Effect of PEM Catalyst Platinum Loading on Fuel Cell Performance

Effect of PEM Catalyst Platinum Loading on Fuel Cell Performance

Over the past two decades, extensive research on the development of low-temperature polymerelectrolyte-membrane fuel cells (PEMFC) resulted in significant increases in the voltage performance of membrane electrode assemblies (MEAs) . These voltage gains were primarily produced by the implementation of thinner membranes, progressing from the originally most common 1100 EW (equivalent weight (gpolymer/molH+)) Nafion® membranes with thickness of 175 μm/125 μm (Nafion 117/115), to 50 μm thick Nafion® 112 , all the way to ultra-thin homogeneous (e.g. 25 μm, 1100 EW membranes extruded in the sulfonylfluoride-form from DuPont and hydrolyzed to proton-form by Ion Power ) or lower-EW PTFE/ionomer composite membranes (either from Asahi Glass (30 μm, 910 ) or Gore (25 μm, <1000 EW )) which produce high cell voltages at current densities ≥1 A/cm². These cell voltage improvements were accompanied by significant reductions in MEA platinum loadings from the high loadings of 5–10 mgPt/cm² per MEA in the early 1990s to <1 mgPt/cm² per MEA in later work , a development which was primarily due to the substitution of Pt-black catalysts with higher surface area carbon-supported Pt catalysts as well as the use of perfluorosulfonic-ionomer binder in thin-film catalyst layers.

Due to these innovations in materials and processing technology, state-of-the-art fuel cells yield cell voltages which surpass older MEA technology, where only up to 0.60 V were achieved at 1.0 A/cm² under high pressure conditions (300 kPaabs) with fully humidified H₂/air reactants (stoichiometric flows of 1.5/2.0) at cell temperatures of 70–80 °C and Pt loadings of <1 mgPt/cm² per MEA . This is, for example, illustrated by reports from UTC Fuel Cells, where 0.68 V are obtained at the same current density (1.0 A/cm²) even at ambient pressure under otherwise similar conditions (65 °C cell temperature, fully humidified H₂/air at stoichiometric flows of 1.25/2.0) . In the latter case, rather low Pt cathode loadings of 0.4 mgPt/cm² were used and Pt loadings on the anode were probably of the same value or lower (not cited). While this represents a major development progress, the Pt-specific power density still equates to ca. 0.9–1.2 gPt/kW (assuming anode Pt loadings of 0.2–0.4 mgPt/cm², i.e. total loadings of 0.6–0.8 mgPt/cm² per MEA), which may be sufficiently low for low-volume applications (e.g. stationary, uninterrupted-power supply, etc.), but is still too high for automotive applications, where less than 0.4 gPt/kW are required for large-scale implementation.

Primarily two approaches may be used to reduce the Pt metal requirement in state-of-the-art fuel cells: (i) reduction of the mass-transport losses particularly at high current densities by improved diffusion media (DM), improved reactant flow-fields, and improved electrode structures and/or (ii) improved catalysts and catalyst utilization. The former approach would allow to increase the stack current density to 1.5–2.0 A/cm² with no or insignificant voltage penalty , thereby reducing the Pt-specific power density by a factor of 1.5–2 (i.e. 0.45–0.6 gPt/kW). Any further reductions would have to be achieved by a reduction of the Pt loading of the MEA below the above 0.6–0.8 mgPt/cm2 per MEA, which may either be done via platinum thrifting or the implementation of alternative catalysts (e.g. Pt-alloy cathode catalysts).

The present work examines in detail the effect of platinum loading reductions (both anode and cathode) on fuel cell performance and seeks to demonstrate the trade-off between Pt-catalyst loading and cell voltage. This will be illustrated by means of 50 cm² single-cell data complemented by full-active-area short stack (250 and 500 cm² active-area, ca. 20 cells) measurements. Owing to the high catalytic activity of Pt toward H₂ electro-oxidation (exchange-current densities, i0, of the order of 10−3 A/cmPt2), we will show that there is a large potential for reducing Pt anode loadings in the case of fuel cell operation with pure H₂, while much less reductions are achievable with current PtRu-anode catalysts in the case of fuel cell operation with CO-contaminated reformate. Unfortunately, the oxygen reduction reaction (ORR) kinetics on Pt are approximately six orders of magnitude slower than the H₂ oxidation kinetics (i0 of the order of 10−9 A/cmPt2 ), and we will show that further reductions in cathode Pt loadings with pure Pt-catalysts result in well predictable voltage losses (these may, however, be avoided by implementing more advanced Pt-alloy cathode catalysts ).