Power from marine sediment fuel cells: the influence of anode material

The effect of anode material on the performance of microbial fuel cells (MFC), which utilise oxidisable carbon compounds and other components present in sediments on ocean floors, estuaries and other similar environments is reported. The MFC anode materials were carbon sponge, carbon cloth, carbon fibre, and reticulated vitreous carbon (RVC). Power was produced through the microbial activity at the anode in conjunction with, principally, oxygen reduction at a graphite cloth cathode. After a period of stabilisation, open circuit voltages up to 700 mV were observed for most cells. Steady state polarisations gave maximum power densities of 55 mW m⁻² using carbon sponge as the anode; which was nearly twice that achieved with carbon cloth. The latter material typically gave power densities of around 20 mW m⁻². The performance of the cell was reduced by operation at a low temperature of 5 °C. Generally, for cells which were capable of generating power at current densities of 100 mA m⁻² and greater, mass transport was found to limit both the anode and the cathode performance, due primarily to the low concentrations of electro-active species present or generated in cells.     

Studies of some operating parameters and cyclic voltammetry for a direct ethanol fuel cell

A direct ethanol fuel cell (DEFC) of 5 cm² membrane-electrode area was studied systematically by varying the catalyst loading, ethanol concentration, temperature and different Pt based electro-catalysts (Pt–Ru/C, Pt-black High Surface Area (HSA) and Pt/C). A combination of 2 M ethanol at the anode, pure oxygen at the cathode, 1 mg cm⁻² of Pt–Ru/C (40%:20%) as the anode and 1 mg cm⁻² of Pt-black as the cathode gave a maximum open circuit voltage (OCV) of 0.815 V, a short circuit current density of 27.90 mA cm⁻² and a power density of 10.3 mW cm⁻². The optimum temperatures of the anode and cathode were determined as 90 °C and 60 °C, respectively. The power density increased with increase in ethanol concentration and catalyst loading at the anode and cathode. However, the power density decreased slightly beyond 2 M ethanol concentration and 1 mg cm⁻² catalyst loading at the anode and cathode. These results were validated using cyclic voltammetry at single electrodes under similar conditions to those of the DEFC.     

In situ active chlorine generation for the treatment of dye-containing effluents

This study examined the possibility to remove colour causing-compounds from synthetic effluent by indirect electrochemical oxidation using iridium oxide anode electrodes. Using a high concentration of chloride ions (17.1 mM) and various current densities, it was possible to produce high concentration of active chlorine with a specific production rate of 2.8 mg min⁻¹ A⁻¹. The best performance for acid methyl violet 2B dye (MV2B) decomposition was obtained using Ti/IrO₂ anodes operated at a current density of 15 mA cm⁻² during 40 min of treatment in the presence of 3.42 mM of chloride ions. Under these conditions, more than 99% of MV2B was removed (with a reaction rate apparent constant of 0.20 min⁻¹), whereas COD and TOC removal were 51% and 75%, respectively. The electrolytic cell was then used for the degradation of three other synthetic dye solutions: Eosin yellowish (EOY), Trypan Blue (TRB), Acridine Orange (ACO). TRB was the most difficult dye to remove from solution with a value reaction rate constant of 0.12 min⁻¹, compared to 0.19 min⁻¹ and 0.24 min⁻¹ recorded for ACO and EOY dyes, respectively. More than 99% of these dyes were removed by electrochemical oxidation.     

DSA electrochemical treatment of olive mill wastewater on Ti/RuO<Subscript>2</Subscript> anode

The electrochemical oxidation of olive mill wastewater (OMW) over a Ti/RuO₂ anode was studied by means of cyclic voltammetry and bulk electrolysis and compared with previous results over a Ti/IrO₂ anode. Experiments were conducted at 300–1,220 mg L⁻¹ initial chemical oxygen demand (COD) concentrations, 0.05–1.35 V versus SHE and 1.39–1.48 V versus SHE potential windows, 15–50 mA cm⁻² current densities, 0–20 mM NaCl, Na₂SO₄, or FeCl₃ concentrations, 80 °C temperature, and acidic conditions. Partial and total oxidation reactions occur with the overall rate being near first-order kinetics with respect to COD. Oxidation at 28 Ah L⁻¹ and 50 mA cm⁻² leads to quite high color and phenols removal (86 and 84%, respectively), elimination of ecotoxicity, and a satisfactory COD and total organic carbon reduction (52 and 38%, respectively). Similar performance can be achieved at the same charge (28 Ah L⁻¹) using lower current densities (15 mA cm⁻²) but in the presence of various salts. For example, COD removal is less than 7% at 28 Ah L⁻¹ in a salt-free sample, while addition of 20 mM NaCl results in 54% COD reduction. Decolorization of OMW using Ti/RuO₂ anode seems to be independent of the presence of salts in contrast with Ti/IrO₂ where addition of NaCl has a beneficial effect on decolorization.     

Preparation and evaluation of RuO2–IrO2, IrO2–Pt and IrO2–Ta2O5 catalysts for the oxygen evolution reaction in an SPE electrolyzer

IrO₂–RuO₂, IrO₂–Pt and IrO₂–Ta₂O₅ electrocatalysts were synthesized and characterized for the oxygen evolution in a Solid Polymer Electrolyte (SPE) electrolyzer. These mixtures were characterized by XRD and SEM. The anode catalyst powders were sprayed onto Nafion 117 membrane (catalyst coated membrane, CCM), using Pt catalyst at the cathode. The CCM procedure was extended to different in-house prepared catalyst formulations to evaluate if such a method could be applied to electrolyzers containing durable titanium backings. The catalyst loading at the anode was about 6 mg cm⁻², whereas 1 mg cm⁻² Pt was used at the cathode. The electrochemical activity for water electrolysis was investigated in a single cell SPE electrolyzer at 80 °C. It was found that the terminal voltage obtained with Ir–Ta oxide was slightly lower than that obtained with IrO₂–Pt and IrO₂–RuO₂ at low current density (lower than 0.15 A cm⁻²). At higher current density, the IrO₂–Pt and IrO₂–RuO₂ catalysts performed better than Ir–Ta oxide.     

The influence of nanocrystalline state of iron on the corrosion inhibitor behavior in aqueous solution

Effect of grain size reduction on the electrochemical and corrosion behavior of iron of different grain sizes (32–320 nm) produced by direct and pulsed current electrodeposition was characterized using Tafel polarization curves and electrochemical impedance spectroscopy (EIS). The grain size of deposits was determined by X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM). The most intensive first-order peak (110) of the XRD patterns was taken for detailed analysis using a Gaussian fitting curve. The electrochemical tests were carried out in electrolyte 30 mg L⁻¹ NaCl + 70 mg L⁻¹ Na₂SO₄ + 250 mg L⁻¹ NaNO₂ aqueous solution. It was found that the corrosion potential and corrosion current density significantly changed as the microstructure morphology was changed. Results obtained from electrochemical tests suggested that the inhibition effect and corrosion protection of sodium nitrite inhibitor in near-neutral aqueous solutions increased as the grain size decreased from submicrocrystalline to nanocrystalline. This was attributed to the excess free energy, and concomitantly the increased number of the active sites caused by higher grain boundary and triple junction content in the nanocrystalline surface, which provides sites for electrochemical activity, and effect of sodium nitrite, was more pronounced.     

Micro-tubular solid oxide fuel cells fabricated from hollow fibres

Recent literature is reviewed on a phase inversion process followed by sintering, used to fabricate ceramic hollow fibres (HFs) as precursors to micro-tubular solid oxide fuel cells (MT-SOFCs) with sub-millimetre inner diameters. These aimed to address the outstanding technological and economic issues that have delayed mass production of SOFCs, by increasing electrode surface areas per unit volume relative to planar structures, increasing power outputs per unit volume/mass, facilitating sealing at high temperatures, and decreasing fabrication costs per kW. Some recent experimental results are presented of the effects of temperature, hydrogen flow rate, thermal cycling and time of NiO reduction with H₂ on the subsequent performance of 25 mm long H₂|Ni–CGO|CGO|LSCF|air MT-SOFCs, incorporating cerium–gadolinium oxide (CGO) electrolyte, nickel anodes and lanthanum strontium cobalt ferrite–CGO (LSCF–CGO) cermet cathodes, designed to operate at 500–600 °C. Maximum power densities of 3–5.5 kW m⁻² were achieved as the temperature was increased from 550–600 °C. The co-extruded MT-SOFCs were resilient to three thermal cycles when heated to operating temperature in ca. 5 min. Their performance was intimately related to the reduction time, suggesting slow conversion of the NiO to Ni within the fabricated anodes. At constant cell voltage, mass transport limited current densities increased from ca. 11 to ca. 13.5 kA m⁻² as hydrogen flow rates were increased from 15 to 60 cm³ min⁻¹, though had residual NiO in the anode been fully reduced, current densities would have been significantly greater.     

Studies on the oxidation rate of metallic inert anodes by measuring the oxygen evolved in low-temperature aluminium electrolysis

The rate of oxygen evolution on metallic inert anodes was measured as a function of current density during electrolysis of a low-melting NaF(12)–KF–AlF₃ bath ([NaF + KF]/[AlF₃] = 1.5 mol mol⁻¹) at 800 °C. The oxidation rate of the anode substrate (A cm⁻²) was calculated. The anode oxidation process was depressed at the potentials of oxygen evolution. The dynamics of the decrease in the oxidation rate, which were obtained in previous study by the change in geometrical size of the metallic part of the specimen, was reproduced both by the technique proposed and also in potentiostatic electrolysis at potentials below that of oxygen evolution, in some cases, depending on prepolarisation.     

Cathodic reaction kinetics and its implication on flow-assisted corrosion of aluminum alloy in aqueous ethylene glycol solution

The cathodic reaction kinetics and anodic behavior of Al alloy 3003 in aerated ethylene glycol–water solution, under well-controlled hydrodynamic conditions, were investigated by various measurements using a rotating disk electrode (RDE). The transport and electrochemical parameters for cathodic oxygen reduction were fitted and determined. The results demonstrate that the cathodic reaction is a purely diffusion-controlled process within a certain potential region. The experimentally fitted value of diffusion coefficient of oxygen is 3.0 × 10⁻⁸ cm² s⁻¹. The dependence of cathodic current on rotation speed was in quantitative agreement with Levich equation. At potentials more positive than the diffusion controlled region, the cathodic process was controlled by both diffusion and electrochemical kinetics. The electrochemical reaction rate constant, k ₀, was determined to be 1.1 × 10⁻⁹ cm s⁻¹. There is little effect of electrode rotation on anodic behavior of Al alloy during stable pitting. However, fluid hydrodynamics play a significant role in formation of the oxide film and the Al alloy passivity. An enhanced electrode rotation would increase the mass-transfer rate of solution, and thus the oxygen diffusion towards the electrode surface for reduction reaction. The generated hydroxide ions are favorable to the formation of Al oxide film on electrode surface.     

Mo oxide modified catalysts for direct methanol, formaldehyde and formic acid fuel cells

Pt black and PtRu black fuel cell anodes have been modified with Mo oxide and evaluated in direct methanol, formaldehyde and formic acid fuel cells. Mo oxide deposition by reductive electrodeposition from sodium molybdate or by spraying of the fuel cell anode with aqueous sodium molybdate resulted in similar performance gains in formaldehyde cells. At current densities below ca. 20 mA cm⁻², cell voltages were 350–450 mV higher when the Pt catalyst was modified with Mo oxide, but these performance gains decreased sharply at higher current densities. For PtRu, voltage gains of up to 125 mV were observed. Modification of Pt and PtRu back catalysts with Mo oxide also significantly improved their activities in direct formic acid cells, but performances in direct methanol fuel cells were decreased.     

Investigation of the response of separate electrodes in a polymer electrolyte membrane fuel cell without reference electrode

The paper presents electrochemical measurements carried out in a PEMFC with a view to determining the separate kinetics of the electrode reactions. For this purpose, the separate response of one electrode (anode or cathode) was magnified by dilution of the reacting gas, respectively hydrogen and oxygen, and comparison of the experimental data in the form of steady voltage-current variations and impedance spectra. Experiments were carried out at 60 °C and ambient pressure. Water management was thoroughly controlled so that the gases leaving the cell had the same relative humidity in all experiments of one series. Hydrogen oxidation, although rapid, corresponds to overpotentials up to 50 mV at high dilution rates and current densities. Assuming a Tafel–Volmer mechanism, the exchange current density of the anode reaction at the Pt surface is of the order of 1 mA cm⁻². The two techniques employed led to Tafel slopes of oxygen reduction ranging from 120 to 150 mV/decade, with an exchange current density near 1 μA cm⁻², in good agreement with published data.     

A parametric study of a platinum ruthenium anode in a direct borohydride fuel cell

Data on the performance of a direct borohydride fuel cell (DBFC) equipped with an anion exchange membrane, a Pt–Ru/C anode and a Pt/C cathode are reported. The effect of oxidant (air or oxygen), borohydride and electrolyte concentrations, temperature and anode solution flow rate is described. The DBFC gives power densities of 200 and 145 mW cm⁻² using ambient oxygen and air cathodes respectively at medium temperatures (60 °C). The performance of the DBFC is very good at low temperatures (ca. 30 °C) using modest catalyst loadings of 1 mg cm⁻² for anode and cathode. Preliminary data indicate that the cell will be stable over significant operating times.     

Underscale corrosion behavior of carbon steel in a NaCl solution using a new occluded cavity cell for simulation

A new occluded corrosion cavity (OCC) simulation cell was designed to study the underscale corrosion behavior of carbon steel (N80) in 0.2 mol L⁻¹ NaCl solution. The chemical components of the solution in the OCC were measured and the electrochemical behavior of the occluded anode and the bulk cathode were studied by electrochemical impedance spectroscopy (EIS). The newly designed OCC cell can easily simulate the auto-catalyzing acidification process and may be used to study the mechanism of underscale corrosion. The corrosion scale exacerbates the underscale corrosion and the area ratio of the bulk cathode to the occluded anode (R = S_c/S_a) determines the development of simulated localized corrosion in the OCC cell. When R was within a certain range, the corrosion rate in the OCC could be kept at a persistently high level. The pH of the solution in the OCC decreased and the chloride ions (Cl⁻) concentrated as the local corrosion developed. The anodic process on the occluded anode was controlled by irreversible charge transfer and the cathodic process on the bulk cathode was controlled mainly by oxygen diffusion.     

Experimental investigation of cell design for the electrolysis of iron oxide suspensions in alkaline electrolyte

Following feasibility studies of iron production by electrolytic reduction of hematite particles suspended in a strong alkaline medium, this article concerns the use of engineering methods to investigate the performance of various cell configurations, in view to designing larger processes: a horizontal flow cell with parallel electrodes and two rotating cylindrical electrodes were used for this purpose. The performance was analyzed in terms of current efficiency at 0.1 A cm⁻² and deposit morphology. The results reveal a negligible role of the mass transfer of Fe (+III) ions from the bulk electrolyte on the process efficiency, as formerly suggested in reaction mechanism studies. Conventional ion-mass transfer theory is therefore not applicable and another approach is proposed. Dispersed phase transport processes, more precisely the mechanical forces acting on both the 10 μm ore particles and the evolved oxygen bubbles, can quantitatively and qualitatively explain the cells performance. The configuration with the external rotating cathode allows both efficient contacts of the particles with the cathode and rapid removal of the produced gas phase; however the two rotating electrode devices are subject to appreciable ohmic losses due to the current lead system. The parallel plate configuration, with the cathode at the bottom appears as the best configuration for the deposition which can be achieved with low energy consumption.     

Electrochemical reduction of oxygen with iron phthalocyanine in neutral media

Recent interest in electricity production using microbial fuel cells makes it important to better understand O₂ reduction in neutral solutions with non-precious metal catalysts. Higher O₂ reduction activity was obtained using iron phthalocyanine supported on Ketjen black carbon (FePc-KJB) than with a platinum catalyst in neutral pH. At low overpotentials, a Tafel slope close to −0.06 V/dec in both acid and neutral pH suggested that the mechanism of O₂ reduction on FePc is not changed with the change of pH, and the reaction is mainly controlled by Fe^II/Fe^III redox couple. This behaviour gives us new insight into catalysis using FePc, and further supports the use of FePc as a promising catalyst for the oxygen reduction applications in neutral media.     

Recycling wood pulping chemicals by molten salt electrolysis: cyclic voltammetry of mixtures containing Na2CO3 and Na2SO4

We are investigating a novel electrochemical pathway for efficient recycling of inorganic chemicals from the kraft pulping process. To analyse this process, cyclic voltammetry was conducted on molten salts containing sodium carbonate, or sodium carbonate and sodium sulfate on gold and nickel/nickel oxide electrodes. Pure sodium carbonate at 860 °C was determined to exhibit oxidation to carbon dioxide and oxygen. Electrochemical reduction proceeded to both sodium metal and to sodium oxide and either carbon or carbon monoxide. A mixture with a 2.36:1 molar ratio of sodium carbonate to sodium sulfate at 860 °C was investigated where the anode again displayed carbonate oxidation. The cathode reactions are the reduction of sulfate ions to form sulfide and oxide ions and carbonate reduction to oxide ions and carbon monoxide. Separated cell operation to avoid oxide ion oxidation appears necessary according to the cyclic voltammetry. Nickel oxide was found to be a stable anode material in a sodium carbonate molten salt. This paper, based on cyclic voltammetry results of oxide ion production, is the first step towards a technical electrolysis process for recausticizing of molten smelt for the kraft pulping process.     

Models of Hypochlorite production in electrochemical reactors with plate and porous anodes

Pseudo two-dimensional finite element models were developed to predict the hypochlorite (chloric(I)) (HOCl + OCl⁻) production by electrolysis of near-neutral aqueous sodium chloride solution, in reactors with (a) an anode and cathode in the form of plates, and (b) a lead dioxide-coated graphite felt anode and titanium plate cathode. The model was used to investigate the feasibility of using a porous anode to achieve high single pass conversions in oxidising chloride ions. For the model reactor with planar anode, the effects of diffusion, migration and convection on the mass transport of the reacting species were considered, whereas with the porous anode, a supporting electrolyte (Na₂SO₄) was notionally present to eliminate the migrational contribution to reactant transport. For an electrolyte flow rate of 10⁻⁶ m³ s⁻¹ (Re = 10 for plate electrodes, Re _porous = 0.76 for porous anode), a cell voltage of 3.0 V and an inlet NaCl of 100 mol m⁻³, the single-pass conversion of Cl⁻ was predicted to increase from 0.45 for the reactor with a planar anode to 0.81 for the reactor with a porous anode. For the same operating conditions, the overall current efficiency was also predicted to increase from 0.71 to 0.77 by replacing the plate with the porous anode.     

High temperature water electrolysis using metal supported solid oxide electrolyser cells (SOEC)

Metal supported cells as developed according to the DLR SOFC concept by applying plasma deposition technologies were investigated for use as solid oxide electrolyser cells (SOEC) for high temperature steam electrolysis. Cells consisting of a porous ferritic steel support, a diffusion barrier layer, a Ni/YSZ hydrogen electrode, a YSZ electrolyte and a LSCF oxygen electrode were electrochemically characterised by means of i-V characteristics and electrochemical impedance spectroscopy measurements including a long-term test over 2000 h. The cell voltage during electrolysis operation at a current density of −1.0 A cm⁻² was 1.28 V at an operating temperature of 850 °C and 1.4 V at 800 °C. A long-term test run over 2000 h with a steam content of 43% at 800 °C and a current density of −0.3 A cm⁻² showed a degradation rate of 3.2% per 1000 h. The impedance spectra revealed a significantly enhanced polarisation resistance during electrolysis operation compared to fuel cell operation which was mainly attributed to the hydrogen electrode.     

Spherical natural graphite coated by a thick layer of carbonaceous mesophase for use as an anode material in lithium ion batteries

A method for coating a thick layer of carbonaceous mesophase was developed to treat spherical natural graphite (SNG) for use as anodes in lithium ion batteries. The carbonaceous mesophase layer was fabricated by heat treatment of a mixture of SNG and coal tar pitch. The thickness of the carbonaceous mesophase on the surface of the SNG was approximately 2.5 μm, which is effective for enhancing the strength of the carbonaceous mesophase shell and for allowing the shell to maintain good integrity at a high anode density (1.6 g cm⁻³). The mesophase layer increased the initial columbic efficiency from approximately 90% to 95%, dramatically improved the capacity retention and reduced the irreversible capacity by greatly decreasing the SNG surface area. The initial efficiency, cycle life and rate capability for the SNG anode covered by a thick mesophase layer gave comparable results as the mesocarbon microbeads (MCMB) anode, while the SNG anode reversible capacity of 341 mAh g⁻¹ was higher than that of MCMB, 319 mAh g⁻¹. Electrochemical measurements showed that SNG particles coated by a thick carbonaceous mesophase layer are strong candidates for use as possible anode materials in high energy density lithium ion batteries.     

Direct Biomass Fuel Cell (BMFC) with Anode/Catalyst Comprising a Nanocomposite of a Mesoporous n-Semiconductor Film and a Metal Thin Layer: A New Concept of Catalyst Design

Abstract  

We designed an efficient direct biomass fuel cell (BMFC) anode and prepared a nanocomposite [base electrode/mesoporous n-semiconductor (SC) thin film/metal thin layer]. A Pt thin layer was photodeposited onto a mesoporous 20-μm thick TiO₂ thin film having a roughness factor of 2000, which was coated on an F-doped tin oxide/glass base electrode (FTO). This anode/catalyst nanocomposite was efficient at decomposing aqueous solutions of glucose and other biomass-related compounds in combination with an O₂-reducing cathode the other side of which was exposed to ambient air. The nanocomposite exhibited sharp optimum conditions at the atomic ratio of Pt/Ti = 0.33 in the BMFC, generating high electrical power of 2 mW cm⁻² without any light irradiation or bias potential when using a 1 M glucose aqueous solution. This output power is 20 times as large as that generated by a mesoporous TiO₂ film anode under UV-light (18 mW cm⁻²) irradiation. At this ratio, the coated Pt specifically exhibited metallic luster, and its average Pt thickness on the mesoporous TiO₂ nanostructure was calculated to be 0.40 nm. The high BMFC activity was interpreted by the simultaneous Schottky-junction/Ohmic contact nature of the nanocomposite. Other biomass compounds such as sucrose, ethanol and polysaccharides were also effective as direct fuels for the BMFC. Immediately after soaking this composite anode without a cathode in a glucose aqueous solution, continuous evolution of H₂ bubbles was observed from the anode surface. The electrical power generation and H₂ production are easily changed by connecting and disconnecting a cathode, respectively. Based on a simple design and calculation, the present system with glucose fuel has the potential to construct a module stack of 2 kW m⁻³. Simultaneous material/energy circulation by using the BMFC with biomass and its waste fuel is proposed for application in future social systems.