In addition to the kinetic activity of the catalyst, the HCD performance is affected by mass transport losses (both proton and oxygen) and ohmic losses. To quantify various voltage loss terms and thus identify the opportunity for improvement, mathematical models with parameters extracted from experiments are commonly employed. Experimental measurements are usually carried out under differential cell conditions operating at high stoichiometry flow with minimal pressure drop down the channel, making it almost one-dimensional across the cell. The cell performance can be described by:
Ecell = Erev -iRΩ - ηHOR - ιηORRι - i (RH+,a + RH+c) - ηtx,O2 (1)
where Ecell is the cell voltage; Erev is the reversible cell voltage, i is the current density; RΩ is the sum of the Ohmic resistances of proton conduction through the membrane and of electron conduction (commonly referred to as high frequency resistance or HFR); ηHOR and ηORR are the charge transfer overpotentials for the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR), respectively; RH+ is the effective resistance to proton conduction in an electrode with subscripts a and c denoting anode and cathode, respectively; and lastly, ηtx,O2 is the oxygen transport loss which consists of two parts: one results from oxygen transport through gas phase, and the other is associated with the oxygen transport resistance local to the Pt surface, namely
ηtx,O2 = ηtx,O2(gas) + ηtx,O2 (Pt-local) (2)
In this study, we analyze the impact of various parameters affecting HCD performance via systematic evaluation of different material and catalyst layer design. Materials selection involves Pt– alloy catalysts on various carbon supports, synthesis routes, ionomer structures. Similarly, design selection would investigate, ionomer content and solvent formulation etc. Measurements in 5 cm2 differential cell with diagnostics such as electrochemical surface area (ECA), mass activity measurements in 100% oxygen, limiting current measurements (to evaluate bulk 2 and local oxygen transport resistance3), electrochemical impedance spectroscopy (EIS) in H2/N2 (to evaluate proton transport losses)4, CO stripping as a function of RH, etc., will be conducted. Measured transport resistances will be correlated to the ex situ properties of catalyst and ionomer microstructures. The measured differential cell data will be used in 1-D model to quantify the performance loss terms.
References
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3) T.A. Greszler, D.A. Caulk, P. Sinha, J. Electrochem. Soc., 159, F831 (2012).
4) R. Makharia, M. F. Mathias, D. R. Baker, J. Electrochem. Soc., 152 (5), A970 (2005).