Engineered Water Highways in Fuel Cells: Radiation Grafting of Gas Diffusion Layers

Polymer electrolyte fuel cells (PEFC) require a sophisticated water management to operate efficiently, especially at high current densities which are needed to reach system cost targets. The description of the complicated two-phase water transport remains a challenge in PEFC models and requires experimental validation on various length scales. In this work, operando X-ray tomographic microscopy (XTM) with scan times of 10 s was used to depict the liquid water at defined conditions at a technically relevant cell temperature of 80 • C. Cells with Toray TGP-H-060 gas diffusion layer (GDL) with microporous layer (MPL) and different rib width were operated with different feed gas humidifications (under-and oversaturated) and current densities between 0.75 to 3.0 A/cm 2 . Based on the quantification of the local and average saturation, the distribution of water cluster size is analyzed. Different categories of the water cluster connectivity are defined and quantified. The analysis is complemented with numerical simulations of the permeability in the liquid phase of the GDL that is correlated to saturation for the different GDL domains. The numerical simulations of the pressure drop of liquid water flow from the catalyst layer toward the gas channels in channel-rib repetition units allows for conclusions on cluster growth mechanisms. In the past two decades, large developments have lead hydrogen fed polymer electrolyte fuel cell (PEFC) technology to the brink of commercialization, e.g. in the stationary sector with more than 100000 deployments in the Enefarm activity 1 as well as in the mobile sector where major car manufacturers have presented first commercial vehicles 2,3 and niche-market commercialization for logistic vehicles. 4While for the automotive market also hydrogen infrastructure is a major barrier for widespread application, in both fields of application, cost of the fuel cell system remains the main hindrance for extensive spread of PEFC technology. Cost is closely tied to materials and manufacturing processes, such as more effective electrocatalyst and more durable membrane materials. These developments are underway and have made significant progress. 5 Cost however is also closely tied to power density of the fuel cell stack. It is obvious that with higher power density less cells or an accordingly smaller cell area with the related reduced material use, is leading to a cost reduction if the high power density materials are of comparable cost.Automotive fuel cells with state of the art materials and cell structures reach today more than 1 W/cm 2 with current densities up to 3 A/cm 2 .5 At such high current densities water management becomes more and more important and has to be properly designed on various scales from the system level to nanoscale structures, including all cell components as flow field plates, gas diffusion layers (GDL), the polymer membrane and the catalyst layer (CL). In order to minimize the ohmic losses, the polymer membrane needs to be well humidified, 6 however at high current den...

Section: Discussionmentioning

confidence: 99%

Operando Properties of Gas Diffusion Layers: Saturation and Liquid Permeability

Eller

Roth

Marone

et al. 2016

“…The use of engineered pathways for water in GDLs 11 was also recently proposed. Despite the interest in water removal and cooling with evaporation, phase change in GDLs is still not well-quantified, as most of the studies related to two phase flow in GDL analyze non-phase change phenomena such as drainage, imbibition and capillary redistribution.…”

mentioning

confidence: 99%

Determination of Water Evaporation Rates in Gas Diffusion Layers of Fuel Cells

Lal

Lamibrac

Eller

et al. 2018

The efficiency of polymer electrolyte fuel cells is closely linked to effective heat and water management. Evaporation of water in the gas diffusion layers (GDL) is an important process influencing water management and can even be used for evaporative cooling. In this study, evaporation rates in Toray-GDL materials are obtained for different temperatures, gas flow rates, carrier gases and water saturation levels. Evaporation rates are correlated with water distribution profiles in the GDL, obtained using X-ray tomographic microscopy imaging. The influence of temperature on the evaporation rates is predicted by considering the effect of temperature on vapor diffusion alone. However, changing the carrier gas type points to a deviation from diffusive-driven vapor transport, which could be due to entrainment of gas inside the GDL, among other factors. © The Author ( Polymer electrolyte fuel cells (PEFC) efficiently convert the chemical energy stored in hydrogen to electrical energy, with water and heat as the byproducts. PEFCs are therefore considered a key technology for future hydrogen-based energy scenarios, especially in the mobility and energy storage sectors, as the usage of renewable electricity gains momentum worldwide. The central component of PEFCs is the membrane-electrode-assembly (MEA), which consists of a membrane at the center that is coated with catalyst layers (CL) on both sides, which are contacted by gas diffusion layers (GDL). Water management in the MEA is a critical topic that is closely related to power density and durability of PEFCs.1 Water management (i.e. having maximum water content in the membrane and a minimum saturation in the porous layers) is a complex issue governed by the fluid and energy flows in the MEA. Most of this complexity arises from the fluxes of heat and water in the GDLs, which are difficult to localize and quantify.Gas diffusion layers are porous structures with porosities in the range of 70-85% and average pore sizes around 10-40 μm.2 The GDL materials have to provide maximum heat and electron conductivity in the solid structure while simultaneously ensuring high permeability of the reactant gases, product water and water vapor in the void. At low operating-temperatures or high current densities, gas transport in the pore space can be significantly hindered by liquid water occupying pore space, especially at the cathode side. Consequently, oxygen transport to the cathode CL is significantly affected, which in turn reduces fuel cell performance or efficiency.

“…A standard dip-coating procedure using water dispersions of FEP particles and a subsequent thermal treatment was followed, which is described in our previous work. 29,34 Coating loads of 5, 30 and 70%wt were targeted (Table I). Calibration curves, relating the FEP concentration in the dispersion to the final coating load, were obtained for the different GDL substrates ( Figure S1).…”

Section: Methodsmentioning

confidence: 99%

“…The details were described in our previous work. 29 An acceleration voltage of 200 kV and a dose of 50 kGy were selected in each case. In order to create patterned materials, radiation-blocking masks were used.…”

Section: Methodsmentioning

confidence: 99%

“…28 Recently, we reported a method to produce GDLs with patterned wettability based on local modifications of the polymer coating's wettability using radiation grafting. 29,30 Contrary to the other approaches, the radiation grafting method allows wettability modifications across the complete material thickness without altering the mechanical properties and, most importantly, it offers realistic scaleup potential. Here, we present a detailed experimental study on the ability of our newly proposed material to confine the liquid water transport to predefined pathways.…”

mentioning

confidence: 99%

See 1 more Smart Citation

Advanced Water Management in PEFCs: Diffusion Layers with Patterned Wettability

Forner‐Cuenca

Biesdorf

Lamibrac

et al. 2016

In this paper, we present an experimental study on the development of gas diffusion layer (GDL) materials for fuel cells with dedicated water removal pathways generated using radiation induced grafting of hydrophilic compounds onto the hydrophobic polymer coating. The impact of several material parameters was studied: the carbon substrate type, the coating load, the grafted chemical compound and the pattern design (width and separation of the hydrophilic pathways). The corresponding materials were characterized for their capillary pressure characteristic during water imbibition experiments, in which we also evidenced the differences between injection from a narrow distribution channel in the center of the material (and thus strongly relying on lateral transport) and homogeneous injection from one face of the material. All materials parameters were observed to have a significant influence on the water distribution. In particular, the type of substrate has a dramatic impact, with results ranging from a nearly perfect separation of water between hydrophilic and hydrophobic domains for substrates having a narrow pore size distribution to a fully random imbibition of the material for substrates having a broad pore size distribution.