The slow kinetics of the oxygen reduction and evolution reactions (ORR and OER) impede the widespread uptake of polymer electrolyte membrane (PEM) fuel cells and electrolysers. In order to improve the kinetics of the ORR and reduce the Pt loading at the fuel-cell cathode, we need to enhance the activity, stability and selectivity of the catalyst. This can be achieved by alteration of the electronic properties of the Pt surface atoms (electronic effects [1,2]) and/or modification of the geometric structure (atomic ensemble effects [3]).

First, I will address atomic ensemble effects in electrocatalysis. We used a self-ordered molecular pattern of cyanide on Pt(111) surfaces to study the role of atomic ensembles for the ORR [3]. Cyanide groups form an ordered structure that blocks all the three-fold hollow sites, effectively blocking the adsorption of spectator anions in the electrolyte. Nonetheless, the holes in this structure are sufficiently large to allow the adsorption of reaction oxygen molecules. As a consequence, cyanide-modified Pt(111) presents a 25-fold enhancement over Pt(111) [3].

Secondly, I will focus on electronic effects by alloying Pt with other metals. In order to reduce the Pt loading, researchers have intensively studied alloys of Pt with late transition metals such as Ni and Co. However, these compounds typically degrade under fuel-cell conditions, due to dealloying. In contrast, Pt-lanthanide alloys present a very negative alloying energy, which should increase their resistance to degradation. We have studied novel Pt-lanthanide and Pt-alkaline earth electrocatalysts for the ORR. Pt-lanthanide and Pt-alkaline earth alloys are amongst the most active polycrystalline Pt-based catalysts ever reported [1,2], presenting up to a 6-fold increase in ORR activity, relative to Pt. The active phase consists of a compressed Pt overlayer of few Pt layers formed onto the bulk alloys by acid leaching. Notably, the oxygen reduction activity versus the bulk lattice parameter follows a volcano relation [1]. We use the lanthanide contraction to control strain effects and tailor the activity, stability and reactivity of Pt alloys.

Finally, I will present new approaches to design and develop more efficient and stable OER electrocatalysts. We have explored the OER activity and stability of RuO₂ nanoparticulate catalysts. Sub-monolayer amounts of IrO_x on top of RuO₂ thin films minimise Ru dissolution [4]. The strategy of tailoring the surface of the catalyst with sub-monolayer amounts of a stable oxide may allow us to tune the stability of active OER catalysts for PEM electrolysers.

References

[1] M. Escudero-Escribano et al., Science 2016, 352, 73.

[2] M. Escudero-Escribano et al., J. Am. Chem. Soc. 2012, 134, 16476.

[3] D. Strmcnik, M. Escudero-Escribano, K. Kodama, V. Stamenkovic, A. Cuesta, N.M. Markovic, Nature Chemistry 2010, 2, 880.

[4] M. Escudero-Escribano et al. J. Phys. Chem. B 2017, DOI: 10.1021/acs.jpcb.7b07047