Jeremy J. Pietron

The influence of acidity on microbial fuel cells containing Shewanella oneidensis

Microbial fuel cells (MFCs) traditionally operate at pH values between 6 and 8. However, the effect of pH on the growth and electron transfer abilities of Shewanella oneidensis MR-1 (wild-type) and DSP10 (spontaneous mutant), bacteria commonly used in MFCs, to electrodes has not been examined. Miniature MFCs using bare graphite felt electrodes and nanoporous polycarbonate membranes with MR-1 or DSP10 cultures generated >8W/m(3) and approximately 400muA between pH 6-7. The DSP10 strain significantly outperformed MR-1 at neutral pH but underperformed at pH 5. Higher concentrations of DSP10 were sustained at pH 7 relative to that of MR-1, whereas at pH 5 this trend was reversed indicating that cell count was not solely responsible for the observed differences in current. S. oneidensis MR-1 was determined to be more suitable than DSP10 for MFCs with elevated acidity levels. The concentration of riboflavin in the bacterial cultures was reduced significantly at pH 5 for DSP10, as determined by high performance liquid chromatography (HPLC) of the filter sterilized growth media. In addition, these results suggest that mediator biosynthesis and not solely bacterial concentration plays a significant role in current output from S. oneidensis containing MFCs.

Characterization of electrochemically active bacteria utilizing a high‐throughput voltage‐based screening assay

Metal reduction assays are traditionally used to select and characterize electrochemically active bacteria (EAB) for use in microbial fuel cells (MFCs). However, correlating the ability of a microbe to generate current from an MFC to the reduction of metal oxides has not been definitively established in the literature. As these metal reduction assays may not be generally reliable, here we describe a four‐ to nine‐well prototype high throughput voltage‐based screening assay (VBSA) designed using MFC engineering principles and a universal cathode. Bacterial growth curves for Shewanella oneidensis strains DSP10 and MR‐1 were generated directly from changes in open circuit voltage and current with five percent deviation calculated between each well. These growth curves exhibited a strong correlation with literature doubling times for Shewanella indicating that the VBSA can be used to monitor distinct fundamental properties of EAB life cycles. In addition, eight different organic electron donors (acetate, lactate, citrate, fructose, glucose, sucrose, soluble starch, and agar) were tested with S. oneidensis MR‐1 in anode chambers exposed to air. Under oxygen exposure, we found that current was generated in direct response to additions of acetate, lactate, and glucose. Biotechnol. Bioeng. 2009;102: 436–444.

Enhanced Oxygen Reduction Activity in Acid by Tin-Oxide Supported Au Nanoparticle Catalysts

Gold nanoparticles supported on hydrous tin-oxide (Au-SnO{sub x}) are active for the four-electron oxygen reduction reaction in an acid electrolyte. The unique electrocatalytic of the Au-SnO is confirmed by the low amount of peroxide detected with rotating ring-disk electrode voltammetry and Koutecky-Levich analysis. In comparison, 10 wt % Au supported on Vulcan carbon and SnO{sub x} catalysts both produce significant peroxide in the acid electrolyte, indicating only a two-electron reduction reaction. Characterization of the Au-SnO{sub x} catalyst reveals a high-surface area, amorphous support with 1.7 nm gold metal particles. The high catalytic activity of the Au-SnO is attributed to metal support interactions. The results demonstrate a possible path to non-Pt catalysts for proton exchange membrane fuel cell cathodes.

Graphitic biochar as a cathode electrocatalyst support for microbial fuel cells

Graphitic biochar (BC) was generated using high temperature gasification and alkaline post-treatment (BCw) of wood-based biomass. The BCw was evaluated as a manganese oxide electrocatalytic support (MnO/BCw) and microbial fuel cell (MFC) air cathode. Nano-structured MnO2 crystals were successfully immobilized on biomass-based graphitic sheets and characterized using physical, chemical, and electrochemical analyses. Cyclic voltammetry of MnO/BCw/Nafion inks showed electrochemical features typical of β-MnO2 with a current density of 0.9 mA cm(-2). BC showed satisfactory maximum power densities of 146.7 mW m(-2) (BCw) and 187.8 W m(-2) (MnO/BCw), compared with Vulcan Carbon (VC) (156.8 mW m(-2)) and manganese oxide VC composites (MnO/VC) (606.1 mW m(-2)). These materials were also tested as oxygen reduction reaction (ORR) catalysts for single chamber MFCs inoculated with anaerobic sludge. Our results demonstrate that BC can serve as an effective, low cost, and scalable material for MFC application.

The utility of Shewanella japonica for microbial fuel cells

Shewanella-containing microbial fuel cells (MFCs) typically use the fresh water wild-type strain Shewanella oneidensis MR-1 due to its metabolic diversity and facultative oxidant tolerance. However, S. oneidensis MR-1 is not capable of metabolizing polysaccharides for extracellular electron transfer. The applicability of Shewanella japonica (an agar-lytic Shewanella strain) for power applications was analyzed using a diverse array of carbon sources for current generation from MFCs, cellular physiological responses at an electrode surface, biofilm formation, and the presence of soluble extracellular mediators for electron transfer to carbon electrodes. Critically, air-exposed S. japonica utilizes biosynthesized extracellular mediators for electron transfer to carbon electrodes with sucrose as the sole carbon source.

Direct methanol oxidation at low overpotentials using Pt nanoparticles electrodeposited at ultrathin conductive RuO2 nanoskins

Small (primarily 2–4 nm) Pt nanoparticles electrodeposited at Ti-supported RuO2 nanoskins, designated as Pt/RuO2(Ti), are highly active for electrocatalytic oxidation of methanol (CH3OH) in acid electrolyte, with peak potentials among the lowest reported anywhere for RuO2-supported Pt. The Pt-modified RuO2 nanoskin is equally effective for CH3OH oxidation whether one or both sides of the Ti-foil substrate is coated or whether the RuO2 nanoskin is electrolessly deposited at the Ti substrate as a single layer or in multiple layers. Current densities for methanol oxidation are ∼2× lower than previously reported, owing to a bimodal distribution of both highly active (2–4 nm) and less active larger (> 5 nm) Pt particles. The methods reported here comprise a means of expressing highly active, nanostructured, bifunctional electrocatalytic coatings on substrates of essentially any geometry and minimizing the quantity of RuO2 necessary.

Electrochemical Observation of Ligand Effects on Oxygen Reduction at Ligand-Stabilized Pt Nanoparticle Electrocatalysts

We investigate the effect of triphenylphosphine triphosphonate, TPPTP, on the electrocatalytic activity for the oxygen reduction reaction (ORR) at ligand-stabilized Pt nanoparticles (Pt 2.1 nm - TPPTP). The Pt 2.1 nm - TPPTP catalyst supported on Vulcan carbon (VC) (Pt 2.1 mm - TPPTP/VC) adsorbs and desorbs oxygen +40 mV vs the potential for bare Pt nanoparticles supported on VC (Pt 2.4 nm /VC), as measured by cyclic voltammetry in Ar-saturated 0.1 M HClO 4 . The area-specific ORR activity (i s ) of Pt in the Pt 2.1 - TPPTP/VC catalyst is -22% higher than that of Pt 2.4 nm /VC, as measured by rotating disk electrode voltammetry. These results suggest that the TPPTP ligand weakens the Pt-O bond, resulting in the observed kinetic enhancement of the ORR.

Electrochemically induced surface modification of titanols in a `nanoglued' titania aerogel–silica aerogel composite film

Titania aerogel, thermally processed into its anatase form, has been bound as submicrometer particles to an electrode with silica sol. The resulting composite gel is supercritically dried to create a titania–silica composite aerogel film on the conducting substrate. Titania aerogel particles present in the mesoporous film are electrochemically addressable. In organic electrolyte containing isopropanol and added water, the electrochemical response negative of the flatband potential of TiO2 indicates that electrogenerated reactants (hydroxides and isopropoxides) induce condenzation reactions with surface titanols (Ti–OH). The data suggest that titania aerogels can be electrochemically modified in a controllable way with a wide variety of compounds that have reducible –OH moieties.

Dual-Pathway Kinetics Assessment of Sulfur Poisoning of the Hydrogen Oxidation Reaction at High Surface-Area Platinum/Vulcan Carbon Electrodes

The kinetics of the hydrogen oxidation reaction (HOR) were measured at high surface-area platinum/Vulcan carbon (Pt/VC) films before and after exposure to aqueous solutions of either sodium sulfide (Na 2 S) or mercaptopropionic acid (MPA, HSCH 2 CH 2 COOH) using rotating disk electrode (RDE) voltammetry. At 45―90% blocking of surface Pt atoms by inorganic sulfur, sulfur appears to adsorb as oligomerized sulfur (S n ). Current―potential RDE data for the HOR at Na 2 S-exposed Pt/VC films were fitted to a recently developed dual-pathway kinetics model. At a submonolayer S n coverage of surface Pt atoms, the exchange current densities for the potential-independent Tafel―Volmer pathway, or two-site HOR pathway, decrease by about 10―50-fold compared to electrochemically cleaned Pt/VC films. Exchange current densities for the single-site, potential-dependent Heyrovsky―Volmer pathway decrease by comparable factors at the same levels of sulfur exposure. The potential-range constant γ increases in the presence of adsorbed S n . The exposure of Pt/VC to aqueous solutions of MPA, an organosulfur species, results in similarly diminished HOR exchange current densities at Pt/VC, as observed for Na 2 S exposure, but yields higher values of γ. The model is sufficiently sensitive to delineate the effects of different catalyst poisons on HOR kinetics at carbon-supported nanoscopic Pt.

Electrocatalysis at Co–poly(difluoropyrrole) electrodeposited on Vulcan carbon supports: demonstration of halogenated polypyrrole as an electrocatalytic material

Halogenated polypyrroles are rendered significantly more stable against oxidative degradation than native polypyrrole and the electron-withdrawing substituents also permit tuning of the electronic state of coordinated catalytic metal ions. Poly(difluoropyrrole), or poly(dfp), was deposited on high-surface-area Vulcan carbon (VC) via bulk electrolytic oxidation of the monomer 3,4-difluoropyrrole at VC slurried in (n-C4H9)4NClO4/acetonitrile; the carbon-supported polymer was subsequently metallated with cobalt(II) ions. As determined by thermal, micrographic, elemental, and chemical state analyses, the Co-poly(dfp)/VC composite is ∼7 wt% poly(dfp) with the polymer inhomogeneously deposited over the Vulcan carbon, and the incorporated Co(II) ions coordinated by nitrogen atoms in the pyrrole moieties. Films of Co-poly(dfp)/VC cast on glassy carbon disk electrodes are electrocatalytically active for the two-electron reduction of oxygen in 0.1 M aqueous perchloric acid as assessed by hydrodynamic voltammetry, thereby demonstrating the viability of halogenated polypyrroles as electrocatalytic materials.