Performance improvement of direct methanol fuel cells via anodic treatment using various organic acids

Performance improvement of direct methanol fuel cells (DMFCs) was achieved via an anodic treatment technique. Previously, anodic treatment was performed using sulfuric acid as acidic media, but various organic acids including formic, acetic, oxalic, and citric acids were employed in this study to avoid the use of toxic sulfuric acid. By replacing sulfuric acid to organic acids, a potential damage to catalyst layers and other components such as polymer electrolyte membrane and bipolar plates are expected to be minimized. The anodic treatment was performed by applying 0.7 V (vs. reversible hydrogen electrode) at the anode of DMFCs flowing the organic acid solutions for 30min. After the anodic treatment, peak power densities of DMFCs were increased by +7, +32, +23, and ?2.6% when formic, acetic, oxalic, and citric acid solutions were employed, respectively. The enhanced catalytic activity of the DMFCs in the acetic and oxalic acid solutions was confirmed by analyzing electrochemical impedance spectroscopy data.     

The relationship between the impedance of corroding electrode and its polarization resistance determined by a linear voltage sweep technique

The polarization resistance of metallic materials in contact with an electrolyte is largely used for the evaluation of its corrosion rate. The experimental value of polarization resistance is very often determined by plotting the polarization curves at the vicinity of the rest potential by a triangular voltage sweep technique. It is found that, for materials having high resistivity against corrosion, the value of polarization resistance is largely dependent on the sweep rate or the period of sweep cycle. If an equivalent circuit of a corroding electrode is to be presented by a parallel connexion of a resistance and a capacitance, such dependence of the polarization resistance cannot be explained at all. In fact, though the impedance of corroding electrode measured within a large frequency range shows one capacitive arc in the complex plane in accordance with parallel R—C circuit, the values of R and C both depend on frequency. The current response to a triangular voltage sweep signal is calculated numerically by Heaviside operational calculus using experimentally determined electrode impedance. A fairly good agreement was found between the experimental and calculated current—voltage cycles for different periods of a triangular voltage sweep signal.     

Pore‐forming technology and performance of MEA for direct methanol fuel cells

BACKGROUND: The commercialization of DMFCs is seriously restricted by its relatively low power density. Lots of work has been concentrated on catalysts with high activity, the optimization of flow path design, development of new kinds of proton exchange membrane and modification of Nafion membrane. Meanwhile, very few reports have involved the structure optimization of the membrane electrode assembly (MEA). To improve the performance of direct methanol fuel cells (DMFCs), the catalyst layer (CL) structures of anode and cathode were optimized by utilizing ammonium carbonate as pore forming agent. RESULTS: The polarization curves showed that in catalyst slurry the optimal content of ammonium carbonate was 50 wt%, and the DMFC performance was enhanced from 75.65 mW cm^?2 to 167.42 mW cm^?2 at 55 °C and 0.2 MPa O₂. Electrochemical impedance spectroscopy and electrochemical active surface area (EASA) testing revealed that the improved performance of optimized MEAs could be mainly attributed to the increasing EASA and the enhanced mass transfer rate of CLs. But poor methanol crossover limited the performance enhancement of MEAs with porous anodes. CONCLUSION: With regard to improving cell performance, this pore‐forming technology is better applied to the cathode catalyst layer to improve its structure rather than the anode catalyst layer. © 2012 Society of Chemical Industry     

Electrochemical behavior of La0.8Sr0.2FeO3 electrode with different porosities under cathodic and anodic polarization

Electrochemical behavior of La_0.8Sr_0.2FeO₃ (LSF) electrode with different porosities under cathodic and anodic current polarization has been investigated by electrochemical impedance spectroscopy and the galvanostatic method. The activation and degradation behavior of the LSF electrode may be related to the partial reduction and oxidation of the Fe ions under cathodic and anodic polarization, especially in the LSF electrodes with high porosities. The performance of the LSF electrode has been found to depend on the oxygen vacancies at the LSF surface, which would promote the transport of oxygen intermediate species at the LSF surface close to the triple-phase boundary (TPB) region. Results show that the polarization resistance (Rp) of the LSF electrode decreases at the beginning with the increase of cathodic polarization time, while Rp always increases with the increase of anodic polarization time. The effect of cathodic and anodic polarization becomes predominant with the increasing of the porosities of the LSF electrode, which is ascribed to the decrease of the interface area between electrode and electrolyte.     

Activity of a novel titanium-supported bimetallic PtSn/Ti electrode for electrocatalytic oxidation of formic acid and methanol

Bimetallic platinum–tin nanoparticles were co-deposited on a titanium surface using a simple one step hydrothermal method process. The electrochemical catalytic activity of this titanium-supported nanoPtSn/Ti electrode towards the oxidation of formic acid and methanol in 0.5 M H₂SO₄ was evaluated by voltammetric techniques, chronoamperometric responses and electrochemical impedance spectra (EIS). According to the cyclic voltammograms of the oxidation of both formic acid and methanol, the nanoPtSn/Ti presents high anodic current densities and low onset potentials. Potential-time transient measurements show that the nanoPtSn/Ti exhibits high steady-state current densities for the oxidation of both formic acid and methanol. The EIS data indicate that the nanoPtSn/Ti presents very low electrochemical impedance values, showing that for the oxidation of both formic acid and methanol, low charge transfer resistances are present on the nanoPtSn/Ti catalyst. This confirms the high electrocatalytic activity of the nanoPtSn/Ti for the formic acid and methanol oxidation.     

Complexities of corrosion behaviour of copper-nickel alloys under liquid impingement conditions in saline water

Flow-assisted electrochemical corrosion of Cu-Ni disc electrodes, subjected to a submerged impinging jet in saline water, was evaluated and quantified using the anodic current densities (j_a), corrosion potentials (E_corr), interfacial capacitances (C) and polarization resistances (R_p). The Reynolds numbers in the range of applied impinging velocities suggested that the flow between the solution and the electrode is turbulent in the systems investigated.The role of Ni content in the resistance of spontaneously formed barrier oxide film and diethyldithiocarbamate inhibitor film on Cu-Ni alloys (10, 20, 30 and 40 at.% Ni) was examined and evaluated using electrochemical impedance spectroscopy in dependence of the impinging velocity and the immersion time.The results are discussed in terms of the corrosion mechanisms and their relevance to the use of Cu-Ni alloys in desalination plants and in the marine engineering field because fluid hydrodynamics plays an important role in Cu-Ni alloys application. Based on the analysis presented in the paper, it is anticipated that the Cu-10Ni alloy is the best choice for application in the marine environment.     

Investigation of direct methanol fuel cell performance of sulfonated polyimide membrane

Sulfonated polyimide (SPI) membranes have been evaluated as electrolyte membranes in direct methanol fuel cells (DMFCs). The membrane-electrode assembly (MEA) was made by hot-pressing the membrane, an anode and a cathode, catalyzed with PtRu/CB (PtRu dispersed on carbon black) and Pt/CB bound with Nafion^® ionomer, respectively. The performance of the cell based on SPI was compared with that of Nafion^® 112 in various operation conditions such as cell temperature (T_cell), cathode relative humidity (RH), and methanol concentration (C_MeOH). The methanol crossover at the cell based on SPI was a half of Nafion^® 112, resulting in the improved cell efficiency. Advantage of the use of SPI became much distinctive from the conventional Nafion^® 112 when the DMFC was operated at a higher T_cell or a higher C_MeOH.     

Effect of bromide ions on the corrosion behavior of tantalum in anhydrous ethanol

The electrochemical behaviors of Ta in Et₄NBr ethanol solutions were investigated using potentiodynamic polarization, cyclic voltammetry, potentiostatic current-time transient and impedance techniques. The potentiodynamic anodic polarization curves did not exhibit active dissolution region due to the presence of thin oxide film on the electrode surface, which was followed by pitting corrosion as a result of passivity breakdown by the aggressive attack of Br⁻ anions. The pitting potential (E_b) decreased with the increase of solution temperature and Br⁻ concentration, but increased with increasing potential scan rate and water concentration. The incubation time derived from potentiostatic current-time transients decreased with increasing potentials. The impedance spectra exhibited two time constants for all the potentials and the resistance of passive layer decreased with increasing potential.     

Kinetic study on the role of an LSGMC5 interlayer in improving the performance of Sm0.5Sr0.5CoO3-La0.8Sr0.2Ga0.8Mg0.15Co0.05O3 (LSGMC5)/LSGMC5 interface

The kinetics of oxygen reduction over various Sm_0.5Sr_0.5CoO₃(SSC)-La_0.8Sr_0.2Ga_0.8Mg_0.15Co_0.05O₃(LSGMC5)/LSGMC5 (interlayer)/LSGMC5 (electrolyte) assemblies were studied, which were essential to find the role of an interlayer in improving the performance of an electrode/electrolyte interface. Two major arcs were identified in the impedance spectra at near equilibrium conditions. The reciprocal of the electrode resistance corresponding to the high frequency arc showed a PO₂ dependency about 0.5 at 1073 K and decreased to one-fourth at 873 K, suggesting that the rate-determining step (rds) changed from the dissociative adsorption of oxygen or diffusion of adsorbed oxygen atoms to charge transfer. The reciprocal of the electrode resistance corresponding to the low frequency arc showed a PO₂ dependency about 1, suggesting an rds involving the gas diffusion of oxygen. DC polarization curves of various assemblies agreed well with the Butler-Volmer equation. Both the cathodic and anodic charge transfer coefficients were about 1, and the PO₂ dependencies of the exchange current densities were about 0.25, especially at low temperatures. The characteristics under polarization corresponded to a charge transfer process. The introduction of an LSGMC5 interlayer between the SSC-LSGMC5 electrode and LSGMC5 electrolyte did not change the reaction mechanism, and the role of the interlayer was to increase the number of active sites for oxygen reduction.     

The influence of oxygen additions to hydrogen in their electrode reactions at Pt/Nafion interface

The influence of oxygen gas added to hydrogen in their electrode reactions at the Pt/Nafion interface was investigated using ac impedance method. The electrochemical cell was arranged in either electrolytic (hydrogen enrichment) or galvanic (fuel cell) mode. The impedance spectra of the electrode reaction of a H₂/O₂ gas mixture were taken in each mode as a function of the gas composition, electrode surface roughness and the cell potential. The spectrum taken for the anodic reaction of electrolytic arrangement confirmed the anodic oxygen reduction reaction (AOR, the local consumption of hydrogen by the added oxygen) by showing an independent arc distinguishable from that for hydrogen oxidation. But the independent arc was not revealed in the spectrum taken on a smooth (low surface area) electrode or on a Pt/C anode of the galvanic cell. At any cell current density, the electrolytic mode showed its anodic overpotential much higher (nearly three times higher at the current density of 100 mA cm⁻²) than the potential registered in galvanic mode implying that the oxygen gas in the mixture engages more active and independent AOR at the anode of the electrolytic cell.     

Double-layer capacity measurements as a method to characterize porous fuel cell electrodes

Growing interest in porous Teflon-bonded carbon and catalyst carbon electrodes has resulted in increasing research efforts to improve performance and lifetime of these electrodes. The impedance method is demonstrated as a useful instrument to investigate these semihydrophobic electrodes. The double-layer capacity, derived by rather simple calculations from impedance spectra, is correlated with important parameters such as Teflon content, fabrication pressure and drying temperature of the electrode as well as to the deterioration processes which are still limiting the lifetime of the electrode. Information needed for improving these electrodes can be deduced from C_D measurements. Further interpretation of the impedance spectra and calculation of kinetic data are possible with the aid of appropriate equivalent circuits.     

Effect of methanol crossover on the cathode behavior of a DMFC: A half-cell investigation

A half-cell consisting of a normal direct methanol fuel cell (DMFC) cathode and a membrane that contacts with an electrolyte solution was developed to investigate the effect of methanol crossover on the cathode behavior. Open circuit potentials, cyclic voltammetry profiles, polarization curves and electrochemical impedance spectroscopy (EIS), resulting from the oxygen reduction reaction (ORR) with/without the effect of methanol oxidation reaction (MOR), were measured. The transient measurements indicated that both current and open circuit potential of the electrode exhibited significant oscillations when the anodic MOR was superposed on the cathodic ORR, which explain the instabilities that may be encountered in the practical DMFC operation. The steady-state results confirmed that the presence of methanol at the cathode led to a significant poisoning effect on the ORR, especially when the DMFC operates at higher methanol concentrations and discharges at lower potentials. More importantly, the half-cell was proved to be ideal for the EIS study of DMFC electrodes because the system not only facilitates an accurate potential control but also reflects the actual mass transport process that occurs in practical DMFCs.     

Cell performances of direct methanol fuel cells with grafted membranes

Cell performances were evaluated with grafted polymer membranes as an electrolyte for a direct methanol fuel cell (DMFC). The membranes were prepared using a poly(ethylene-tetrafluoroethylene), or ETFE, film. The base polymer film was added to sulfonic groups using γ-radiation activated grafting technique as ion-exchange groups. These membranes had more suitable properties for DMFCs, i.e. higher electric conductivity and lower methanol permeability than perfluorinated ionomer membrane (Nafion). Nevertheless, the cell performance with the grafted membrane was inferior to that with Nafion. The analysis of electrode potentials vs. reversible hydrogen electrode showed larger activation overpotential for both the electrodes on the grafted membranes. We concluded that this is due to poor bonding of the catalyst layers to the grafted membranes.     

EIS-assisted performance analysis of non-noble metal electrocatalyst (Fe-N/C)-based PEM fuel cells in the temperature range of 23-80 °C

A carbon-supported non-noble metal catalyst, Fe-N/C, was used as the cathode catalyst to construct membrane electrolyte assemblies (MEAs) for a proton exchange membrane (PEM) fuel cell. The performance of such a fuel cell was then tested and diagnosed using electrochemical impedance spectroscopy (EIS) in the temperature range of 23-80 °C. Based on the EIS measurements, individual resistances, such as charger transfer resistance and membrane resistance, were obtained and used to simulate polarization curves (current-voltage (I-V) curves). A close agreement between the simulated I-V curves and the measured curves demonstrates consistency between the polarization and EIS data. The temperature-dependent parameters obtained via EIS, such as apparent exchange current densities and electrolyte membrane conductivities, were also used to acquire activation energies for both the oxygen reduction reaction (ORR) catalyzed by an Fe-N/C catalyst and the proton transport process across the electrolyte membrane. In addition, the maximum power densities for such a fuel cell were also analyzed.     

Anodic dissolution of amorphous Ni-P alloys

Anodic behaviour of amorphous Ni-P alloys containing 23 and 27 at.% P was compared to that of pure Ni. Measurements were performed in 0.1 M sulphate solutions of different pH by means of potentiodynamic polarization and impedance spectroscopy at selected anodic potentials. Significant discrepancies in the course of anodic polarisation curves and impedance spectra between amorphous Ni-P and pure Ni were seen. Amorphous Ni-P alloys exhibit a suppression of dissolution in the active range of Ni, whereas they show a faster dissolution in the passive domain of Ni. From the other side, there are minor modifications in impedance spectra taken for Ni-P over a wide range of anodic potentials, whereas Ni exhibit essential changes of impedance spectra within the same potential region, related to the active-passive transition of this metal. The charge transfer resistance of the anodic dissolution for amorphous Ni-P decreases gradually with increasing anodic potential in contrast to a sharp increase of this parameter for pure Ni in the region of its passivation. In contrast to Ni, the anodic dissolution of amorphous Ni-P exhibits a slight dependence on pH. These findings prove that the anodic dissolution suppression of amorphous Ni-P cannot be associated with the oxide passivity typical of pure Ni.     

Effect of electrochemical polarization of PtRu/C catalysts on methanol electrooxidation

To determine the influence of electrochemical polarization of PtRu/C catalysts on methanol electrooxidation, this work investigated methanol electrooxidation on as received and different electrochemically polarized PtRu/C catalysts. Thermogravimetric analysis (TGA) and X-ray diffraction (XRD) were used to characterize the redox state of PtRu/C after different electrochemical polarization. The methanol electrooxidation activity was measured by cyclic voltammetry (CV), Tafel steady state plot and electrochemical impedance spectroscopy (EIS). The results indicate that the metallic state Pt⁰Ru⁰ can be formed during cathodic polarization and contribute to electrooxidation of methanol, while the formation of inactive ruthenium oxides during anodic polarization cause the negative effect on methanol electrooxidation. Different Tafel slopes and impedance behaviors in different potential regions also reveal a change of the mechanism and rate-determining step in methanol electrooxidation with increasing potentials. The kinetic analysis from Tafel plots and EIS reveal that at low potentials indicate the splitting of the first CH bond of CH₃OH molecule with the first electron transfer is rate-determining step. However, at higher potentials, the oxidation reaction of adsorbed intermediate CO_ads becomes rate-determining step.     

Variations in interfacial properties during cell conditioning and influence of heat-treatment of ionomer on the characteristics of direct methanol fuel cells

Variations in interfacial properties in the anode catalyst layer during cell conditioning were characterized, and influence of the heat-treatment of ionomer on the characteristics of direct methanol fuel cells was investigated in this work. The anode catalyst layer was made by mixing a solvent-substituted Nafion solution with unsupported Pt/Ru black and curing the mixture in an oven with an inert environment. Materials characterization (SEM and optical microscopy) and electrochemical characterization (cell polarization, anode polarization, electrochemical impedance spectroscopy, and CO-stripping cyclic voltammetry) were performed. During cell conditioning, the enhanced kinetics of MeOH electrochemical oxidation and severe limiting current phenomenon are due to the combination of variations in interfacial properties and swelling of ionomer in the anode catalyst layer over time. Ru oxides at the catalyst surface are reduced continuously during cell conditioning. The nearly constant integrated areas under the CO-stripping CV peaks and broadened peak shapes indicate a stable number of Pt/Ru bimetallic alloy surface sites, yet the surface composition distribution is broadened. Heat-treatment influences ionomer crystallinity, altering its swelling behavior and hence affecting the characteristics of the direct methanol fuel cell (DMFC) anode.     

Anodic polarization induced performance loss in GdBaCo2O5+δ oxygen electrode under solid oxide electrolysis cell conditions

Double perovskite oxide GdBaCo₂O_5+δ (GBCO) is widely investigated as promising cathode for solid oxide fuel cells (SOFCs), but it remains unclear whether GBCO is suitable for application in solid oxide electrolysis cells (SOECs) anode. In this study, the effect of anodic polarization on electrochemical activity and microstructure of GBCO electrode are investigated under SOECs operation conditions. Both polarization and impedance spectra results clearly demonstrate that anodic bias treatment leads to substantial performance degradation and higher anodic current passage causes more serious activity loss. The deactivation behavior can be mainly ascribed to the formation of surface BaO precipitates in harsh oxidation atmosphere, as revealed by microstructural observation. Our study suggests that GBCO oxide is not suitable for SOECs anode application, but significant change of surface chemistry enables it a good model electrode for segregation related studies.     

The pore texture of raney-nickel determined by impedance measurements

The pore texture of Raney-nickel was determined by impedance measurements carried out over a wide frequency range. The impedance obtained could be characterized by three resistances, R, Re and Rt and one capacitor C. R is the high frequency limit of the electrode impedance, and is equal to the electrolyte resistance between the reference capillary-tip and the upper-surface of catalyst layer. Re is the electrolyte resistance through the cavity of catalyst layer. Rt is linked in parallel with the capacitor C.C value allows to evaluate the surface area of catalyst in contact with electrolyte provided that the double layer capacitance of nickel electrode per unit surface is known. R value, measured with and without catalyst layer allows to estimate its thickness, hence the total pore volume when the density of catalyst-metal is known. The Raney-nickel has a double pore-structure: the one related to micro-pores inside catalyst grain and the other to a heaping of grains. If the volumes of these two types of pore are known, the pore radius of micro-pore can be evaluated. Therefore, impedance measurements determine, in situ, the pore texture of Raney-nickel in liquid phase. Results were in good agreement with those determined in gas phase by current methods.     

Optimization of cathode catalyst layer for direct methanol fuel cells: Part I. Experimental investigation

The cathode catalyst layer (CL) in direct methanol fuel cells (DMFCs) has been optimized through a balance of ionomer and porosity distributions, both playing important roles in affecting proton conduction and oxygen transport through a thick CL of DMFC. The effects of fabrication procedure, ionomer content, and Pt distribution on the microstructure and performance of a cathode CL under low air flowrate are investigated. Electrochemical methods, including electrochemical impedance, cyclic votammetry and polarization curves, are used in conjunction with surface morphology characterization to correlate electrochemical characteristics with CL microstructure. CLs in the form of catalyst-coated membrane (CCM) have higher cell open circuit voltages (OCVs) and higher limiting current density; while catalyzed-diffusion-media (CDM) CLs display better performance in the moderate current density region. The CL with a composite structure, consisting both CCM and CDM, shows better performance in both kinetic and mass-transport limitation region, due to a suitable ionomer distribution across the CL. This composite cathode is further evaluated in a full DMFC and the cathode performance loss due to methanol crossover is discussed.