Best Practice for Evaluating Electrocatalysts for Hydrogen Economy.

Screening new electrocatalysts is key to the development of new materials for next-generation energy devices such as fuel cells and electrolyzers. The counter electrodes used in such tests are often made from materials such as Pt and Au, which can dissolve during testing and deposit onto test electrocatalysts, resulting in inaccurate results. The most common strategy for preventing this effect is to separate the counter electrode from the test material using an ion-transporting Nafion membrane. Here, we use X-ray photoelectron spectroscopy, energy-dispersive X-ray analysis, mass spectrometry, and voltammetry to demonstrate the limitations of this approach during constant-current, extended stability testing of electrocatalysts for H2 evolution. We show that Nafion membranes cannot prevent contamination of carbon electrocatalysts by Pt and Au counter electrodes, leading to an apparent increase in the electrocatalytic activity of the carbon. We then demonstrate that carbon counter electrodes in undivided cells can contaminate and deactivate Pt and Au electrocatalysts for H2 evolution. We show that use of a setup composed of a glass frit separating a carbon counter electrode from the test electrocatalyst can prevent these effects. Finally, we discuss these phenomena using H2 evolution at MoS2 and at a K6[P2W18O62](H2O)14/carbon nanotube composite as test reactions.

Electroanalysis of Neutral Precursors in Protic Ionic Liquids and Synthesis of High-Ionicity Ionic Liquids.

Protic ionic liquids (PILs) are ionic liquids that are formed by transferring protons from Brønsted acids to Brønsted bases. While they nominally consist entirely of ions, PILs can often behave as though they contain a significant amount of neutral species (either molecules or ion clusters), and there is currently a lot of interest in determining the degree of "ionicity" of PILs. In this contribution, we describe a simple electroanalytical method for detecting and quantifying residual excess acids in a series of ammonium-based PILs (diethylmethylammonium triflate [dema][TfO], dimethylethylammonium triflate [dmea][TfO], triethylammonium trifluoroacetate [tea][TfAc], and dimethylbutylammonium triflate [dmba][TfO]). Ultra-microelectrode voltammetry reveals that some of the accepted methods for synthesizing PILs can readily result in the formation of nonstoichiometric PILs containing up to 230 mM excess acid. In addition, vacuum purification of PILs is of limited use in cases where nonstoichiometric PILs are formed. Although excess bases can be readily removed from PILs under ambient conditions, excess acids cannot be removed, even under high vacuum. The effects of excess acid on the electrocatalytic oxygen reduction reaction (ORR) in PILs have been studied, and the onset potential of the ORR in [dema][TfO] increases by 0.8 V upon addition of acid to PIL. On the basis of the results of our analyses, we provide some recommendations for the synthesis of highly ionic PILs.