Considerations of Oxide Species Coverage on the Kinetic Parameters of Pt-Based Catalysts

In order for hydrogen polymer electrolyte membrane fuel cell (PEMFC) technology to be economically competitive, overall system costs still need to be lowered. A key component of cost is the amount (grams) of Pt used in the membrane electrode assembly (MEA). Significant advances in electrocatalyst research, especially focused on the oxygen reduction reaction (ORR) for PEMFC cathodes, have thus been made and have led to the development of highly active catalysts capable of delivering target power at reduced Pt loadings.1 Unfortunately, as the Pt loading i.e. electrode roughness factor, decreases, an additional resistance emerges leading to voltage loss. This resistance, associated with phenomena at or near the Pt surface, is poorly understood and has been shown to have the greatest impact on low loaded Pt electrodes ( 1.0 A/cm2). Therefore, an understanding of the kinetic parameters governing the electrochemical reactions, specifically at the cathode, is essential in order to distinguish between voltage losses that are kinetically derived from those originating from other sources. An understanding of the nature and mechanisms of performance losses in PEMFCs is essential in order for industry to best direct R&D resources. Discrepancies observed in theoretical kinetic predictions and real performance polarization curves for low Pt loaded MEAs (>0.1 mg Pt/cm2) have led to alterations in the simple Tafel kinetic model2 by including an additional term related to oxide coverage.3 While the operating cell potential inherently drops as the catalyst loading decreases, oxide-coverage kinetics become increasingly important in modeling performance especially when transitioning between catalyst surfaces with little to no oxide coverage to those with high oxide coverage. Since some kinetic parameter values change considerably when incorporating an oxide coverage term into the simple tafel model, it is important to know how dependent these changes are in regards to catalyst type and support, as well as in how oxide coverage is defined. Because specific oxide species generated at the catalyst surface can differ with changing conditions e.g. Pt-OH (1e- process), Pt-O (2e- process), it is difficult to obtain a unified model which accurately describes oxide formation. Subsurface oxide generation has also been shown to occur,4 which can skew interpretations of oxide measurements. Additionally, oxide formation has been shown to differ substantially among different catalyst surfaces and supports.This work attempts to clarify the impact that oxide species have on kinetic parameters (exchange current density (ios), reaction order (γ), activation energy (EA) etc.) for Pt-based catalysts. Experiments were performed under pure oxygen using a 5cm2 differential cell with MEAs consisting of Pt/V, Pt/HSC, and Pt alloy/HSC at loadings of 0.2 – 0.05 mg Pt/cm2. Sub-ambient system pressures were employed by means of a vacuum panel in order to lower pure oxygen partial pressure while minimizing gas transport effects. An example measurement is shown in Figure 1, with the oxide and transport free surface potential and current region highlighted. Interpretations of oxide measurement techniques as well as the impact of different catalyst surfaces and supports are discussed. As catalyst activity continues to improve, future implications and significance of oxide kinetics are also considered. Figure 1. iR-free potential vs log current density as a function of oxygen partial pressure for Pt/Vu electrocatalyst. The authors would like to acknowledge funding from the U.S. Department of Energy under CRADA #CRD-14-539. References 1. A. Kongkanand and M. F. Mathias, The Journal of Physical Chemistry Letters, 7, 1127 (2016). 2. K. C. Neyerlin, W. Gu, J. Jorne and H. A. Gasteiger, Journal of the Electrochemical Society, 153, A1955 (2006). 3. N. Subramanian, T. Greszler, J. Zhang, W. Gu and R. Makharia, Journal of The Electrochemical Society, 159, B531 (2012). 4. B. Conway, B. Barnett, H. Angerstein‐Kozlowska and B. Tilak, The Journal of chemical physics, 93, 8361 (1990). Figure 1