Recent Progress in Nanostructured Thin Film (NSTF) ORR Electrocatalyst Development for PEM Fuel Cells

Development of highly active, durable, and low cost ORR electrocatalysts and supports for PEM fuel cells remains a key challenge for large-scale commercial deployment.  These factors are highly coupled, especially so when incorporated into membrane electrode assemblies (MEAs) at low Platinum Group Metal (PGM) content approaching mid-term U.S. Department of Energy (DOE) targets [1].  3M Nanostructured Thin Film (NSTF) electrocatalysts and electrodes are a unique approach towards addressing these challenges, due to the thin film electrocatalyst structure and non-carbon, high-aspect ratio, oriented support, and ultrathin (<1µm) NSTF electrodes which do not require ionomer for proton conduction [2].

Our presentation will address recent progress towards development of next generation NSTF ORR electrocatalysts and our efforts to integrate advanced NSTF electrodes into robust, durable, high performance, and low-cost MEAs. 

Current generation Pt₃Ni₇/NSTF ORR electrocatalysts have high specific and mass activities (2.7mA/cm²_Pt and 0.75A/mg_PGM, 0.900V v. RHE, RDE [3]) and in MEA electrodes have exhibited exceptional tolerance to corrosion and good cyclic durability.  The high mass activity of Pt₃Ni₇/NSTF is in part due to formation of a nanoporous Pt alloy thin film structure upon dealloying, which yields substantially increased specific area and specific activity over nonporous Pt/NSTF cathodes [4].  When dealloyed Pt₃Ni₇/NSTF electrodes are integrated into an optimized MEA, very high MEA ¼ power, rated power and specific power densities are achieved at low total PGM content (Fig. 1(A)), which approach or exceed these DOE 2020 targets [5]. 

Development of highly active nanoporous electrocatalysts depends strongly upon simultaneous optimization of the electrocatalyst structure and composition via catalyst deposition, annealing, and dealloying process parameters.  Fig. 1(B) summarizes MEA ORR mass activity data for “Pt_xM_1-x” NSTF cathodes over a range of compositions with three different catalyst structures induced by process set variation, all at 0.117-0.123mg_PGM/cm², and includes Pt₃Ni₇ data for reference.  At certain compositions, mass activity can increase over 2.5 times as the catalyst process conditions are varied, whereas at other compositions no significant effect is observed.  Through process and composition optimization, mass activities of “Pt_xM_1-x” are nearly 1.3 times higher than Pt₃Ni₇. 

Development of high performance MEAs requires not only high cathode mass activity, but also low residual leachable transition metal content to minimize contamination of the PFSA PEM.  Fig. 1(C) compares MEA H₂/Air performance of otherwise identical MEAs with either a 0.129mg_Pt/cm² Pt₃Ni7/NSTF cathode or a 0.075mg_Pt/cm² Pt₃Ni₇/NSTF cathode, dealloyed by different methods.  H₂/Air performance of the lower loaded electrode with dealloy method “B” is similar or superior to the higher loaded cathode which was dealloyed with method “A”, even with 40% lower PGM loading.

 References

1. U.S. Department of Energy, Office of Energy Efficiency & Renewable Energy, “FUEL CELL TECHNOLOGIES OFFICE MULTI-YEAR RESEARCH, DEVELOPMENT, AND DEMONSTRATION PLAN”, Section 3.4 Table 3.4.13, retrieved from http://energy.gov/sites/prod/files/2014/12/f19/fcto_myrdd_fuel_cells.pdf on April 6th, 2015.
2. M. K. Debe, ECS Transactions, 45(2) 47–68 (2012).
3. D. A. Cullen and S. Kocha, private correspondence.
4. D. A. Cullen et al., J. Mater. Chem. A, accepted, (2015).
5. A. J. Steinbach, U.S. Department of Energy EERE FCTO Annual Merit Review (2015), Project FC104, submitted.