Automotive Fuel Cell Subzero Cold-Starts in a Single Cell Quasi-Adiabatic Fixture
Authors: Rice Cynthia A. , Pistono Antonio 
Published: February 2020

Proton exchange membrane fuel cells (PEMFC) mirror current amenities available from the internal combustion engine such as: rapid start-up time, refueling time, and travel range. For an automotive PEMFC stack to attain 80 kW of power, typically greater than 300 repeat cells are required. A single cell is comprised of 7 layers between the anode and cathode flow fields/current collectors and include layers of macro/microporous diffusion media, ionomer bound carbon supported platinum catalyst layers, and a central proton exchange membrane (PEM). In the presence of partially humidified fuel and oxidant feeds (H2 and 21%O2/79%N2 on the anode and cathode, respectively) energy is produced under applied load as electrons and protons are formed from H2 oxidation on the anode and water is formed by the exothermic reduction of the protons and electrons with O2 on the cathode. The hygroscopic PEM inherently retains humidified reactant gas and oxygen reduction reaction product water which facilitates proton conduction. Four main issues limit commercial viability of automotive fuel cells – (i) cathode catalyst durability, (ii) fuel cell cost, (iii) subzero cold-starts, and (iv) refueling infrastructure. Automotive subzero cold-starts are challenging due to residual water inventory of the PEMFC and product water formation and retention. Water retained within the ionomer of the membrane and catalyst layers partially remains in a liquid phase above -40°C due to pore size and acidity induced freezing point depression.1 For successful cold-start, the temperature of the PEMFC must increase above 0°C before the accumulation of product water in the cathode catalyst layer significantly restricts O2 mass transport.
Catalyst layer fabrication affects the water storage capacity of the PEMFC at subzero temperatures. Directly spraying the cathode catalyst layer on the membrane vs. decal transfer of a screen printing results in a nearly 60 mV increase in the initial on load voltage that would translate to a high initial power output, but a 2-2.5x decrease in the water storage capacity at -20°C.2 Partially dehydrating the PEMFC to lower initial water contents (λinitial) increases that water storage capacity at the expense of the initial on load voltage.3

A prototype quasi-adiabatic cold-start single cell PEMFC fixture was designed in-house to address several of the initial issues found in UTRC’s4 original design. The insulating materials selected for permeability and mechanical stability were polycarbonate and balsa wood, respectively. Internal Kapton heaters were used to emulate adjacent cell heading. Figure 1 show the impact of λinitial of 3.2 and 6.2, heater output levels equivalent to 1x and 2x adjacent cell heating (to compensate for non-adiabatic losses), and constant current (600 mV cm-2) vs. constant potential (0.1V) starts. The initial temperature rise increased with higher λinitial and under constant potential control. Isothermal water fill tests were performed on the same membrane electrode assembly after cold starts. Performance degradation due to subzero testing was higher for isothermal water fill tests compared to cold starts.

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Pistono, A. and C.A. Rice, 'The Effect of Material Properties on the Subzero Water Storage Capacity of Cathode Catalyst Layers for Proton Exchange Membrane Fuel Cells', J. Electrochem. Soc., 2017, 164, F582.

Pistono, A. and C.A. Rice, 'Subzero Water Storage Capacity in Proton Exchange Membrane Fuel Cells: Effects of Preconditioning Method', The Journal of Power Souces, 2018
Under Review.

Rice-York, C., J. Needham, N. Gupta, and P.L. Hagans, 'Platform for Rapid Prototyping of PEM Fuel Cell Designs with Enhanced Cold-Start Performance and Durability', ECS Transactions, 2006, 1, 383.

Figure 1