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) and X-ray diffraction (XRD) confirm incorporation of nitrogen into the graphene framework during co-reduction of GO, KMnO4 and NH3 to form the electrocatalyst, while transmission electron microscopy (TEM) and 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 only observed 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

Room temperature ionic liquid electrolytes for redox flow batteries

Redox flow batteries (RFBs) usually contain aqueous or organic electrolytes. The aim of this communication is to explore the suitability of room temperature ionic liquids (RTILs) as solvents for RFBs containing metal complexes. Towards this aim, the electrochemistry of the metal acetylacetonate (acac) complexes Mn(acac)3, Cr(acac)3, and V(acac)3 was studied in imidazolium-based RTILs. The V2+/V3+, V3+/V4+, and V4+/V5+ redox couples are quasi-reversible in 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, [C2C1Im][N(Tf2)]. The Mn(acac)3 and Cr(acac)3 voltammetry, on the other hand, is irreversible in [C2C1Im][N(Tf2)] at glassy carbon (GC) but the rate of the Mn2+/Mn3+ reaction increases if Au electrodes are used. Charge–discharge measure- ments show that a coulombic efficiency of 72% is achievable using a V(acac)3/[C2C1Im][N(Tf2)]/GC cell

Developing energy efficient lignin biomass processing: towards understanding mediator behaviour in ionic liquids

Environmental concerns have brought attention to the requirement for more efficient and renewable processes for chemicals production. Lignin is the second most abundant natural polymer, and might serve as a sustainable resource for manufacturing fuels and aromatic derivatives for the chemicals industry after being depolymerised. In this work, the mediator 2,2′-azino-bis(3-ethylbenthiazoline-6-sulfonic acid) diammonium salt (ABTS), commonly used with enzyme degradation systems, has been evaluated by means of cyclic voltammetry (CV) for enhancing the oxidation of the non-phenolic lignin model compound veratryl alcohol and three types of lignin (organosolv, Kraft and lignosulfonate) in the ionic liquid 1-ethyl-3-methylimidazolium ethyl sulfate, ([C2mim][C2SO4]). The presence of either veratryl alcohol or organosolv lignin increased the second oxidation peak of ABTS under select conditions, indicating the ABTS-mediated oxidation of these molecules at high potentials in [C2mim][C2SO4]. Furthermore, CV was applied as a quick and efficient way to explore the impact of water in the ABTS-mediated oxidation of both organosolv and lignosulfonate lignin. Higher catalytic efficiencies of ABTS were observed for lignosulfonate solutions either in sodium acetate buffer or when [C2mim][C2SO4] (15 v/v%) was present in the buffer solution, whilst there was no change found in the catalytic efficiency of ABTS in [C2mim][C2SO4]–lignosulfonate mixtures relative to ABTS alone. In contrast, organosolv showed an initial increase in oxidation, followed by a significant decrease on increasing the water content of a [C2mim][C2SO4] solution

The role of adsorbed ions during electrocatalysis in ionic liquids

The effects of electrode–adsorbate interactions on electrocatalysis at Pt in ionic liquids are described. The ionic liquids are diethylmethylammonium trifluoromethanesulfonate, [dema][TfO], dimethylethylammonium trifluoromethanesulfonate, [dmea][TfO], and diethylmethylammonium bis(trifluoromethanesulfonyl)imide, [dema][Tf2N]. Electrochemical analysis indicates that a monolayer of hydrogen adsorbs onto Pt during potential cycling in [dema][[TfO] and [dmea][TfO]. In addition, a prepeak is observed at lower potentials than that of the main oxidation peak during CO oxidation in the [TfO]−-based liquids. In contrast, hydrogen does not adsorb onto Pt during potential cycling in [dema][Tf2N] and no prepeak is observed during CO oxidation. By displacing adsorbed ions on Pt surfaces with CO at a range of potentials, and measuring the charge passed during ion displacement, the potentials of zero total charge of Pt in [dema][TfO] and [dmea][TfO] were measured as 271 ± 9 and 289 ± 10 mV vs RHE, respectively. CO displacement experiments also indicate that the [Tf2N]− ion is bound to the Pt surface at potentials above −0.2 V and the implications of ion adsorption on electrocatalysis of the CO oxidation reaction and O2 reduction reaction in the protic ionic liquids are discussed

Understanding the Electrochemistry of “Water-in-salt” Electrolytes: Basal Plane Highly Ordered Pyrolytic Graphite as a Model System

A new approach to expand the accessible voltage window of electrochemical energy storage systems, based on so-called “water-in-salt” electrolytes, has been expounded recently. Although studies of transport in concentrated electrolytes date back over several decades, the recent demonstration that concentrated aqueous electrolyte systems can be used in the lithium ion battery context has rekindled interest in the electrochemical properties of highly concentrated aqueous electrolytes. The original aqueous lithium ion battery conception was based on the use of concentrated solutions of lithium bis(trifluoromethanesulfonyl)imide, although these electrolytes still possess some drawbacks including cost, toxicity, and safety. In this work we describe the electrochemical behavior of a simple 1 : 1 electrolyte based on highly concentrated aqueous solutions of potassium fluoride (KF). Highly ordered pyrolytic graphite (HOPG) is used as well-defined model carbon to study the electrochemical properties of the electrolyte, as well as its basal plane capacitance, from a microscopic perspective: the KF electrolyte exhibits an unusually wide potential window (up to 2.6 V). The faradaic response on HOPG is also reported using K(3)Fe(CN)(6) as a model redox probe: the highly concentrated electrolyte provides good electrochemical reversibility and protects the HOPG surface from adsorption of contaminants. Moreover, this electrolyte was applied to symmetrical supercapacitors (using graphene and activated carbon as active materials) in order to quantify its performance in energy storage applications. It is found that the activated carbon and graphene supercapacitors demonstrate high gravimetric capacitance (221 F g(−1) for activated carbon, and 56 F g(−1) for graphene), a stable working voltage window of 2.0 V, which is significantly higher than the usual range of water-based capacitors, and excellent stability over 10 000 cycles. These results provide fundamental insight into the wider applicability of highly concentrated electrolytes, which should enable their application in future of energy storage technologies

Electrocatalytic oxidation of methanol and carbon monoxide at platinum in protic ionic liquids

Oxidation of H2O, CO and CH3OH was investigated at Pt as a function of temperature in the protic ionic liquid diethylmethylammonium trifluoromethanesulfonate. Trace H2O oxidation in the ionic liquid results in coverage of the Pt with adsorbed oxides. Increasing the temperature significantly reduces the potential at which this reaction occurs. CH3OH and CO oxidation kinetics increased significantly with increasing temperature and oxidation of each species coincided with coverage of the Pt surface by the oxide. These observations indicate that surface oxides are required for complete oxidation of CH3OH to CO2 in the protic ionic liquid, in a similar way to that observed in conventional aqueous electrolytes. While the overpotential for CH3OH oxidation was drastically higher than that observed in purely aqueous electrolytes, it decreased with increasing water content of the ionic liquid. The results described here have implications for the development of protic ionic liquid electrolyte fuel cells and these implications are discussed. Keywords: Direct methanol fuel cells, Electrocatalysis, Protic ionic liqui

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

Single Stage Simultaneous Electrochemical Exfoliation and Functionalization of Graphene

Development of applications for graphene are currently hampered by its poor dispersion in common, low boiling point solvents. Covalent functionalization is considered as one method for addressing this challenge. To date, approaches have tended to focus upon producing the graphene and functionalizing subsequently. Herein, we describe simultaneous electrochemical exfoliation and functionalization of graphite using diazonium salts at a single applied potential for the first time. Such an approach is advantageous, compared to postfunctionalization of premade graphene, as both functionalization and exfoliation occur at the same time, meaning that monolayer or few-layer graphene can be functionalized and stabilized <i>in situ</i> before they aggregate. Furthermore, the N₂ generated during <i>in situ</i> diazonium reduction is found to aid the separation of functionalized graphene sheets. The degree of graphene functionalization was controlled by varying the concentration of the diazonium species in the exfoliation solution. The formation of functionalized graphene was confirmed using Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy. The functionalized graphene was soluble in aqueous systems, and its solubility was 2 orders of magnitude higher than the nonfunctionalized electrochemically exfoliated graphene sheets. Moreover, the functionalization enhanced the charge storage capacity when used as an electrode in supercapacitor devices with the specific capacitance being highly dependent on the degree of graphene functionalization. This simple method of <i>in situ</i> simultaneous exfoliation and functionaliztion may aid the processing of graphene for various applications