Closed Bipolar Electrodes for Spatial Separation of H₂ and O₂ Evolution during Water Electrolysis and the Development of High-Voltage Fuel Cells

Electrolytic water splitting could potentially provide clean H₂ for a future “hydrogen economy”. However, as H₂ and O₂ are produced in close proximity to each other in water electrolyzers, mixing of the gases can occur during electrolysis, with potentially dangerous consequences. Herein, we describe an electrochemical water-splitting cell, in which mixing of the electrogenerated gases is impossible. In our cell, separate H₂- and O₂-evolving cells are connected electrically by a bipolar electrode in contact with an inexpensive dissolved redox couple (K₃Fe­(CN)₆/K₄Fe­(CN)₆). Electrolytic water splitting occurs in tandem with oxidation/reduction of the K₃Fe­(CN)₆/K₄Fe­(CN) redox couples in the separate compartments, affording completely spatially separated H₂ and O₂ evolution. We demonstrate operation of our prototype cell using conventional Pt electrodes for each gas-evolving reaction, as well as using earth-abundant Ni₂P electrocatalysts for H₂ evolution. Furthermore, we show that our cell can be run in reverse and operate as a H₂ fuel cell, releasing the energy stored in the electrogenerated H₂ and O₂. We also describe how the absence of an ionically conducting electrolyte bridging the H₂- and O₂-electrode compartments makes it possible to develop H₂ fuel cells in which the anode and cathode are at different pH values, thereby increasing the voltage above that of conventional fuel cells. The use of our cell design in electrolyzers could result in dramatically improved safety during operation and the generation of higher-purity H₂ than available from conventional electrolysis systems. Our cell could also be readily modified for the electrosynthesis of other chemicals, where mixing of the electrochemical products is undesirable

Synergistic Catalyst–Support Interactions in a Graphene–Mn₃O₄ Electrocatalyst for Vanadium Redox Flow Batteries

The development of vanadium redox flow batteries (VRFBs) is partly limited by the sluggishness of the electrochemical reactions at conventional carbon-based electrodes. The VO²⁺/VO₂⁺ redox reaction is particularly sluggish, and improvements in battery performance require the development of new electrocatalysts for this reaction. In this study, synergistic catalyst–support interactions in a nitrogen-doped, reduced-graphene oxide/Mn₃O₄ (N-rGO-Mn₃O₄) composite electrocatalyst for VO²⁺/VO₂⁺ electrochemistry are described. X-ray photoelectron spectroscopy (XPS) confirms incorporation of nitrogen into the graphene framework during co-reduction of graphene oxide (GO), KMnO₄, and NH₃ to form the electrocatalyst, while transmission electron microscopy (TEM) and X-ray diffraction (XRD) confirm the presence of ca. 30 nm Mn₃O₄ nanoparticles on the N-rGO support. XPS analysis shows that the composite contains 27% pyridinic N, 42% pyrrolic N, 23% graphitic N, and 8% oxidic N. Electrochemical analysis shows that the electrocatalytic activity of the composite material is significantly higher than those of the individual components due to synergism between the Mn₃O₄ nanoparticles and the carbonaceous support material. The electrocatalytic activity is highest when the Mn₃O₄ loading is ∼24% but decreases at lower and higher loadings. Furthermore, electrocatalysis of the redox reaction is most effective when nitrogen is present within the support framework, demonstrating that the metal–nitrogen–carbon coupling is key to the performance of this electrocatalytic composite for VO²⁺/VO₂⁺ electrochemistry

Hydrogen Oxidation and Oxygen Reduction at Platinum in Protic Ionic Liquids

H₂ oxidation and O₂ reduction have been studied as a function of temperature at Pt electrodes in the protic ionic liquid diethylmethylammonium trifluoromethanesulfonate. Hydrodynamic voltammetry showed that the H₂ oxidation reaction (HOR) became hindered at positive potentials (>1.0 V). Electrochemical analysis and X-ray photoelectron spectroscopy revealed that this drop in HOR activity was due to the formation of an adsorbed blocking oxide layer, which formed on the Pt surface due to trace H₂O oxidation at positive potentials. Electrochemical analysis also revealed that the O₂ reduction reaction (ORR) occurred at an appreciable rate only when pre-existing surface oxides were reduced. As the temperature increased, the potential at which the surface oxides were reduced shifted to more positive potentials and the reduction peak narrowed. The net result was significantly higher rates of the ORR at positive potentials at higher temperatures. Finally, even when Pt surfaces were not initially covered with an oxide adlayer, the rate of the ORR increased significantly upon increasing the temperature and some possible reasons for this temperature dependence are discussed

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