Mathematical model for a direct propane phosphoric acid fuel cell

A mathematical model was developed and used to predict the performance of direct propane phosphoric acid (PPAFC) fuel cells, utilizing both Pt/C state-of the-art electrodes and older Pt black electrodes. It was found that the overpotential caused by surface processes on the platinum catalyst in the anode is much greater than the potential losses caused by either ohmic resistance or propane diffusion in gas-filled and liquid-filled pores. In one comparison, the anode overpotential (0.5 V) was larger than the cathode overpotential (0.3 V) at a current density of 0.4 A cm⁻² for Pt loadings 4 mg Pt cm⁻². The need for sufficient water concentration at the anode, where water is a reactant, was indicated by the large effect of H₃PO₄ concentration. In another comparison, the model predicted that at 0.2 A cm⁻², modern carbon supported Pt catalysts would produce 0.35 V compared to 0.1 V for unsupported Pt black catalysts that were used several decades ago, when the majority of the research on direct hydrocarbon fuel cells was performed. The propane anode and oxygen cathode catalyst layers were modeled as agglomerates of spherical catalyst particles having their interior spaces filled with liquid electrolyte and being surrounded by gas-filled pores. The Tafel equation was used to describe the electrochemical reactions. The model incorporated gas and liquid-phase diffusion equations for the reactants in the anode and cathode and ionic transport in the electrolyte. Experimental data were used for propane and oxygen diffusivities, and for their solubilities in the electrolyte. The accuracy of the predicted electrical potentials and polarization curves were normally within ±0.02 V of values reported in experimental investigations of temperature and electrolyte concentration. Polarization curves were predicted as a function of temperature, pressure, electrolyte concentration, and Pt loading. A performance of 0.45 V at 0.5 A cm⁻² was predicted at some conditions.     

Three-dimensional non-isothermal modeling of carbon monoxide poisoning in high temperature proton exchange membrane fuel cells with phosphoric acid doped polybenzimidazole membranes

The performance of proton exchange membrane fuel cell (PEMFC) degrades when carbon monoxide (CO) is present in the supplied fuel, which is referred to as CO poisoning. Even though the high temperature PEMFC (HT-PEMFC) with a typical operating temperature range from 100 °C to 200 °C features higher CO tolerance than the conventional PEMFC operating at lower than 100 °C, the performance degradation of HT-PEMFC is still significant with high CO concentrations (e.g. ?0.5% CO by volume at 130 °C) in the supplied fuel. In this study, a CO poisoning model is developed for HT-PEMFCs with phosphoric acid doped polybenzimidazole (PBI) membranes. The present three-dimensional non-isothermal model compares well with published experimental data at various operating temperatures and CO concentrations in the supplied fuel. It is found that the CO adsorption/desorption processes follow Langmuir kinetics in HT-PEMFCs instead of the well-known Temkin kinetics in conventional PEMFCs. The results indicate that a HT-PEMFC can operate with sufficiently good performance at 130 °C or higher with hydrogen gas produced by methanol reforming with selective oxidation process, and at 160 °C or higher even without the selective oxidation process. At high current densities, it is also observed that severe performance degradation due to CO poisoning only occurs if the volume averaged hydrogen coverage is lower than 0.1 in the anode catalyst layer (CL).     

Heat transfer in a phosphoric acid fuel cell stack

The electricity production in PAFC (Phosphoric Acid Fuel Cell) is accompanied by approximately equal amount of heat generation. For the best performance and efficient operation of the fuel cell stack, temperature distribution in the fuel cell stack should be controlled. Because the temperature gradient, both in the direction of stacking and in the cell plate, affects the cell performance, endurance, eletrolyte loss, and corrosion. In this study, a 30-plate phosphoric acid fuel cell stack was analyzed and modelled by using electrochemical equations and heat and mass transfer equations. To obtain more realistic profiles, local values of temperature and current density were used in the calculation. Obtained results agreed well with experimentally measured values : the temperature deviation from the measured values was less than 1.5°C.     

Estimation of corrosion conditions of a phosphoric acid fuel cell

The changes in the anode and cathode potentials in the horizontal plane of a phosphoric acid fuel cell (PAFC), under various conditions of reactant gas pressure and its utilization, were studied using a single cell with twelve reference electrodes located around the cathode. Pressure-utilization (P-U) potential maps were obtained from the data at various reactant gas partial pressures (PO₂, PH₂) and their utilization (UO₂, UH₂). These maps show the corrosion conditions clearly. A PO₂-UO₂ potential map of maximum cathode potential showed that the cathode is corroded at high oxygen partial pressures and at low oxygen utilization. Cathode corrosion can occur over the entire cell surface. A PH₂-UH₂ potential map of maximum cathode potential showed that the cathode is corroded at high hydrogen utilization and at any hydrogen partial pressure. However, in this case, cathode materials corrodes at the fuel outlet; the potential does not climb to high values at the fuel inlet area. Fuel gas flowing in series resulted in a lower possibility for corrosion than parallel gas flow.     

Recent advances in polybenzimidazole/phosphoric acid membranes for high‐temperature fuel cells

Proton exchange membrane fuel cells are one of the most promising technologies for sustainable power generation in the future. In particular, high‐temperature proton exchange membrane fuel cells (HT‐PEMFCs) offer several advantages such as increased kinetics, reduced catalyst poisoning and better heat management. One of the essential components of a HT‐PEMFC is the proton exchange membrane, which has to possess good proton conductivity as well as stability and durability at the required operating temperatures. Amongst the various membrane candidates, phosphoric acid‐impregnated polybenzimidazole‐type polymer membranes (PBI/PA) are considered the most mature and some of the most promising, providing the necessary characteristics for good performance in HT‐PEMFCs. This review aims to examine the recent advances made in the understanding and fabrication of PBI/PA membranes, and offers a perspective on the future and prospects of deployment of this technology in the fuel cell market. © 2014 Society of Chemical Industry     

Phosphoric acid doped polybenzimidazole membranes: Physiochemical characterization and fuel cell applications

A polymer electrolyte membrane fuel cell operational at temperatures around 150–200 °C is desirable for fast electrode kinetics and high tolerance to fuel impurities. For this purpose polybenzimidazole (PBI) membranes have been prepared and H₃PO₄-doped in a doping range from 300 to 1600 mol %. Physiochemical properties of the membrane electrolyte have been investigated by measurements of water uptake, acid doping level, electric conductivity, mechanical strength and water drag coefficient. Electrical conductivity is found to be insensitive to humidity but dependent on the acid doping level. At 160 °C a conductivity as high as 0.13 S cm^–1 is obtained for membranes of high doping levels. Mechanical strength measurements show, however, that a high acid doping level results in poor mechanical properties. At operational temperatures up to 190 °C, fuel cells based on this polymer membrane have been tested with both hydrogen and hydrogen containing carbon monoxide.     

Investigation of phosphoric acid and water transport in the high temperature proton exchange membrane fuel cells using a multiphase model

A three-dimensional, nonisothermal, and multiphase model of high temperature proton exchange membrane fuel cells is built to investigate water and phosphoric acid transportation, in which a spherical agglomerate model considering catalyst layer structure and liquid saturation is applied to determine the electrochemical kinetics in the cathode catalyst layer. Experimental polarization curve, water proportion in the anode outlet gas, and phosphoric acid distribution are selected for validation. It is found that the simulated results can represent the experimental data with reasonable accuracy. Based on the model, the effects of current density and stoichiometry on the variable distributions are analyzed. The results show that water in anode is mainly from cathode by concentration diffusion of liquid water, and the proportion of anode outlet water to the total produced water decreases slightly with the increase of current density. A higher current density leads to a greater electromigration of phosphoric acid from cathode to anode and a higher liquid phase fraction in anode, while a lower phosphoric acid concentration in the fuel cells.     

Air and fuel starvation of phosphoric acid fuel cells: A study using a single cell with multi-reference electrodes

The changes in the anode and cathode potentials in the horizontal plane of a phosphoric acid fuel cell (PAFC) under starving conditions for either air or fuel were studied using a single cell furnished with twenty-four reference electrodes which were located around the anode or the cathode. When air starvation occurred, both the anode and cathode potentials became nearly 0 V against RHE, and hydrogen generation began to occur on the cathode side. Fuel starvation occurred when fuel utilization became more than 95%, and the cathode potential in the fuel outlet area shifted significantly toward the positive and, simultaneously, CO and CO₂ were detected in the air exhaust gas, indicating the occurrence of carbon corrosion of the cathode components. By further increasing the fuel utilization, the cell voltage changed to negative and the anode potential in the fuel outlet area became the highest. At that time, significant amounts of CO and CO₂ were detected in the fuel exhaust gas, indicating the occurrence of carbon corrosion of the anode components. Immediately after the termination of fuel starvation, the cathode potential in the fuel outlet area shifted quickly and remarkably toward the positive, and exceeded 1 V against RHE in a few seconds.     

Design of an “all solid-state” supercapacitor based on phosphoric acid doped polybenzimidazole (PBI) electrolyte

The effectiveness of phosphoric acid doped polybenzimidazole as a polymer electrolyte membrane to fabricate an all solid-state super capacitor has been explored using hydrous RuO₂/carbon composite electrodes (20 wt.%) of surface area 250 m² g⁻¹ with many intrinsic advantages. The electrochemical evaluation of these super capacitors through cyclic voltammetry, charge/discharge and impedance measurements demonstrate the utility of this type of thin, compact and flexible supercapacitor capable of functioning at 150 °C to yield a maximum capacitance of about 290 F g⁻¹ along with a life of more than 1,000 cycles. A power density of 300 W kg⁻¹ and energy density of 10 Wh kg⁻¹ have been accomplished although the equivalent series resistance (ESR) of about 3.7 Ω needs to be reduced further for high rated applications.     

Effect of operational potential on performance decay rate in a phosphoric acid fuel cell

The effect of operational potential on the cell voltage decay rate in a phosphoric acid fuel cell was estimated using the calculation of the platinum dissolution rate. The voltage loss, due to deterioration in activation polarization, was used in calculating the cell voltage decay rate. The voltage loss due to activation polarization was estimated using the relationship between the activation polarization and the platinum surface area. The change in the Pt surface area, arising from the dissolution of platinum, was obtained under accelerated open circuit voltage (o.c.v.) conditions. The study was focused on typical voltage decay rates from 0.1 to 10 mV per 1000 h. It was found that the cell voltage decay became significant even at low temperature during long-term operation (at o.c.v.) and that the cell had to be operated below 840 mV (iR-free) at 200 mA cm^–2) 200°C for a decay rate of 1 mV per 1000 h. From the present estimation, operational conditions such as temperature, cathode potential, and holding time at a given potential, can be roughly determined for a given decay rate. It was concluded that the voltage loss due to platinum dissolution may be negligible at a rated power operation.     

Proton conductors of cerium pyrophosphate for intermediate temperature fuel cell

The crystal structure and proton conductivity of cerium pyrophosphate are investigated to explore its potential electrolyte applications for intermediate temperature fuel cell. Among the CeP₂O₇ thin plates, which are sintered at 300–900 °C, the 450 °C CeP₂O₇ sample exhibits superior proton conductivity under humidified conditions. Its conductivity, measured with impedance spectroscopy, is higher than 10⁻² S cm⁻¹ in the intermediate temperature range, with a maximum value 3.0 × 10⁻² S cm⁻¹ at 180 °C. When 10 mol% Mg is doped on the Ce site of CeP₂O₇, the maximum conductivity is raised to 4.0 × 10⁻² S cm⁻¹ at 200 °C. The Mg doping not only raises the conductivity, but also shifts and widens its temperature window for electrolyte applications. Ce_0.9Mg_0.1P₂O₇ is considered a more appropriate composition, with conductivity >10⁻² S cm⁻¹ between 160 and 280 °C. Accordingly, a hydrogen–air cell is built with the Ce_0.9Mg_0.1P₂O₇ electrolyte and its performance is measured. The fuel cell generates electricity up to 122 mA cm⁻² at 0.33 V using 50% H₂ at 240 °C.     

Trimethoxymethane as an alternative fuel for a direct oxidation PBI polymer electrolyte fuel cell

The oxidation of trimethoxymethane (TMM) (trimethyl orthoformate) in a direct oxidation PBI fuel cell was examined by on-line mass spectroscopy and on-line FTIR spectroscopy. The results show that TMM was almost completely hydrolyzed in a direct oxidation fuel cell which employs an acid doped polymer electrolyte to form a mixture of methylformate, methanol and formic acid. It also found that TMM was hydrolyzed in the presence of water at 120°C even without acidic catalyst. The anode performance improves in the sequence of methanol, TMM, formic acid/methanol, and methylformate solutions. Since formic acid is electrochemically more active than methanol, these results suggest that formic acid is probably a key factor for the improvement of the anode performance by using TMM instead of methanol under these conditions.     

Functional nanoceramics for intermediate temperature solid oxide fuel cells and oxygen separation membranes

This work reviews results of research aimed at design and characterization of mixed ionic–electronic conducting perovskite–fluorite nanocomposite oxide ceramics. Nanocrystalline oxides were prepared via Pechini route, nanocomposites – via ultrasonic dispersion of their mixture in organic solvents with addition of surfactants. Genesis of the real structure of nanocomposites at sintering by conventional as well as advanced (microwave or e-beam treatment) techniques was studied in details by structural methods. Applied preparation procedures ensured nano-sizes of perovskite/fluorite domains even in dense ceramics and a high spatial uniformity of their distribution. Redistribution of elements between perovskite and fluorite domains without formation of new phases was revealed. Characterization of nanocomposite transport properties by oxygen isotope heteroexchange and conductivity or weight relaxation demonstrated that perovskite–fluorite interfaces are paths for fast oxygen diffusion. Best perovskite–fluorite combinations tested as cathode layers or dense oxygen separation layers in asymmetric supported membranes demonstrated performance promising for the practical application.     

燃料电池高温质子交换膜研究进展

高温质子交换膜燃料电池具有反应动力学快、CO耐受性高等特点,但磷酸掺杂的高温质子交换膜因磷酸的流失和聚合物的降解等原因导致燃料电池的输出功率发生衰减。本文通过介绍聚苯并咪唑衍生物的高温质子交换膜、聚苯并咪唑的复合型质子交换膜、新型芳基聚合物的高温质子交换膜,阐明聚合物的主链结构、官能团结构以及复合填料对高温质子交换膜性能的影响。在近期的研究报道中,提高膜性能的主要策略包括提升自由体积、建立交联结构、嵌段共聚、复合掺杂（ILs、MOFs、PIMs、MO_x）、阳离子官能团修饰等。文章指出,在未来的研究中应该加强对磷酸基高温质子交换膜质子传输通道结构的进一步理解,关注聚合物化学降解和物理性能衰败的原因,并开发更多的新型聚合物材料。     

Changes in cathode catalyst structure and activity in phosphoric acid fuel cell operation

Changes in the cathode catalyst structure and activity obtained from a small size phosphoric acid fuel cell (PAFC) operated for various times up to 12 000 h, were examined. It was confirmed that the platinum surface oxide reduction potential in cyclic voltammograms (CV) shifted in the positive direction with cell operation. This may be one of the manifestations of the activity enhancement for the oxygen reduction reaction (ORR). It was assumed that this activity increase for the ORR was caused by an increase in the surface roughness, due to the dissolution of the alloyed base metals. Changes in the platinum chemical state of the alloy surface, from PtO to Pt, and growth of the Pt (1 1 0) plane would also contribute to this effect.     

Electricity generation from terephthalic acid using a microbial fuel cell

BACKGROUND: Pure terephthalic acid (PTA) is a petrochemical product of global importance and is widely applied as an important raw material in making polyester fiber and polyethylene terephthalate (PET) bottles. In this work, a single‐chamber microbial fuel cell (MFC) was constructed using terephthalic acid (TA) with a chemical oxygen demand (COD) concentration range from 500 mg L^?1 to 3500 mg L^?1 as the electron donor and strain PA‐18 as the biocatalyst. RESLUTS: In the single chamber MFC, several factors were examined to determine their effects on power output, including COD concentration and electrode spacing. The characteristic of the strain PA‐18 was further studied. Cyclic voltammetry showed that electrons were directly transferred onto the anode by bacteria in biofilms, rather than self‐produced mediators of bacteria in the solutions. Scanning electron microscopy (SEM) observation showed that the anodic electrode surface was covered by bacteria which were responsible for electron transfer. Direct 16s‐rDNA analysis showed that the PA‐18 bacteria shared 99% 16SrDNA sequence homology with Pseudomonas sp. CONCLUSIONS: Electricity generation from TA in MFC was observed for the first time. The maximum power density produced by TA was 160 mW m^?2, lower than that achieved using domestic wastewater. This novel technology provided an economical route for electricity energy recovery in PTA wastewater treatment. High internal resistance was the major limitation. To further improve the power output, the electron transfer rate was accelerated by overexpression of membrane the protein gene of the strain PA‐18 and by reducing the electrolyte and mass transfer resistance by optimizing reactor configuration. Copyright © 2008 Society of Chemical Industry     

The role of phosphoric acid in the anodic electrocatalytic layer in high temperature PEM fuel cells

The poisoning effect and the role of H₃PO₄ (PA) at the anodic electrocatalytic layer of a high temperature polymer electrolyte membrane (HT PEM based on ADVENT TPS^®) fuel cell are discussed under the light of cyclic voltammetry, CO stripping, and X-ray photoelectron spectroscopy (XPS) experiments. The catalytic layer was based on both the pyridine-modified multi-wall carbon nanotubes, 30 wt% Pt/(ox.MWCNT)–Py, and on commercial 30 wt% Pt/C, with varying PA loadings on the electrode. At low PA loadings (<3 gPA/gPt), the electrochemically active surface area of Pt decreases significantly under H₂ anode long-term operation, approaching surface Pt utilization <10 %. This degradation is attributed to the formation of pyrophosphoric or triphosphoric acid as well as catalytically H₂ reduced PA species, which block the Pt surface area. As was explicitly detected by means of XPS PA species were displaced from the Pt surface under H₂ or CO exposure. The poisoning effect is reversible as these species can be hydrated back to orthophosphoric acid. The reduced species can be reoxidized into PA at 750 mV versus RHE. On the other hand, the electrochemical interface is stable at PA loadings exceeding 3 gPA/gPt, thus approaching Pt surface utilization >80 % in the long term. This is believed to be a consequence of the more uniform distribution of PA, thus eliminating the PA displacement from the Pt interphase. It is hypothesized that the minimization of the PA poisoning effect at PA > 3 gPA/gPt, may also be a result of more efficient hydration of the catalytic layer that is being achieved through the hydration of the PA in the membrane and in the catalyst layer by the cathodically produced water vapors.