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.Keywords: bipolar electrochemistry; electrocatalysis; electrolyzer; hydrogen economy; regenerative fuel cell;

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.

The hydrogen oxidation reaction (HOR), an electrocatalytic reaction of fundamental and applied interest, is 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 rates of mass transport to the UMEs 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 polarization 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 reaction in an ionic liquid, a factor that may become important as the technological applications of these liquids increase.

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.Keywords: cyclic voltammetry; electrocatalysis; energy; graphene; redox flow battery

•The electrochemistry of Cr(acac)3, Mn(acac)3, and V(acac)3 in imidazolium-based room temperature ionic liquids is described•The ionic liquid 1-ethyl-3-methylimidazolium trifluoromethanesulfonate, [C2C1Im][N(Tf)2], stabilizes various Vn+ species•A cell containing V(acac)3, [C2C1Im][N(Tf2)], and glassy carbon electrodes exhibits a coulombic efficiency of 72%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 measurements show that a coulombic efficiency of 72% is achievable using a V(acac)3/[C2C1Im][N(Tf2)]/GC cell.

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.

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, LiClO4 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−1 for PANI/CNXL; 358 F g−1 for PANI; 69 F g−1 for PEDOT/CNXL; 58 F g−1 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 cm−2 and corresponding specific capacitances of 440 F g−1, demonstrating that this particular nanocomposite may be promising for the construction of high-performance supercapacitors.

The oxygen reduction reaction (ORR) has been studied at Pt surfaces in the protic ionic liquid diethylmethylammonium trifluoromethanesulfonate. Water content measurements suggested that the ORR proceeded in the ionic liquid predominantly via a 4-electron reduction to water. A mechanistic analysis using rotating ring-disk electrode (RRDE) voltammetry confirmed that negligible amounts of hydrogen peroxide were formed during the ORR. A kinetic analysis of the ORR was performed using rotating disk electrode (RDE) voltammetry and the importance of correcting for ohmic (iR) drop prior to performing kinetic measurements in the ionic liquid is demonstrated. A Tafel analysis of the RDE voltammetry data revealed a change in the ORR Tafel slope from 70 mV per decade at low ORR overpotentials to 117 mV per decade at high overpotentials, and the reason for this change is discussed. The change in the Tafel slope for the ORR with increasing overpotential meant that the exchange current density for the ORR varied from 0.007 nA cm−2 to 10 nA cm−2, depending on the applied potential. Finally, the implications of these results for the development of protic ionic liquid fuel cells are discussed.

Concentric microring-disk tips for scanning electrochemical microscopy (SECM) were fabricated and used in a new “tip generation–substrate collection–tip collection” (TG–SC–TC) mode to determine the activity of an Au electrocatalyst for the oxygen reduction reaction (ORR), while simultaneously monitoring hydrogen peroxide produced during the reaction. ORR electrocatalysis/hydrogen peroxide detection measurements were performed by evolving O2 at the Au microring of the SECM tip while it was positioned close to an Au substrate. The O2 diffused to the Au substrate where it was reduced generating a substrate current. At the same time, hydrogen peroxide generated during the ORR was detected by oxidation at the SECM tip microdisk. The effects of the microring current, the tip-substrate distance and the substrate potential during the TG–SC–TC experiment have been determined and are described. The ability to use these SECM tips to scan electrocatalyst surfaces while generating maps containing electrocatalytic and mechanistic data is then demonstrated and the prospects for the use of these SECM tips in screening arrays of ORR electrocatalysts discussed.Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Novel microring-disk probes have been fabricated for SECM. ► Probes have been used to study O2 reduction at Au substrates. ► Electrochemically generated H2O2 can be detected while O2 reduction is studied.

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.

Pt nanoparticles have been synthesized at relatively low temperatures in aqueous solution from hexachloroplatinic acid using cellulose nanocrystals (CNXLs) from cotton as reducing agents. The Pt nanoparticles were characterised using X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy and thermogravimetric analysis. X-ray diffraction and X-ray photoelectron spectroscopy showed that the particles have a metallic Pt core and an oxidised surface layer. TEM analysis showed that the nanoparticles have an average diameter of approximately 2 nm, which is independent of the reactant concentrations. By performing the reduction reaction in the presence of a carbon-black support (Vulcan XC-72R), and removing the cellulosic material by heating in air, it was possible to produce carbon black supported Pt nanoparticles. Electrochemical analysis revealed that this Pt/C was highly active towards electrocatalysis of the oxygen reduction reaction, suggesting that this method may be very useful for fabricating Pt/C electrocatalysts.

There is considerable interest in the development of nanostructured metal surfaces for applications in chemical sensing, optics, catalysis and magnetics. For a number of years, the use of templates based on polystyrene nanospheres has been particularly successful for the construction of meso- and macroporous surfaces. We have discovered that redox groups on the surfaces of commercially available polystyrene nanospheres reduce Ag+ ions directly. When the nanospheres are arranged as a close packed template on a glassy carbon surface, the reduced Ag preferentially deposits in the pores of the polystyrene template. After removing the template, very well defined Ag nanobowl arrays remain on the carbon surface. This discovery may open the door to the production of highly inexpensive polystyrene-templated metal nanostructures for photonic and sensing applications.

The electrochemistry of I−/I3− was studied in ionic liquids using a combination of cyclic voltammetry, chronoamperometry and scanning electrochemical microscopy (SECM). The electrolytes were 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide [CnC1Im][Tf2N], ionic liquids (where n = 2, 4 and 8) and I− was typically added at a concentration of approximately 11 mM. During cyclic voltammetry, two sets of peaks were observed in each ionic liquid due to oxidation and reduction of the I−/I3− redox couple and oxidation/reduction of the I3−/I2 redox couple. The diffusion coefficients of I− and I3−, as determined using chronoamperometry, increased with increasing temperature and decreased with increasing ionic liquid viscosity. The effect of ionic liquid viscosity on ultramicroelectrode (UME) voltammetry was also determined using the I−/I3− redox couple. Steady-state behaviour was observed at 1.3 μm UMEs at slow voltammetric scan rates and steady-state SECM feedback approach curves were also obtained at a 1.3 μm Pt SECM tips, provided that the tip approach speed was sufficiently low.Highlights► Electrochemical behavior of I−/I3− was studied in ionic liquids using a range of electrochemical techniques. ► The ionic liquid viscosity has a drastic effect on the electrochemical behavior of the system. ► Mass transport properties of the ionic liquid electrolytes depend on viscosity. ► Steady-state electrochemical responses can be obtained in ionic liquids if the experimental parameters are carefully controlled.

The electrochemical behaviour of ferrocenemethanol (FcMeOH) has been studied in a range of room-temperature ionic liquids (RTILs) using cyclic voltammetry, chronoamperomery and scanning electrochemical microscopy (SECM). The diffusion coefficient of FcMeOH, measured using chronoamperometry, decreased with increasing RTIL viscosity. Analysis of the mass transport properties of the RTILs revealed that the Stokes–Einstein equation did not apply to our data. The “correlation length” was estimated from diffusion coefficient data and corresponded well to the average size of holes (voids) in the liquid, suggesting that a model in which the diffusing species jumps between holes in the liquid is appropriate in these liquids. Cyclic voltammetry at ultramicroelectrodes demonstrated that the ability to record steady-state voltammograms during ferrocenemethanol oxidation depended on the voltammetric scan rate, the electrode dimensions and the RTIL viscosity. Similarly, the ability to record steady-state SECM feedback approach curves depended on the RTIL viscosity, the SECM tip radius and the tip approach speed. Using 1.3 μm Pt SECM tips, steady-state SECM feedback approach curves were obtained in RTILs, provided that the tip approach speed was low enough to maintain steady-state diffusion at the SECM tip. In the case where tip-induced convection contributed significantly to the SECM tip current, this effect could be accounted for theoretically using mass transport equations that include diffusive and convective terms. Finally, the rate of heterogeneous electron transfer across the electrode/RTIL interface during ferrocenemethanol oxidation was estimated using SECM, and k0 was at least 0.1 cm s−1 in one of the least viscous RTILs studied.

The effect of thermal spraying on the electrochemical activity of an anti-corrosion superalloy was studied quantitatively using scanning electrochemical microscopy (SECM). The superalloy used was Inconel 625 (a Ni base superalloy) and thin coatings of the alloy were formed on mild steel using high velocity oxy-fuel (HVOF) thermal spraying. The kinetics of electron transfer (ET) across the Inconel 625 coating/electrolyte interface were studied using SECM using ferrocenemethanol as the redox mediator. For comparison, the kinetics of ET across stainless steel/electrolyte and bulk wrought Inconel 625/electrolyte interfaces were also studied using SECM. The standard heterogeneous ET rate constant, k°, for ferrocenemethanol reduction at stainless steel was 1.0 ± 0.5 × 10−3 cm s−1, compared to 2.6 ± 1.8 × 10−2 cm s−1 at the wrought Inconel 625 surface. However, at the HVOF-sprayed Inconel 625 surface, the kinetics of ET varied across the surface and k° for ferrocenemethanol reduction ranged between ∼2.2 × 10−4 cm s−1 and ∼2.6 × 10−3 cm s−1. These results clearly demonstrate that SECM can be used to quantify the effect of thermal spraying on the electrochemical properties of Inconel 625 and that thermal spraying results in an electrochemically-heterogeneous surface.Highlights► The electrochemical activity of thermal spInconel 625 was studied using SECM. ► Kinetics of electron transfer across the Inconel 625/water interface were measured. ► Electron transfer at Inconel 625 coating was compared with that at bulk Inconel 625 and stainless steel. ► Thermal spraying of Inconel 625 results in electrochemically-heterogeneous surfaces.

The high viscosity and unusual properties of room temperature ionic liquids (RTILs) present a number of challenges when performing steady-state voltammetry and scanning electrochemical microscopy in RTILs. These include difficulties in recording steady-state currents at ultramicroelectrode surfaces due to low diffusion coefficients of redox species and problems associated with unequal diffusion coefficients of oxidised and reduced species in RTILs. In this tutorial review, we highlight the recent progress in the use of RTILs as electrolytes for ultramicroelectrode voltammetry and SECM. We describe the basic principles of ultramicroelectrode voltammetry and SECM and, using examples from the recent literature, we discuss the conditions that must be met to perform steady-state voltammetry and SECM measurements in RTILs. Finally, we briefly discuss the electrochemical insights that can be obtained from such measurements.

Crystalline cellulose nanofibrils from cotton were used as reducing agents for the synthesis of nanostructured silver. The hydrothermal synthesis involved heating an AgNO3 solution containing suspended cellulose nanofibrils at 80 °C for 2 h. The formation of metallic silver was verified using UV/Visible spectroscopy, X-ray diffraction and transmission electron microscopy (TEM). Cellulose/silver nanocomposite films were formed at glassy carbon surfaces by drop coating with the product suspension and scanning electron microscopy (SEM) was used to characterise the modified surfaces. The film morphology depended on the ratio of silver to cellulose in the films. Cyclic voltammetry and rotating-disk electrode voltammetry were used to study the electrochemical and electrocatalytic behavior of these films. The nanocomposite films formed using this approach were highly active electrocatalysts for the reduction of oxygen in alkaline media.

A carbon-supported platinum electrocatalyst for the oxygen reduction reaction (ORR) in acidic medium was synthesised using supercritical fluid impregnation. The catalytic performance and electrochemical stability of this catalyst were studied using voltammetric and microscopic analysis. The results from these analyses were compared with those obtained using a Pt/C catalyst formed using a traditional wet chemical method (reduction of H2PtCl6 in aqueous solution using NaBH4). In the supercritical impregnation method, carbon black (Vulcan XC72R) was impregnated with (1,5-cyclooctadiene)dimethyl platinum(II), PtMe2COD, in supercritical carbon dioxide (scCO2) at 1200 p.s.i and 40 °C. After depressurisation, carbon-supported Pt nanoparticles were formed by thermally decomposing the adsorbed PtMe2COD. An accelerated stability test was performed, which revealed morphological changes in the electrocatalysts, which were significantly more pronounced in the Pt/C formed using the wet chemical method. Electrochemical analysis also showed that the Pt/C formed using wet chemistry lost a significant proportion of its electrochemical surface area after potential cycling, whereas that formed using scCO2 did not. Furthermore, the Pt/C formed in scCO2 retained its electrocatalytic activity to a significantly larger extent. The stability of the electrocatalysts formed using scCO2 processing suggests that this approach may be promising for the fabrication of more stable fuel cell cathodes than are currently available.

Porous nanocomposites consisting of cellulose nanocrystals (CNXLs) and polypyrrole (PPY) were fabricated using electrochemical co-deposition. The CNXLs were extracted from cotton using sulfuric acid hydrolysis and were subjected to 2,2,6,6-tetramethylpiperidine-1-oxyl-mediated oxidation, in which primary hydroxyls were oxidized to carboxylate moieties. The PPY/CNXL composites were electrodeposited from a solution of the carboxylated CNXLs and pyrrole (PY) monomers, and the negatively charged CNXLs were incorporated as the counteranion during electrodeposition. The resulting PPY/CNXL nanocomposites were characterized using scanning electron microscopy, cyclic voltammetry, and electrochemical impedance spectroscopy (EIS). Cyclic voltammetry and EIS analysis of the PPY/CNXL nanocomposites showed that the stability and specific capacitance of the nanocomposite material were higher than that of PPY containing Cl− anions. The electrochemical performance of the PPY/CNXL nanocomposites was also compared to that of a PPY/carbon nanotube (CNT) composite deposited under the same conditions, which revealed that the PPY/CNXL nanocomposites had a capacitance similar to that of the PPY/CNT nanocomposite and was at least equally as stable as the PPY/CNT nanocomposite.

The electrochemical properties of a series of room temperature ionic liquids (RTILs) were studied using voltammetric methods and scanning electrochemical microscopy (SECM). The RTILs consisted of 1-alkyl-3-methylimidazolium cations, [CnC1Im]+, and either bis[(trifluoromethyl)sulfonyl]imide anions, [Tf2N]−, or hexafluorophosphate anions, [PF6]−. The effect of RTIL viscosity on mass transfer dynamics within each RTIL was studied electrochemically using ferrocene as a redox probe. In the case of the [CnC1Im][Tf2N] RTILs, the viscosity was altered by changing the alkyl chain length. [C4C1Im][PF6] was used for comparison as its viscosity is significantly higher than that of the [CnC1Im][Tf2N] RTILs. The RTIL viscosity affected the ability to record steady-state voltammograms at ultramicroelectrodes (UMEs). For example, it was possible to record steady-state voltammograms at scan rates up to 10 mV s−1 in [C2C1Im][Tf2N] using 1.5 μm radius disk UMEs, but non-steady-state behavior was observed at 50 mV s−1. However, at 12.5 μm radius UMEs, steady-state voltammetry was only observed at 1 mV s−1 in [C2C1Im][Tf2N]. The RTIL viscosity also affected the ability to record SECM feedback approach curves that agreed with conventional SECM theory. In the most viscous [CnC1Im][Tf2N] RTILs, feedback approach curves agreed with conventional theory only when very slow tip approach speeds were used (0.1 μm s−1). These observations were interpreted using the Péclet number, which describes the relative contributions of convective and diffusive mass transfer to the tip surface. By recording feedback approach curves in each RTIL at a range of tip approach speeds, we describe the experimental conditions that must be met to perform SECM in imidazolium-based RTILs. The rate of heterogeneous electron transfer across the RTIL/electrode interface was also studied using SECM and the standard heterogeneous electron transfer rate constant, k0, for ferrocene oxidation recorded in each RTIL was higher than that determined previously using voltammetric methods.

Nanostructured thin films of cellulose nanowhiskers derived from cotton were formed using a simple drop-coating procedure. The hydrogen-bonded cellulose films were stable in aqueous solutions and their permselective properties were probed using voltammetric techniques. The nanowhisker extraction procedure produces cellulose nanowhiskers with negatively-charged sulfate surface groups that inhibit the transfer of negatively-charged species through the nanowhisker membrane, while the diffusion of neutral species is only slightly hindered. Using rotating-disk electrode measurements, the diffusion of various species within the film was studied and it was shown that the positively-charged species, Ru(NH3)63+, was adsorbed by the film, whereas the negatively-charged species, IrCl63−, was excluded by the film. The thermodynamics of adsorption of the positively-charged species by the cellulose nanoparticles were then studied using isotherm data. These observations open up new possibilities in electrochemical sensor development using renewable cellulosic materials as building blocks. Furthermore, charge-based permselective membranes can also be formed using free standing cellulose nanowhisker films, which offer the promise of renewable, selective membranes for separation technologies.

Scanning electrochemical microscopy was used to study the electrochemical activity of anti-corrosion coatings formed from Inconel 625, a Ni–Cr–Mo alloy commonly used in engineering applications. The coatings were formed using a high velocity oxygen fuel thermal spraying technique. Upon spraying the alloy onto mild steel substrates, clear splat boundaries were formed at the interface between adjacent droplets as they cooled on the substrate surface. Scanning electrochemical microscopy in the feedback mode, employing ferrocenemethanol as redox mediator, was used to study the local electrochemical activity of samples of the wrought alloy, the sintered alloy and the thermal sprayed coating. The wrought and sintered materials showed responses typical of that expected for a purely insulating material. However, SECM approach curve data showed that the electrochemical activity of the thermal sprayed material was higher than that of the bulk alloy. Local variations in the coating's electrochemical activity were then visualised using SECM imaging, which appear to be related to the splat boundaries formed during the thermal spray process.

The electrochemical behavior of a redox-active, ferrocene-modified ionic liquid (1-ferrocenylmethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) in acetonitrile and in an ionic liquid electrolyte (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) is reported. Reversible electrochemical behavior was observed in each electrolyte with responses typical of those for unmodified ferrocene observed in each medium. In the ionic liquid electrolyte, the diffusion coefficient of the redox-active ionic liquid increased by a factor of 5 upon increasing the temperature from 27 to 90 °C. The kinetics of electron transfer across the ionic liquid/electrode interface were studied using cyclic voltammetry, and the standard heterogeneous electron transfer rate constant, k0 was determined to be 4.25 × 10−3 cm s−1. Scanning electrochemical microscopy was then also used to probe the heterogeneous kinetics at the interface between the ionic liquid and the solid electrode and conventional kinetic SECM theory was used to determine k0. The k0 value obtained using SECM was higher than that determined using cyclic voltammetry. These results indicate that SECM is a very useful technique for studying electron transfer dynamics in ionic liquids.

The electrochemical behaviour of ferrocenemethanol (FcMeOH) has been studied in a range of room-temperature ionic liquids (RTILs) using cyclic voltammetry, chronoamperomery and scanning electrochemical microscopy (SECM). The diffusion coefficient of FcMeOH, measured using chronoamperometry, decreased with increasing RTIL viscosity. Analysis of the mass transport properties of the RTILs revealed that the Stokes–Einstein equation did not apply to our data. The “correlation length” was estimated from diffusion coefficient data and corresponded well to the average size of holes (voids) in the liquid, suggesting that a model in which the diffusing species jumps between holes in the liquid is appropriate in these liquids. Cyclic voltammetry at ultramicroelectrodes demonstrated that the ability to record steady-state voltammograms during ferrocenemethanol oxidation depended on the voltammetric scan rate, the electrode dimensions and the RTIL viscosity. Similarly, the ability to record steady-state SECM feedback approach curves depended on the RTIL viscosity, the SECM tip radius and the tip approach speed. Using 1.3 μm Pt SECM tips, steady-state SECM feedback approach curves were obtained in RTILs, provided that the tip approach speed was low enough to maintain steady-state diffusion at the SECM tip. In the case where tip-induced convection contributed significantly to the SECM tip current, this effect could be accounted for theoretically using mass transport equations that include diffusive and convective terms. Finally, the rate of heterogeneous electron transfer across the electrode/RTIL interface during ferrocenemethanol oxidation was estimated using SECM, and k0 was at least 0.1 cm s−1 in one of the least viscous RTILs studied.

The oxygen reduction reaction (ORR) has been studied at Pt surfaces in the protic ionic liquid diethylmethylammonium trifluoromethanesulfonate. Water content measurements suggested that the ORR proceeded in the ionic liquid predominantly via a 4-electron reduction to water. A mechanistic analysis using rotating ring-disk electrode (RRDE) voltammetry confirmed that negligible amounts of hydrogen peroxide were formed during the ORR. A kinetic analysis of the ORR was performed using rotating disk electrode (RDE) voltammetry and the importance of correcting for ohmic (iR) drop prior to performing kinetic measurements in the ionic liquid is demonstrated. A Tafel analysis of the RDE voltammetry data revealed a change in the ORR Tafel slope from 70 mV per decade at low ORR overpotentials to 117 mV per decade at high overpotentials, and the reason for this change is discussed. The change in the Tafel slope for the ORR with increasing overpotential meant that the exchange current density for the ORR varied from 0.007 nA cm−2 to 10 nA cm−2, depending on the applied potential. Finally, the implications of these results for the development of protic ionic liquid fuel cells are discussed.

The high viscosity and unusual properties of room temperature ionic liquids (RTILs) present a number of challenges when performing steady-state voltammetry and scanning electrochemical microscopy in RTILs. These include difficulties in recording steady-state currents at ultramicroelectrode surfaces due to low diffusion coefficients of redox species and problems associated with unequal diffusion coefficients of oxidised and reduced species in RTILs. In this tutorial review, we highlight the recent progress in the use of RTILs as electrolytes for ultramicroelectrode voltammetry and SECM. We describe the basic principles of ultramicroelectrode voltammetry and SECM and, using examples from the recent literature, we discuss the conditions that must be met to perform steady-state voltammetry and SECM measurements in RTILs. Finally, we briefly discuss the electrochemical insights that can be obtained from such measurements.

Crystalline cellulose nanofibrils from cotton were used as reducing agents for the synthesis of nanostructured silver. The hydrothermal synthesis involved heating an AgNO3 solution containing suspended cellulose nanofibrils at 80 °C for 2 h. The formation of metallic silver was verified using UV/Visible spectroscopy, X-ray diffraction and transmission electron microscopy (TEM). Cellulose/silver nanocomposite films were formed at glassy carbon surfaces by drop coating with the product suspension and scanning electron microscopy (SEM) was used to characterise the modified surfaces. The film morphology depended on the ratio of silver to cellulose in the films. Cyclic voltammetry and rotating-disk electrode voltammetry were used to study the electrochemical and electrocatalytic behavior of these films. The nanocomposite films formed using this approach were highly active electrocatalysts for the reduction of oxygen in alkaline media.

There is considerable interest in the development of nanostructured metal surfaces for applications in chemical sensing, optics, catalysis and magnetics. For a number of years, the use of templates based on polystyrene nanospheres has been particularly successful for the construction of meso- and macroporous surfaces. We have discovered that redox groups on the surfaces of commercially available polystyrene nanospheres reduce Ag+ ions directly. When the nanospheres are arranged as a close packed template on a glassy carbon surface, the reduced Ag preferentially deposits in the pores of the polystyrene template. After removing the template, very well defined Ag nanobowl arrays remain on the carbon surface. This discovery may open the door to the production of highly inexpensive polystyrene-templated metal nanostructures for photonic and sensing applications.