Membraneless laminar flow-based micro fuel cells operating in alkaline, acidic, and acidic/alkaline media

The lack of a polymer electrolyte membrane (PEM, e.g. Nafion) in membraneless, laminar flow-based micro fuel cells (LF-FCs) eliminates several PEM-related issues such as fuel crossover, cathode flooding, and anode dry-out, as we reported previously. This paper explores the media flexibility of LF-FCs by working in acidic and alkaline media, as well under “mixed-media” conditions in which the anode is in acidic media while the cathode is in alkali, or vice versa. Operating a fuel cell under alkaline conditions has positive effects on the reaction kinetics, both at the anode and cathode, while the cell performance under “mixed-media” conditions offers an opportunity to increase the maximum achievable open cell potential (OCP). The lack of media-related constraints and the simplicity of the LF-FC design allow for these experiments to be performed consecutively in a single LF-FC without changing the system, except for altering the composition/pH of the fuel and oxidant stream. The performance of LF-FCs operated with different media is described and compared.     

An alkaline microfluidic fuel cell based on formate and hypochlorite bleach

An alkaline microfluidic fuel cell is demonstrated employing an alkaline version of a formic acid anode and a sodium hypochlorite cathode. Both sodium formate fuel and sodium hypochlorite oxidant are available and stable as highly concentrated solutions, thereby facilitating fuel cell systems with high overall energy density. Sodium hypochlorite is commonly available as hypochlorite bleach. The alkaline anodic half-cell produces carbonate rather than the less-desirable gaseous CO₂, while sustaining the rapid kinetics associated with formic acid oxidation in acidic media. Both half-cells provide high current densities at relatively low overpotentials and are free of gaseous products that may otherwise limit microfluidic fuel cell performance. The microfluidic fuel cell takes advantage of a recently developed membraneless architecture with flow-through porous electrodes. Power densities up to 52 mW cm⁻² and overall energy conversion efficiencies up to 30% per single pass are demonstrated at room temperature using 1.2 M formate fuel and 0.67 M hypochlorite oxidant. The alkaline formate/hypochlorite fuel and oxidant combination demonstrated here, or either one of its individual half-cells, may also be useful in conventional membrane-based fuel cell designs.     

Study of direct type ethanol fuel cells: Analysis of anode products and effect of acetaldehyde

The anode products are observed when ethanol fuel is circulated in the direct ethanol fuel cell system using Nafion^® as an electrolyte. The main products are CO₂ and acetaldehyde. I-V characteristics of a direct type fuel cell using ethanol and acetaldehyde as fuels are investigated. Anode and cathode overpotentials are also measured to analyze the characters of the polarization curves obtained for both fuels. The MEA consisted of PtRu anode catalyst. The voltage drops as the concentration of acetaldehyde solution increases. In the case of ethanol solution, the voltage increases as the concentration increases. The anode overpotential increases as the concentration of acetaldehyde increases although the increase of cathode overpotential is smaller than that of anode overpotential. The opposite result is observed for ethanol solutions, i.e., the anode overpotential increases as the concentration of ethanol decreases. This result shows that the voltage drop observed for acetaldehyde solution results from the anode overpotential. Rotating disc electrode (RDE) measurements and polarization curve measurements were also performed to confirm the relation between acetaldehyde concentration and overpotentials. It is supposed that the electrocatalytic oxidation mechanism of acetaldehyde on PtRu catalyst is different from that of ethanol.     

Miniature fuel cells fabricated on silicon substrates

Novel miniature fuel cells were fabricated from micromachined silicon wafers. The cells used methanol and air as reactants, and a thin polymer electrolyte as separator. The assembled cells had a working volume of 12 mm³ and could be scaled down in size by three orders of magnitude by simple adjustments of the masking and etching procedures. Electrodeposition of Pt-Ru as the anode catalyst (oxidation of methanol) was successful in lowering the loading to 0.25 mg/cm² without loss of performance. Cell performance approached that of the best, state-of-the-art, large fuel cells, when scaled for size. In particular, single miniature cells yielded 822 Wh/kg and 924 Wh/L when operated at 70°C. The same chip design was also used for the hydrogen/air system, and the cell current, power, and specific energy density were higher than those of methanol/air, Further tailoring of the chips for spefici fuels could lead to further improvements.     

Novel approach to membraneless direct methanol fuel cells using advanced 3D anodes

A conventional membrane electrode assembly (MEA) for a direct methanol fuel cell (DMFC) consists of a polymer electrolyte membrane (PEM) compressed between an anode and cathode electrode. Limitations with this conventional design include: cost, fuel crossover, membrane degradation or contamination, ohmic losses and reduced active triple phase boundary (TPB) sites for catalyst located away from the electrode/membrane interface. In this work, ex situ and in situ characterization of a novel electrode assembly based on a membraneless architecture and advanced 3D anodes was investigated. The approach was shown to be fuel independent and scaleable to a conventional bi-polar fuel cell arrangement. The membraneless configuration exhibits comparable performance to a conventional ambient (25 °C, 1 atm) air-breathing DMFC. However, it has the additional advantages of a simplified design, the elimination of the membrane (a significant component expense) and enhanced fuel and catalyst utilization through the extension of the active catalyst zone.     

Role of the diffuse layer in acidic and alkaline fuel cells

A numerical model is developed to study electrolyte dependent kinetics in fuel cells. The model is based on the Poisson–Nernst–Planck (PNP) and generalized-Frumkin–Butler–Volmer (gFBV) equations, and is used to understand how the diffuse layer and ionic transport play a role in the performance difference between acidic and alkaline systems. The laminar flow fuel cell (LFFC) is used as the model fuel cell architecture to allow for the appropriate comparison of equivalent acidic and alkaline systems. We study the overall cell performance and individual electrode polarizations of acidic and alkaline fuel cells for both balanced and unbalanced electrode kinetics as well as in the presence of transport limitations. The results predict cell behavior based on electrolyte composition that strongly correlates with observed experimental results from literature and provides insight into the fundamental cause of these results. Specifically, it is found that the working ion concentration at the reaction plane plays a significant role in fuel cell performance including activation losses and the response to different kinetic rates at an individual electrode. The working ion and the electrode where its consumed are different for acidic and alkaline fuel cells. Therefore, we compare the role of the diffuse region in both acidic and alkaline fuel cells. From this we conclude that oxidant reduction at the cathode and slow fuel oxidation (such as alcohol oxidation) can be improved with an alkaline electrolyte.     

The effect of methanol and ethanol cross-over on the performance of PtRu/C-based anode DAFCs

In the present work, the cross-over rates of methanol and ethanol, respectively, through Nafion^®-115 membranes at different temperatures and different concentrations have been measured and compared. The changes of Nafion^®-115 membrane porosity in the presence of methanol or ethanol aqueous solutions were also determined by weighing vacuum-dried and alcohol solution-equilibrated membranes. The techniques of anode polarization and adsorption stripping voltammetry were applied to compare the electrochemical activity and adsorption ability, respectively. To investigate the consequences of methanol and ethanol permeation from the anode to the cathode on the performance of direct alcohol fuel cells (DAFCs), single DAFC tests, with methanol or ethanol as the fuel, have been carried out and the corresponding anode and cathode polarizations versus dynamic hydrogen electrode (DHE) were also performed. The effect of alcohol concentration on the performance of PtRu/C anode-based DAFCs was investigated.It was found that ethanol shows lower cross-over rates than methanol through the Nafion^® membrane in spite of the higher membrane porosity resulted in presence of ethanol aqueous solutions. Furthermore, it was found that ethanol presents less negative effect on the cathode performance due to both its smaller permeability through Nafion^® membrane and its slower electrochemical oxidation kinetics over Pt/C cathode.     

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

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

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

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

Performance Study of Direct Borohydride Fuel Cells Employing Polyvinyl Alcohol Hydrogel Membrane and Nickel‐Based Anode

A direct borohydride fuel cell (DBFC) employing a polyvinyl alcohol (PVA) hydrogel membrane and a nickel‐based composite anode is reported. Carbon‐supported platinum and sputtered gold have been employed as cathode catalysts. Oxygen, air and acidified hydrogen peroxide have been used as oxidants in the DBFC. Performance of the PVA hydrogel membrane‐based DBFC was tested at different temperatures and compared with similar DBFCs employing Nafion® membrane electrolytes under identical conditions. The borohydride–oxygen fuel cell employing PVA hydrogel membrane yielded a maximum peak power density of 242 mW cm^–2 at 60 °C. The peak power densities of the PVA hydrogel membrane‐based DBFCs were comparable or a little higher than those using Nafion® 212 membranes at 60 °C. The fuel efficiency of borohydride–oxygen fuel cell based on PVA hydrogel membrane and Ni‐based composite anode was found to be between 32 and 41%. The cell was operated for more than 100 h and its performance stability was recorded.     

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.     

Performance improvement of a micro borohydride fuel cell operating at ambient conditions

In this study, aqueous borohydride solutions were employed to fuel a micro cell. Electrochemical performance of the micro borohydride fuel cell was tested at ambient conditions without any auxiliary facilities. Electrochemical impedance spectroscopy (EIS) analyses were performed to characterize the cell performance. Both anion and cation exchange membranes were tried to separate the fuel from the cathode. Membrane properties were found to be a decisive factor for cell performance. A maximum power density of 40 mW/cm² at room temperature was achieved when the Nafion NRE211 membrane was used. Hydrogen evolution at the anode side resulted from the competitive hydrolysis reaction influenced cell performance by obstructing transfer of the electrolyte. The cell also demonstrated promising performance even when an Ag cathode was used.     

Ethanol crossover and direct ethanol PEM fuel cell performance modeling and experimental validation

In the present work, a one-dimension, steady-state and single phase model is developed with the purpose of describing the mass transport within a PtRu/Nafion^®-115/Pt membrane-electrode assembly and the performance of a direct ethanol proton exchange membrane fuel cell (DE-PEMFC). The effect of the most important cell operating parameters on the ethanol crossover rate and the fuel cell performance is investigated. According to the results, in the case of low current density values and high concentrations of ethanol aqueous solutions, ethanol crossover could pose serious problems to the DEFC operation. Moreover, it was pointed out that the ethanol crossover rate dependence on the ethanol feed concentration is an almost linear function presenting a maximum at about . A further increase of the ethanol feed concentration leads to a steep decrease of ethanol crossover rate. This behavior could be attributed to the membrane swelling which is responsible for the membrane volume fraction decrement. It was also found that by the aid of the same model the performance of a direct ethanol PEM fuel cell over three different anode catalysts can be predicted. A relatively good agreement between theory and experimental results related to both ethanol crossover rates and direct ethanol fuel cell performance was found.     

Preparation of a PVA/HAP composite polymer membrane for a direct ethanol fuel cell (DEFC)

A novel PVA/Hydroxyapatite (HAP) composite polymer membrane was prepared by the direct blend process and solution casting method. The characteristic properties of the PVA/HAP composite polymer membranes were investigated using thermal gravimetric analysis (TGA), X-ray diffraction (XRD), scanning electron microscopy (SEM), micro-Raman spectroscopy and the AC impedance method. An alkaline direct ethanol fuel cell, consisting of an air cathode with MnO₂ carbon inks based on Ni-foam, an anode with PtRu black on Ni-foam, and the PVA/HAP composite polymer membrane, was assembled and investigated. It was found that the alkaline direct ethanol fuel cell comprising of a novel cheap PVA/HAP composite polymer membrane showed an improved electrochemical performance in ambient temperature and air. As a result, the maximum power density of the alkaline DEFC, using a PtRu anode based on Ni-foam (10.74 mW cm⁻²), is higher than that of DEFC using an E-TEK PtRu anode based on carbon (7.56 mW cm⁻²) in an 8M KOH + 2M C₂H₅OH solution at ambient temperature and air. These PVA/HAP composite polymer membranes are a potential candidate for alkaline DEFC applications.     

Direct methanol alkaline fuel cells with catalysed anion exchange membrane electrodes

Membrane electrodes prepared by chemical deposition of platinum directly onto the anion exchange membrane electrolyte were tested in direct methanol alkaline fuel cells. Data on the cell voltage against current density performance and anode potentials are reported. The relatively low fuel cell performance was probably due to the low active surface area of Pt deposits on the membrane comparing to other membrane electrode assembly (MEA) fabrication methods. However, the catalysed membrane electrode showed good performance for oxygen reduction. A reduction in cell internal resistance was also obtained for the catalysed membrane electrode. By combining the catalysed membrane electrodes with a catalysed mesh, maximum current density of 98 mA cm^–2 and peak power density of 18 mW cm^–2 were achieved.     

Study of oxygenated biomass fuel blends on a diesel engine

Vegetable methyl ester was added in ethanol–diesel fuel to prevent separation of ethanol from diesel in this study. The ethanol blend proportion can be increased to 30% in volume by adding the vegetable methyl ester. Engine performance and emissions characteristics of the fuel blends were investigated on a diesel engine and compared with those of diesel fuel. Experimental results show that the torque of the engine is decreased by 6%–7% for every 10% (by volume) ethanol added to the diesel fuel without modification on the engine. Brake specific fuel consumption (BSFC) increases with the addition of oxygen from ethanol but equivalent brake specific fuel consumption (EBSFC) of oxygenated fuels is at the same level of that of diesel. Smoke and particulate matter (PM) emissions decrease significantly with the increase of oxygen content in the fuel. However, PM reduction is less significant than smoke reduction. In addition, PM components are affected by the oxygenated fuel. When blended fuels are used, nitrogen oxides (NO_x) emissions are almost the same as or slightly higher than the NO_x emissions when diesel fuel is used. Hydrocarbon (HC) is apparently decreased when the engine was fueled with ethanol–ester–diesel blends. Fuelling the engine with oxygenated diesel fuels showed increased carbon monoxide (CO) emissions at low and medium loads, but reduced CO emissions at high and full loads, when compared to pure diesel fuel.     

A simple method for the synthesis of PtRu nanoparticles on the multi-walled carbon nanotube for the anode of a DMFC

We report the synthesis of PtRu nanoparticles on the multi-walled carbon nanotubes (MWCNTs) by a simple sodium borohydride reduction method. Transmission electron microscopy (TEM) analysis indicated that well-dispersed small (2-3 nm) PtRu particles were formed on the MWCNTs. X-ray diffraction (XRD) analysis confirmed the formation of the PtRu alloy on the MWCNTs. X-ray photoelectron spectroscopy (XPS) measurements revealed that 70.4% Pt and 61.0% Ru are present in their metallic states. Cyclic voltammetry (CV) and chronoamperometry results demonstrated that the PtRu/MWCNT synthesized by this method exhibited a higher methanol oxidation current than did the PtRu/MWCNT synthesized by the more complex method using sodium borohydride as the reducing agent and tetraoctyl ammonium bromide as the stabilizer. Finally, the direct methanol fuel cell (DMFC) performance test showed that the PtRu/MWCNT nanocatalyst used at the anode of the fuel cell yielded higher performance than did the commercial E-TEK PtRu/C catalyst.     

Emission characteristics using methyl soyate–ethanol–diesel fuel blends on a diesel engine

A blend of 20% (v/v) ethanol/methyl soyate was prepared and added to diesel fuel as an oxygenated additive at volume percent levels of 15 and 20% (denoted as BE15 and BE20). We also prepared a blend containing 20% methyl soyate in diesel fuel (denoted as B20). The fuel blends that did not have any other additive were stable for up to 3 months. Engine performance and emission characteristics of the three different fuels in a diesel engine were investigated and compared with the base diesel fuel. Observations showed that particulate matter (PM) emission decreased with increasing oxygenate content in the fuels but nitrogen oxides (NO_x) emissions increased. The diesel engine fueled by BE20 emitted significantly less PM and a lower Bosch smoke number but the highest NO_x among the fuel blends tested. All the oxygenate fuels produced moderately lower CO emissions relative to diesel fuel. The B20 blend emitted less total hydrocarbon (THC) emissions compared with base diesel fuel. This was opposite to the fuel blends containing ethanol (BE15, BE20), which produced much higher THC emission.     

Performance of a direct methanol fuel cell

The performance of a direct methanol fuel cell based on a Nafion® solid polymer electrolyte membrane (SPE) is reported. The fuel cell utilizes a vaporized aqueous methanol fuel at a porous Pt–Ru–carbon catalyst anode. The effect of oxygen pressure, methanol/water vapour temperature and methanol concentration on the cell voltage and power output is described. A problem with the operation of the fuel cell with Nafion® proton conducting membranes is that of methanol crossover from the anode to the cathode through the polymer membrane. This causes a mixed potential at the cathode, can result in cathode flooding and represents a loss in fuel efficiency. To evaluate cell performance mathematical models are developed to predict the cell voltage, current density response of the fuel cell.     

直接硼氢化物燃料电池阳极电催化剂研究进展

以硼氢化物作为燃料电池的燃料因其高的理论电动势和比能量而引起研究者的广泛关注。理论上,BH-4的电氧化反应为八电子反应,但实际上由于所用阳极电催化剂的不同,BH-4电氧化释放出的电子数也不同。如何抑制BH-4在阳极的水解反应,促进其八电子氧化反应一直是直接硼氢化物燃料电池研究中的核心问题。综述了近几年来国内外在直接硼氢化物燃料电池阳极电催化剂方面所取得的研究进展,并对这一领域中需要深入研究的主要问题进行了论述。