinsic fracture energy of regular polymer materials, it is several orders of magnitude lower than those obtained with the other two methods, where process-dependent viscous and plastic dissipation dominate over the intrinsic material property.
Temperature and humidity fluctuations in operating fuel cells impose significant biaxial stresses in the constrained proton exchange membranes (PEMs) of a fuel cell stack. The strength of the PEM, ...and its ability to withstand cyclic environment-induced stresses, plays an important role in membrane integrity and consequently, fuel cell durability. In this study, a pressure loaded blister test is used to characterize the biaxial strength of Gore-Select
® series 57 over a range of times and temperatures. Hencky's classical solution for a pressurized circular membrane is used to estimate biaxial strength values from burst pressure measurements. A hereditary integral is employed to construct the linear viscoelastic analog to Hencky's linear elastic exact solution. Biaxial strength master curves are constructed using traditional time–temperature superposition principle techniques and the associated temperature shift factors show good agreement with shift factors obtained from constitutive (stress relaxation) and fracture (knife slit) tests of the material.
Commercial proton exchange membrane heavy-duty fuel cell vehicles will require a five-fold increase in durability compared to current state-of-the art light-duty fuel cell vehicles. We describe a new ...composite membrane that incorporates silicotungstic heteroply acid (HPA),
α
-K
8
SiW
11
O
40
▪13H
2
O, a radical decomposition catalyst and when acid-exchanged can potentially conduct protons. The HPA was covalently bound to a terpolymer of tetrafluoroethylene, vinylidene fluoride, and sulfonyl fluoride containing monomer (1,1,2,2,3,3,4,4-octafluoro-4-((1,2,2-trifluorovinyl)oxy)butane-1-sulfonyl fluoride) by dehydrofluorination followed by addition of diethyl (4-hydroxyphenyl) phosphonate, giving a perfluorosulfonic acid-vinylidene fluoride-heteropoly acid (PFSA-VDF-HPA). A composite membrane was fabricated using a blend of the PFSA-VDF-HPA and the 800EW 3M perfluoro sulfonic acid polymer. The bottom liner-side of the membrane tended to have a higher proportion of HPA moieties compared to the air-side as gravity caused the higher mass density PFSA-VDF-HPA to settle. The composite membrane was shown to have less swelling, more hydrophobic properties, and higher crystallinity than the pure PFSA membrane. The proton conductivity of the membrane was 0.130 ± 0.03 S cm
−1
at 80 °C and 95% RH. Impressively, when the membrane with HPA-rich side was facing the anode, the membrane survived more than 800 h under accelerated stress test conditions of open-circuit voltage, 90 °C and 30% RH.
Highlights
Heteropoly acid functionalized perfluorinated sulfonic acid - vinylidene fluoride material.
Decrease in size of heteropoly acid clusters on hydration..
Composite membrane was more crystalline and hydrophobic compared to the control membrane.
Chemical durability improvement with heteropoly acid-dense side of the membrane faced to anode.
Water uptake activities and transport properties are critical for water management in fuel cell membranes. In this work, three perflourosulfonic acid (PFSA) fuel cell membranes, including ...Nafionregistered sign-117 and two Gore membranes, were evaluated at different relative humidity controlled conditions. These fuel cell membranes were studied using variable temperature 1H spin-lattice relaxation times (T1) and pulsed field gradient (PFG) NMR techniques in the temperature range of 298 to 239 K. Water self-diffusion coefficients and proton transport activation energies in the fuel cell membranes were obtained from the PFG-NMR experiments. The results show that the water self-diffusion coefficients increase with increasing hydration level, and decrease with decreasing temperature. The water molecular motion is significantly slowed at low temperatures; however, the water molecules in these membranes are not frozen, even at 239 K. The water uptake activity and diffusivity in these membranes were compared as a function of temperature and hydration level. At the same temperature and hydration level, the water self-diffusion coefficients of two Gore fuel cell membranes are higher than that of Nafionregistered sign-117. This is attributed to the lower EW of the Gore membranes. The presence of an expanded polytetrafluoroethylene (ePTFE) reinforcing layer in the membrane also has an impact on water diffusivity.
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