Electron transfer in two and three dimensions

A number of osmium bipyndyl complexes have been synthesised and characterised using spectroscopic, chromatographic and electrochemical techniques. The complexes formed are [Os(bpy)2 4-tet C1]C104, [Os(bpy)2 4-bpt C1]PF6 and [Os(bpy)2 Cl 4-bpt Os(bpy) 2 C1](PF6)2, where bpy is 2 ,2 ’-bipyridyl, 4-tet is 3,6-bis(4-pyndyl)-l,2,455-tetrazine and 4- bpt is 3,5-bis(pyridin-4-yl)-l,2,4-tnazole Monolayers of [Os(bpy) 2 4-tet C1]C1 0 4 have been formed by spontaneous adsorption onto clean gold microelectrodes. The tetrazine bridge between the [Os(bpy)2Cl]+ head group and the metal electrode surface undergoes a reversible protonation/deprotonation reaction depending on the pH of the contacting electrolyte solution High speed cyclic voltammetry reveals that the redox switching mechanism is best described as a non-adiabatic, through-bond tunnelling mechanism Significantly, while protonating the bridging ligand does not influence the free energy of activation, 10 3±1 1 kJ mol *, k° decreases by 1 order of magnitude from 1 1 x 104 s 1 to 1 2 x 103 s 1 upon going from a deprotonated to a protonated bridge. These observations are interpreted in terms of a through-bond tunnelling mechanism m which protonation decreases the electron density on the bridge and reduces the strength of electronic coupling between the redox centre and the electrode. Solid deposits of the dimeric complex [Os(bpy) 2 Cl 4-bpt Os(bpy)2 C1](PF6)2 have been deposited on platinum microelectrodes by mechanical attachment. The electrochemical response exhibited by these deposits is unusually ideal over a wide range of electrolyte compositions and pH values Dct, the charge transport diffusion coefficient, is independent of the electrolyte concentration, indicating that electron self-exchange between adjacent redox centres limits the overall rate of charge transport through the solid In 1 0 M L1CIO4 and 1 0 M HCIO4, Dct values are 2 0±0 lxlO10 and 1 7±0 4x10 10 cm2 s corresponding to second order electron transfer rate constants of 18 x l07 and 3xl07 M 1 s 1. The standard rate of heterogeneous electron transfer across the electrode/deposit interface is 1 08+0 05x10 cm s. This value is approximately one order of magnitude lower than that found for a similar monomeric complex in which the bridging ligand is attached directly to the electrode surface, indicating that the 4-bpt ligand does not promote strong electronic communication between the [Os(bpy)2CI]+ head group and the electrode surface. Monolayers of [Os(bpy)2 4-bptCl]PF6 have been formed by spontaneous adsorption onto platinum microelectrodes. These monolayers are extremely stable under a wide range of electrolyte compositions and pH values Significantly, the 4-bpt ligand is capable of undergoing a protonation/deprotonation reaction depending on the pH of the contacting electrolyte solution. High speed chronoamperometry reveals that protonation of the 4-bpt bridging ligand causes the standard rate of heterogeneous electron transfer to decrease by at least an order of magnitude from 2 67 x 106 to 4 5 x 104 s' 1 for the oxidation process and from 1 60 x 106 to 1 9 x 105 for the reduction process Consistent with a superexchange mechanism, these observation are interpreted in terms of a hole superexchange process, the rate of which decreases with increasing energy gap between the osmium metal dn orbitals and the highest occupied molecular orbital of the bridge

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 electrolysers, 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 electrolysers 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

The nature of proton shuttling in protic ionic liquid fuel cells

It has been proposed previously that protic ionic liquids (PILs) such as diethylmethylammonium triflate could be used as the electrolytes in nonhumidified, intermediate temperature H2 fuel cells, potentially offering the prospect of high conductivity and performance, even under anhydrous conditions. In this contribution, a combination of electroanalytical chemistry and fuel?cell polarization analyses is used to demonstrate for the first time that the pure PILs cannot support proton shuttling between the electrodes of fuel cells. Only through the inclusion of dissolved acidic or basic proton shuttles can viable protic ionic fuel cells be fabricated, which has major consequences for the use of these neoteric electrolytes in fuel cells

Valorization of lignin waste: high electrochemical capacitance of lignin-derived carbons in aqueous and ionic liquid electrolytes

This report describes the utilization of waste lignin-derived activated carbons (LACs) as high-energy/high-power electrode materials for electric double layer capacitors(EDLCs). The influence of carbon-pore structure on the capacitance of the LACs in two aqueous (H2SO4 and KCl) and two ionic liquid (1-ethyl-3-methylimidazolium ethylsulfate, [EMIm][EtSO4], and 1-butyl-3-methylimidazolium tetrafluoroborate, [BMIm][BF4]) electrolytes is evaluated. In EDLCs containing aqueous H2SO4 as electrolyte, the LACs exhibit specific capacitances of up to 223 F/g and good cycling stability, with energy density of 5.0 Wh/kg at a power density of 200 W/kg. EDLCs containing KCl achieved a specific capacitance of 203 F/g, and energy density of 7.1 Wh/kg at a power density of 510 W/kg. The specific capacitances of the LACs in [EMIm][EtSO4] and [BMIm][BF4] were up to 147 F/g and 175 F/g, respectively. The energy density in the IL electrolytes, is up to 25 Wh/kg at power density of 500 W/kg, and 16.4 Wh/kg at 15 kW/kg. We demonstrate that the electrochemical performance of the LACs depends not only the surface area and pore size, but also on the pore-wall thickness

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

Polyaniline- and poly(ethylenedioxythiophene)-cellulose nanocomposite electrodes for supercapacitors

The formation and characterisation of films of polyaniline (PANI) and poly(ethylenedioxythiophene) (PEDOT) containing cellulose nanocrystals (CNXLs) from cotton are described. PANI/CNXL films were electrodeposited from a solution containing CNXLs, HCl and aniline, while PEDOT/CNXL films were electrodeposited from a solution containing CNXLs, lithium perchlorate and ethylenedioxythiophene. In each case, incorporation of CNXLs into the electrodepositing polymer film led to the formation of a porous polymer/CNXL nanocomposite structure. The films were characterised using scanning electron microscopy, cyclic voltammetry, electrochemical impedance spectroscopy (EIS) and galvanostatic charge-discharge analysis. The specific capacitances of the nanocomposite materials were higher than those of the CNXL-free counterparts (488 F/g for PANI/CNXL; 358 F/g for PANI; 69 F/g for PEDOT/CNXL; 58 F/g for PEDOT). The durability of the PANI/CNXL film under potential cycling was slightly better than that of the CNXL-free PANI, while the PEDOT film was slightly more durable than the PEDOT/CNXL film. Using electrodeposition, it was possible to form thick PANI/CNXL films, with total electrode capacitances of 2.07 F farads per squared cm (and corresponding specific capacitances of 440 F/g), demonstrating that this particular nanocomposite may be promising for the construction of high performance supercapacitors

Biomass-derived activated carbon with simultaneously enhanced CO2 uptake for both pre and post combustion capture applications

We report on the synthesis and CO2 uptake capabilities of a series of activated carbons derived from biomass raw materials, Jujun grass and Camellia japonica. The carbons were prepared via hydrothermal carbonization of the raw materials, which yielded hydrochars that were activated with KOH at temperature between 600 and 800 °C. Carbons activated at KOH/hydrochar ratio of 2 have moderate to high surface area (1050 – 2750 m2 g-1), are highly microporous (95% of surface area arises from micropores, and 84% of pore volume from micropores of size between 5 and 7 Å), and exhibit excellent CO2 uptake capacity at 25 oC of up to 1.5 mmol g-1 at 0.15 bar and 5.0 mmol g-1 at 1 bar, which is amongst the highest reported so far for biomass-derived carbons. On the other hand, activation at KOH/hydrochar ratio of 4 generates carbons with surface area and pore volume of up to 3,537 m2 g-1 and 1.85 cm3 g-1, and which, depending on level of activation, simultaneously exhibit high CO2 uptake at both 1 bar (4.1 mmol g-1) and 20 bar (21.1 mmol g-1), i.e. under conditions that mimic, respectively, post combustion and pre combustion CO2 capture from flue gas streams. The present carbons are the first examples of biomass derived porous materials with such allround CO2 uptake performance, which arises due to the pore size distribution of the carbons being shifted towards small micropores even for samples with very high surface area. Thus the carbons satisfy the requirements for both low pressure (presence of small micropores) and high pressure (high surface area) CO2 uptake

Hydrogen electrooxidation under conditions of high mass transport in room-temperature ionic liquids and the role of underpotential-deposited hydrogen

The hydrogen oxidation reaction (HOR), an electrocatalytic reaction of fundamental and applied interest, was studied in the protic ionic liquid (PIL) diethylmethylammonium trifluoromethanesulfonate, [dema][TfO], at Pt electrodes using rotating disk electrode (RDE) and ultramicroelectrode (UME) voltammetry. A steady-state HOR current is observed during RDE voltammetry at overpotentials > 50 mV but an additional plateau is observed in the overpotential region 50-200 mV when using UMEs. The difference in voltammetric responses is attributed to higher rate of mass transport to the UME than to the RDE. Three models have been used to fit the experimental data. The first is a dual-pathway model, which assumes that the Tafel-Volmer and Heyrovsky-Volmer pathways are both active over the potential range of interest and no blockage of catalytic sites occurs during the reaction. The second is a dual-pathway model, which assumes that reaction intermediates block access of H2 to catalytic sites. The third is based on the premise that underpotential-deposited hydrogen atoms (Hupd) can block adsorption and electrooxidation of H2 at the Pt surface. While each model fits the polarisation curves reasonably well, detailed analysis suggests that the Hupd- blocking model describes the responses better. To the best of our knowledge, this work is the first to demonstrate the advantages of UME voltammetry over RDE voltammetry for studying electrocatalytic reactions in PILs, and the first to show that Hupd can inhibit an electrocatalytic reactions in an ionic liquid, a factor that may become important as the technological applications of these liquids increase

Bridging the performance gap between electric double-layer capacitors and batteries with high-energy/high-power carbon nanotube-based electrodes

Electric double-layer capacitors (EDLCs) store electrical energy at the interface between charged electrodes and electrolytes and are higher-power devices than batteries. However, the amount of energy stored in EDLCs cannot compete with that in batteries. In this contribution, we describe the development of new EDLCs that can store about as much energy as lead-acid and nickel metal hydride (NiMH) batteries but operate at much higher power densities than achievable using batteries. The electrode materials are derived from carbon nanotubes (CNTs) synthesised from CCl4 and ferrocene at 180 °C, which is drastically lower than the temperatures usually used to synthesise CNTs. By chemically activating the CNTs using KOH, Bruneuer-Emmett-Teller (BET) surface areas reach ~3000 m2/g, which is orders of magnitude higher than those typical of CNTs, and exceeds even that of pristine graphene. Gas sorption analysis shows that the samples activated at 900 °C contain a mix of micropores and small mesopores, while the samples activated at lower temperatures are predominantly microporous. In EDLCs containing aqueous H2SO4 as the electrolyte, the mesoporous carbons exhibit mass-specific capacitances up to 172 F/g, while in the presence of the ionic liquids 1-ethyl-3-methylimidazolium tetrafluoroborate, [EMIM][BF4], and 1-butyl-3-methylimidazolium tetrafluoroborate, [BMIM][BF4], capacitances up to 150 F/g are measured. Due to the wide potential window of the ionic liquid electrolytes and the unique morphology of the electrode materials, 3-V devices with volume-specific energy densities of the order of 6 Wh/L and mass specific energy densities up to about 15 Wh/kg can be fabricated. The energy stored can be delivered at power densities >1 kW/kg meaning that the performance of these devices bridges the performance gap between those of EDLCs and batteries. The use of this novel electrode material not only allows the fabrication of high- energy/high-power energy storage systems, the methods used to fabricate the electrode materials are inexpensive and can readily be scaled to industrial levels

Tuning the Reactivity of TEMPO during Electrocatalytic Alcohol Oxidations in Room-Temperature Ionic Liquids

2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) is a promising, sustainable, metal-free mediator for oxidation of alcohols. In this contribution, we describe how the selectivity of TEMPO for electrocatalytic alcohol oxidations in room-temperature ionic liquids (RTILs) can be changed by design of the solvent medium. Cyclic voltammetry of TEMPO in a series of ammonium-, phosphonium-, and imidazolium-based RTILs reveals that the potential at which TEMPO is oxidized increases from 677 mV (vs. the potential of the decamethylferrocene/ decamethylferrocinium, dmFc/dmFc+, redox couple) to 788 mV as the H-bond basicity of the RTIL anions decreases. The increase in potential is accompanied by an increase in the rate constant for oxidation of benzyl alcohol from about 0.1 dm3 mol−1 s−1 to about 0.7 dm3 mol−1 s−1, demonstrating the ability to manipulate the reactivity of TEMPO by judicious choice of the RTIL anions. The rate of alcohol oxidation in a series of RTILs increases in the order 2-butanol < 1phenylethanol < octanol < benzyl alcohol, and the RTIL 1-octyl-3-methylmidazolium bis(trifluoromethanesulfonyl)imide ([NTf2]–) shows clear selectivity towards the oxidation of primary alcohols. In addition, the reaction kinetics and selectivity are better in [NTf2]–-based RTILs than in acetonitrile, often the solvent-of-choice in indirect alcohol electrooxidations. Finally, we demonstrate that electrolytic TEMPO-mediated alcohol oxidations can be performed using RTILs in a flow-electrolysis system, with excellent yields and reaction selectivity, demonstrating the opportunities offered by such systems