Synthesis, characterization and application of platinum based bi-metallic catalysts for direct glucose alkaline fuel cell

In the context of development of direct glucose fuel cell (DGFC), low metal loading (ca. 15 wt.%) bi-metallic platinum–bismuth (PtBi/C) and platinum–gold (PtAu/C) catalysts are synthesized by immobilizing metal sols on carbon substrate (Vulcan XC 72R). Physical characterization of electro-catalysts, studied using TEM, SEM, EDX and XRD, reveals the formation of nano-sized metal particles on carbon substrate. The cyclic voltammetry and chronoamperometry of the prepared catalysts point out that PtAu/C is more active and stable than PtBi/C and commercial PtRu/C towards glucose electro-oxidation in alkaline medium. The catalysts are tested as anode in batch DGFC using activated charcoal as cathode in different glucose and electrolyte (KOH solution) concentrations at ambient temperature (30 °C). Open-circuit voltage of ∼0.9 V is obtained for PtAu/C and commercial PtRu/C and 0.8 V for PtBi/C anode in 0.2 M glucose and in 1 M KOH. However, the peak power density per unit metal loading or specific peak power density obtained is 1.6 mW cm⁻² mg⁻¹ for PtAu/C followed by PtBi/C (1.25 mW cm⁻² mg⁻¹) and commercial PtRu/C (1.13 mW cm⁻² mg⁻¹). For PtBi/C and PtRu/C, the cell performance increases up to 0.2 M glucose concentration and then decreases. However, for PtAu/C catalyst the cell performance increases up to 0.3 M glucose concentration and then decreases. A prominent transition zone is observed in which current density sharply decreases with the decrease in voltage (increase in overpotential) for PtBi/C and PtRu/C at 0.3 M glucose concentration, which is not observed in the case of PtAu/C. The transition zone for PtAu/C is insignificant and at higher glucose concentration (0.4 M) pointing out that PtAu/C is much stable catalyst than PtBi/C and commercial PtRu/C.     

Ethanol oxidation on a high temperature PBI-based DEFC using Pt/C, PtRu/C and Pt3Sn/C as catalysts

A high temperature ethanol-fed polymer electrolyte membrane fuel cell has been implemented by using H₃PO₄-doped m-polybenzimidazole as polymeric electrolyte. Commercial Pt/C, PtRu/C and Pt₃Sn/C catalysts are used in the anode. The performance was assessed in terms of polarization curves at different temperatures, feeding the cell with a high concentration ethanol solution (water/ethanol mass ratio of 2). The product distribution was measured with the support of a gas chromatograph. The use of bimetallic catalysts increased the current density. PtRu/C showed the best performance up to 175 °C, but it is outperformed by Pt₃Sn/C at 200 °C. In terms of oxidation products, higher temperatures and current densities favour the oxidation of ethanol. However, Pt₃Sn/C promoted the generation of more oxidized products compared to PtRu/C (in which most of the ethanol is oxidized to acetaldehyde), especially at high temperature. This accounts for the large current density. In terms of complete oxidation of ethanol to CO₂, Pt/C was by far the most efficient catalyst for C–C scission, achieving percentages of 56 % of CO₂, although operating above 175 °C dramatically boosted an undesirable methanation process that slashed the efficiency. The combination of fuel cell results and product distribution helped to suggest the different oxidation routes on the surface of the different catalysts.     

Design of a reference electrode for high-temperature PEM fuel cells

In this work, we present the design of an external reference electrode for high-temperature PEM fuel cells. The connection between the reference electrode with one of the fuel cell electrodes is realized by an ionic connector. Using the same material for the ionic connection as for the fuel cell membrane gives us the advantage to reach temperatures above 100 °C without destroying the reference electrode. This configuration allows for the separation of the anode and cathode overpotential in a working fuel cell system. In addition to the electrode overpotentials in normal hydrogen/air operation, the influence of CO and CO + H₂O in the anode feed on the fuel cell potentials was investigated. When CO poisons the anode catalyst, not only the anode potential increased, but also the cathode overpotential, due to fewer protons reaching the cathode. By the use of synthetic reformate containing hydrogen, carbon monoxide and water on the anode, fuel cell voltage oscillations were observed at high constant current densities. The reference electrode measurements showed that the fuel cell oscillations were only related to reactions on the anode side influencing the anode overpotential. The cathode potential, in contrast, was only negligibly affected by the oscillations under the applied conditions.     

NiCoFe/C cathode electrocatalysts for direct ethanol fuel cells

A direct ethanol fuel cell (DEFC), which is less prone to ethanol crossover, is reported. The cell consists of PtRu/C catalyst as the anode, Nafion^® 117 membrane, and Ni–Co–Fe (NCF) composite catalyst as the cathode. The NCF catalyst was synthesized by mixing Ni, Co, and Fe complexes into a polymer matrix (melamine-formaldehyde resins), followed by heating the mixture at 800 °C under inert atmosphere. TEM and EDX experiments suggest that the NCF catalyst has alloy structures of Ni, Co and Fe. The catalytic activity of the NCF catalyst for the oxygen reduction reaction (ORR) was compared with that of commercially available Pt/C (CAP) catalyst at different ethanol concentrations. The decrease in open circuit voltage (V_oc) of the DEFC equipped with the NCF catalysts was less than that of CAP catalyst at higher ethanol concentrations. The NCF catalyst was less prone to ethanol oxidation at cathode even when ethanol crossover occurred through the Nafion^®117 film, which prevents voltage drop at the cathode. However, the CAP catalyst did oxidize ethanol at the cathode and caused a decrease in voltage at higher ethanol concentrations.     

Modeling and experimental validation of overpotentials of a direct ethanol fuel cell

Direct ethanol fuel cell (DEFC) is a promising power source for future use in portable electronic equipments. In general, the power density obtained in DEFC is lower than that of direct methanol fuel cell. In the present study, various losses in DEFC are estimated by performing experiments with the prepared membrane electrode (MEA) to obtain current–voltage characteristics and comparing it with the prediction of mathematical model. MEA for the DEFC is prepared using Pt–Ru (40:20 by wt.%)/C as anode catalyst, Pt–black as cathode catalyst with 1 mg/cm² of loadings and cast Nafion^® (SE5112, DuPont) ionomer as proton exchange membrane. The mathematical model for DEFC is developed considering different overpotentials. The activation overpotential term is formulated considering ethanol electrooxidation mechanism proposed in literature and Butler–Volmer equation. The ohmic overpotential is modeled based on proton conductivity of Nafion^® membrane and ohmic losses at the electrodes, current collectors and electrode–current collector interfaces. The concentration overpotential is formulated using Fick's law, modified Butler–Volmer equation and transport process through electrodes and electrocatalyst layers. The experiment data on current–voltage characteristics is predicted by the model with reasonable agreement and the influence of ethanol concentration and temperature on the performance of DEFC is captured by the model.     

A Study on Process Parameters of Direct Ethanol Fuel Cell

Ethanol is one of the promising future fuels of Direct Alcohol Fuel Cells (DAFC). The electro‐oxidation of ethanol fuel on anode made of carbon‐supported Pt‐Ru electrode catalysts was carried out in a lab scale direct ethanol fuel cell (DEFC). Cathode used was Pt‐black high surface area. The membrane electrode assembly (MEA) was prepared by sandwiching the solid polymer electrolyte membrane, prepared from Nafion^® (SE‐5112, DuPont USA) dispersion, between the anode and cathode. The DEFC was fabricated using the MEA and tested at different catalyst loadings at the electrodes, temperatures and ethanol concentrations. The maximum power density of DEFC for optimized value of ethanol concentration, catalyst loading and temperature were determined. The maximum open circuit voltage (OCV) of 0.815 V, short circuit current density (SCCD) of 27.90 mA/cm² and power density of 10.30 mW/cm² were obtained for anode (Pt‐Ru/C) and cathode (Pt‐black) loading of 1 mg/cm² at a temperature of 90°C anode and 60°C cathode for 2M ethanol.     

Investigation of fuel and media flexible laminar flow-based fuel cells

We investigate the performance of air-breathing laminar flow-based fuel cells (LFFCs) operated with five different fuels (formic acid, methanol, ethanol, hydrazine, and sodium borohydride) in either acidic or alkaline media. The membraneless LFFC architecture enables interchangeable operation with different fuel and media combinations that are only limited by the actual anode catalyst used. Furthermore, operating under alkaline conditions significantly improves methanol and ethanol oxidation kinetics and stabilizes sodium borohydride. LFFCs operated with hydrazine and sodium borohydride as fuels exhibit power densities of 80 and 101 mW/cm², respectively. To optimize anode performance, particularly for ethanol electro-oxidation, we introduced a hydrogen cathode to the membraneless LFFC design which renders the cell an ideal platform for anode investigation. Here, we highlight two simple diagnostic methods, in situ single electrode studies and electrochemical impedance spectroscopy (EIS), for characterizing and optimizing the performance of a direct ethanol LFFC anode.     

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.     

The influence of sodium ion as a potential fuel impurity on the direct methanol fuel cells

Na⁺ is a likely intrinsic impurity in water and is a sort of common cation impurity in the direct methanol fuel cells (DMFCs). In this paper, the effect of Na⁺ on the DMFC electrochemical response is studied by adding Na⁺ into the methanol water solution fed in the anode of DMFC. The dynamic variation of cell voltage results shows that the DMFC performance degraded by the presence of Na⁺ impurity, and the higher concentration of Na⁺ impurity, the higher poisoning rate is observed. In the meantime, an external reference electrode is used to measure the potential and impedance of the cathode and anode. It is found that the dramatic decrease of the cell voltage is mainly ascribed to the increase of the cathode overpotential which is caused by Na⁺ exchange with protons in the cathode catalyst layer. The electrochemical impedance measurements suggest that the lack of available protons and low oxygen concentration at the cathode catalytic sites contributed to this degradation. Furthermore, the recovery strategy is introduced and it is found that the poisoned MEA could be partly recovered by immersing in 0.5 M H₂SO₄ solution for 4 h.     

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.     

A study on direct glucose and fructose alkaline fuel cell

Direct glucose fuel cell (DGFC) has huge potential as a power source in low power long term portable devices. Electro-oxidation of glucose and fructose on PtRu/C catalyst are studied using cyclic voltammetry in alkaline medium to study the reason for deactivation of glucose fuel cell. A simple direct glucose fuel cell with PtRu/C as anode and activated charcoal as cathode was constructed and operated to study the effect of different temperature and concentration of glucose and KOH. An open-circuit voltage (OCV) of 0.91 V is obtained using 0.3 M glucose in 1 M KOH solution. OCV increased with the increase in glucose concentration. The maximum peak power density of 1.38 mW cm⁻² is obtained using 0.2 M glucose in 1 M KOH at 30 °C and it decreases with further increase in glucose concentration and temperature. In order to determine the reason for decrease in performance of glucose fuel cell due to conversion of glucose to fructose, the fuel cell was operated using 0.2 M fructose in 1 M KOH. The peak power density delivered is 0.57 mW cm⁻². The DGFC is continuously operated for 260 h at constant load of 500 Ω produces final constant voltage of 0.21 V.     

Improved electrochemical CO removal via potential oscillations in serially connected PEM fuel cells with PtRu anodes

The polarization performance of two PEM fuel cells (with anode PtRu/C catalyst) connected either in parallel or serial, was compared to the performance of a single PEM fuel cell in galvanostatic operation using CO-free H₂ or 200 ppm CO-containing H₂ stream as anode feed at ambient temperature. Spontaneous potential oscillations were observed experimentally for the coupled configuration with two cells connected in serial or parallel using CO-containing H₂ feed at various current densities applied. The potential oscillations are ascribed by the dynamic CO adsorption and subsequent electrochemical CO oxidation on the anode. The measured anode outlet CO concentration was found to decrease with the order: single cell > parallel cells > serial cells at various current densities and anodic flow rates. The low anode outlet CO concentration (<10 ppm) at high current densities applied showed that CO in the anode feed was removed efficiently by the electrochemical CO oxidation occurring on the PtRu anode. The anode outlet CO concentration decreased as follows: a single cell > the parallel cells > the serial cells at broad range of current densities and anodic flow rates. The highest CO conversion and the highest average power output at equal hydrogen recovery degree were obtained with serially coupled fuel cells.     

Studies of some operating parameters and cyclic voltammetry for a direct ethanol fuel cell

A direct ethanol fuel cell (DEFC) of 5 cm² membrane-electrode area was studied systematically by varying the catalyst loading, ethanol concentration, temperature and different Pt based electro-catalysts (Pt–Ru/C, Pt-black High Surface Area (HSA) and Pt/C). A combination of 2 M ethanol at the anode, pure oxygen at the cathode, 1 mg cm⁻² of Pt–Ru/C (40%:20%) as the anode and 1 mg cm⁻² of Pt-black as the cathode gave a maximum open circuit voltage (OCV) of 0.815 V, a short circuit current density of 27.90 mA cm⁻² and a power density of 10.3 mW cm⁻². The optimum temperatures of the anode and cathode were determined as 90 °C and 60 °C, respectively. The power density increased with increase in ethanol concentration and catalyst loading at the anode and cathode. However, the power density decreased slightly beyond 2 M ethanol concentration and 1 mg cm⁻² catalyst loading at the anode and cathode. These results were validated using cyclic voltammetry at single electrodes under similar conditions to those of the DEFC.     

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.     

Overpotential Behavior of Carbon Monoxide Fuel in a Molten Carbonate Fuel Cell

The overpotential of carbon monoxide (CO) fuel was analyzed with a 100‐cm² class molten carbonate fuel cell. The overpotential at the anode was measured using the steady state polarization, inert gas step addition, and reactant gas addition methods. Then, the overpotential was compared between normal hydrogen fuel (H₂:CO₂:H₂O = 0.69:0.17:0.14 atm, inlet composition) and CO fuels (CO:CO₂:H₂O = 0.5:0.5:0 atm and 0.43:0.43:0.14 atm, inlet compositions). The CO fuel without H₂O showed a much greater overpotential at 150 mA cm^–2 than the CO fuel with H₂O. This implies that the water‐gas‐shift reaction prevails at the anode and humidification of CO fuel is an efficient way to reduce anodic overpotential. The anodic overpotential with CO:CO₂:H₂O = 0.43:0.43:0.14 atm was about 73% of that of the H₂ fuel at 150 mA cm^–2. The anode showed gas‐phase mass‐transfer limitations with CO fuels.     

Effects of crossover on product yields measured for direct ethanol fuel cells

Products yields from a direct ethanol fuel cell (DEFC) have been measured in the normal operating mode with O₂ at the cathode, and with N₂/H₂ at the cathode. Apparent yields of both acetic acid and CO₂ are significantly higher with O₂ at the cathode, and this has been attributed to crossover effects. It is shown that ethanol crossing through the membrane reacts with oxygen at the cathode to produce acetic acid, which then crosses to the anode. In contrast, the main role of CO₂ produced from ethanol at the cathode appears to be to inhibit CO₂ crossover from the anode. The effects of acetaldehyde crossover from the anode to the cathode have also been investigated, and it has been shown that loss of acetaldehyde in this way is very high at elevated temperatures. Recommendations are made for how best to measure product yields from a DEFC.     

Catalysed titanium mesh electrodes for ethylene glycol fuel cells

Electrodes comprising thermally deposited Pt, PtRu and PtRuW on titanium mesh were evaluated for the oxidation of ethylene glycol in acidic electrolyte. The electrodes were characterised using cyclic voltammetry, scanning electron microscopy and X-ray diffraction and the effect of reactant concentration and temperature were examined. Single fuel cell tests employing the titanium mesh anode with the PtRuW catalyst showed better performance than that of the PtRu catalyst. A peak power density of 15 mW cm⁻² was obtained at a temperature of 90 °C with 1.0 M ethylene glycol solution. The performance of the catalysed PtRu mesh electrode was comparable to that of a commercial, alcohol oxidation, PtRu carbon supported catalyst.     

Modelling and simulation of a direct ethanol fuel cell considering multistep electrochemical reactions, transport processes and mixed potentials

In this work a one-dimensional mathematical model of a direct ethanol fuel cell (DEFC) is presented. The electrochemical oxidation of ethanol in the catalyst layers is described by several reaction steps leading to surface coverage with adsorbed intermediates (CH₃CO, CO, CH₃ and OH) and to the final products acetaldehyde, acetic acid and CO₂. A bifunctional reaction mechanism is assumed for the activation of water on a binary catalyst favouring the further oxidation of adsorbates blocking active catalyst sites. The chemical reactions are highly coupled with the charge and reactant transport. The model accounts for crossover of the reactants through the membrane leading to the phenomenon of cathode and anode mixed potentials due to the parasitic oxidation and reduction of ethanol and oxygen, respectively. Polarisation curves of a DEFC were recorded for various ethanol feed concentrations and were used as reference data for the simulation. Based on one set of model parameters the characteristic of electronic and protonic potential, the relative surface coverage and the parasitic current densities in the catalyst layers were studied.     

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.     

Reliability and accuracy of measured overpotential in a three-electrode fuel cell system

Numerical simulation was conducted to study the potential and current density distributions at the active electrode surface of a solid oxide fuel cell. The effects of electrode deviation, electrolyte thickness and electrode polarization resistance on the measurement error were investigated. For a coaxial anode/electrolyte/cathode system where the radius of the anode is greater than that of cathode, the cathode overpotential is overestimated while the anode overpotential is underestimated. Although the current interruption method or impedance spectroscopy can be employed to compensate/correct the error for a symmetric electrode configuration, it is not useful when dealing with the asymmetric electrode system. For the purpose of characterizing the respective overpotentials in a fuel cell, the cell configuration has to be carefully designed to minimize the measurement error, in particular the selection of the electrolyte thickness, which may cause significant error. For the anode-support single fuel cell, it is difficult to distinguish the polarization between the anode and cathode with reference to a reference electrode. However, numerical results can offer an approximate idea about the source/cause of the measurement error and provide design criteria for the fuel cell to improve the reliability and accuracy of the measurement technique.