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.
The automated process of coating catalyst layers on gas diffusion electrodes (GDEs) for high-temperature proton exchange membrane fuel cells results inherently into a number of defects. These defects ...consist of agglomerates in which the platinum sites cannot be accessed by phosphoric acid and which are the consequence of an inconsistent coating, uncoated regions, scratches, knots, blemishes, folds, or attached fine particles—all ranging from μm to mm size. These electrochemically inactive spots cause a reduction of the effective catalyst area per unit volume (cm2/cm3) and determine a drop in fuel cell performance. A computational fluid dynamics (CFD) model is presented that predicts performance variation caused by manufacturing tolerances and defects of the GDE and which enables the creation of a six-sigma product specification for Advent phosphoric acid (PA)-doped polybenzimidazole (PBI)-based membrane electrode assemblies (MEAs). The model was used to predict the total volume of defects that would cause a 10% drop in performance. It was found that a 10% performance drop at the nominal operating regime would be caused by uniformly distributed defects totaling 39% of the catalyst layer volume (~0.5 defects/μm2). The study provides an upper bound for the estimation of the impact of the defect location on performance drop. It was found that the impact on the local current density is higher when the defect is located closer to the interface with the membrane. The local current density decays less than 2% in the presence of an isolated defect, regardless of its location along the active area of the catalyst layer.
Characterization of transport properties in gas diffusion layers (GDLs) for proton exchange membrane fuel cells (PEMFCs) is presented. The permeability coefficients are determined by strictly ...controlling the direction of flow through the GDL sample. The permeability coefficients are measured initially for the entire GDL containing macro-porous substrate and microporous layer, and for the macro-porous substrate alone. It is concluded that the polytetrafluoroethylene (PTFE) loading increases the volume of the intra-agglomerate pores, which further influences the permeability after sintering. The results also show that the loading increases the material rigidity, increasing its capability to maintain higher porosity and permeability under compressive load.
Electrochemical Hydrogen Pumps (EHP) provide a unique highly efficient means of separating and compressing hydrogen with continuous steady-state operation. In this paper, we demonstrate the ...performance of a commercially available, polybenzimidazole (PBI) membrane based platform as a benchmark for ultra-high efficiency performance. A primary gas mixture of CO
2
and H
2
with a ratio of 4:1 respectively was selected to demonstrate the performance of
EHP
s with near theoretical Faradaic efficiency with negligible CO poisoning due to reverse water gas shift reaction (RWGS). It was found that humidification of the feed gas at room temperature improved polarization performance while also improving energy efficiency, thus reducing the need for a tightly controlled relative humidity of feed gas. A new perspective on EHP energy efficiency calculation methodology is also provided by including the cell heating requirement in the calculation. In this manner, an overall improvement to the energy efficiency of nearly 20% was realized by dropping the cell temperature to 120 °C while paying no significant penalty to electrochemical performance. Nearly 99.99% pure H
2
and 99.93% pure CO
2
were produced with a hydrogen yield of 99.34%.
A characterization of the ionic conduction of the active layer of a polymer electrolyte membrane fuel cell (PEMFC) cathode by ac impedance measurement at open-circuit potential conditions was ...conducted. Porous electrode theory was used to derive a compact equation, *d(2)*Q(2) / *dy(2) + *d In f(y)/*dy X *d*Q(2) /*dy - R/f(y)(1 + j*W) *Q(2) = 0, to solve for the impedance response of a cathode at open-circuit potential conditions. This equation includes a parameter R, the ratio of an ionic resistance (evaluated at the active layer/membrane interface), to the total charge-transfer resistance of the active layer. The influence of an assumed ionic conductivity distribution profile f(y) on the error in the estimation of total double-layer capacitance of the active layer from the -1/(Z(Im)*w) vs. Z(Re) plot was also investigated in this work. The increase of ionic conductivity in the active layer of an air cathode with an increase in the ionomer loading was revealed from both impedance data and surface area measurements. A nonlinear parameter estimation method was used to extract the ionic resistance from the high-frequency region of the impedance data at open-circuit potential conditions. The assumed ionic conductivity distribution profile in the active layer was found to vary with ionomer loadings.
This is the first in a series of papers in which we present state-of-the-art methods demonstrated at Case for the estimation of transport properties in gas diffusion layers (GDLs) for proton exchange ...membrane fuel cells (PEMFCs). Most of the methods used today for measuring wettability properties of GDLs are related to the external contact angle to water. The external contact angle however does not describe adequately capillary forces acting on the water inside the GDL pores. We show as well that the direct method of estimation of the internal contact angle using goniometry on micrographs is impractical. We propose and describe in this paper a method for estimating the internal contact angle to water and the surface energy of hydrophobic and hydrophilic gas diffusion media. The method was applied to GDLs having different contents of hydrophobic agent and carbon types. The method can be applied separately to different components of the GDL including macro-porous substrates and micro-porous layers. The uncertainty estimates using this method are usually within 3% of the measured value.
Phosphoric acid loss poses immense hurdles for the durability of high-temperature polymer electrolyte membrane fuel cells (HT-PEMFCs). Here we report quaternary ammonium-biphosphate ion-pair ...HT-PEMFCs that do not lose phosphoric acids under normal and accelerated stress conditions. Our energetics study explains the acid loss behavior of the conventional phosphoric acid-polybenzimidazole (PA-PBI) system by two mechanisms. If PA loss occurs via acid evaporation, the acid loss is constant over time. On the other hand, when water activity in the PA-PBI system is high, exponential decay of PA loss occurs via the water replacement mechanism. Combined 31P NMR and computational studies show that the proposed ion-pair system has six times higher interaction energy, which allows for containing all PAs in the membrane electrode assemblies under a broad range of operating conditions. In addition, polar interactions between the phosphonic acid ionomer and phosphoric acid explain acid retention in the electrodes of the ion-pair HT-PEMFCs.
This is the second in a series of papers in which we present methods demonstrated in our group for the estimation of transport properties in gas diffusion layers (GDLs) for proton exchange membrane ...fuel cells (PEMFCs). Here we describe a method for determining separately the in-plane (
x,
y-directions) and through-plane (
z-direction), viscous and inertial permeability coefficients of macro-porous substrates and micro-porous layers by controlling the direction of the gas flow through the porous sample. The method is applied initially to the macro-porous substrate of the GDL alone and subsequently to the macro-porous substrate with different micro-porous layers applied on it. The permeability coefficients of the micro-porous layer are calculated from the two measurements. The permeability coefficients are calculated from the Darcy–Forchheimer equation by application of the method of least squares. The method was applied to GDLs having different contents of polytetrafluoroethylene (PTFE) and carbon types. The samples with a higher PTFE content have in-plane and through-plane viscous permeability coefficients higher than those of the samples with lower PTFE content. The in-plane and through-plane viscous permeability coefficients also depend on the carbon type.
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