Electrochemical Power Generation Research Articles

Biomass integrated gasification fuel cell systems–Concept development and experimental results

The link-up of wood gasification with high temperature solid oxide fuel cells (Biomass-Integrated Gasification Fuel Cell System, B-IGFC) is a promising approach to reach high electrical efficiencies in small-scale biomass fuelled combined heat and power plants (CHP). The main technical challenge is the adjustment of the three main system components gasification, gas processing and fuel cell. A B-IGFC concept has been developed based on the findings that producer gas originating from the updraft gasification of wood can be electrochemically converted in a SOFC whereby tars are degraded to hydrogen and carbon monoxide and contribute to the electrochemical reactions and power generation. Important unit operations of the B-IGFC system were characterised experimentally. During a long-term test of the complete B-IGFC demonstration unit, the gasifier and the catalytic partial oxidation could be operated without problems, delivering a fuel gas to the SOFC system with relatively constant composition and properties. Compared to methane operation, the SOFC system delivered approx. 40% less current. With the chosen operating conditions, carbon deposition was effectively prevented. Ash deposits were identified as major obstacle for a smooth SOFC system operation.

Chronopotentiometric Investigation of Borohydride Oxidation at a Gold Electrode

Sodium borohydride presents interesting options for electrochemical power generation acting either as a hydrogen source or an anodic fuel for direct borohydride fuel cells. Though there have been several papers concerning electrochemical determination of relevant thermodynamic and kinetic parameters to the borohydride system, there are many basic aspects that have not yet been systematically studied. This paper reports chronopotentiometric studies of the electro-oxidation of at a gold electrode in 2 M NaOH solutions at temperatures ranging from 25 to . Gold displays a rather good oxidation activity, and the overall oxidation process is shown to be irreversible involving a number of exchanged electrons close to the theoretically expected value of 8. For the specific potential and concentration ranges observed, the results suggest that the rate-determining step is an irreversible, diffusion-controlled, one-electron oxidation step for which several key kinetic parameters (α, , and ) are calculated.

In situ infrared (FTIR) study of the borohydride oxidation reaction

The direct borohydride fuel cell (DBFC) is an interesting alternative for the electrochemical power generation at lower temperatures due to its high anode theoretical specific capacity ( 5 A h g - 1 ) . However, the borohydride oxidation reaction (BOR) is a very complex eight-electron reaction, influenced by the nature of the electrode material (catalytic or not with respect to BH 4 - hydrolysis), the [ BH 4 - ] / [ OH - ] ratio and the temperature. In order to understand the BOR mechanism, we performed in situ infrared reflectance spectroscopy measurements (SPAIRS technique) in 1 M NaOH/1 M NaBH 4 with the aim to study intermediate reactions occurring on a gold electrode (a poor BH 4 - hydrolysis catalyst). We monitored several bands in B–H (1184 cm −1) and B–O bond regions (1326 and 1415 cm −1), appearing sequentially with increasing electrode polarisation. Thanks to these experimental findings, we propose possible initial elementary steps for the BOR.

Anodic Reactions of Borohydride in Sodium Hydroxide Solutions

Borohydrides present interesting options for electrochemical power generation acting either as hydrogen sources or anodic fuels for direct borohydride fuel cells and batteries. Though there have been several papers concerning electrochemical determination of relevant thermodynamic and kinetic parameters to the borohydride system, there are a number of basic aspects that have not been yet systematically studied. This paper reports chronopotentiometric studies of the electrooxidation of sodium borohydride at a gold sphere electrode in 2 M NaOH solutions, at temperatures ranging from 25 to 65 ºC. Gold displayed a rather good borohydride oxidation activity, and the overall oxidation process was shown to be irreversible involving a number of electrons very close to the theoretically expected value of 8. The results suggested that the rate-determining step is an irreversible, diffusion controlled, one-electron oxidation step, for which the transfer coefficients were calculated.

Chronopotentiometric Study of the Electrooxidation of Borohydride Anion in Alkaline Medium

Borohydrides present interesting options for electrochemical power generation acting either as hydrogen sources or anodic fuels for direct borohydride fuel cells and batteries. Though there have been several papers concerning electrochemical determination of relevant thermodynamic and kinetic parameters to the borohydride system, a number of its basic aspects have not been yet systematically studied. In this paper we report chronopotentiometric studies of the electrooxidation of sodium borohydride at a gold sphere electrode in 2M NaOH solutions, at temperatures ranging from 25 to 55 °C. Gold displayed a rather good BH4 - oxidation activity, and the overall oxidation process was shown to be irreversible involving a number of electrons very close to the theoretically expected value of 8. The results suggested that the rate-determining step is an irreversible, diffusion controlled, one-electron oxidation step, for which the transfer coefficients were calculated.

Electrochemical Power Generation Directly from Combustion Flame of Gases, Liquids, and Solids

The performance of a solid oxide fuel cell fueled with flames of combustible gases, liquids, and solids was studied. A cell structure in which cells were serially integrated within a disk was also examined. Open-circuit voltages indicated with a single cell and the integrated cell were ∼0.8 and ∼3.5 V, respectively. Maximum power density obtained with the flames of n-butane, kerosine, paraffin wax (candle), and wood were respectively 75, 65, 62, and 5 mW/cm2. The integrated cell gave a maximum power density of 318 mW/cm2 with n-butane flame. Fuel to air ratio distinctly influenced the output. © 2004 The Electrochemical Society. All rights reserved.

Highly Ion Conductive Poly(ethylene oxide)-Based Solid Polymer Electrolytes from Hydrogen Bonding Layer-by-Layer Assembly

We report the development of a solid polymer electrolyte film from hydrogen bonding layer-by-layer (LBL) assembly that outperforms previously reported LBL assembled films and approaches battery integration capability. Films were fabricated by alternating deposition of poly(ethylene oxide) (PEO) and poly(acrylic acid) (PAA) layers from aqueous solutions. Film quality benefits from increasing PEO molecular weight even into the 10(6) range due to the intrinsically low PEO/PAA cross-link density. Assembly is disrupted at pH near the PAA ionization onset, and a potential mechanism for modulating PEO:PAA ratio within assembled films by manipulating pH is discussed. Ionic conductivity of 5 x 10(-5) S/cm is achievable after short exposure to 100% relative humidity (RH) for plasticization. Adding free ions by exposing PEO/ PAA films to lithium salt solutions enhanced conductivity to greater than 10(-5) S/cm at only 52% RH and tentatively greater than 10(-4) S/cm at 100% RH. The excellent stability of PEO/PAA films even when exposed to 1.0 M salt solutions led to an exploration of LBL assembly with added electrolyte present in the adsorption step. Fortuitously, the modulation of PEO/PAA assembly by ionic strength is analogous to that of electrostatic LBL assembly and can be attributed to electrolyte interactions with PEO and PAA. Dry ionic conductivity was enhanced in films assembled in the presence of salt as compared to films that were merely exposed to salt after assembly, implying different morphologies. These results reveal clear directions for the evolution of these promising solid polymer electrolytes into elements appropriate for electrochemical power storage and generation applications.

CO 2 removal from air for alkaline fuel cells operating with liquid hydrogen — Heat exchanger development

Two independent problems when using hydrogen as an energy source in vehicles are: (i) gaseous hydrogen boil-off from stored liquid hydrogen, its associated pressure rise and hence necessary venting; and (ii) CO 2 removal from air for hydrogen-air alkaline fuel cells. An experimental program has been undertaken to see if in a system using an alkaline fuel cell running on hydrogen stored as liquid, both these problems may be solved concurrently. For vehicular hydrogen storage the most suitable system in terms of low mass and volume, is cryogenic storage of liquid hydrogen. Unfortunately, with liquid hydrogen there is always a small amount of boil-off, which necessitates venting of gaseous hydrogen from the storage vessel. Vented boil-off is hazardous and wasteful, and so it is essential that it be rendered safe, and preferably utilized efficiently. For electrochemical power generation, alkaline fuel cells offer the highest efficiency, as well as the possibility of using electrocatalysts that do not include platinum. However, since they operate at low temperatures they do not support hydrogen production by reformation of carbonaceous fuels. Further, they are rendered inoperative by the CO 2 content of the (reformed) fuel and or air. Their application to vehicular use is impaired by this inability to use other than pure hydrogen and complexity of on-board CO 2 removal. This paper describes a novel and simple process whereby H 2 boil-off can be used to continuously sublime and remove CO 2 from air, prior to the purified air and hydrogen being supplied to an alkaline fuel cell. It includes a schematic description of the process and an energy balance for refrigeration purification for the CO 2 removal. It is shown that the process relies on high effectiveness heat exchangers and water re-vaporization. An overview of thermal analysis of matrix heat exchangers is presented along with experimental results of heat exchanger effectiveness tests, which indicate that heat exchangers of the required effectiveness are easily designed and manufactured.

Electron delocalisation during oxidation-reduction cycle offad: Design and fabrication of a coenzyme immobilized bioanode

A biochemical fuel cell is an electrochemical power generation device that converts the chemical energy of a fuel (alcohol, glucose, hydrocarbons, etc) directly into electrical energy through enzyme-catalysed oxidation-reduction reactions. These systems possess several advantages over the conventional processes and are ideal for rural electrification in conjunction with biogas plants. The major bottleneck in the design of such power sources is the electron transport from the substrate to the electrode. Biochemical systems which use coenzymes such asfad ornad seem to be the most promising in circumventing these difficulties. We have made systematic molecular orbital studies of the electron-flow diagrams of riboflavin during its oxidation-reduction cycle and it is possible to obtain very efficient electron transport from the coenzyme to the electrodes by immobilising flavin through semiconducting side chains, at certain selected positions, to electrodes such as graphite. Based on these studies, we have immobilisedfad on graphite electrodes. The chemical steps involve creating active centres on graphite which are reacted withfad to form covalent linkages between the electrode and the coenzyme. Cyclic voltamograms of the modified electrode show thatfad is active and undergoes the expected redox cycles. We hope that such electrodes will form suitable bioanodes forfad-linked enzymatic reactions.