Hydrogen production by photoelectrolytic decomposition of H2O using solar energy

Photoelectrochemical systems for the efficient decomposition of water are discussed. Semiconducting d band oxides which would yield the combination of stability, low electron affinity, and moderate band gap essential for an efficient photoanode are sought. The materials PdO and Fe-xRhxO3 appear most likely. Oxygen evolution yields may also be improved by mediation of high energy oxidizing agents, such as CO3(-). Examination of several p type semiconductors as photocathodes revealed remarkable stability for p-GaAs, and also indicated p-CdTe as a stable H2 photoelectrode. Several potentially economical schemes for photoelectrochemical decomposition of water were examined, including photoelectrochemical diodes and two stage, four photon processes

Aqueous polyoxometalates: design and analysis of electrochemical catalysts for the indirect reduction of oxygen in PEM fuel cells

The applicability of aqueous, mixed addenda polyoxometalates with the general formula [PMo12-xVxO40](3+x)- as catalysts for FlowCath® technology has been demonstrated. These compounds were used as a platinum substitute in the PEM fuel cell cathode for the indirect reduction of oxygen. The effect of increased vanadium substitution within the Keggin structure upon the diffusion coefficient (Do) and the standard rate constant for electron transfer (ko) was investigated via simulation and electrochemical analysis. The apparent decrease in electrode kinetics linked with increased vanadium substitution is explained via simulation modelling, with the VxPOMs systems demonstrating multiple redox processes. The effects of solvent and electrode material upon the voltammetry are also discussed. Self supporting conditions analogous to the in fuel cell were employed to the VxPOM catalysts and their behaviour compared to the [Fe(CN)6]4-/3- redox couple via CV, simulation and RDE analysis. The resulting self-supported [Fe(CN)6]3-/4- system demonstrated significantly increased currents, but less than theoretically expected due to increases in cell resistance. The self-supported VxPOM system electrode processes are hindered due to the formation of a VO20 driven blocking layer reducing the actual potential experienced by the redox active species at the electrode surface. The resulting blocking layer prevented the VxPOM from approaching the electrode surface thus not experiencing the actual potential applied at the electrode surface. Tafel plots based upon the VxPOM systems showed characteristics not resembling ‘classical’ Tafel analysis with curvature preventing extrapolation for exchange current density. An ‘alternative’ analysis method involving the interpolation of the raw rotating disc electrode data to determine the required overpotential to generate a desired current was developed. The regeneration of the V4POM catalysts was investigated which demonstrated a possible change in speciation and a more ordered structure based upon single crystal X-ray analysis. The effects of formulation development of the lead V4POM catalyst upon its electrochemical and fuel cell performance were investigated. Substitution of Na+ counter ions with H+ (HV4POM) showed a decrease in charge transfer resistance (Rc) as well increase in membrane resistance (Rs) and cathodic current. The affects of adding stoichiometric quantities of H3PO4, HBF4 and VOSO4 were investigated with RDE and fuel cell testing indicating improved performance for the HBF4 formulation at fuel cell conditions. The effects of current developments in FlowCath® technology upon the H3PO4 formulation are also discussed

The Incorporation and Controlled Release of Anionic Drugs from a Polypyrrole Membrane Film

In this thesis, results are presented and discussed on the synthesis and characterisation of polypyrrole (PPy) doped with various anionic drugs. The drugs, which were small to medium‐large in size, included the aqueous soluble dexamethasone 21‐phosphate disodium (NaDex) and valproic acid sodium (NaVPA) salts, and the less soluble diclofenac sodium (NaDF) salt. Two other drugs, indomethacin sodium (NaIndo) and sulindac sodium (NaSul) with very imited solubility in aqueous solution, were chosen. r l The incorporation of dexamethasone (Dex2‐) and diclofenac anions (DF‐) within the PPy membrane was achieved by a potentiostatic mode of growth from an aqueous solution of pyrrole and the drug under investigation. For the PPy doped with Dex2‐, characterisation and release studies found the doping level of the polymer to be about 0.30, and approximately 31 μmol cm‐2 of Dex2‐ was incorporated into the polymer upon polymerisation. Furthermore, it was observed that the rate of release could be controlled by the potential applied with approximately 89 % of the Dex2‐ released within 60 min at an applied potential of – 0.900 V vs SCE. For the PPy doped with DF‐, unusual patterns in growth and morphology were observed. During the deposition of the polymer, the rate of polymerisation decreased with increasing time and higher applied potentials. The polymer had features of an insulating film, as evident from electrochemical impedance measurements, while SEM confirmed the presence of crystal‐like shards on the surface of the polymer. These findings suggest that insoluble drug crystals are formed during polymerisation. DF‐ displays a limited solubility in aqueous solutions and during the oxidation of the monomer and drop of pH at the surface, the equilibrium is shifted from the soluble DF‐ towards the insoluble HDF causing insoluble crystals of the drug to deposit on the surface of the polymer, which hinders further polymerisation. The incorporation of two insoluble medium sized drugs, NaIndo and NaSul, into the PPy film was also investigated. Deposition of PPy doped with either of the drugs in question was carried out in ethanol and tetrabutylammonium perchlorate (TBAP) was added to increase the conductivity. It was found that the PPy films formed in this organic medium were not as conducting as those formed in the aqueous solution, and doping levels were considerably lower than those previously reported for PPy. The addition of the TBAP introduces the small and mobile, ClO4‐, anion which is well known to dope PPy. UV‐visible spectroscopy was used to calculate that approximately 2.19 x 10‐6 mol cm‐2 of ClO4‐ was present in the polymer. However, this is quite minute compared to the estimated amount of drug doped within the polymer; 260 μmol cm‐2 and 60 μmol cm‐2 of Indo‐ and Sul‐, respectively. As seen with the other polymers the rate of release was controlled by the applied potential. Finally, the formation of PPy doped with a small soluble anionic drug, VPA‐, was studied. Although this was the smallest of all the drugs studied, it was not possible to incorporate this drug into the PPy membrane electrochemically. This was explained in terms of the solubility of the anion at low pH values. At pH values below 5.6 the equilibrium of the VPA‐ is shifted towards the insoluble HVPA. As the monomer is oxidised, there is a decrease in the local pH in the vicinity of the electrode and this causes the HVPA to precipitate from solution. This, in turn, prevents any PPy from being deposited at the electrode. Vapour phase polymerisation is offered as an alternative approach to immobilise this drug into the PPy film

Electrocatalytic and catalytic oxygen reduction utilising transition metal and heteroatom doped carbon materials

The commercialisation of polymer electrolyte fuel cells (PEFCs) is partly delayed due to the use of expensive and scarce precious metal catalysts for the oxygen reduction reaction (ORR). Transition metal and nitrogen containing carbon materials (M-N/C) could potentially replace Pt. However, the activity and stability is still too low. This is due to a lack in the understanding of the active site, a difficulty in determining intrinsic catalyst parameters and the challenges posed by the higher catalyst loading in fuel cells. A new precursor was identified. It can be doped with different metal centres and readily forms self-supporting ORR active carbon catalysts upon pyrolysis. Physicochemical characterisations of the Fe-N/C material suggests atomic metal centres as active sites. A proton coupled electron transfer is presumably the rate determining step. The catalyst is exceptionally poison tolerant against a wide range of compounds that affect Pt based materials. It was found that nitrite and nitric oxide interact with the active site(s). Fundamental insight was gained and it seems that two different types of metal centred active sites are present within Fe-N/C catalysts. A methodology was developed to electrochemically count one type of those active sites by means of reductive nitrite stripping in a conventional rotating disk electrode (RDE) setup. It is possible to estimate the turnover frequency and active site density. The material also catalyses the epoxidation of alkenes at room temperature and ambient pressure, suggesting a similar working principle as transition metal macrocycles. A study of M-N/C catalysts in operating PEFCs has been conducted. The catalyst layer was investigated by means of impedance spectroscopy. The peculiar 45 degree feature and its deviation in the impedance spectrum can be used to determine the optimal ionomer content in the catalyst layer and therefore speed up the investigation in single cells.Open Acces

Corrosion Behaviour of Advanced Fe-Based Bulk Metallic Glasses

Early developed non-bulk Fe-based glasses, e.g. Fe-Cr(-Mo)-metalloid(s), exhibit extraordinary corrosion resistance, but low glass formation ability (GFA). Newly developed bulk glass-forming Fe-based alloys have on the contrary high GFA, but also very different compositions and therefore their corrosion behaviour is expectedly not similar. Fundamental investigations regarding corrosion behaviour were performed for one of the most prominent bulk glassy alloy, namely (Fe44.3Cr5Co5Mo12.8Mn11.2C15.8B5.9)98.5Y1.5. Particularly, the free corrosion and the anodic polarization behaviour, the passivation ability and the pitting susceptibility have been assessed in electrolytes with varying pH values and anion species concentrations. Due to its monolithic single phase microstructure this alloy has a much lower corrosion rate in acids than a two-phase conventional steel (DIN X210Cr12) with much higher content of passivating Cr, i.e. 11.4 at. %. However, the high concentration of electrochemically active Mn and B as well as the unfavourably high Mo to Cr concentration ratio determine a higher corrosion rate of this bulk glassy alloy in strong alkalis and also a very poor passivation ability in acids. On the contrary, the high content of Mo has a positive influence on the pitting resistance by inhibiting very effectively the propagation of pits occurring at Y2O3 inclusions. Detailed microscopic analysis investigations by HRSEM and in-situ AFM revealed the formation of characteristic morphological features at the micro- and nanometre scale on the surface of samples exposed to acidic solutions. These were explained by selective dissolution of active elements, e.g. Mn, B. This study demonstrated the necessity to investigate the corrosion properties of newly developed bulk glass-forming Fe-based alloys – they are not per-se highly corrosion resistant, but their corrosion behaviour depends on their particular chemical composition.Früh entwickelte, nicht-massive amorphe Eisenbasislegierungen, z.B. Fe-Cr(-Mo)-Metalloid(e), zeigen bemerkenswerte Korrosionsbeständigkeit, aber niedrige Glasbildungsfähigkeit (englisch: glass-forming ability, GFA). Neu entwickelte massiv-glasbildende Eisenbasislegierungen haben im Gegenteil eine höhere GFA, aber auch sehr unterschiedliche Zusammensetzungen und deshalb ist ihr Korrosionverhalten ist wie zu erwarten nicht änlich. Grundlegende Untersuchungen des Korrosionsverhaltens einer der bekanntesten massiven amophen Legierung, nämlich (Fe44.3Cr5Co5Mo12.8Mn11.2C15.8B5.9)98.5Y1.5, wurden vorgenommen. Insbesondere wurde das Augenmerk auf das freie Korrosions- und das anodische Polarisationsverhalten, die Passivierungseigenschaften und die Anfälligkeit gegenüber Lochfraß in Elektrolyten mit verschiedenen pH-Werten und Anionenkonzentrationen gerichtet. Aufgrund ihres einphasig monolitischen Gefüges zeigt diese Legierung in Säuren eine viel niedrigere Korrosionsgeschwindigkeit als die eines zweiphasigen herkömmlichen Stahls (DIN X210Cr12) mit viel höherem Gehalt an passivierendem Cr, d.h. 11.4 at.%. Der höhere Gehalt an electrochemisch aktivem Mn und B sowie das nachteilige Verhältnis von Mo zu Cr Konzentration sind für eine höhere Korrosionsgeschwindigkeit dieser massiven amorphen Legierung in konzentrierten Alkalien sowie eine geringere Passivierungsfähigkeit in Säuren verantwortlich. Der hohe Gehalt an Mo hat jedoch einen positiven Einfluss auf die Lochfraßbeständigkeit – er hindert sehr wirksam das Wachstum der an Y2O3-Einschlüssen gebildeten Löcher. Detaillierte mikroskopische Untersuchungen durch HRSEM und in-situ AFM zeigten die Bildung charakteristischer Morphologien im Mikrometer- und Nanometerbereich auf der Oberfläche von Proben, die starken Säure ausgesetzt waren. Dieses wurde durch selektive Auflösung aktiver Elemente, z.B. Mn, B, erklärt. Diese Arbeit unterstreicht die Notwendigkeit, die Korrosionseigenschaften der neu entwickelten, massivglasbildenden Eisenbasislegierungen zu untersuchen – diese sind nicht per-se „hochkorrosionsbeständig“, stattdessen hängt ihr Korrosionsverhalten vielmehr von ihrer besonderen chemischen Zusammensetzung ab

Advances in vanadium and polyoxometalate redox flow batteries

PhD ThesisElectrochemical energy storage is one of the few options to store the energy from intermittent renewable energy sources like wind and solar. Redox flow batteries are such an energy storage system, which has favourable features over other battery technologies, for example, solid state batteries, due to their inherent safety and the independent scaling of energy and power content. However, because of their low energy density, low power density, and the cost of components such as redox species and membranes, commercialised RFB systems like the all-vanadium chemistry cannot make full use of the inherent advantages over other systems. This thesis shows a comparison of promising cell chemistries with the aim to elucidate which redox system is most favourable in terms of energy density, power density and capital cost. Additionally, the choice of solvent and the selection of inorganic or organic redox couples with the entailing consequences are discussed. The sluggish redox kinetics of the VO2+/VO2 + couple limit the power density of the VRFB, which increases the footprint of the power converters and increases capital costs. During recent years, much research has been carried out in the field of heterogeneous catalysis, but this is now approaching its limits. In this work, homogeneous catalysis was conducted to improve the system. The kinetics of the VO2+/VO2 + redox reaction have been investigated in 1M sulphuric and 1 M phosphoric acid by cyclic voltammetry, chronoamperometry, electrochemical impedance spectroscopy and flow battery tests. It was found that in 1 M phosphoric acid the electron transfer constant k0 is up to 67 times higher than in 1 M sulphuric acid and an over-potential dependent difference in electron transfer constant was observed and explained. This study shows that the redox kinetics of the VO2+/VO2 + can be considerably accelerated by altering the chemical environment of the vanadium ions, and that this effect can also be transferred into a flow battery. However, the prevailing technology, the all-vanadium system, comprises low energy and low power densities, therefore, the chemistry of polyoxometalates, [SiW12O40] 4- and [PV14O42] 9- , as nano-sized electron shuttles was investigated. It is shown that these POMs exhibit fast redox kinetics (electron transfer constant k0 ≈ 10-2 cm s-1 for [SiW12O40] 4- ), thereby enabling high power densities; in addition, they feature multi-electron transfer, realizing a high capacity per molecule; they do not cross cation exchange membranes, thus eliminating self-discharge through the separator; and they are chemically and electrochemically stable as shown by in- iv situ nuclear magnetic resonance spectroscopy. In flow battery studies the theoretical capacity (10.7 Ah L-1 ) could be achieved under operating conditions with Coulombic efficiency of 94%. Very small losses occurred due to residual oxygen in the system. Options to improve the energy density of the system are discussed.SIEMENS A

Bioanalytical applications of boron doped diamond electrodes

Maintaining pH homeostasis is one of the most important biological processes, and variations in tissue or fluid pH can often be related to diseases. Hence, detecting pH changes accurately could be crucial for early diagnosis. This thesis investigates electrochemical methods for accurate biological pH measurements. Of importance is characterising protein fouling on the electrode surfaces and the implications for accurate pH evaluation. Chapter 3 investigates the ex vivo pH profiling of the gastrointestinal (GI) tract using an electrochemical pH sensor. Also, the variation caused by omeprazole and melatonin, known to alter the pH in upper GI tract. Three pH electrodes, glass electrode, iridium oxide electrodeposited on boron doped diamond (BDD-IrOx), and quinone surface integrated BDD (BDD-Q), were assessed in a GI-tract mimic solution to determine suitability before ex vivo application. When assessing the electrodes in terms of response time, robustness and minimal electrode fouling, the BDD-Q was determined to be the most suitable sensor. Chapter 4 explores the effect of protein adsorption on: BDD, a BDD that has the whole surface laser ablated to sp2 carbon (sp2-BDD), and a glassy carbon. The performance of these electrodes was assessed in 0.5% mucin (GI-tract mimic), and 5% bovine serum albumin (most abundant protein in blood). Fouling was studied using three different methods: (i) Time-dependent change in cyclic voltammograms of metal redox couples (Ru(NH3)63+, IrCl62-); (ii) Changes in double layer capacitance in response to protein solutions; and (iii) the effect of applied electrode potential on protein adsorption and the subsequent silver electrodeposition. Chapter 5 describes the development of a 3D-printed flow cell capable of housing a BDD-Q pH electrode to conduct pH measurements under controlled flow. We hypothesise that flow can be used to reduce fouling from mucin and albumin. The BDD-Q sensor was flow-independent in the range from 0.1 to 100 mL/min. The pH of solutions containing proteins were measured under stationary and flow (100 mL/min) conditions, while alternating with buffer solution. Conducting pH measurements under flow improved the accuracy of the measurement. A similar experiment was conducted in blood, a more complex media with multiple proteins. Measurements conducted at 100 mL/min showed low variation in pH using the BDD-Q