Characterization of direct formic acid fuel cells by Impedance Studies: In comparison of direct methanol fuel cells

We characterized direct liquid fuel cells by electrochemical impedance spectroscopy (EIS) combined with reversible hydrogen electrode (RHE) under fuel cell operating conditions. EIS has been successfully implemented as an in-situ diagnostic tool using an impedance setup with RHE, capable of singling out individual contributions to the overall polarization of fuel cells and separating the anode and cathode contributions. While a direct methanol fuel cell (DMFC) anode was subject to substantial poisoning by reaction intermediates due to better accessibility of methanol to catalyst surface regardless of anode diffusion media, a direct formic acid fuel cell (DFAFC) anode suffered from significant mass transfer limitation depending on the anode diffusion media property and formic acid concentration. The high frequency resistance of a DFAFC cathode increased linearly with an increase of formic acid concentration by membrane dehydration effect. Interestingly, on both the DMFC and DFAFC cathodes, decrease in the mixed charge transfer resistance with an increase of fuel crossover was observed together with a drop in the cathode potential.     

An impedance study of an operating direct methanol fuel cell

An electrochemical impedance spectroscopy (EIS) technique was developed to characterize a direct methanol fuel cell (DMFC) under various operating conditions. A silver/silver chloride electrode was used as an external reference electrode to probe the anode and cathode during fuel cell operation and the results were compared to the conventional anode or cathode half-cell performance measurement using a hydrogen electrode as both the counter and reference electrode. The external reference was sensitive to the anode and the cathode as current was passed in a working DMFC. The impedance spectra and DMFC polarization curves were systematically investigated as a function of air and methanol flow rates, methanol concentration, temperature, and current density. Water flooding in the cathode was also examined.     

Fabrication and characterization of gold nanoparticles and fullerene-C60 nanocomposite film at glassy carbon electrode as potential electro-catalyst towards the methanol oxidation

Development of low cost anodic materials and high efficient electro-kinetics of methanol in direct methanol fuel cell (DMFC) has been a promising approach. However it has not been successfully reached to market from laboratory due to its high cost and low kinetic oxidation. Both issues encounter from one of its main components, the catalyst. Therefore, present work focuses upon the development of new catalyst material and optimization of various most significant influencing parameters of a high performance DMFC. We have developed a nanocomposite material employing gold nanoparticles and fullerene-C₆₀ at glassy carbon electrode (AuNP@reduced-fullerene-C₆₀/GCE) as anode for high performance oxidation of methanol. Fullerene-C₆₀ was manually dropped on pre treated GCE and partially electro-reduced in KOH to make it more conductive. Gold nanoparticles (AuNPs) were deposited on reduced-fullerene-C₆₀ modified electrode using cyclic voltammetry (CV). Electrochemical characterization techniques such as CV, electrochemical impedance spectroscopy (EIS) and chronocoulometry were used to characterize modified electrode. Modified electrode was also characterized by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) for morphological properties. The electrochemical behavior of methanol was performed in alkaline medium using CV and chronoamperometry methods. The results revealed good electrocatalytic performance and better stability than previously reported catalysts using AuNP@reduced-fullerene-C₆₀ catalyst, suggesting making promising anodic material for direct methanol oxidation fuel cell.     

Numerical analysis of the electrochemical impedance spectra of the cathode of direct methanol fuel cells

A mathematical model is developed to simulate the electrochemical impedance spectra (EIS) of the cathode of a direct methanol fuel cell (DMFC) based on the electrode kinetics and mass transports. Successful simulation of the impedance spectra confirms the usefulness of the model as a diagnostic tool for interpreting the impedance characteristics of the cathode. Numerically, the capacitive semicircle in the impedance pattern is ascribed to the charge transfer process and the inductive semicircle is mainly due to the CO adsorption relaxation. Results show that the impedance pattern is strongly dependent on the electrode potential, which can be used as a criterion for judging the relative effect of the methanol permeation on the cathode. Another capacitive semicircle appears and the charge transfer resistance is changed when the oxygen transport is limited. The effects of the methanol permeation on the impedance pattern are also delineated, indicating that the methanol permeation often leads to larger oxygen transport impedance and the charge transfer resistance of the DMFC cathode depends on the methanol permeation rate.     

Long-term durability test for direct methanol fuel cell made of hydrocarbon membrane

A long-term durability test has been conducted for a direct methanol fuel cell (DMFC) using the commercial hydrocarbon membrane and Nafion ionomer bonded electrodes for 500 h. Membrane electrode assembly (MEA) made by a decal method has experienced a performance degradation about 34% after 500 h operation. Cross-sectional analysis of the MEA shows that the poor interfacial contact between the catalyst layers and membrane in the MEA has further deteriorated after the durability test. Therefore, the internal resistance of a cell measured by electrochemical impedance spectroscopy (EIS) has considerably increased. The delamination at the interfaces is mainly attributed to incompatibility between polymeric materials used in the MEA. Furthermore, X-ray diffraction (XRD) analysis reveals that the catalyst particles have grown; thereby decreasing the electrochemical surface area. Electron probe micro analysis (EPMA) shows a small amount of Ru crossover from anode to cathode; and its effect on the performance degradation has been analyzed.     

Carbon nanotube supported Pt electrodes for methanol oxidation: A comparison between multi- and single-walled carbon nanotubes

This work presents a detailed comparison between multi-walled (MWNT) and single-walled carbon nanotubes (SWNT) in an effort to understand which can be the better candidate of a future supporting carbon material for electrocatalyst in direct methanol fuel cells (DMFC). Pt particles were deposited via electrodeposition on MWNT/Nafion and SWNT/Nafion electrodes to investigate effects of the carbon materials on the physical and electrochemical properties of Pt catalyst. The crystalloid structure, texture (surface area, pore size distribution, and macroscopic morphology), and surface functional groups for MWNT and SWNT were studied using XRD, BET, SEM and XPS techniques. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were employed to characterize the electrochemically accessible surface area and charge transfer resistances of the MWNT/Nafion and SWNT/Nafion electrodes. CO stripping voltammograms showed that the onset and peak potentials on Pt-SWNT/Nafion were significantly lower that those on the Pt-MWNT/Nafion catalyst, revealing a higher tolerance to CO poisoning of Pt in Pt-SWNT/Nafion. In methanol electrooxidation reaction, Pt-SWNT/Nafion catalyst was characterized by a significantly higher current density, lower onset potentials and lower charge transfer resistances using CV and EIS analysis. Therefore, SWNT presents many advantages over MWNT and would emerge as an interesting supporting carbon material for fuel cell electrocatalysts. The enhanced electrocatalytic properties were discussed based on the higher utilization and activation of Pt metal on SWNT/Nafion electrode. The remarkable benefits from SWNT were further explained by its higher electrochemically accessible area and easier charge transfer at the electrode/electrolyte interface due to SWNT's sound graphitic crystallinity, richness in oxygen-containing surface functional groups and highly mesoporous 3D structure.     

An impedance study for the anode micro-porous layer in an operating direct methanol fuel cell

This study presents the benefit to an operating direct methanol fuel cell (DMFC) by coating a micro-porous layer (MPL) on the surface of anode gas diffusion layer (GDL). Taking the membrane electrode assembly (MEA) with and without the anodic MPL structure into account, the performances of the two types of MEA are evaluated by measuring the polarization curves together with the specific power density at a constant current density. Regarding the cell performances, the comparisons between the average power performances of the two different MEAs at low and high current density, various methanol concentrations and air flow rates are carried out by using the electrochemical impedance spectroscopy (EIS) technique. In contrast to conventional half cell EIS measurements, both the anode and cathode impedance spectra are measured in real-time during the discharge regime of the DMFC. As comparing each anode and cathode EIS between the two different MEAs, the influences of the anodic MPL on the anode and cathode reactions are systematically discussed and analyzed. Furthermore, the results are used to infer complete and reasonable interpretations of the combined effects caused by the anodic MPL on the full cell impedance, which correspond with the practical cell performance.     

Metamorphosis of the mixed phase PtRu anode catalyst for direct methanol fuel cells after exposure of methanol: In situ and ex situ characterizations

The change in the mixed phase heavily oxidized PtRu anode with the exposure of methanol in a direct methanol fuel cell (DMFC) has been investigated by electrochemical impedance spectroscopy (EIS) and X-ray diffraction (XRD). The investigation had two major objectives: (i) to explore the original state of the active catalyst and (ii) to understand if alloying of Pt and Ru is a requirement for higher methanol oxidation activity. It was found that the methanol oxidation activity gradually improved for ∼2 h of exposure. The impedance spectra were taken at different times within this time of improvement of activity. The impedance spectra were deconvoluted in different contributions like membrane resistance (R_m), charge transfer resistance (R_Ct), adsorption resistance (R_ad), and oxidation resistance (R_ox). The improvement of the activity was explained in terms of the effect of the pretreatment on different contributions. XRD was done on the virgin and methanol exposed sample as a possible mean to identify the difference. It was postulated that the reduction of the as prepared PtRu after exposure was responsible for the activity improvement. Also, it was shown that bulk alloy formation is not a necessary condition for higher methanol activity of PtRu catalysts.     

Electrochemically synthesized polypyrrole/MWCNTs-Al2O3 ternary nanocomposites supported Pt nanoparticles toward methanol oxidation

As known, a good support enhances the activity and durability of any catalyst. In the current study, polypyrrole (PPY)/nanocomposite (MWCNTs and Al₂O₃) films were fabricated by electrochemical polymerization of pyrrole solution with a certain amount of nanoparticles on titanium substrates and were used as new support materials for Pt catalyst. The modified electrodes were characterized by Fourier transform infrared (FT-IR) spectroscopy, field-emission scanning electron microscopy (FE-SEM) and energy dispersive X-ray analysis (EDX) techniques. High catalytic activity and long-time stability toward methanol oxidation of Pt/PPY–MWNTs-αAl₂O₃ catalyst have also been verified by cyclic voltammetry results and chronoamperometric response measurements. This catalyst exhibits a vehemently high current density (345.03 mA cm^?2) and low peak potential (0.74 v) for methanol oxidation. Other electrochemical measurements (electrochemical impedance spectroscopy (EIS), CO stripping voltammetry and Tafel test) clearly confirmed that Pt/PPY–MWNTs-αAl₂O₃/Ti electrode has a better performance toward methanol oxidation compared to the other electrodes and that can be used as a promising electrode material for application in direct methanol fuel cells (DMFCs).     

Excellent electrocatalysis of methanol oxidation on platinum nanoparticles supported on carbon-coated silicon

The prepared carbon-coated silicon (Si@C) material was blended with graphite powder together to form the specific carbon paste electrode with different mass percent X% of Si@C (CPE-Si@C(X%)). The electrochemical impedance spectroscopy (EIS) was performed on the prepared CPE-Si@C and the pure carbon paste electrode (CPE), and the results show that the CPE-Si@C (X%) electrode has a smaller charge transfer resistance. Pt/CPE and Pt/CPE-Si@C(X%) electrodes were prepared by electrodepositing Pt particles on CPE-Si@C and CPE, and the obtained electrodes were used for electrocatalytic oxidation of methanol in acid media. The results show that the activity of Pt/CPE-Si@C(X%) electrode for electrocatalytic oxidation of methanol was higher than that of Pt/CPE electrode, and the mass peak current density of Pt/CPE-Si@C(10%) electrode for electrocatalytic oxidation of methanol reached 321 mA mg^?1, which was 1.8 times higher than that of Pt/CPE electrode. The Pt/CPE-Si@C (10%) electrode and the Pt/CPE electrode were characterized by chronoamperometry. The results show that Pt/CPE-Si@C (10%) has a better stability of activity and stronger tolerance against CO poisoning.     

Pt–Ru nanoparticles anchored on poly(brilliant cresyl blue) as a new polymeric support: Application as an efficient electrocatalyst in methanol oxidation reaction

The glassy carbon electrode is modified by poly(brilliant cresyl blue) (PBCB) to be applied as a new green and efficient platform for Pt and Pt–Ru alloy nanoparticles deposition. Surface composition, morphology and catalytic activity of these modified electrodes towards methanol oxidation are assessed by applying X-ray diffraction, field emission scanning electron microscopy, cyclic voltammetry and electrochemical impedance spectroscopy techniques. The X-ray diffraction patterns reveal that the highly crystalline Pt and Pt–Ru alloy and RuO₂ nanoparticles with low crystallinity are deposited on the PBCB modified glassy carbon electrodes. The microscopic images indicate smaller size and better distribution of deposited nanoparticles on the surface of PBCB modified electrodes. Cyclic voltammetry and electrochemical impedance spectroscopy results reveal that PBCB supported Pt and Pt–Ru nanoparticles have better electrocatalytic performance and durability towards methanol oxidation rather than the unsupported nanoparticles. From the obtained results it can be concluded that the presence of PBCB not only improves the stability of nanoparticles on the surface, but also leads to the formation of smaller size and more uniform distribution of nanoparticles on the surface, which, in turn, cause the nanoparticles to provide a higher accessible surface area and more active centers for the oxidation of methanol. The results will be valuable in extending the applications of this polymer in surface modification steps and in developing promising catalyst supports to be applied in direct methanol fuel cells.     

On the electrochemical impedance spectroscopy of direct methanol fuel cell

The direct methanol fuel cell (DMFC) was operated under a variety of current densities to monitor the electrochemical impedance spectroscopy (EIS) for understanding its reaction mechanism. Based on the EIS analysis, the impedance of the cell reaction is divided into three components, two of them are current dependent and the remainder is current independent. Through detailed exploration of the impedance components, the high-frequency impedance was attributed to interfacial behavior, the medium-frequency impedance to electrochemical reactions, and the low-frequency impedance to the adsorption/relaxation of CO. Based on EIS analysis, a qualitative model is proposed to delineate the reaction mechanisms of DMFC, which is confirmed quantitatively by one set of equivalent circuit elements. The experimental data are satisfactorily consistent with the results simulated from the proposed model.     

Simplified model for the direct methanol fuel cell anode

A kinetic model for the anode of the direct methanol fuel cell (DMFC) is presented. The model is based on the generally accepted dual site mechanism of methanol oxidation, in aqueous solution, on well characterized Pt–Ru catalyst and it can predict the performance of the electrode as a function of cell temperature, anode potential and methanol concentration. In addition the model also generates data regarding the surface coverage of significant adsorbates involved in methanol oxidation on the dual site catalyst.     

Development of a direct methanol fuel cell stack fed with pure methanol

Two passive fuel cell stacks with the same four MEAs in a series connection have been fabricated, tested, and compared. The dilute-stack was filled with 30 mL dilute methanol solutions (1–3 M), whereas the pure-stack was driven by 3 mL pure methanol. In the pure-stack, porous components were added on both sides of the MEAs to modify its mass transfer characteristics so that the stack could directly use pure methanol as fuel without having severe methanol crossover. The performance, fuel efficiency, energy efficiency, and electrochemical impedance spectroscopy (EIS) responses of the passive dilute-stack and pure-stack were measured at room temperature with different fuels. The pure-stack using pure methanol showed similar performance with the dilute-stack using 1 M methanol solution. The measured fuel efficiency and energy efficiency of the pure-stack were 53.6% and 13.3%, respectively, at 1.2 V. Since 100% methanol, instead of the less than 10% methanol solutions, was used as fuel, the energy density of the pure-stack per weight of fuel was more than 10 times higher than that of the dilute stack.     

Analysis of performance losses of direct methanol fuel cell with methanol tolerant PtCoRu/C cathode electrode

The long-term stability of PtCoRu/C to methanol crossover has been evaluated in a direct methanol fuel cell (DMFC) configuration. The DMFC has been subjected to continuous operation under potential step cycles. The degradation of the DMFC with PtCoRu/C has been followed by comparison of the power density curves recorded after 0, 60 and 312 h of continuous operation, and compared to that recorded for a DMFC with Pt/C. Electrochemical Impedance Spectra (EIS) were recorded directly from the DMFCs and used to identify the main degradation phenomena responsible for the loss of performance of the used fuel cell. AC impedance spectra show that the resistance of the anode reaction increases while resistance associated to the cathode reaction decreases after the long-term stability tests; however, the analysis of the power density curves unequivocally show that the performance of the DMFCs goes down during the stability tests. This apparent contradiction can be explained by taking into account the changes between the fresh and used PtCoRu/C observed by X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses. During the potential step cycles Ru dissolves form PtCoRu/C leading to Pt-enriched catalysts which are more active for the oxygen reduction reaction (lower resistance) but less tolerant to methanol (lower power density).     

Electrochemical impedance studies on carbon supported PtRuNi and PtRu anode catalysts in acid medium for direct methanol fuel cell

The anodic Pt–Ru–Ni/C and the Pt–Ru/C catalysts for potential application in direct methanol fuel cell (DMFC) were prepared by chemical reduction method. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) measurements were carried out by using a glassy carbon working electrode covered with the catalyst powder in a solution of 0.5 mol L⁻¹ CH₃OH and 0.5 mol L⁻¹ H₂SO₄ at 25 °C. EIS information discloses that the methanol electrooxidation on the Pt–Ru–Ni/C catalyst at various potentials shows different impedance behaviors. The mechanism and the rate-determining step of methanol electrooxidation are changed with increasing potential. Its rate-determining steps are the methanol dehydrogenation and the oxidation reaction of adsorbed intermediate CO_ads and OH_ads in low (400–500 mV) and high (600–800 mV) potentials, respectively. The catalytic activity of the Pt–Ru–Ni/C catalyst is higher for methanol electrooxidation than that of the Pt–Ru/C catalyst. Its tolerance performance to CO formed as one of the intermediates of methanol dehydrogenation is also better than that of the Pt–Ru/C catalyst.     

Basic model for membrane electrode assembly design for direct methanol fuel cells

This research proposes a model that predicts the effect of the anode diffusion layer and membrane properties on the electrochemical performance and methanol crossover of a direct methanol fuel cell (DMFC) membrane electrode assembly (MEA). It is an easily extensible, lumped DMFC model. Parameters used in this design model are experimentally obtainable, and some of the parameters are indicative of material characteristics. The quantification of these material parameters builds up a material database. Model parameters for various membranes and diffusion layers are determined by using various techniques such as polarization, mass balance, electrochemical impedance spectroscopy (EIS), and interpretation of the response of the cell to step changes in current. Since the investigation techniques cover different response times of the DMFC, processes in the cell such as transport, reaction and charge processes can be investigated separately. Properties of single layers of the MEA are systematically varied, and subsequent analysis enables identification of the influence of the layer's properties on the electrochemical performance and methanol crossover. Finally, a case study indicates that the use of a membrane with lower methanol diffusivity and a thicker anode micro-porous layer (MPL) yields MEAs with lower methanol crossover but similar power density.     

Study of pulsed-current loading of direct methanol fuel cells using a new time-domain model based on bi-functional methanol oxidation kinetics

This work describes a non-linear time-domain model of a direct methanol fuel cell (DMFC) and uses that model to show that pulsed-current loading of a direct methanol fuel cell does not improve average efficiency. Unlike previous system level models, the one presented here is capable of predicting the step response of the fuel cell over its entire voltage range. This improved model is based on bi-functional methanol oxidation reaction kinetics and is derived from a lumped, four-step reaction mechanism. In total, six states are incorporated into the model: three states for intermediate surface adsorbates on the anode electrode, two states for the anode and cathode potentials, and one state for the liquid methanol concentration in the anode compartment. Model parameters were identified using experimental data from a real DMFC. The model was applied to study the steady-state and transient performance of a DMFC with the objective to understand the possibility of improving the efficiency of the DMFC by using periodic current pulses to drive adsorbed CO from the anode catalyst. Our results indicate that the pulsed-current method does indeed boost the average potential of the DMFC by 40 mV; but on the other hand, executing that strategy reduces the overall operating efficiency and does not yield any net benefit.     

A flow field enabling operating direct methanol fuel cells with highly concentrated methanol

In this work, an anode flow field that allows a direct methanol fuel cell (DMFC) to operate with highly concentrated methanol is developed and tested. The basic idea of this flow field design is to vaporize methanol solution in the flow field by utilizing the heat generated from the fuel cell so that the methanol concentration in the anode catalyst layer can be controlled to an appropriate level. The flow field is composed of two parallel flow channel plates, separated with a gap. The upper plate with a grooved serpentine flow channel is to vaporize a highly concentrated methanol solution to ensure the fuel to be completely vaporized before it enters the gap, while the lower plate, perforated to form a serpentine flow channel and located between the gap and the membrane electrode assembly (MEA), is to uniformly distribute the fuel onto the anode surface of the MEA. The test results show that this unique flow field design enables the DMFC operating with 16.0-M methanol to yield a power output similar to that with the conventional flow field design with 2.0-M methanol, significantly increasing the specific energy of the DMFC system. Finally, the effects of methanol solution flow rates and operating temperature on cell performance are investigated.     

A one dimensional model of a methanol fuel cell anode

A model of a direct methanol fuel cell anode is presented which considers electrocatalysts to have a distribution of over potential and current density in the structure. The model is applicable to an anode based on a metal mesh supported electrocatalysts structure. Methanol oxidation is described by dual site mechanisms involving adsorbed CO and OH intermediates. The model is used to predict the electrode potential–current density behaviour of the anode. The concentration of methanol is shown to influence overall electrode polarisation characteristics and critically the selection of the mechanism for methanol oxidation has a major impact in this respect. The model gives good correspondence with experimentally observed cell polarisation behaviour.