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Silica sol-gel matrices and its organically modified analogues that contain aqueous electrolytes, ionic liquids, or other ionic conductors constitute stand-alone solid-state electrochemical cells when hosting electrodes or serve as modifying films on working electrodes in conventional cells. These materials facilitate a wide variety of analytical applications and are employed in various designs of power sources. In this review, analytical applications are the focus. Solid-state cells that serve as gas sensors, including in chromatographic detectors of gas-phase analytes, are described. Sol-gel films that modify working electrodes to perform functions such as hosting electrochemical catalysts and acting as size-exclusion moieties that protect the electrode from passivation by adsorption of macromolecules are discussed with emphasis on pore size, structure, and orientation. Silica sol-gel chemistry has been studied extensively; thus, factors that control its general properties as frameworks for solid-state cells and for thin films on the working electrode are well characterized. Here, recent advances such as the use of dendrimers and of nanoscale beads in conjunction with electrochemically assisted deposition of silica to template pore size and distribution are emphasized. Related topics include replacing aqueous solutions as the internal electrolyte with room-temperature ionic liquids, using the sol-gel as an anchor for functional groups and modifying electrodes with silica-based composites.

Platinum–rhodium nanoparticles modified with cesium salt of phosphotungstic acid, CsPTA, have been tested for electro-oxidation of 2-propanol in acidic medium. The resulting Pt-based nanoparticle-containing materials have been carried out using X-ray diffraction (XRD), microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and different electrochemical techniques combining with differential electrochemical mass spectrometry (DEMS). The experimental results such as cyclic voltammetry and chronoamperometry exhibit that films on carbon electrodes composed of CsPTA-modified PtRh/Vulcan nanoparticles improve the catalytic activity, in terms of potential and current densities recorded during electro-oxidation of 2-propanol, relative to the CsPTA-free PtRh/Vulcan nanoparticles. Particularly, the boost in the catalytic current densities is observed at low potential value (below 0.25 V vs. RHE). The differential electrochemical mass spectrometry measurements were utilized to recognize the reaction intermediates as well as products created during the electro-oxidation of 2-propanol. The results indicate that contributions from a straightforward oxidation of 2-propanol to CO2 are small in comparison to acetone yield.

In the recent research years is observed huge interest in design of new functional nanomaterials for developing new electrocatalysts for oxygen reduction reaction (ORR) in acid and alkaline media. The research has been focused on developing carbon nanostructures materials with the different transition metals (e.g. Co, Ni, Ag, Au, Cu, Mn) analogue of polynuclear Prussian Blue for fuel cell applications. The development of electrocatalytic material prepared by heat-treating a composite of a Prussian Blue analogue - cobalt hexacyanocobaltate immobilized on a reduced graphene oxide. The pyrolysis process, forms a new type of catalyst with efficient active cobalt centers (Co-N-C and Co-C-N) of electrochemical oxygen reduction reaction in alkaline media. Physicochemical properties of this catalytic material containing cobalt in differing oxidation states were elucidated using microscopic, spectroscopic methods and various electrochemical diagnostic techniques. Experimental results showed that the cobalt analogue of Prussian Blue displayed comparable electrocatalytic activity (e.g., a half-wave potential and overpotential) towards oxygen reduction reaction in alkaline media to Vulcan-supported platinum nanoparticles. Furthermore, the presence of cyanide-bridged cobalt ionic sites tended to decrease formation of undesirable peroxide intermediate species, as demonstrated by monitoring ring currents during RRDE measurements. Furthermore, the proposed electrocatalytic material exhibits great stability and tolerance to alcohols (e.g. methanol). Acknowledgement The study was supported by the National Science Center (NCN), Poland, Project Number 2015/19/B/ST4/03758

The crucial point of this study was to verify the hypothesis that both thallium species are not affected by the procedure of photolysis when it is applied as a method of simplification of sample matrix containing surfactants, and that this method of sample pretreatment leads to reliable results of Tl speciation analysis. It is important especially because of high instability of Tl(III), which makes partial reduction of Tl(III)DTPA complex unavoidable. After conservation of Tl speciation by addition of DTPA the photolysis assisted with nanostructured hierarchically organized film was performed using “solar lamp” (Fe2O3/WO3/Fe2O3 film, pH 6, 380–800 nm). The results imply that under proposed photolysis conditions Tl(I) is not oxidized in the presence of water matrix, SDS nor DTPA. Also, even 100-fold excess of Fe(III) ions over Tl(I) ions does not accelerate the oxidation of Tl(I) to Tl(III), and already 2 h of heterogeneous photolysis allow to limit the interferences caused by SDS and obtain reliable results. The data obtained by anodic stripping voltammetry (ASV) were verified using an inter-method comparison with SEC ICP MS. Moreover, after 2 h irradiation the reduction rate of Tl(III)DTPA standard solution increased only slightly from 2-3% to 4–6%. The presence of river water matrix causes a slightly higher (9%) reduction of the trivalent form. These changes are irrelevant in the context of instability of Tl(III) compounds. This study indicates that “soft decomposition” can be applied in speciation analysis of thallium in water matrix.

The catalytic activity of commercial carbon-supported PtFe (PtFe/C) nanoparticles admixed with mesoporous polyoxometalate Cs3H3PMo9V3O40, (POM3-3–9), has been evaluated towards oxygen reduction reaction (ORR) in acid medium. The polyoxometalate cesium salt co-catalyst/co-support has been prepared by titration using the aqueous solution of phosphovanadomolibdic acid. The synthesized material has been characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The results confirm formation of the polyoxometalate salt with the characteristic Keggin-type structure. The composite catalyst has been prepared by mixing the POM3-3–9 sample with the commercial PtFe/C by sonication. The diagnostic rotating ring-disk voltammetric studies are consistent with good performance of the system with low Pt loading during ORR. The fuel cell membrane electrode assembly (MEA) utilizing the PtFe/POM-based cathode has exhibited comparable or better performance (at relative humidity on the level of 100, 62, and 17%), in comparison to the commercial MEA with higher Pt loading at the cathode. Furthermore, based on the cell potential and power density polarization curves, noticeable improvements in the fuel cell behavior have been observed at the low relative humidity (17%). Finally, the accelerated stress test, which uses the potential square wave between 0.4 V and 0.8 V, has been performed to evaluate MEA stability for at least 100 h. It has been demonstrated that, after initial losses, the proposed catalytic system seems to retain stable performance and good morphological rigidity.

Platinum nanoparticles have been deposited onto hybrid carriers composed of iron hexacyanoferrate (Prussian Blue) pretreated or functionalized reduced graphene oxide (rGO), and the resulting system’s electrocatalytic activity has been investigated during oxygen reduction reaction in acid solution. Physicochemical properties of the Pt nanoparticle-containing materials have been obtained using transmission electron microscopy, X-ray diffraction and various electrochemical diagnostic techniques including cyclic voltammetry and rotating ring-disk electrode (RRDE) voltammetry. Although functionalization of rGO carriers with Prussian Blue, followed by the system’s thermal treatment at 300°C, does not change activity of Pt catalytic centers in terms of the electroreduction potential of oxygen in 0.5 mol dm−3 H2SO4 (under rotating disk voltammetric conditions), the presence of cyanide-bridged iron ionic sites tends to decrease formation of the undesirable hydrogen peroxide intermediate, as demonstrated by monitoring ring currents during RRDE measurements. The results imply usefulness of PB-modified rGO supports for dispersed Pt nanoparticles at the low loading of 10 μg cm−2 in the oxygen reduction electrocatalysis. In addition to the possible metal-support activating interactions, the mechanism, in which oxygen reduction is initiated at Pt centers and the hydrogen peroxide intermediate is reductively decomposed at reactive PB-modified rGO supports, is feasible.

Hybrid systems composed of the reduced graphene oxide-supported platinum and multiwalled carbon nanotube-supported iridium (both noble metals utilized at low loadings on the level of 15 and &le, 2 &micro, g cm&minus, 2, respectively) were considered as catalytic materials for the reduction of oxygen in acid media (0.5-mol dm&minus, 3 H2SO4). The electrocatalytic activity toward reduction of oxygen and formation of hydrogen peroxide intermediate are tested using rotating ring&ndash, disk electrode (RRDE) voltammetric experiments. The efficiency of the proposed catalytic systems was also addressed by performing galvanodynamic measurements with gas diffusion electrode (GDE) half-cell at 80 &deg, C. The role of carbon nanotubes is to improve charge distribution at the electrocatalytic interface and facilitate the transport of oxygen and electrolyte in the catalytic systems by lowering the extent of reduced graphene oxide restacking during solvent evaporation. The diagnostic electrochemical experiments revealed that&mdash, in iridium-containing systems&mdash, not only higher disk currents, but also somehow smaller ring currents are produced (when compared to the Ir-free reduced graphene oxide-supported platinum), clearly implying formation of lower amounts of the undesirable hydrogen peroxide intermediate. The enhancement effect originating from the addition of traces of iridium (supported onto carbon nanotubes) to platinum, utilized at low loading, may originate from high ability of iridium to induce decomposition of the undesirable hydrogen peroxide intermediate.

The applicability of photolysis in the speciation analysis of arsenic is investigated. The use of nano scale semiconductors (Fe2O3/WO3/Fe2O3 at pH 6) as an active film during solar light irradiation of a water sample, containing some surfactants (SDS), results in the simplification of the organic matter and gives no speciation change in the arsenic. The reproducibility of active layer is shown to be high and the surface roughness of each photoactive sample and photocurrent do not differ by more than 6 and less than 8%, respectively. The procedure of sample pretreatment caused a minimum (8–10%) amount of speciation change, whilst the irradiation is no longer that 2 h. The study indicates that “soft decomposition” can be performed for as long as 4 h, and still give photostable arsenates (III) and methylarsenate species. However, the saturation of the water sample with Ar is required (to reduce the oxygen content) for the longer the decomposition time being applied.

This work reports the preparation and electrochemical characterization of PtSn/C and PtRh/C modified with a cesium salt of phosphotungstic acid, CsPTA, for use as electrochemical oxidation catalysts in acidic medium. The primary test system is the oxidation of ethylene glycol. Electrode with these modified nanoparticles confined as surface layers are used in studies not only on ethylene glycol but also on all side products of its oxidation in H2SO4 electrolyte. The experimental results reveal that films on carbon electrodes of nanoparticulate PtSn/C and PtRh/C modified with CsPTA enhance the catalytic activity relative to those lacking the CsPTA towards ethylene glycol oxidation. Higher anodic current densities are especially evident at low potentials, particularly below 0.35 V (vs. RHE). Additionally, a preliminary single fuel cell test with the CsPTA-modified PtSn/Vulcan anode shows a better performance than does the PtSn/Vulcan catalyst.

Voltammetry is a sensitive method for metal determination and one alternative to ICP MS, but its limitation is the influence of the organic matrix on the measurements. To avoid those interferences, wet digestion with H2O2 accelerated with UV irradiation is applied and evaporation of excess of reagents is required. In this study, photolytic oxidation in quartz tubes with anatase was carried out, using as object of study wastewater samples with high amount of organic matter. Cadmium and lead determination was carried out in order to test metal recoveries. Lead recoveries reached c.a 85% for UV digestion with hydrogen peroxide, 98% for digestion with anatase immobilized with polyvinylidene fluoride (N-metylo-2-pirolidon) and c.a 103% for digestion with immobilized anatase (anatase in gum). The results obtained showed the usefulness of TiO2 layer as an oxidation medium. An easy, cheap and eco-friendly digestion method of surfactants without any reagent has been developed, with equal sensitivity but and the same detection limit of traditional digestion methods.

In this paper we present the application of “sunlight” assisted digestion in the presence of WO 3 to the decomposition of dissolved organic matter, using the anionic surfactant sodium dodecyl sulfate (SDS) and the nonionic surfactant (1,1,3,3-tetramethylbutyl)phenyl-polyethylene glycol (Triton™X-114) in natural water samples, prior to the determination of traces residues of lead by stripping voltammetry methods. The results of the study showed firstly that the preparation of reproducible WO 3 layers characterized by high mechanical and chemical resistance was possible, and secondly that it was also possible to obtain a high efficiency of decomposition, equal in efficiency to that of the reference method, which was the hydrogen peroxide oxidation assisted by UV, with evaporation nearly to dryness. The developed procedure is suggested to be a no-reagents method for the decomposition of added SDS, leading to 100% recovery of added Pb (II). The anodic stripping voltammetric curves recorded in solution after 4 h irradiation with UV assisted by WO 3 were repeatable and increased linearly with standard additions, but the data finally obtained were incorrect. The curves recorded in solution after “sunlight” assisted digestion in the presence of WO 3 were repeatable, and increased linearly with an increasing of concentration of standard additions (100% recovery of Pb). In the case of a nonionic surfactant, the decomposition time is at least 6 h. The advantage of the proposed method is the fact that the digestion process does not need the addition of any chemicals for the complete decomposition of organic matter.

The unique feature of Keggin-type dodecatungstosilicate (SiW12O404−) to undergo adsorption on silver surfaces is explored here to fabricate, modify and stabilize silver nanoparticles in acid medium, as well as to link them to different reduced graphene oxide (rGO) substrates: SiO2-doted rGO, carboxylate-derivatized rGO, and conventional rGO (obtained through the chemical reduction method). Upon medium transfer to alkaline electrolyte, the inorganic polyoxometalate-capping-ligand disappears but silver nanostructures remain on rGO supports. The electrocatalytic activity of the resulting systems are probed during the oxygen reduction reaction (ORR). It is apparent from XRD data that silver is crystalline in nature and, particularly in a case of SiO2-doted rGO supports, metallic silver nanocrystals have face-centered-cubic structures enriched with high-index Ag(111) facets known to facilitate ORR. Functionalization of graphene with SiO2 seems to provide an efficient way to keep the nanosheets of graphene exfoliated thus facilitating unimpeded flux of reactants and making the system easily accessible for intercalation with catalytic Ag. Silver nanoparticles supported onto SiO2-doted rGO are relatively the most active toward ORR when examined under rotating ring disk conditions in terms of the observed highest catalytic current density, the most positive ORR onset potential, and the lowest percent formation of the H2O2 reaction intermediate.

This study describes the utilization of hierarchical photoactive surface films for the decomposition of surfactants in water samples (with different pH). Photoactive films, containing tungsten (VI) oxide and iron (III) oxide (hematite), were deposited in a systematic and controlled manner using a layer-by-layer method. Physicochemical properties of the photoactive materials were developed and characterized using XRD analysis, Raman spectroscopy, water contact angle, voltammetry, and microscopic (SEM) techniques. The resulting multilayer films showed attractive performances in the photodegradation of the anionic surfactant sodium dodecyl sulfate (SDS) and the nonionic surfactant (1,1,3,3-tetramethylbutyl)phenyl-polyethylene glycol (Triton&trade, X-144) under solar light irradiation. The efficiency of the surfactants&rsquo, photodegradation was evaluated with a &ldquo, test&rdquo, based on a method, which is extremely sensitive to surfactants&rsquo, interference, with trace analysis of Pb using anodic stripping voltammetry on mercury electrodes (recovery study). The usefulness of hierarchical photoactive systems in the photodegradation of both surfactants is demonstrated in the presence and absence of the applied bias voltage. The maximum decomposition times were 2&ndash, 3 h and 30 min, respectively. Furthermore, a properly designed layer system may be proposed, matching the pH of the water sample (depending on the treatment on the sampling side).

Electrocatalysis can be enhanced either by using more active catalysts or by means of external process intensifications exploiting different energy types. The improvement of oxygen reduction reaction (ORR) efficiency by using an external magnetic field is technologically practical yet poorly scrutinized. An iron/nitrogen/sulfur-co-doped carbon gel (Fe-N-C/S) is studied as a non-precious metal catalyst for the ORR under an external magnetic field generated by a permanent magnet. The application of a magnetic field enhances the ORR process in both acidic and alkaline environments and yields higher selectivity towards 4e− electroreduction of O2. To elucidate the origin of the observed phenomena and show the role of the catalyst’s components, samples either free of sulfur (Fe-N-C) or iron (N-C/S) are studied as references. Moreover, the highly porous carbon gel constitutes an attractive substrate for enzyme immobilization. Modification of the catalyst with laccase (a multicopper oxidase enzyme) additionally strengthens the efficiency of O2 electroreduction.

We report on the development of two new Pt-free electrocatalysts (ECs) for the oxygen reduction reaction (ORR) process based on graphene nanoplatelets (GNPs). We designed the ECs with a core-shell morphology, where a GNP core support is covered by a carbon nitride (CN) shell. The proposed ECs present ORR active sites that are not associated with nanoparticles of metal/alloy/oxide but are instead based on Fe and Sn subnanometric clusters bound in coordination nests formed by carbon and nitrogen ligands of the CN shell. The performance and reaction mechanism of the ECs in the ORR are evaluated in an alkaline medium by cyclic voltammetry with the thin-film rotating ring-disk approach and confirmed by measurements on gas-diffusion electrodes. The proposed GNP-supported ECs present an ORR overpotential of only ca. 70 mV higher with respect to a conventional Pt/C reference EC including a XC-72R carbon black support. These results make the reported ECs very promising for application in anion-exchange membrane fuel cells. Moreover, our methodology provides an example of a general synthesis protocol for the development of new Pt-free ECs for the ORR having ample room for further performance improvement beyond the state of the art.

Graphene is, in principle, a promising material for consideration as component (support, active site) of electrocatalytic materials, particularly with respect to the reduction of oxygen, an electrode reaction of importance to low-temperature fuel cell technology. Different concepts of utilization, including nanostructuring, doping, admixing, preconditioning, modification or functionalization of various graphene-based systems for catalytic electroreduction of oxygen are elucidated, as well as important strategies to enhance the systems’ overall activity and stability are discussed.

A typical procedure based on sol-gel aggregation for the fabrication of mesoporous WO 3 photoanodes was modified and used produce modified tungsten oxide films by utilizing the tungstic acid sol admixed with Keggin-type borotungstic acid (H 5 BW 12 O 40 ) or the borotungstic acid–stabilized hematite (Fe 2 O 3 ). Physicochemical properties (structure, morphology and spectroscopic identity) of the resulting hybrid (composite) WO 3 films were assessed using X-ray diffraction, scanning electron microscopy, as well as UV-Vis and Raman spectroscopies. During photoelectrochemical experiments with a solar cell system operating in 0.5 mol dm −3 H 2 SO 4 in three-electrode configuration, the composite WO 3 films acting as photoanodes yielded (following illumination with visible light) significantly (up to 80%) larger water oxidation photocurrents (at potentials higher than 0.75 V vs. RHE), relative to the performance of analogous bare (pristine) mesoporous tungsten oxide films. Independent voltammetric diagnostic experiments with Pt microdisk electrode implied the appearance of oxygen during photooxidation of water. The enhancement effect was the most pronounced for WO 3 films admixed with borotungstic acid–stabilized hematite. The observed phenomena could be rationalized either in terms of the presence of new conduction band structures in composite WO 3 films or a marked increase in the degree of hydration of the composite oxide structures regardless of the annealing at high temperatures (450 °C). It was also apparent from our experiments in 0.5 mol dm −3 NaCl, the systems were also applicable to sea-water-type environments.

An electrochemical cell comprising a silica sol-gel solid electrolyte, a working electrode that protrudes into a gas phase, and reference and counter electrodes that contact the solid electrolyte comprises an amperometric detector for gas chromatography. Under potentiostatic conditions, a current related to the concentration of an analyte in the gas phase is produced by its oxidation at the three-phase boundary among the sol-gel, working electrode, and the gas phase. The sol-gel is processed to contain an electrolyte that also serves as a humidistat to maintain a constant water activity even in the presence the gas chromatographic mobile phase. Response was demonstrated toward a diverse set of analytes, namely hydrogen, 1,2-ethandithiol, phenol, p-cresol, and thioanisole. Using flow injection amperometry of hydrogen with He as the carrier gas, 90% of the steady-state current was achieved in < 1s at a flow rate of 20mLmin-1. A separation of 1,2-ethandithiol, phenol, p-cresol, and thioanisole at a 2.2mLmin-1 flow rate was achieved with respective detection limits (k = 3 criterion) of 4, 1, 3, and 70 ppmv when the working electrode potential was 800mV.

Chemically-reduced graphene-oxide-supported gold nanoparticles with a diameter, of 40–60 nm are considered here as catalytic materials for the reduction of oxygen in alkaline medium in comparison to analogous systems based on conventional Vulcan carbon carriers. Gold nanoparticles are prepared by the chemical reduction method, in which the NaBH 4 -prereduced Keggin-type phosphomolybdate heteropolyblue acts as the reducing agent for the precursor (HAuCl 4 ). Polyoxometallate (PMo 12 O 40 3− ) capping ligands stabilize gold nanoparticle deposits, facilitate their dispersion and attachment to carbon supports. Indeed, it is apparent from the independent diagnostic voltammetric experiments (in 0.5 mol dm −3 H 2 SO 4 ) that heteropolymolybdates form readily stable adsorbates on nanostructures of both gold and carbon (reduced graphene oxide and Vulcan). It is reasonable to expect that the polyoxometallate-assisted nucleation of gold has occurred in the proximity of oxygenated defects existing on carbon substrates. Under conditions of electrochemical diagnostic experiments (performed in 0.1 mol dm −3 KOH): ( i ) the phosphomolybdate adsorbates are removed from the interface as they undergo dissolution in alkaline medium; and ( ii ) the Au nanoparticles (Au loading, 30 μg cm −2 ) remain well-dispersed on the carbon as evident from transmission electron microscopy. High electrocatalytic activity of the reduced-graphene oxide-supported Au nanoparticles toward reduction of oxygen in alkaline medium is demonstrated using cyclic and rotating ring-disk voltammetric experiments. The latter system could also act as the active support for Pt nanoparticles during the reduction of oxygen. Among important issues are possible activating interactions between gold and the support, as well as presence of structural defects existing on poorly organized graphitic structure of reduced graphene oxide (as evident from Raman spectroscopy).

In the recent research years is observed huge interest in design of new functional nanomaterials for developing new electrocatalysts for oxygen reduction reaction (ORR) in acid and alkaline media. The research has been focused on developing carbon nanostructures especially graphene/graphene oxide materials with the different transition metals (e.g. Co, Ni, Ag, Au, Cu, Mn) analogue of polynuclear Prussian Blue, namely with ultra-thin Co, and Ni hexacyanoferrate layers and trace amount of platinum for fuel cell applications. Electrocatalysts based on graphene and graphene-oxide (GO) are more homogeneous and possess properties such as excellent conductivity, good chemical stability and can be functionalized in a controlled manner. Following the heat-treatment step at higher temperatures, some thermal decomposition of the cyanometallate network occurs and, consequently, metallic sites are generated. Their formation and distribution are facilitated by the voltammetric potential cycling in acid and alkaline electrolytes. The most promising electrocatalytic results with respect to the reduction of oxygen (the highest currents and the most positive electroreduction potentials) have been obtained when graphene nanostructures are combined with analogue of polynuclear Prussian Blue and trace amount of Pt nanoparticles. What is even more important that, due to the presence of the polynuclear cyanoferrate modifier or linker, the amounts of the undesirable hydrogen peroxide intermediate are significantly decreased. An electrocatalytic system, that utilizes metal hexacyanometallates nanoparticles with trace amount of Pt modified graphene and graphene related materials, is developed and characterized here using transmission electron microscopy and such electrochemical diagnostic techniques as cyclic volammetry and rotating ring-disk voltammetry in a 0.5 M H2SO4 electrolyte and in a 0.1 M KOH electrolyte and upon introduction (as cathode) to the low-temperature hydrogen-oxygen fuel cell. Comparative measurements have been performed against the model noble metal (Vulcan-supported platinum nanoparticles) catalyst.

In this article, we characterized tungsten oxide-decorated carbon-supported PtIr nanoparticles and tested it for the electrooxidation reactions of ethylene glycol and ethanol. Phase and morphological evaluation of the proposed electrocatalytic materials are investigated employing various characterization techniques including X-ray diffraction (XRD) and transmission electron microscopy (TEM). Electrochemical diagnostic measurements such as cyclic voltammetry, chronoamperometry, and linear sweep voltammetry revealed that the tungsten oxide-modified PtIr/Vulcan nanoparticles have higher catalytic activity for ethylene glycol and ethanol electrooxidation than that of PtIr/Vulcan. A significant enhancement for electrooxidation of CO-adsorbate monolayers occurred in the presence of a transition metal oxide relative to that of pure PtIr/Vulcan electrocatalyst. The likely reasons for this are modification on the Pt center electronic structure and/or increasing the population of reactive oxo groups at the PtIr/Vulcan electrocatalytic interface in different potential regions.

In the recent research years is observed huge interest in design of new functional nanomaterials for developing new electrocatalysts for oxygen reduction reaction (ORR) in acid and alkaline media. The research has been focused on developing carbon nanostructures materials with the different transition metals (e.g. Co, Ni, Ag, Au, Cu, Mn) analogue of polynuclear Prussian Blue for fuel cell applications. Electrocatalysts based on graphene and graphene-oxide (GO) are more homogeneous and possess properties such as excellent conductivity, good chemical stability and can be functionalized in a controlled manner. Following the heat-treatment step at higher temperatures, some thermal decomposition of the cyanometallate network occurs and, consequently, metallic sites are generated. Their formation and distribution are facilitated by the voltammetric potential cycling in acid and alkaline electrolytes. The most promising electrocatalytic results with respect to the reduction of oxygen (the highest currents and the most positive electroreduction potentials) have been obtained when carbon nanostructures materials are combined with transition metal hexacyanometallates. An electrocatalytic system, that utilizes transition metal hexacyanometallates nanoparticles modified carbon nanostructures materials (e.g. reduced graphene oxide, carbon nanotubes), is developed and characterized here using transmission electron microscopy and such electrochemical diagnostic techniques as cyclic volammetry and rotating ring-disk voltammetry in a 0.5 M H2SO4 electrolyte and in a 0.1 M KOH electrolyte and upon introduction (as cathode) to the low-temperature hydrogen-oxygen fuel cell. Comparative measurements have been performed against the model noble metal (Vulcan-supported platinum nanoparticles) catalyst.

We are going to address practical issues related to the principles and analytical aspects of electrochemical studies (typically done at room temperature) of solid, rigid or semi-rigid (nonfluid) systems in the absence of a liquid solution phase. Representative examples include redox and conducting organic polymers, melts and solid solutions of redox centers in solid ionic conductors, mixed-valence polynuclear inorganic materials, transition metal salts, oxides, and zeolites. The emphasis is on the elements of dynamics for the efficient delivery of charge and on reactivity of the ‘redox conducting’ materials. The effective (apparent) diffusional mechanism is critical to the success of most analytical measurements in solid-state. Historically, electrochemistry in the absence of a bulk liquid phase has focused on the problems of energy storage and production. Recently, it has become possible to apply conventional electrochemical methods to characterize solid-state type redox electrolytes with respect to analytical characterization of materials, measurement of electron transfer rates, and determination of analytes using amperometric sensing. The characterization of materials by solid-state voltammetry primarily has focused on mixed-valence, ionically conducting compounds and on diverse organic ionomers hosting identifiable molecular (redox) sites. Potential materials for solid-state electrochemical measurements are expected to contain three-dimensionally distributed highly concentrated redox centers between which fast electron self-exchange (hopping) is feasible. Among inorganic systems, single crystals of heteropolyacids of molybdenum and tungsten, as well thin films of Prussian Blue type metal hexacyanoferrates will be considered. These redox centers are fixed and, although they may have short range mobility about an equilibrium position, they classically are macroscopically immobile. The applicable materials also must host mobile counter-ions that are capable of providing charge balance during electron transfers, thereby serving the same purpose as the supporting electrolytes in conventional electrochemistry. The population of these ions must be sufficient to support diffusive mass transport of electrons and to minimize ohmic effects. We will describe typical electrochemical cells and experimental tactics to overcome the relatively slow dynamics of transport in solid (non-fluid) systems. Application of microdimensional electrodes leads to an improvement in the quality of solid-state electrochemical data and provides new diagnostic and analytical possibilities. Basic concepts of mechanistic studies, which are relevant to the development of novel analytical methods, together with trends towards possible future applications will also be addressed.

We report a modification of nanostructured WO3 films by doping with boron. The films were obtained by a direct one-step sol–gel route involving tungstic acid/polyethelene glycol precursor. Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) showed that the incorporation of boron results in the retention of a substantial amount of water and/or hydroxyl groups in the WO3 lattice and at the surface of nanoparticles occurring despite high temperature (550 °C) annealing of the films. Another consequence of boron doping is the largely increased roughness factor revealed by atomic force microscopy (AFM) imaging. Both kinds of films are highly porous and consist of partly sintered particles with sizes in the range of tens of nanometers. The photoelectrochemical (PEC) studies performed under simulated solar AM 1.5 illumination showed significantly enhanced water oxidation photocurrents for B-WO3 photoanodes, by about 25% higher than those for the undoped WO3 films of similar thickness. The low extent of recombination of photogenerated charges was confirmed by incident photon-to-current conversion efficiencies (IPCEs) reaching 70% in the region of visible wavelengths at 420 nm. The improved PEC properties were attributed to the increased surface hydroxylation of B-WO3 nanoparticles favoring water photo-oxidation reaction and to the larger surface area of the film exposed to the electrolyte.

Unsupported Pt/Sn nanoparticles, that had been prepared via polyol method, were modified with ultra-thin (monolayer-type) films of Keggin heteropolyacids of molybdenum and tungsten. The resulting nanostructured catalysts were investigated using both electrochemical and spectroscopic (XPS) methods. Modification of unsupported Pt/Sn with phosphotungstates or phosphomolybdates led to the improvement of the system’s electrocatalytic activity in terms of relatively higher currents observed during oxidation of ethanol, especially when they were recorded and compared at such low potentials as 0.2 or 0.3 V vs. RHE. It can be concluded from the XPS spectroscopic analysis that, following modification with polyoxometallate, the oxidation state of tin has changed. An attempt was also made to distinguish and correlate distinct oxidation states of tin with the system’s overall electrocatalytic activity and to explain the observed enhancement effect during ethanol electrooxidation.

RuSex electrocatalytic nanoparticles were deposited onto hybrid carriers composed of Black Pearl carbon-supported tungsten oxide; and the resulting system's electrochemical activity was investigated during oxygen reduction reaction. The tungsten oxide-utilizing and RuSex nanoparticle-containing materials were characterized using transmission electron microscopy, X-ray diffraction and electrochemical diagnostic techniques such as cyclic voltammetry and rotating ring-disk voltammetry. Application of Black Pearl carbon carriers modified with ultra-thin films of WO3 as matrices (supports) for RuSex catalytic centers results during electroreduction of oxygen in 0.5 mol dm−3 H2SO4 (under rotating disk voltammetric conditions) in the potential shift of ca. 70 mV towards more positive values relative to the behavior of the analogous WO3-free system. Also the percent formation (at ring in the rotating ring-disk voltammetry) of the undesirable hydrogen peroxide has been decreased approximately twice by utilizing WO3-modified carbon carriers. The results are consistent with the bifunctional mechanism in which oxygen reduction is initiated at RuSex centers and the hydrogen peroxide intermediate is reductively decomposed at reactive WO3-modified Black Pearl supports. The electrocatalytic activity of the system utilizing WO3-modified Black Pearl supports has been basically unchanged upon addition of acetic acid, formic acid or methyl formate to the sulfuric acid supporting electrolyte.

A comparative study describing immobilization of the Dawson type mixed addenda heteropolyanion, [P2W17VO62]8− into conducting polymer films of poly(3,4-ethylenedioxythiophene), PEDOT, and poly(2,2′-bithiophene), PBT, is reported. Electrosynthesis of these hybrid films was performed using a micellar aqueous solution of the nonionic surfactant, polyethylene glycol tert-octylphenyl ether (Triton X-100). Deposited composite films were characterised electrochemically and, on the whole, they exhibited fast electron transfer (ET) properties and relatively high stability towards continuous potential cycling in acidic media. In particular, PEDOT composite showed relatively faster ET properties in comparison to PBT composite. Their permeability was investigated in the presence of cationic and anionic redox probes. Our results implied good mediating capabilities of the [P2W17V4+O62]8− anion (within the [P2W17V4+O62]8−–PEDOT hybrid film) towards the iron (III) reduction. The specific electrocatalytic (reductive) capabilities of hybrid films were also studied by probing the reduction of bromate. The films were further characterised by X-ray photoelectron spectroscopy to establish their interfacial elemental composition. Moreover, their surface morphology was imaged by atomic force microscopy and scanning electron microscopy. Results have shown that physicochemical properties of the investigated hybrid films were affected by polymer hydrophobicity.

A novel electrocatalytic system for oxidation of ethanol, in which carbon-supported PtSn nanoparticles were modified with ultra-thin films/deposits of tungsten oxide, was proposed, fabricated and characterized here using electrochemical as well as spectroscopic (X-ray diffraction) and microscopic (transmission electron) techniques. The enhancement effect was evident from comparative diagnostic electrochemical experiments utilizing WO3-modified and bare (unmodified) PtSn nanoparticles in acid medium (0.5 mol dm−3 H2SO4) at room temperature in the presence of ethanol. It is noteworthy that, the chronoamperometric electrocatalytic currents measured at potentials as low as 0.3 V (vs. RHE) were significantly larger for WO3-modified PtSn/C relative to bare PtSn/C. In another diagnostic “stripping” experiment, it was found that oxidation of CO-adsorbate occurred in the presence of WO3 at potentials almost 100 mV lower in comparison to the unmodified system. The overall activation effect may have origin in interactions of tungsten oxide with tin (from PtSn alloy nanoparticles) leading to stabilization of the catalytic tin oxo species. WO3 may also provide large population of reactive oxo groups at the Pt/Sn-based electrocatalytic interface.

Se/Ru nanoparticles – a potent non-platinum catalyst towards oxygen reduction reaction – were modified by hydrated WO3 and investigated using the rotating disk/ring electrode methods and by synchrotron X-ray photoelectron spectroscopy. The modification resulted in an enhanced catalytic activity towards oxygen reduction reaction (ORR). Our data indicate that the oxygen reduction current starts ca. 70 mV more positive and formation of undesirable hydrogen peroxide has significantly decreased following the modification of Se/Ru with WO3. X-ray photoelectron spectroscopy reveals that WO3 interacts electronically with Se/Ru as the W 4f and Se 3d line-shapes change. We therefore conclude that the electronic interactions between Se/Ru and WO3 are primarily responsible for the increase in activity and selectivity of the WO3-modified Se/Ru towards ORR.

As evidenced from the increase of electrocatalytic currents measured under voltammetric and chronoamperometric conditions, the activity of bimetallic Pt–Ru and Pt–Sn nanoparticles towards oxidation of ethanol is increased by modification of their surfaces with ultra-thin films of phosphododecamolybdic acid (H 3 PMo 12 O 40 ). The enhancement effect has been most pronounced in a case of heteropolymolybdate-modified carbon-supported Pt–Sn catalysts. Independent high-resolution XPS measurements indicate the ability of heteropolymolybdates to stabilize tin (in bimetallic Pt–Sn particles) at higher oxidation states (presumably as tin oxo species). The overall activation effect may also be ascribed to changes in the morphology of catalytic films following modification with heteropolymolybdates. Presence of the polyoxometallate is also likely to increase of the interfacial population of reactive oxo groups in the vicinity of platinum centers.

Hybrid systems composed of the reduced-graphene-oxide supported gold, iridium or bimetallic (AuIr) nanoparticles (at loadings typically below 5 µg cm-2) have been considered as active matrices for platinum catalysts utilized at low loadings (-2) during the reduction of oxygen in mostly acid but also alkaline media. Comparison is made to the analogous systems based on conventional Vulcan carbon carriers. Gold nanoparticles are prepared both by the electrochemical and chemical reductionapproaches in which the pre-reduced Keggin-type phosphomolybdate heteropolyblue species act as the reducing agent for the HAuCl4 precursor. Polyoxmetallate (PMo12O40 3-) adsorbates or “capping ligands” tend to stabilize gold (iridium or bimetallic) nanoparticle deposits, facilitate their dispersion and attachment to carbon supports. Indeed, it is apparent from the independent diagnostic voltammetric experiments (in 0.5 mol dm-3 H2SO4) that heteropolymolybdates form readily stable adsorbates on nanostructures of gold, irridium and carbon (reduced graphene oxide and Vulcan). It is reasonable to expect that the polyoxometallate-assisted nucleation of gold has occurred in the proximity of oxygenated defects existing on carbon substrates. Under conditions of the electrochemical experiments performed in 0.1 mol dm-3 KOH) the phosphomolybdate adsorbates are removed from the interface because they undergo dissolution and decomposition in alkaline medium. High electrocatalytic activity of the reduced-graphene oxide-supported catalytic systems toward the reduction of oxygen is demonstrated using chronoamperometry and gas-diffusion electrode, in addition to the conventional and rotating ring-disk electrode (RDE) voltammetry. Among important issues is the presence of structural defects existing on poorly organized graphitic structure of reduced graphene oxide (as evident from Raman spectroscopy). When using the reduced graphene oxide carriers for Au (Ir or AuIr) nanostructures together with Pt nanoparticles, the resulting catalytic systems have exhibited typically higher (certainly not lower) O2-reduction currents (relative to those recorded at conventional Vulcan-supported Pt at the same loading) in acid medium (0.5 M H2SO4). The RDE data are consistent with the lower formation of the hydrogen peroxide intermediate (

Reduced graphene-oxide-supported the various metals analogue of polynuclear Prussian Blue are considered here together with traces of platinum as well as low loadings of platinum (10-15 mg cm-2) as catalytic materials for reduction of oxygen in acid media. Comparison is made to the analogous systems based on conventional Vulcan carbon carriers. Electrocatalysts based on graphene and graphene-oxide (GO) are more homogeneous and possess properties such as excellent conductivity, good chemical stability and can be functionalized in a controlled manner. Following the heat-treatment step at higher temperatures, some thermal decomposition of the cyanometallate network occurs and, consequently, metallic sites are generated. The resulting material has occurred to show highly potent electrocatalytic properties toward electroreductions of oxygen in acid solution (the highest currents and the most positive electroreduction potentials) using cyclic and rotating ring-disk electrode (RDE) voltammetric experiments. Among important issues are possible activating interactions between analogue of Prussian Blue, platinum nanoparticles and the support, as well as presence of structural defects existing on poorly organized graphitic structure of reduced graphene oxide (as evident from Raman spectroscopy). An electrocatalytic system, that utilizes metal hexacyanometallates nanoparticles with trace amount of Pt modified reduced graphene-oxide-supported is also characterized here by additional diagnostic experiments, namely galvanodynamic measurements with use of so called gas diffusion electrode (GDE) operating under conditions resembling those characteristic of the cathode in a real fuel cell.

Modification of multi-walled carbon nanotubes with ultra-thin films of oxocyanoruthenium structures resulted in their improved capacitive characteristics, as evident from increased specific capacitances and energy densities determined using diagnostic charging-discharging electrochemical experiments (involving cyclic voltammetry, galvanostatic and AC impedance methods). The largest specific capacitance (123 F g-1) was found when the oxocyanoruthenium modified carbon nanotubes were further covered or linked with conducting polymer, namely poly (3,4-ethylenedioxythiophene) or PEDOT, structures. Among important issues are high stability of both inorganic and organic components, attractive electrostatic interactions between negatively charged oxocyanoruthenate and positively charged PEDOT, fast redox transitions of oxocyanoruthenium and high dynamics of charge propagation in PEDOT species, as well as ability to improve distribution of charge by mul ti-walled carbon nanotubes.

The ability of Keggin-type phosphododecamolybdate (PMo 12 O 40 3− , PMo 12 ) to undergo chemisorption on solid surfaces (including gold) is explored here to convert (by ligand place-exchange and phase transfer to aqueous solution) the alkanothiolate-modified Au nanoparticles of controlled size (prepared in toluene) into a stable colloidal solution of PMo 12 -protected gold nanoparticles, PMo 12 –AuNPs, the sizes of which are ca. 4–5 nm as determined by transmission electron microscopy. By dip-coating, PMo 12 –AuNPs were assembled on carbon electrode substrates. The step-by-step assembly, by which alternate exposures to the solutions of PMo 12 –AuNPs and either anilinium cations or pyrrole monomers, was utilized to grow in controlled manner hybrid network films in which the negatively charged PMo 12 –AuNP deposits were linked, or electrostatically attracted, by ultra-thin, positively charged conducting polymer (polyaniline or polypyrrole) structures. The three-dimensionally distributed PMo 12 –AuNPs immobilized within the polypyrrole-based composite film exhibited some electrocatalytic reactivity towards reduction of hydrogen peroxide.

Combination of multi-walled carbon nanotubes, cobalt porphyrin, and peroxidase (horseradish, cabbage) enzyme in the film (deposited onto glassy carbon electrode substrate) produces a bio-electrocatalytic system capable of effective reduction of oxygen in such neutral media as 0.1 mol dm −3 KCl and 0.1 mol dm −3 KCl + 0.01 citrate buffer (pH 6). Carbon nanotubes have been modified with ultra-thin layers of 4-(pyrrole-1-yl) benzoic acid, or phosphododecamolybdate, to form stable colloidal suspensions of carbon nanostructures. The resulting inks have been utilized during sequential deposition of components. Co-existence of cobalt porphyrin, peroxidase enzyme together with dispersed carbon nanotubes leads to synergistic effect that is evident from some positive shift of the oxygen reduction voltammetric potentials (more than 50 mV in citrate buffer) and significant (ca. twice) increase of voltammetric currents (relative to those of the enzyme-free system). The multi-component bio-electrocatalytic film has also exhibited relatively higher activity towards reduction of hydrogen peroxide. It is reasonable to expect that the reduction of oxygen is initiated at cobalt porphyrin redox centers, and the undesirable hydrogen peroxide intermediate is further reduced at the horseradish or cabbage peroxidase enzymatic sites. An important function of carbon nanotubes is to improve transport of electrons within the bio-electrocatalytic multi-component film.

The ability of 4-(pyrrole-1-yl) benzoic acid (PyBA) to form monolayer-type carboxylate-derivatized ultra-thin organic films on solid electrode surfaces was explored here to attract coordinatively and immobilize N1 2+ ions at the electrode/electrolyte interface. In the next step, the system was exposed to Fe(CN) 6 3- or Fe(CN) 6 4- solution to form a robust nickel hexacyanoferrate (NiHCF) layer. By repeated and alternate treatments in solutions of PyBA, N1 2+ cations, and Fe(CN) 6 3- or Fe(CN) 6 4- anions, the amount of the material could be increased systematically in a controlled fashion to form three-dimensional multilayered NiHCF-based assemblies. The layer-by-layer method was also extended to the growth of hybrid conducting polymer stabilized NiHCF films in which the initial PyBA-anchored NiHCF layer (formed on glassy carbon) was subsequently exposed (a desired number of times) through alternate immersions to the monomer (3,4-ethylenedioxythiophene), Fe(CN) 6 3- and N1 2+ solutions. During voltammetric potential cycling (electropolymerization) in the external supporting electrolyte solution, poly(3,4-ethylenedioxythiophene) or PEDOT linked NiHCF-based multilayered films were produced. They were characterized by good stability and high dynamics of charge transport.

Combination of polyoxometallate-modified multi-walled carbon nanotubes, cobalt porphyrin, and peroxidase (horseradish, cabbage) enzyme within the film (deposited on glassy carbon) produces a bio-electrocatalytic system capable of effective reduction of oxygen in neutral media such as 0.1 mol dm-3 KCl and 0.1 mol dm-3 KCl + 0.01 citrate buffer (pH=6). An important function of carbon nanotubes is to improve transport of electrons within the film. Co-deposition of the peroxidase enzyme together with the cobalt porphyrin catalytic centers leads to bifunctional synergistic effect that is evident from sizeable positive shift of the oxygen reduction potentials and significant increase of voltammetric currents. The bio- electrocatalytic system also exhibits relatively much higher reductive activity towards hydrogen peroxide. It is reasonable to expect that the reduction of oxygen is initiated at cobalt porphyrin redox centers, and the undesirable hydro gen peroxide intermediate is further reduced at the horseradish or cabbage peroxidase enzymatic sites.

Fabrication of structured film (on glassy carbon substrate) composed of compact Prussian Blue (that has been prepared by alternate immersions and through assembling within ultra-thin layers of 4(pyrrole-1-yl)-benzoic acid, PPyBA) and poly(3,4-ethylendioxythiophene), PEDOT, is described. This functionalized film has been characterized by fast charge propagation, and it has served as a redox conducting template for permanent attachment of a model enzyme, horseradish peroxidase, HRP. The resulting organic–inorganic system acts as an effective hybrid bioelectrocatalyst for electroreduction of hydrogen peroxide, a model reactant for biosensors and biofuel cells. Among important issues are rigidity, permanence of enzyme attachment, morphology, hydrophilicity, and attractive mediating capabilities of the PEDOT-stabilized Prussian Blue based structured film.

WO3-modified carbon-supported bi-component ruthenium–selenium, RuSex (Ru, 20; Se, 1 wt%), nanoparticles were dispersed in the form of Nafion-containing inks on glassy carbon electrodes to produce electrocatalytic interfaces reactive towards electroreduction of dioxygen in acid medium (0.5 mol dm−3 H2SO4). It was apparent from the rotating disk voltammetric experiments that the reduction of oxygen proceeded at WO3-modified electrocatalyst at more than 100 mV more positive potential when compared to bare (WO3-free) RuSex system (that had been prepared under analogous conditions and deposited at the same loading of 156 μg cm−2). The ring-disk rotating voltammetric measurements show that, while the production of hydrogen peroxide intermediate was significantly lower, the kinetic parameter (heterogeneous rate constant) for the oxygen reduction was higher for WO3-modified RuSex (relative to bare RuSex). Comparison was also made to highly-efficient Vulcan-supported Pt or Pt/Co nanoparticles: while the half-wave potential for the oxygen reduction at WO3-modified carbon-supported RuSex was still more negative relative to the potentials characteristic of Pt-based electrocatalysts, the oxygen reduction rotating disk voltammetric current densities (measured at 1600 rpm) were almost identical.