(56r) Controlling and Optimizing Activity/Stability Balance in Electrocatalytic Materials

Electrocatalysis plays a key role in the energy conversion processes central to several renewable energy technologies that have been developed to reduce our reliance on fossil fuels¹. The biggest challenge in developing electrochemical energy conversion technologies, including fuel cells and water electrolyzers, is the development of catalysts that are active, selective, and durable. In the design of efficient electrocatalysts, however, catalyst stability is too often overshadowed by the pursuit of ever increasing activities. By studying the relationships between electrocatalytic activity and stability, we aim to identify the relevant mechanisms of these competing performance metrics and develop insight to aid the optimization of a sufficient balance for practical application and device integration.

The development of efficient and stable catalysts for the cathodic oxygen reduction reaction (ORR) is one of the greatest challenges for the exploitation of polymer electrolyte membrane (PEM) fuel cells². After electrochemical dealloying, the open, bicontinuous, three-dimensional nanoporous nanoparticle electrocatalysts exhibit dramatically enhanced electrocatalytic properties³. Here, using a combination of in-situ and ex-situ experimental techniques, we studied the instability of this nanoporous materials which can lose electrochemically active surface area (ECSA) through surface smoothening driven coarsening⁴. With a better understanding of the interplay between nanoporous structure coarsening and transition metal loss, we have developed strategies to mitigate coarsening and improve catalyst stability through slowing electrochemically enhanced surface diffusion and limiting step edge movement. The first approach uses ionic liquids (IL) strategically placed at the metal/electrolyte interface to limit electrochemically enhanced surface diffusion. Taking advantage of the free volume within the nanoporous nanoparticles and creating a composite catalyst architecture through the incorporation of [MTBD][beti] IL, in addition to the improved oxygen reduction reaction kinetics by chemically biasing reactants to the surface and changing the degree of solvation/stabilization of adsorbed intermediates⁵, significant improvements in retention of ECSA during accelerated durability testing is observed. The interfacial IL acts to limit the charge dependent formation of a surface metal/electrolyte anion complex which is responsible for the potential dependent enhancement in surface diffusion. A second approach involves the minimization of morphology evolution by impeding step edge movement through the use of foreign adsorbates on the surface. We show that partial monolayer decoration of np-NiPt with Ir, possessing a significantly lower rate of surface diffusion than Pt, acts to pin step edges and results in significant enhancement in catalyst durability as measured by ECSA and ORR activity retention⁴. With these strategies we will show how more detailed insight into the atomic processes that govern electrocatalytic material instability can begin to break the inverse correlation between activity and durability.

As an ideal clean energy carrier, hydrogen can be produced through PEM electrolysis by generating H₂ from H₂O. However, noble platinum group metals (PGM) remain the most common electrocatalysts fnor hydrogen evolution reaction (HER). The expense and scarcity of these catalytic materials obstruct the wide-spread adoption of electrochemical fuel generation technologies⁶. Sulfur based transition metal dichalcogenides (TMDs) have emerged as a promising alternative to PGM HER electrocatalysts as they are abundant, inexpensive, and exhibit a low HER overpotential in acidic environment⁷. Here we present a systemic assessment of the compositional dependent HER activity and stability for Co-based mixed chalcogen, CoS_xSe_2-x, pyrite TMDs⁸. We observe a decrease in HER activity from the single chalcogen TMDs, CoS₂ and CoSe₂, to mixed chalcogen TMDs. This observed compositional trend in HER activity can be explained by the unique combination of compositional dependent hydrogen adsorption free energy (ΔG_Had) and bulk resistivity/conductivity of the pyrite TMD. With near thermoneutral ΔG_Had, the highest resistivity among the compositions was observed. The following increase in Se content leads to the decrease in HER activity due to a steady movement away from optimal ΔG_Had. As the composition approaches CoSe₂, however, it is observed that HER activity again increases at higher Se contents, with CoSe₂ exhibiting similar activity to CoS₂. This highlights the convolution of ΔG_Had and material conductivity in determining the HER activity. Furthermore, through stability tests under constant potential HER electrolysis, Se-rich Co-based pyrite TMDs are found to be more durable than S-rich samples. Therefore, with an HER activity matching that of CoS₂, but with a dramatic improvement in stability, CoSe₂ breaks away from the traditional inverse activity/stability relationship and represents a promising material for non-PGM HER electrocatalysis in acidic based PEM electrolyzers.

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