Density Functional Theory Study of Oxygen Reduction Reaction Mechanism on Pt Alloy Catalysts

Power generation is one of the key challenges in the current century due to decreasing natural energy resources, population growth and the goal to reduction of carbon emissions. Proton exchange membrane fuel cells (PEMFCs) have attracted significant attention for potential power generation technology especially for mobile application. However, PEMFC currently faces several challenges: high cost of Platinum group metals catalysts, the slow kinetics of oxygen reduction reaction (ORR) at the cathode and the durability of cathode catalysts in harsh acidic environment.

Alloying Pt modifies the surface electronic structures and at the same time reduces the use of expensive Pt from the catalyst design. Recently, 3d-transition metals (TMs) alloyed Pt catalysts have been reported to show superior ORR activity as compared to pure Pt catalyst.^1-5 For example, the ORR activity enhancement for bimetallic Pt₃Ni(111) extended single crystal surface is reported to be 10 times (10´) higher than pure Pt(111) surface and 90´ higher than the commercial Pt/C nanoparticle catalysts.¹ With these encouraging studies, catalysts design by controlling composition, sizes and shapes of Pt-based alloy nanoparticle are now under extensive investigations.

Within this contribution, we employed an atomistic Monte Carlo (MC) simulation method to predict the equilibrium surface structures of Pt alloy surfaces. Based on the calculated surface composition profile, we built Pt surface-segregated surface slab models for Pt and Pt alloy catalysts. The possible ORR mechanism on the (111) and (100) surfaces of Pt and Pt alloys are explained by first-principles density functional theory (DFT) calculations. 

Our DFT computations predict that alloying Pt with transition metal alters the adsorption energetics of the ORR intermediates and changes the mechanism of ORR compared to pure Pt catalyst. The ORR on Pt and Pt alloy catalysts can proceed via one of the following ORR mechanisms: O₂ dissociation, OOH dissociation or H₂O₂ dissociation ORR mechanism.

Our calculations show that the ORR occurs via an OOH dissociation mechanism on the pure Pt(111) surface. In contrast, we predict that the ORR on Pt(100) surface proceeds via an O₂ dissociation ORR mechanism. Our calculated activation energy for the rate determining step (RDS) on the Pt(111) surface (0.79 e V)³ is very similar to that (0.80 eV)⁴ on the Pt(100) surface. This suggests that the (111) and (100) facets of Pt nanocrystals show similar catalytic activity for ORR.

We further studied the mechanism of ORR on Pt surface segregated modified Pt/TM(111) (TM =Fe, Co, Ni) and Pt₃TM(111) (TM=Ti, V, Fe, Ni) surfaces. We found that the activation energy of the RDS on modified Pt/TM(111) and Pt₃TM(111) surfaces is always smaller than that on the pure Pt(111) surface. Thus modified Pt/TM(111) and Pt₃TM(111) surfaces have superior catalytic activity for ORR compared to the ORR activity of pure Pt(111) surface. The DFT calculations further predict that the most favorable ORR mechanism on the modified Pt(111) surfaces is H₂O₂ dissociation ORR mechanism. We found that Ni modified Pt/TM(111) and Pt₃TM(111) surfaces show highest catalytic activity for ORR. Contrarily, our DFT calculations demonstrates that the catalytic activity of modified Pt/Ni(100) surface is very similar to that of pure Pt(100) surface. All these predictions are in excellent agreement with previous experimental measurements.

Acknowledgements

This work was funded by Chemical Sciences Research Programs, Office of Basic Energy Sciences, U.S. Department of Energy (Grant no. DE-FG02-09ER16093). We also acknowledge the research grant from the EERE program of the U.S. Department of Energy (Grant no. DE-AC02-06CH11357).

References

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