Since the electrode/catalyst materials contribute to nearly half of a fuel cell stack cost, there is an urgent need to reduce or replace PGM usage. Among all the non-PGM candidates explored so far, transition metal doped nitrogen-carbon (TM-N-C) composites appear to be the most promising ones.  Generally, these materials are prepared by forming TM-N₄ molecular complex over amorphous carbon support, followed by thermal activation.  Since non-PGM catalysts are known to have lower turn-over frequency per catalytic site comparing to platinum, their active site densities must be substantially higher to deliver a comparable performance. Using carbon support dilutes the active site density. The new approaches that could circumvent such limitation without the need of inert carbon support include the use of metal-organic framework (MOF) [1-3] and porous organic polymer (POP) [4] as the catalyst precursors. The materials derived from these rationally designed precursors can serve as high efficiency non-PGM catalysts directly or be further modified to low-PGM catalysts. To maximize the utilization of non-PGM or low-PGM catalytic centers requires an electrode structure that ensures efficient mass transport of reactant and product to and from the active site. Furthermore, the catalyst must be durable under accelerated aging conditions.  .     

At Argonne National Laboratory, we pioneered the research of using MOFs and POPs as the precursors for non-PGM catalyst synthesis [1, 3, 4, 5]. MOFs have clearly-defined lattice structures through the metal-ligand coordination chemistry. The TM-N₄ entities can be grafted into MOFs with very high ligation site densities. During thermal activation, the organic linkers are converted to electro-conductive carbon while maintaining porous framework, leading to catalysts with high surface areas and uniformly distributed catalytic sites.  We demonstrated that zeolitic imidazolate framework, subclass of MOF, can used to prepare low-cost, highly active non-PGM catalysts. More recently, we developed a “one-pot” solid-state synthesis method which produces non-PGM catalyst using low-cost material without the need of solvent and separation [5]. Parallel to MOFs, we also developed another class of non-PGM catalysts using the porous organic polymers (POPs). [4] POPs are prepared by cross-linking the monomers containing strong TM-N binding site through polymerization.  Similar to MOFs, POPs have high surface areas and uniformly distributed active sites inside the pours framework.  The cathode catalysts prepared by MOF or POP have produced some major catalytic activity breakthroughs, generating the highest current and power densities among those reported in the literature.

To improve mass and charge transfer for non-PGM catalysts, we have recently invented a new method to produce non-PGM catalyst with nano-network electrode architecture. [6] The MOF-based catalysts are incorporated into individual nano-fibers connected by a graphitic network. High micro-pore volume and surface area are maintained whereas the meso-pores in the conventional powder catalysts were no longer necessary and eliminated. Mass transport is improved through macro-pores inside the nano-network while the charge transfer is accomplished through the network of graphitic fibers.  The new nano-network non-PGM catalyst has achieved excellent fuel cell performances in both activity and durability. Catalyst durability under fuel cell operating condition represents another critical challenge facing non-PGM and low-PGM catalyst development. We have recently developed a new approach to stabilize the catalyst performance.  The new catalyst demonstrated less than 12% loss of mass activity at 0.9 V after 30,000 voltage cycles in a fuel cell test following DOE test protocol.

Acknowledgement: The authors wish to thank Chen Chen and Lauren Grabstanowicz for her assistance in the MEA preparation.  The work performed at Argonne is supported by DOE Fuel Cell Technologies Office and Office of Science.

References:

[1]  Shengqian Ma, Gabriel Goenaga, Ann Call and Di-Jia Liu, Chemistry: A European Journal 17, 2063 (2011)

[2]  Eric Proietti, Frédéric Jaouen, Michel Lefèvre, Nicholas Larouche, Juan Tian, Juan Herranz & Jean-Pol Dodelet, Nature Comm. 2, 416 (2011)

[3]  D. Zhao, J.-L. Shui, C. Chen, X. Chen, B. M. Reprogle, D. Wang and D.-J. Liu, Chem. Sci., 2012, 3 (11), 3200 – 3205

[4]  S. Yuan, J.-L. Shui, L. Grabstanowicz, C. Chen, S. Commet, B. Reprogle, T. Xu,  L. Yu and D.-J. Liu, Angew. Chem. Int. Ed., 2013, 52(32), 8349–8353

[5]  Dan Zhao, Jiang-Lan Shui, Lauren R. Grabstanowicz, Chen Chen, Sean M. Commet, Tao Xu, Jun Lu, and Di-Jia Liu, Advanced Materials, 2014, 26, 1093–1097

[6] J. Shui, C. Chen, L. R. Grabstanowicz, D. Zhao and D.-J. Liu, Proceedings of National Academy of Sciences, 2015, vol. 112, no. 34, 10629