Előd L. Gyenge

Characterizing the Structural Degradation in a PEMFC Cathode Catalyst Layer: Carbon Corrosion

In this study, the structural degradation of a polymer electrolyte membrane fuel cell (PEMFC) cathode catalyst layer due to carbon corrosion was investigated. To oxidize the catalyst carbon support, the PEMFC catalyst layer was subjected to a 30 h accelerated stress test that cycled the cathode potential from 0.1 to 1.5 V RHE (where RHE denotes reversible hydrogen electrode) at 30 and 150 s intervals. Carbon dioxide release was measured in the gas exhaust to establish the rate and amount of carbon loss. Cyclic voltammetry, electrochemical impedance spectroscopy (EIS), scanning electron microscopy, and polarization analyses were completed to characterize and correlate the structural degradation of the catalyst layer to the PEMFC performance. The results showed a clear thinning of the cathode catalyst layer and the gas diffusion layer carbon sublayer and a reduction in the effective platinum surface area due to carbon support oxidation. The degradation of the cathode catalyst layer also altered the water management, as evidenced by an increase in the voltage losses associated with oxygen mass transport and catalyst layer ohmic resistance. There was an emphasis on the EIS measurement to further develop and verify this methodology for other degradation mechanisms.

Ionomer Degradation in Polymer Electrolyte Membrane Fuel Cells

The structural degradation of the polymer electrolyte in both the bulk membrane and the cathode catalyst layer (CL) was investigated. An accelerated stress test (AST) was developed to degrade the ionomer in two membrane electrode assembly (MEA) designs, with cathode catalyst structures comprised of 23 and 33 wt % Nafion, respectively. During the AST the air cathode potential was held at 1.0 V RHE (where RHE is reference hydrogen electrode) at 90°C and 100% relative humidity for up to 440 h. The MEA with 33 wt % Nafion had a greater platinum content in the membrane and a higher fluoride washout rate, suggesting the higher ionomer content in the cathode CL facilitated the mass transfer of contaminants (such as dissolved platinum) into the membrane. It is proposed that H 2 O 2 was produced at the anode, diffused into the membrane, and decomposed at the platinum and iron sites bound in the membrane structure. The decomposition products attacked the ionomer both in the bulk phase and CL, causing (i) membrane thinning, which exacerbated the H 2 crossover, (ii) lower membrane conductivity, and (iii) CL structure degradation manifested by enhanced reaction penetration depth into the CL and decreased effective oxygen diffusivity due to the changes in CL water content. These effects acting in synergy had profound negative repercussions on the fuel cell polarization for the MEA with 33 wt % Nafion in the cathode CL.

Influence of surfactants on the electroreduction of oxygen to hydrogen peroxide in acid and alkaline electrolytes

The effect of surfactants on the electroreduction of O2 to H2O2 was investigated by cyclic voltammetry and batch electrolysis on vitreous carbon electrodes. The electrolytes were either 0.1 M Na2CO3 or 0.1 M H2SO4 at 295 K, under 0.1 MPa O2. Electrode kinetics and mass transport parameters showed the influence of surfactants on the O2 electroreduction mechanism. The cationic surfactant (Aliquat 336®, tricaprylmethylammonium chloride), at mM levels, increased the standard rate constant of O2 electroreduction to H2O2 15 times in Na2CO3 and 1900 times in H2SO4, to 1.8 × 10−6 m s−1 and 9.9 × 10−10 m s−1, respectively. This effect on the reaction rate might be due to an increase of the surface pH, induced by the Aliquat 336® surface film. The nonionic (Triton X-100) and anionic (sodium dodecyl sulfate) surfactants retarded the O2 electroreduction, presumably by forming surface structures, which blocked the access of O2 to the electrode. Ten hour batch electrosynthesis experiments performed at 300 A m−2 superficial current density, 0.1 MPa O2, 300 K, on reticulated vitreous carbon (30 ppi), showed that compared to the values obtained in the absence of surfactant, mM concentrations of Aliquat 336® increased the current efficiency for peroxide from 12% to 61% (0.31 M H2O2) in 0.1 M Na2CO3 and from 14% to 55% (0.26 M H2O2) in 0.1 M H2SO4, respectively.

Electrosynthesis of hydrogen peroxide in acidic solutions by mediated oxygen reduction in a three-phase (aqueous/organic/gaseous) system Part II: Experiments in flow-by fixed-bed electrochemical cells with three-phase flow

The second part of the work concerned with mediated electrosynthesis of H2O2 in acidic solutions (pH 3) deals with investigations using divided flow-by fixed bed electrochemical cells operated with co-current three-phase flow (aqueous/organic emulsion and O2 gas at 0.1 MPa). Graphite felt (GF) and reticulated vitreous carbon (RVC) were evaluated as cathodes at superficial current densities up to 3000 A m−2. Typically, at current densities above 600 A m−2 graphite felt yielded higher peroxide concentrations per pass and current efficiencies, most likely due to the almost an order of magnitude higher organic liquid to solid mass transfer capacity for 2-ethyl-9,10-anthraquinone (EtAQ) mediator, that is, 0.13 s−1 in the case of GF vs 0.015 s−1 for RVC with 39 ppc (pores per cm). Factorial experiments revealed a positive interaction effect between superficial current density and emulsion load with respect to the current efficiency for H2O2 electrosynthesis. Thus, at the highest investigated superficial current density of 3000 A m−2, the current efficiency was 84% when the emulsion load was at the highest explored level of 11.7 kg m−2 s−1, whilest for the lowest level of emulsion load, 2.8 kg m−2 s−1, the current efficiency for H2O2 was 18%. Furthermore, the presence of 1 mM cationic surfactant, tricaprylmethylammonium chloride (CH3(C8H17)3N+Cl−, A336), had a positive main effect of about 12% on H2O2 current efficiency and there was also a positive synergistic effect between surfactant and emulsion load, estimated at about 7%. The aqueous to organic phase volume ratio, in the range of 0.9/1 and 3/1, had a statistically insignificant effect on the current efficiency for H2O2 generation. A decrease of the aqueous to organic phase volume ratio from 3 to 0.9 increased the cell voltage from about 6.5 to 7.3 V.

Electrosynthesis attempts of tetrahydridoborates

The aim of the present study was to verify the possibility (claimed by a number of patents) of electroreduction of borate compounds to BH 4 - under diverse experimental conditions. No measurable amount of BH 4 - are obtained

Electrochemically exfoliated graphene anodes with enhanced biocurrent production in single-chamber air-breathing microbial fuel cells

Microbial fuel cells (MFCs) present promising options for environmentally sustainable power generation especially in conjunction with waste water treatment. However, major challenges remain including low power density, difficult scale-up, and durability of the cell components. This study reports enhanced biocurrent production in a membrane-free MFC, using graphene microsheets (GNs) as anode and MnOx catalyzed air cathode. The GNs are produced by ionic liquid assisted simultaneous anodic and cathodic electrochemical exfoliation of iso-molded graphite electrodes. The GNs produced by anodic exfoliation increase the MFC peak power density by over 300% compared to plain carbon cloth (i.e., 2.85Wm(-2) vs 0.66Wm(-2), respectively), and by 90% compared to conventional carbon black (i.e., Vulcan XC-72) anode. These results exceed previously reported power densities for graphene-containing MFC anodes. The fuel cell polarization results are corroborated by electrochemical impedance spectroscopy indicating three times lower charge transfer resistance for the GN anode. Material characterizations suggest that the best performing GN samples were of relatively smaller size (~500nm), with higher levels of ionic liquid induced surface functionalization during the electrochemical exfoliation process.

Electrosynthesis of hydrogen peroxide in acidic solutions by mediated oxygen reduction in a three-phase (aqueous/organic/gaseous) system Part I: Emulsion structure, electrode kinetics and batch electrolysis

The mediated electrosynthesis of H2O2 in acidic solutions (pH 0.9–3.0) was investigated in a three-phase, aqueous/organic/gaseous system using 2-ethyl-9,10-anthraquinone (EtAQ) as mediator (redox catalyst). The main hydrogen peroxide producing route is the in situ mediating cycle: EtAQ electroreduction–homogeneous oxidation of anthrahydroquinone (EtAQH2). The organic phase was composed of tributylphosphate solvent (TBP) with 0.2 M tetrabutylammonium perchlorate (TBAP) supporting electrolyte, 0.06 M tricaprylmethylammonium chloride (A336) surface active agent, and 0.1–0.2 M EtAQ mediator. Part I of this two part work deals with the physico-chemical characteristics of the emulsion electrolyte (e.g., ionic conductivity, emulsion type, H2O2 partition between the aqueous and organic phases), and kinetic aspects (both electrode and homogenous) of the mediation cycle. Furthermore, batch electrosynthesis experiments are presented employing reticulated vitreous carbon cathodes (specific surface area 1800 m2 m−3) operated at superficial current densities of 500–800 A m−2. During 10 h batch electrolysis involving the emulsion mediated system with O2 purge at 0.1 MPa pressure, H2O2 concentrations in the range 0.53–0.61 M were obtained in 0.1 M H2SO4 (pH 0.9) and 2 M Na2SO4(acidified to pH3). The corresponding apparent current efficiencies were from 46 to 68%. Part II of the present work describes investigations using flow-by fixed-bed electrochemical cells with co-current upward three-phase flow.

Platinum- and membrane-free swiss-roll mixed-reactant alkaline fuel cell.

Eliminating the expensive and failure-prone proton exchange membrane (PEM) together with the platinum-based anode and cathode catalysts would significantly reduce the high capital and operating costs of low-temperature (<373 K) fuel cells. We recently introduced the Swiss-roll mixed-reactant fuel cell (SR-MRFC) concept for borohydride-oxygen alkaline fuel cells. We now present advances in anode electrocatalysis for borohydride electrooxidation through the development of osmium nanoparticulate catalysts supported on porous monolithic carbon fiber materials (referred to as an osmium 3D anode). The borohydride-oxygen SR-MRFC operates at 323 K and near atmospheric pressure, generating a peak power density of 1880 W m(-2) in a single-cell configuration by using an osmium-based anode (with an osmium loading of 0.32 mg cm(-2)) and a manganese dioxide gas-diffusion cathode. To the best of our knowledge, 1880 W m(-2) is the highest power density ever reported for a mixed-reactant fuel cell operating under similar conditions. Furthermore, the performance matches the highest reported power densities for conventional dual chamber PEM direct borohydride fuel cells.

Electrochemically Produced Graphene for Microporous Layers in Fuel Cells.

The microporous layer (MPL) is a key cathodic component in proton exchange membrane fuel cells owing to its beneficial influence on two-phase mass transfer. However, its performance is highly dependent on material properties such as morphology, porous structure, and electrical resistance. To improve water management and performance, electrochemically exfoliated graphene (EGN) microsheets are considered as an alternative to the conventional carbon black (CB) MPLs. The EGN-based MPLs decrease the kinetic overpotential and the Ohmic potential loss, whereas the addition of CB to form a composite EGN+CB MPL improves the mass-transport limiting current density drastically. This is reflected by increases of approximately 30 and 70 % in peak power densities at 100 % relative humidity (RH) compared with those for CB- and EGN-only MPLs, respectively. The composite EGN+CB MPL also retains the superior performance at a cathode RH of 20 %, whereas the CB MPL shows significant performance loss.

Synthesis and Characterization of Diverse Pt Nanostructures in Nafion

With the aid of TEM characterization, we describe two distinct Pt nanostructures generated via the electroless reduction of Pt(NH3)4(NO2)2 within Nafion. Under one set of conditions, we produce bundles of Pt nanorods that are 2 nm in diameter and 10-20 nm long. These bundled Pt nanorods, uniformly distributed within 5 μm of the Nafion surface, are strikingly similar to the proposed hydrated nanomorphology of Nafion, and therefore strongly suggestive of Nafion templating. By altering the reaction environment (pH, reductant strength, and Nafion hydration), we can also generate nonregular polyhedron Pt nanoparticles that range in size from a few nanometers in diameter up to 20 nm. These Pt nanoparticles form a dense Pt layer within 100-200 nm from the Nafion surface and show a power-law dependence of particle size and distribution on the distance from the Nafion membrane surface. Control over the distribution and the type of Pt nanostructures in the surface region may provide a cost-effective, simple, and scaleable pathway for enhancing manufacturability, activity, stability, and utilization efficiency of Pt catalysts for electrochemical devices.