Increased Electrocatalytic Activity of Colloidal Nanoparticles through Anionic Substitution and Chemical-Free Binding

One of the largest factors limiting the commercialization of air-breathing electrochemical energy storage technologies lies in the requirement of rare, precious-metal electrocatalysts. The adoption of Pt and other rare metal catalysts by industry is hindered by their high costs. Transition-metal-containing compounds such as oxides, chalcogenides, and pnictides exhibit high activities and are attractive alternatives to rare, precious metals. In recent years, intensive work has been applied to developing alternative catalytic materials and to developing methods to implement bare nanoparticles, without binders and conductive additives. In this talk I’ll discuss two of our recent results in these areas of new materials and methods to test and improve the naked nanoparticle catalyst. I’ll discuss our work using chemical transformations to tailor nanoparticles for electrocatalysis and other applications. Chemical transformations in nanoparticles are performed on as-synthesized nanoparticles to alter their composition, morphology, and/or phase. By tuning the anion stoichiometry, we have recently found a method to optimize the hydrogen evolution reaction (HER kinetics) in cobalt oxysulfide nanoparticles. We use cobalt oxysulfide nanoparticles with controlled substitution of sulfur in the anion site as our experimental testbed. We find that a small addition of sulfide in the cobalt oxide (CoOx) nanoparticles produces an oxysulfide catalyst which is more active towards the HER than either the pure cobalt oxide or a sulfur-rich oxysulfide. Specifically, we find that when sulfur is dilutely substituted for oxygen the enhancement can be as large as 5x for HER. The restructuring that produces the higher activity is unique since the cobalt oxysulfide phase is not thermodynamically stable: we relax our top-performing composition by annealing it but the catalytic activity decreases. Our chemical transformation method to induce the sulfur addition, stabilizes and enables the formation of this metastable oxysulfide phase. We apply a density functional theory (DFT) calculation to understand the observed activity difference. Our calculation reveals that a small sulfur addition strengthens the H* binding energy on the Co site, which we believe underlies the increased HER activity. Interestingly, further incorporation of sulfur decreases the surface H* binding energy. This binding energy decrease is consistent with the observed reduction in the HER activity for sulfur-rich oxysulfides. I will also present a comparative study of electrophoretically deposited and dropcast nanoparticulate cobalt oxide thin films for the oxygen reduction/oxygen evolution reactions (ORR and OER).  One issue that has yet to be adequately addressed in nanoparticle catalysis is the adherence of the nanoparticles to the electrically conductive support.  The fundamental activity of the naked nanoparticle can be obfuscated by incorporating binders and carbon-black, which are commonly used for nanoparticle catalysts. In our work, we compare the conventional dropcast method versus electrophoretic deposition (EPD). In examining the film’s catalytic properties, we find — much to our surprise — that the electrophoretically deposited nanoparticles outperform the dropcast films by as much as 2.5 for the oxygen reduction reaction (ORR) and 2.6 and the oxygen evolution reaction (OER) when accounting for both surface area and mass.   The critical solution-processing technique that we use to obtain high cataltyic performance is EPD.  This process results in this significant catalytic improvement in oxygen reduction/evolution performance which cannot be duplicated by dropcast films with the same loading. This route also indicates a new method for the catalytic community to test the naked nanoparticle under optimized assembly conditions. Andrew Nelson*, Kevin E. Fritz*, Shreyas Honrao, Richard G. Hennig, Richard D. Robinson †, and Jin Suntivich†, “Increased Activity in Hydrogen Evolution Electrocatalysis for Partial Anionic Substitution in Cobalt Oxysulfide Nanoparticles,” Submitted 2015 M. Fayette, A. Nelson, and R.D. Robinson, “Electrophoretic Deposition Improves Catalytic Performance of Co3O4 Nanoparticles for Oxygen Reduction/Oxygen Evolution Reactions,” Journal of Materials Chemistry A 3, 4274–4283 (2015). DOI: 10.1039/C4TA04189E Figure 1