Investigation of a proton-conducting SOFC with internal autothermal reforming of methane

This study presents a performance analysis of a proton-conducting SOFC (SOFC-H⁺) with internal reforming of methane. The autothermal reforming within the SOFC-H⁺ stack is considered to be a potential solution of the carbon formation problem facing in operation of internal steam reforming SOFC-H⁺. A one-dimensional, steady-state model of the SOFC-H⁺ coupled with a detailed electrochemical model is employed to investigate its performance in terms of power density and fuel cell efficiency. The simulation results show that when SOFC-H⁺ is operated under an autothermal reforming environment, the presence of carbon monoxide, which is a major cause of carbon formation, in the fuel cell stack decreases. Effect of key operating parameters, such as temperature, steam-to-carbon and oxygen-to-carbon feed ratios, current density and fuel utilization, on the SOFC-H⁺ performance in terms of electrical efficiencies and energy demand is also investigated. The results indicate that operating temperatures have strong influence on SOFC-H⁺ performance, carbon monoxide production and heat generation.     

Development of electrochemical cell with layered composite of the Gd-doped ceria/electronic conductor system for generation of H2–CO fuel through oxidation–reduction of CH4–CO2 mixed gases

This paper shows the recent results on the development of layered composite promoting two types of electrochemical reactions (oxidation and reduction) in one cell. This cell consisted of porous Ni–Gd-doped (GDC) ceria cathode/thin porous GDC electrolyte (50 μm)/porous SrRuO₃–GDC anode. The external electric current was flowed in this cell at the electric field strength of 1.25 and 6.25 V/cm. The mixed gases of CH₄ (30–70%) and CO₂ (70–30%) were fed at the rate of 50 ml/min to the cell heated at 400–800 °C under the electric field. In the cathode, CO₂ was reduced to CO (CO₂ + 2e^? → CO + O^2?) and the formed CO and O^2? ions were transported to the anode through the pores and surface and interior of grains of GDC film. On the other hand, CH₄ was oxidized in the anode to form CO and H₂ through the reaction with diffusing O^2? ions (CH₄ + O^2? → CO + 2H₂ + 2e^?). As a result, H₂–CO mixed fuel was produced from the CH₄–CO₂ mixed gases (CH₄ + CO₂ → 2H₂ + 2CO). This electrochemical reaction proceeded completely at 800 °C and no blockage of gases was measured for long time (>10 h). Only H₂–CO fuel was generated in the wide gas compositions of starting CH₄–CO₂ gases.     

Performance of yttria-stabilized zirconia fuel cell using H2–CO2 gas system and CO–O2 gas system

This paper reports the performance of an yttria-stabilized zirconia fuel cell (YSZ) using five kinds of gas systems. The final target of this research is to establish the combined fuel cell systems which can produce a H₂ fuel and circulate CO₂ gas in the production process of electric power. A large electric power was measured in the H₂–O₂ gas system and the CO–O₂ gas system at 1073 K. The formation process of O²⁻ ions in the endothermic cathodic reaction (1/2O₂+2e⁻→O²⁻) controlled the cell performance in both the gas systems. The electric power of the H₂–CO₂ gas system, which allowed to change CO₂ gas into a CO fuel (H₂+CO₂→H₂O+CO) in the cathode, was 1/31–1/11 of the maximum electric power for the H₂–O₂ gas system. This result is related to the larger endothermic energy for the formation of O²⁻ ions from CO₂ molecules at the cathode (CO₂+2e⁻→CO+O²⁻) than from O₂ molecules. The CO–H₂O gas system and the H₂–H₂O gas system was expected to produce a H₂ fuel in the cathode (CO+H₂O→H₂+CO₂, H₂+H₂O→H₂+H₂O). Although relatively high OCV values (open circuit voltage) were measured in these gas systems, no electric power was measured. At this moment, it was difficult to apply H₂O vapor as an oxidant to the cathodic reaction in a YSZ fuel cell.     

Hydrogen formation from a real biogas using electrochemical cell with gadolinium-doped ceria porous electrolyte

The electrochemical cell consisting of a gadolinium-doped ceria (GDC, Ce_0.9Gd_0.1O_1.95) porous electrolyte, Ni–GDC cathode and Ru–GDC anode was applied for the dry-reforming (CH₄+CO₂→2H₂+2CO) of a real biogas (CH₄ 60.0%, CO₂ 37.5%, N₂ 2.5%) produced from waste sweet potato. The composition of the supplied gas was adjusted to CH₄/CO₂=1/1 volume ratio. The supplied gas changed continuously into a H₂–CO mixed fuel with H₂/CO=1/0.949–1/1.312 vol ratios at 800 °C for 24 h under the applied voltage of 1–2 V. The yield of the mixed fuel was higher than 80%. This dry-reforming reaction was thermodynamically controlled at 800 °C. The application of external voltage assisted the reduction of NiO and the elimination of solid carbon deposited slightly in the cathode. The decrease of heating temperature to 700 °C reduced gradually the fraction of the H₂–CO fuel (61.3–18.6%) within 24 h. Because the Gibbs free energy change was calculated to be negative values at 700–600 °C, the above result at 700–600 °C originated from the gradual deposition of carbon over Ni catalyst through the competitive parallel reactions (CH₄→C+2H₂, 2CO→C+CO₂). The application of external voltage decreased the formation temperature of carbon by the disproportionation of CO gas. At 600 °C, the H₂–CO fuel based on the Faraday's law was produced continuously by the electrochemical reforming of the biogas.     

Cathode catalyst layer design for proton exchange membrane fuel cells

To improve water management and enhance the catalyst utilization of the cathode catalyst layer of proton exchange membrane (PEM) fuel cells, the effects of polytetrafluoroethylene (PTFE) addition in the catalyst ink and the loading pattern of the catalyst layer were investigated. Two types of catalyst ink were used: a typical one without PTFE (Pt on carbon support + Nafion) and another type added with PTFE (Pt on carbon support + Nafion + PTFE). In exploring the effect of PTFE addition into the conventional full loading pattern of catalyst layer, the presence of 10% PTFE in the catalyst layer improved the cell performance (34% increase of maximum power density) and the optimum Pt loading for the PTFE-added catalyst layer was 0.25 mg/cm². Two catalyst layer loading patterns created in this work were the strip and chess patterns. Each pattern consists of equal areas of several hydrophilic and hydrophobic segments. The hydrophilic segments were formed by using the ink with PTFE while the hydrophilic had no PTFE. For the catalyst loading pattern effect, the cell achieved the highest performance with the chess pattern, followed by the strip and full loading pattern for the case of 0.5 mg/cm² Pt loading having a thick catalyst layer of 50-μm thickness. On the other hand, for the case of 0.25 mg/cm² Pt loading forming a thin catalyst layer of ～30-μm thickness, the catalyst loading pattern had no effect on the cell’s performance.     

Polymer-assistant ceramic nanocomposite materials for advanced fuel cell technologies

In this study,nanocomposites of LaCePr-oxide (LCP) and Ni_0.8Co_0.15Al_0.05LiO_2-δ (NCAL) with different contents of polyvinylidene fluoride (PVDF) were prepared and applied to solid oxide fuel cells. The composite materials were characterized by X-ray diffraction analysis (XRD), scanning electron microscope (SEM), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and electrochemical impedance spectrum (EIS). The effect of PVDF concentration on the conductivity and performance of the fuel cells was investigated. It was found that PVDF plays a template role of pore forming in the nanocomposites, and the changed microstructure by as-formed pores greatly influences the electrochemical property of the nanocomposites. The cell with 3 wt% PVDF heat-treated at 210 °C achieved the highest power density of 982 mW cm⁻² at 520 °C, which enhanced performance by more than 57% than when no heat-treatment was implemented. It is 66% higher than the cell with no PVDF and no heat-treatment. Pores formed by PVDF after heat-treatment enlarged the triple phase boundary (TPB), which results in improved fuel cell performance.     

Deep desulfurization of model gasoline by selective adsorption on Ag+/Al-MSU-S

The desulfurization of model gasoline containing 600 ppmw thiophene or dibenzothiophene (DBT) by selective adsorption over Ag⁺ exchanged mesoporous material Al-MSU-S was studied in a fixed adsorbent bed at ambient temperature and pressure. The results showed that the sulfur capacity increased with Al content incorporated in the silicate framework and Ag⁺ exchange can effectively improve the desulfurization performance. The best adsorbent, Ag⁺/20%Al-MSU-S, has adsorption capacity of 5 or 20 ml model gasoline containing thiophene or DBT per gram adsorbent, respectively, before the detection limit in our experiments, as a result of π-complexation. The adsorbent can be regenerated more than six times by simple calcination in air at 350 °C without obvious losing the sulfur adsorption capacity.     

Liquid-phase synthesis of SrCo0.9Nb0.1O3-δ cathode material for proton-conducting solid oxide fuel cells

A novel liquid-phase synthesis strategy is demonstrated for the preparation of the Nb-containing ceramic oxide SrCo_0.9Nb_0.1O_3-δ (SCN). In comparison with the traditional solid-state reaction (SSR) method, the liquid-phase synthesis route offers a couple of advantages, including a lower phase formation temperature and a smaller particle size of the SCN materials that are beneficial for applications as proton-conducting fuel cell cathode. With BaCe_0.4Zr_0.4Y_0.2O_3-δ (BCZY442) as the electrolyte and the SCN synthesized in this work as the cathode, a proton-conducting solid oxide fuel cell (SOFC) shows a peak power density of 348 mW cm^?2 at 700 °C, significantly higher than that of a SOFC fabricated with SCN cathode prepared using the SSR method, which can only deliver 204 mW cm^?2 at the same temperature. Additionally, this new synthesis strategy allows impregnation of Sr²⁺, Co³⁺and Nb⁵⁺ on the solid backbone in aqueous solution, further improving cell performance to reach a peak power density of 488 mW cm^?2 at 700 °C.     

The development of a fully integrated micro-channel fuel processor using low temperature co-fired ceramic (LTCC)

A fully integrated micro-channel fuel processor system consisting of vaporizer, steam reformer, heat exchanger and preferential CO oxidation (PROX) was developed using low temperature co-fired ceramic (LTCC). To fabricate a compact all-in-one system, each substrate was stacked to build a multilayered type fuel processor. A CuO/ZnO/Al₂O₃ catalyst and Pt-based catalyst prepared by wet impregnation were deposited inside the micro-channel of steam reformer and PROX, respectively. The performance of the fully integrated micro-channel reformer was measured at various conditions such as the ratio of the feed flow rate, the ratio of H₂O/CH₃OH and the operating temperature of the reactor. In parallel with the experiments, 3-D fluid dynamics simulation (Fluent) was conducted to verify the micro-reformer performance. The fully integrated micro-channel reformer has the dimensions of W: 130 mm × D: 50 mm × H: 3 mm. The fuel processor produced the gas composition of 71% H₂ and 25% CO₂, and more than 93% of methanol conversion was achieved at 300 °C and 2 cm³/h of the feed flow rate when CO concentration was maintained below 100 ppm by PROX.     

Effect of increasing anode surface area on the performance of a single chamber microbial fuel cell

The anode material and its configuration represent an important parameter in a microbial fuel cell (MFC), as it influences the development of the microbial community involved in the electrochemical bio-reactions.The aim of this work was to evaluate single chamber microbial fuel cells (SCMFCs) with high anode surface area, achieved by using packed beds of irregular graphite granules. The performance of the SCMFC with the packed bed anode configuration was studied using a mixed microorganism culture from real wastewaters in batch and continuous mode operation.The current output was found to increase with the increase in thickness of the anode bed and with the approximate anode area. The best performance was obtained with the 3 cm anode bed depth SCMFC. When the latter was operated in batch mode, Coulombic efficiencies varied from 30% to 74%, depending upon feed COD. In continuous mode operation, the COD removal was 89% and Coulombic efficiency 68% with a feed COD of 50 ppm, and at a flow rate of 0.0028 cm³ min^?1. Power performance was also reasonable with a volumetric power density of 1.3 W m^?3, with respect to the net anodic volume (12.5 cm³). Comparable performance was achieved with real wastewater. Over the duration of tests current output was stable. The investigation performed in this study represent a step forward for implementing real applications of MFC technology. A model of the current distribution in the packed bed electrode was applied, which correlates the effective utilization of the electrode to its specific area, solution conductivity and slope of the polarization curve. This model could function as a starting point in designing appropriate electrode geometries.     

Mechanism of NO reduction by CO over Pt/SBA-15

The mechanism of the CO + NO reaction catalyzed by Pt/SBA-15 was studied via independent investigations of CO oxidation and NO disproportionation. Below 400 °C, both CO + O₂ and CO + NO reactions approach 100 % conversion, while the catalyst shows negligible activity for NO disproportionation. These results suggest that CO oxidation by atomic oxygen arising from NO dissociation is not a major route for CO₂ formation in the CO + NO reaction. In situ IR spectra reveal the formation of isocyanates (NCO⁻) adsorbed on silica. Their surface concentration changes with the extent of the CO + NO reaction. A mechanism is proposed in which isocyanates are reaction intermediates.     

Effect of TiO2 on hydrodenitrogenation performances of MCM-41 supported molybdenum phosphides

The effect of TiO₂ on the hydrodenitrogenation (HDN) performance of MoP/MCM-41 was investigated using quinoline and decahydroquinoline as the model molecules. The catalysts were characterized by XRD, CO chemisorption, TEM, TPR and pyridine FT-IR. Addition of TiO₂ enhanced the C–N bond cleavage activity of MoP/MCM-41 but inhibited its dehydrogenation activity. A maximum HDN activity was observed when the TiO₂ loading was 5 wt%. The characterization results indicated that introduction of TiO₂ did not affect the formation of MoP phase. The TiO₂-containing catalysts possessed higher CO uptake than MoP/MCM-41, but no significant differences in the acid properties and particle size distributions were observed for all the catalysts. XPS results revealed a surface enrichment of TiO₂ in Ti-containing catalysts and small amount of these surface TiO₂ can be partially reduced to Tiⁿ⁺ (n < 4). It is suggested that these Tiⁿ⁺ (n < 4) species may be responsible for the promoting effect of TiO₂ on the HDN performance of MoP/MCM-41.     

Reforming of raw fuel gas from biomass gasification to syngas over highly stable nickel–magnesium solid solution catalysts

The gasification of biomass to obtain a syngas provides a competitive means for clean energy from renewable resources. The feasibility of the process depends on the performance of catalyst for upgrading of the raw fuel gas from gasifier. The highly stable NiO–MgO catalyst (Ni / (Ni + Mg) = 15, atomic ratio) was prepared by co-precipitation method for the reforming of raw fuel gas. Its performance was investigated under practical conditions of biomass gasification. The Ni_0.03Mg_0.97O and Ni / MgO catalysts (Ni / (Ni + Mg) = 15, atomic ratio) were also prepared for comparison. The NiO–MgO catalyst exhibited excellent reducibility and highly stable activity for the reforming of raw fuel gas without pre-reduction. No deactivation and very little carbon deposition were observed during 100 h lifetime test. The results of characterization (H₂–TPR, TGA, XRD, XPS) indicated that the formation of nickel–magnesium solid solution inhibited the sintering of nickel particles for high temperature reaction. Due to Ni²⁺ ions diffusion, the Ni / Mg atomic ratio decreased gradually with increasing depth. The highly stable activity was attributed to the small nickel particles size, high dispersion of nickel particles in the solid solution structure, and the promotion by catalyst reducibility.     

Anode-supported SOFCs based on Sm0.2Ce0.8O2−δ electrolyte thin-films fabricated by co-pressing using microwave combustion synthesized powders

Decreasing the electrolyte thickness is an effective approach to improve solid oxide fuel cells (SOFCs) performance for intermediate-temperature applications. Sm_0.2Ce_0.8O_2−δ (SDC) powders with low apparent density of 32±0.3 mg cm⁻³ are synthesized by microwave combustion method, and SDC electrolyte films as thin as ~10 μm are fabricated by co-pressing the powders onto a porous NiO–SDC anode substrate. Dense SDC electrolyte thin films with grain size of 300–800 nm are achieved at a low co-firing temperature of 1200 °C. Single cells based on SDC thin films show peak power densities of 0.86 W cm⁻² at 650 °C using 3 vol% humidified H₂ as fuel and ambient air as oxidant. Both the thin thickness of electrolyte films and ultra-fine grained anode structure make contributions to the improved cell performance.     

The comparison between the polyol process and the impregnation method for the preparation of CNT-supported nanoscale Cu catalyst

Carbon nanotubes (CNTs) are relatively potential materials for catalyst supports. CNT-supported Cu catalysts were prepared by an impregnation method and a polyol process. The catalytic activity was examined under different reaction atmospheres, Cu contents, and sizes of supports for CO oxidation. The experimental results showed that the active phase on the catalyst prepared by the impregnation method (10–20 nm) was smaller than that on the catalyst prepared by the polyol process (30–50 nm). Furthermore, the smaller active phase showed better performance for CO oxidation. Therefore, catalysts prepared by the impregnation method had a lower activation energy (57.47 kJ mol^?1) than those prepared by the polyol process. The optimum CNT-supported Cu catalyst prepared by the impregnation method using 10–20 nm CNTs had a Cu content of 13.4 wt.%, and a CO conversion of 33% achieved at 125 °C with a total space velocity of 1.56 × 10⁵ h^?1.     

Water-gas shift reaction over Cu–Zn,Cu–Fe,and Cu–Zn–Fe composite-oxide catalysts prepared by urea-nitrate combustion

Water-gas shift reaction was investigated over Cu–Zn, Cu–Fe and Cu–Zn–Fe composite-oxide catalysts at atmospheric pressure from 200 to 375 °C in terms of reducing the CO content with maximal H₂ yield. The Cu_0.15ZnFe₂ spinel catalyst expressed a higher CO conversion level and H₂ yield at a lower temperature compared to the Cu_0.15Zn and Cu_0.15Fe catalysts. Adding H₂O to the feed up to 30% (v/v), but not above, increased the CO reduction level, presumably by increasing the hydroxyl species to react with the adsorbed CO. Increasing the W/F ratio to 0.24 g s cm^?3 increased the CO conversion level to 0.76 at 275 °C with the Cu_0.15ZnFe₂ catalyst, and could be further increased to 0.86 at 350 °C by increasing the Cu molar ratio to 0.30 (Cu_0.30ZnFe₂). Nevertheless, increasing the Cu molar content to 0.50 reduced the CO conversion level. No requirement for adding O₂ when using the Cu_0.30ZnFe₂ catalyst at >260 °C was observed. Increasing the CO content in the reactant decreased its conversion level. The performance of the Cu_0.30ZnFe₂ catalyst was stable over a test period in a CO-rich condition. No undesired product was detected, suggesting a higher selectivity for hydrogen production with a low CO content.     

Improved electrochemical performances of reduced graphene oxide based supercapacitor using redox additive electrolyte

Reduced graphene oxide (rGO) is prepared by simple and eco-friendly hydrothermal reduction method. X-ray photoelectron spectroscopy and Ultraviolet–visible analysis corroborated the reduction of graphene oxide into rGO in basic medium. The flexibility of the prepared rGO is inferred from transmission electron micrographs. Further, the identification of suitable electrolyte is carried out using different anions (SO₄²⁻, Cl⁻, OH⁻) and cations (K⁺, Na⁺) for the superior performance of the rGO based supercapacitors. The electrochemical performance revealed that K⁺ and OH⁻ ions are more active species in aqueous solutions. Subsequently, an effort was taken to improve the specific capacitance in the optimized 1 M KOH electrolyte by KI as redox additive at different concentrations (0.025, 0.05, 0.075 and 0.1 M). The calculated specific capacitance and energy density of rGO electrode in the optimized 1 M KOH + 0.05 M KI electrolyte is 500 F g⁻¹ and 44 Wh kg⁻¹, respectively. On the other hand, it exhibited the specific capacitance of 298 F g⁻¹ at 0.83 A g⁻¹ in non-aqueous polymer gel (PVA + KOH + KI) electrolyte. Finally, the charged aqueous device is utilized to glow the light emitting diode.     

Dissolution of platinum in acidic media

To understand the degradation of the Pt/C cathode for use in polymer electrolyte fuel cells, the solubility and the dissolution mechanism of platinum in acidic media was very important fundamentally. In this study, the platinum solubility has been determined in acidic electrolytes as a function of temperature and pH. The solubility was 3 × 10^?6 mol dm^?3 at 25 °C in 1 mol dm^?3 of H₂SO₄, HClO₄, and CF₃SO₃H under air. It increased at higher temperature and decreased in pH. The apparent enthalpy of the dissolution reaction was ca. 25 kJ mol^?1, and the solubility was proportional to [H⁺] in these acids. The platinum solubility in oxygen was slightly higher than that in air. The detected dissolved Pt species was 4 valent. On the other hand, the solubility of platinum in nitrogen was much lower than the oxygen-containing atmosphere. Based on these results, the platinum solubility in an oxygen-containing atmosphere would mainly be the following acidic dissolution reaction: PtO₂ + H⁺ + H₂O = Pt(OH)₃⁺.     

Benzene electro-oxidation in a PEMFC for phenol and electricity cogeneration

In the present investigation, the electrochemically-assisted oxidation of benzene in a H₂–O₂ proton exchange membrane fuel cell (PEMFC), for electricity and phenol cogeneration is studied. Experiments were carried out in a PEMFC electrochemical reactor using Pd black as cathode electrocatalyst at 60 and 80 °C, respectively and 1 atm back pressure. Indeed, it was found that the only product detected under the examined experimental conditions was phenol. The online GC product analysis revealed that it is impossible to produce phenol when the fuel cell circuit is open (I = 0) under all the examined experimental conditions. When the fuel cell circuit was closed, however, the phenol yield was found to follow a volcano-type dependence on the current of the external circuit. It was found that the maximum phenol yield was 0.35% at 100 mA/cm² at 80 °C. At the same time, the PEMFC performance was also investigated during the phenol generation process. Furthermore, experiments with the rotating ring disc electrode (RRDE) technique showed that the intermediate oxidation product, i.e. H₂O₂ existed during the oxygen electro-reduction process. The cyclic voltammograms showed that benzene was strongly adsorbed on the Pd surface, leading to a degradation of the PEMFC performance.