(50a) Metal Decorated Graphene Catalysts for the ORR: Effects of Nanoparticle Size, Composition, and Support Dopant on Activity

The development of next-generation materials and energy sources/fuels is driven by rising population, CO2 emissions, and standards of living. A promising technology to help alleviate these concerns is the development of the Proton Exchange Membrane Fuel Cell (PEMFC). Unfortunately, current state-of-the-art PEMFC technology requires catalyst materials that are platinum or platinum-group-metal (PGM) based at the electrode to facilitate reduction of oxygen via the Oxygen Reduction Reaction (ORR). To bring PEMFC technology to worldwide commercial development requires reducing the PGM loading (amount of PGM atoms in the catalyst) , as well as improve their efficiency. Recent advances in the literature have shown that formation of advanced Pt-based intermetallics and advanced structures (such as Kagome lattice materials) can both lower the Pt content and improve the overpotential and efficiency of the catalyst. In this work we present a competing approach- to use graphene as a doped/tuned support to modify the electronic structure and – hence- catalytic activity of small intermetallic transition metal nanoparticle (NP) catalysts of size: 4, 7, and 19 atoms.1-3 Our results to date show that the most promising combinations of size, composition, and dopant can fall near the predictive Volcano Plot derived for ORR activity on these systems.

The work described and presented was performed using modern, state-of the art plane-wave DFT calculations as implemented in the VASP 5.4x code deployed in the Materials Design MEDEA environment 2.21. Calculations and supercells for surface-mediate adsorption and catalytic phenomena involved dipole corrections parallel to the surface normal, vacuum spacing of at least 65% of the overall calculation cell height, and slab models of at least 6-8 Ã… or equivalence to 4 crystalline layers of the material(s) studied. Typical electronic convergence of the SCF was performed to at least 10-6 eV, and atomic force relaxations of the uppermost slab model atoms and adsorbed species was performed to less than 0.035 eV/ Å . Specific functional(s) used for the calculations in this study were the Van der Waals corrected opt-PBE.

Our results have indicated that ORR activity for 4,7, and 19 atom transition metal intermetallic nanoparticles on graphene and doped-graphene supports spans a *wide* range of descriptor values on the corresponding Volcano Plot. Careful selection of size, composition, and support-doping can cause predicted catalyst descriptor values to fall very close the to predicted volcano peak with similar or better overpotential than current state-of-the-art Pt-based ORR catalysts, while not using any PGM , or much less PGM atoms per proton-electron transfer. Our results form the basis of further ongoing study for stability and ability to be synthesized.

We will present results from DFT studies on the effects of composition, size, solvation, and support doping for the catalytic activity of Metal-Decorated-Graphene (MDG) nanocatalysts for the Oxygen Reduction Reaction (ORR). Conclusions and recommendations based on Volcano Plots are given, and counter-intuitive and non-linear results are discussed. To date our group is unique in publishing comprehensive studies on the possible activity of novel MDG catalysts for the ORR. This presentation summarizes our work in seeing the broad range to which the possible catalytic activity of these systems – if they can be synthesized- can yield for ORR electrochemistry; our results show equivalent or higher performance to state of the art Pt-based alloys and Kagome systems while theoretically using less Pt and other precious metals.

References
1. Wang, Z.; Ping, Y.; Fu, Q.; Pan, C., Preparation of Metal Nanoparticle Decorated Graphene Hybrid Composites: A Review. MRS Advances 2018, 3 (15-16), 849-854.
2. Lozano, T.; Rankin, R. B., Computational predictive design for metal-decorated-graphene size-specific subnanometer to nanometer ORR catalysts. Catalysis Today 2018, 312, 105-117.

3. Hu, M.; Yao, Z.; Wang, X., Graphene-Based Nanomaterials for Catalysis. Industrial & Engineering Chemistry Research 2017, 56 (13), 3477-3502

4. Rankin, R.B, Lozano T; accepted/in print https://doi.org/10.3390/met9020227

5. Rankin, R.B. Lozano T; submitted April 2019.