The Random Bimetallic Database

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It is essential to promote alternative energy sources as our energy demands continue to increase. One such form of alternative energy is the hydrogen fuel cell, which converts hydrogen and oxygen gas into water and energy, but its viability is limited due to the expense of the necessary platinum catalyst ($847.77/oz, updated 7/18/19). Our research aims to replace platinum with a more economical catalyst via high-throughput computation and analysis of material candidates.

In the search for a suitable catalyst, binding energies, the energy released when the adsorbate attaches to the material, are explored. Oxygen is explored for the Oxygen Reduction Reaction (ORR) that occurs within the fuel cell to release energy and the Hydrogen Evolution Reaction (HER) that occurs outside the fuel cell to produce hydrogen gas from water. The equation used to calculate binding energy is displayed below.

Binding Energy = EnergyAdsorbent + Adsorbate - EnergyAdsorbent - ½ EnergyAdsorbate

There is an established trend between binding energy and catalytic activity for both ORR and HER, as displayed in the figures below. Evidently, platinum as a catalyst is near-ideal for both reactions. The idea in the bimetallic project is to combine two FCC metals in hopes of creating a more ideal or cheaper alternative. Eight FCC metals were selected for this project are Ni, Pd, Rh, Pt, Ir, Cu, Au, and Ag, allowing for 28 possible bimetallic combinations.

A computational approach was utilized due to the number of calculations that are required. Two structures were analyzed in 50:50 compositions: 79-atom nanoparticles arranged in a face-centered cubic (FCC) truncated octahedron and four-layer FCC(111) slabs with the bottom two layers frozen. Five randomly generated structures of each combination were generated to reflect the randomness of how these catalysts would be created for practical use. On each surface, four unique 3-atom hollow binding sites were explored with both oxygen and hydrogen adsorbates, as shown below. This results in 2240 calculations, not accounting for errors. Surrounding each hollow site are a few other possible sites: the 2-atom bridge site and 1-atom top site. On occasion, the adsorbate favored these alternate sites or even a neighboring hollow site as opposed to the designated hollow site. Thus, the distance moved statistic is measured to determine which structures observed this adsorbate movement, indicating instability at the hollow site. The hollow site is explored specifically because the ensemble effect dictates that the surrounding composition of the site generally reflects the overall composition of a structure. So when the adsorbate optimizes to a different binding site, this effect cannot be analyzed properly and affects a property called tunability. If a material exhibits tunability, its binding energy scales linearly with its overall composition, so it is important to know whether or not altering the structure will generate the expected results.

Along with tunability as a factor for predicting binding energies, a binding energy prediction algorithm is under development. The idea is to be able to predict binding energies of other metal combinations not explored in the bimetallic project without having to undergo the lengthy calculation process.