Performance of a direct ethylene glycol fuel cell with an anion-exchange membrane

This paper reports on the development and performance test of an alkaline direct ethylene glycol fuel cell. The fuel cell consists of an anion-exchange membrane with non-platinum electrocatalysts at both the anode and cathode. It is demonstrated that this type of fuel cell with relatively cheap membranes and catalysts can result in a maximum power density of 67 mW cm⁻² at 60 °C, which represents the highest performance that has so far been reported in the open literature. The high performance is mainly attributed to the increased kinetics of both the ethylene glycol oxidation reaction and oxygen reduction reaction rendered by the alkaline medium with the anion-exchange membrane.     

Performance of air-breathing direct methanol fuel cell with anion-exchange membrane

This report details development of an air-breathing direct methanol alkaline fuel cell with an anion-exchange membrane. The commercially available anion-exchange membrane used in the fuel cell was first electrochemically characterized by measuring its ionic conductivity, and showed a promising result of 1.0 × 10⁻¹ S cm⁻¹ in a 5 M KOH solution. A laboratory-scale direct methanol fuel cell using the alkaline membrane was then assembled to demonstrate the feasibility of the system. A high open-circuit voltage of 700 mV was obtained for the air-breathing alkaline membrane direct methanol fuel cell (AMDMFC), a result about 100 mV higher than that obtained for the air-breathing DMFC using a proton exchange membrane. Polarization measurement revealed that the power densities for the AMDMFC are strongly dependent on the methanol concentration and reach a maximum value of 12.8 mW cm⁻² at 0.3 V with a 7 M methanol concentration. A durability test for the air-breathing AMDMFC was performed in chronoamperometry mode (0.3 V), and the decay rate was approximately 0.056 mA cm⁻² h⁻¹ over 160 h of operation. The cell area resistance for the air-breathing AMDMFC was around 1.3 Ω cm² in the open-circuit voltage (OCV) mode and then is stably supported around 0.8 Ω cm² in constant voltage (0.3 V) mode.     

A microfabricated low cost enzyme-free glucose fuel cell for powering low-power implantable devices

In the past decade the scientific community has showed considerable interest in the development of implantable medical devices such as muscle stimulators, neuroprosthetic devices, and biosensors. Those devices have low power requirements and can potentially be operated through fuel cells using reactants present in the body such as glucose and oxygen instead of non-rechargeable lithium batteries. In this paper, we present a thin, enzyme-free fuel cell with high current density and good stability at a current density of 10 μA cm⁻². A non-enzymatic approach is preferred because of higher long term stability. The fuel cell uses a stacked electrode design in order to achieve glucose and oxygen separation. An important characteristic of the fuel cell is that it has no membrane separating the electrodes, which results in low ohmic losses and small fuel cell volume. In addition, it uses a porous carbon paper support for the anodic catalyst layer which reduces the amount of platinum or other noble metal catalysts required for fabricating high surface area electrodes with good reactivity. The peak power output of the fuel cell is approximately 2 μW cm⁻² and has a sustainable power density of 1.5 μW cm⁻² at 10 μA cm⁻². An analysis on the effects of electrode thickness and inter electrode gap on the maximum power output of the fuel cell is also performed.     

Enhanced performance of a direct methanol alkaline fuel cell (DMAFC) using a polyvinyl alcohol/fumed silica/KOH electrolyte

A novel polymer-inorganic composite electrolyte for direct methanol alkaline fuel cells (DMAFCs) is prepared by physically blending fumed silica (FS) with polyvinyl alcohol (PVA) to suppress the methanol permeability of the resulting nano-composites. Methanol permeability is suppressed in the PVA/FS composite when comparing with the pristine PVA membrane. The PVA membrane and the PVA/FS composite are immersed in KOH solutions to prepare the hydroxide-conducting electrolytes. The ionic conductivity, cell voltage and power density are studied as a function of temperature, FS content, KOH concentration and methanol concentration. The PVA/FS/KOH electrolyte exhibits higher ionic conductivity and higher peak power density than the PVA/KOH electrolyte. In addition, the concentration of KOH in the PVA/FS/KOH electrolytes plays a major role in achieving higher ionic conductivity and improves fuel cell performance. An open-circuit voltage of 1.0 V and a maximum power density of 39 mW cm⁻² are achieved using the PVA/(20%)FS/KOH electrolyte at 60 °C with 2 M methanol and 6 M KOH as the anode fuel feed and with humidified oxygen at the cathode. The resulting maximum power density is higher than the literature data reported for DMAFCs prepared with hydroxide-conducting electrolytes and anion-exchange membranes. The long-term cell performance is sustained during a 100-h continuous operation.     

Understanding mass and charge transports to create anion-ionomer-free high-performance alkaline direct formate fuel cells

The disadvantage of anion ionomer that possesses low hydroxide conductivity, and thermal and chemical instability hinders the development of the high-performance anion-exchange membrane direct liquid fuel cells. Instead of adding additional base and synthesizing high-conductivity ionomer material, by gaining insight into species transports, herein, we propose an anion-ionomer-free anion-exchange membrane direct formate fuel cell (AEM DFFC). Experimental result reveals that this conceptual anion-ionomer-free AEM DFFC can operate stably within a 6-h constant-current discharge at 10 mA cm⁻², mainly because formate hydrolysis renders a high OH⁻ conductivity. It was also found that the anion-ionomer-free AEM DFFC yields a peak power density as high as 41 mW cm⁻² at 40 °C, 40% higher than that of the conventional quaternary ammonia polysulfone anion-ionomer AEM DFFC. This can be attributed to the fact that the OH⁻-containing formate solution facilitates the mass and charge transports, thereby enlarging the triple-phase boundary for both anodic formate oxidation reaction and cathodic oxygen reduction reaction.     

An abiotically catalyzed glucose fuel cell for powering medical implants: Reconstructed manufacturing protocol and analysis of performance

Although the first abiotically catalyzed glucose fuel cells have already been developed as sustainable power supply for medical implants in the 1970s, no detailed information concerning the fabrication of these devices has been published so far. Here we present a comprehensive manufacturing protocol for such a fuel cell, together with a detailed analysis of long-term performance in neutral buffer containing physiological amounts of glucose and oxygen. In air saturated solution a power density of (3.3 ± 0.2) μW cm⁻² is displayed after 10 days of operation that gradually decreases to a value of (1.0 ± 0.05) μW cm⁻² in the course of 224 days. A novelty of this work is the characterization of fuel cell performance with individually resolved electrode potentials. Using this technique, we can show that the major part of performance degradation originates from a positive shift of the anode potential, indicating that a more poisoning-resistant glucose oxidation catalyst would improve the degradation behavior of the fuel cell. As further factors influencing performance an incomplete reactant separation and a mass transfer governed cathode reaction under the relatively low oxygen partial pressures of body tissue have been identified. Consequently we propose an oxygen depleting electrode interlayer and the application of more effective oxygen reduction catalysts as promising strategies to further improve the fuel cell performance under physiological concentrations of glucose and oxygen.     

Performance of alkaline electrolyte-membrane-based direct ethanol fuel cells

A single alkaline direct ethanol fuel cell (alkaline DEFC) with an anion-exchange membrane and non-platinum (non-Pt) catalysts is designed, fabricated, and tested. Particular attention is paid to investigating the effects of different operating parameters, including the cell operating temperature, concentrations of both ethanol and the added electrolyte (KOH) solution, as well as the mass flow rates of the reactants. The alkaline DEFC yields a maximum power density of 60 mW cm⁻², a limiting current density of about 550 mA cm⁻², and an open-circuit voltage of about 900 mV at 40 °C. The experimental results show that the cell performance is improved on increasing the operating temperature, but there exists an optimum ethanol concentration under which the fuel cell has the best performance. In addition, cell performance increases monotonically with increasing KOH concentration in the region of low current density, while in the region of high current density, there exists an optimum KOH concentration in terms of cell performance. The effect of flow rate of the fuel solution is negligible when the ethanol concentration is higher than 1.0 M, although the cell performance improves on increasing the oxygen flow rate.     

An onboard hydrogen generation method based on hydrides and water recovery for micro-fuel cells

Micro-proton exchange membrane fuel cells are considered to be the next generation power sources for micro-scale power applications, but onboard hydrogen storage and generation with high energy density at the small scale is still a technical barrier. This paper introduces a hydrogen generation method based on an onboard hydride fuel and a byproduct water recovery mechanism for micro-hydrogen PEM fuel cells. The water recovery is carried out by water diffusion from the more humid cathode side to the less humid anode side through the proton exchange membrane. The micro-fuel cells based on this water recovery method were constructed and tested. The results demonstrate that the relative humidity has a significant affect on the fuel cell performance as well as the opening area on the cover layer, the type of hydrides, and the thickness of the Nafion membrane also can affect the fuel cell performance. A 10 mm³ prototype water recovery micro-fuel cell has been built and tested, and the device has produced a maximum power density of 104 W L⁻¹ and a maximum energy density of 313 W h L⁻¹.     

Direct hybrid glucose-oxygen enzymatic fuel cell based on tetrathiafulvalene-tetracyanoquinodimethane charge transfer complex as anodic mediator

TTF-TCNQ has been used for the first time as a mediator in a direct glucose fuel cell operating on gas-phase oxygen. It has been shown that TTF-TCNQ forms highly irregular porous structure, which emphasizes the importance of optimization of mass transport and kinetic resistance in the catalyst layer. Kinetics resistance can be optimized by variation of the mediator and/or enzyme loading, while mass transport resistance mainly by the variation of other structural parameters such as electrode thickness. The optimized anode reached limiting current densities of nearly 400 μA cm⁻² in presence of 5 mM glucose under rotation. The enzymatic fuel cell exhibited unexpectedly high OCV values (up to 0.99 V), which were tentatively ascribed to different pH conditions at the anode and the cathode. OCV was influenced by glucose crossover and was decreasing with an increase of glucose concentration or flow rate. Although the performance of the fuel cell is limited by the enzymatic anode, the long-term stability of the fuel cell is mainly influenced by the Pt cathode, while the enzymatic anode has higher stability. The fuel cell delivered power densities up to 120 μW cm⁻² in presence of 5 mM glucose, depending on the glucose flow rate.     

Plasma-grafted alkaline anion-exchange membranes based on polyvinyl chloride for potential application in direct alcohol fuel cell

Plasma grafting is employed to prepare alkaline anion-exchange membranes in this study. The attenuated total reflection Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and thermo gravimetric analysis demonstrate that the benzyltrimethylammonium cationic groups are successfully introduced into the polyvinyl chloride matrix via plasma grafting, quaternization and alkalization. The plasma-grafted alkaline anion-exchange membrane exhibits a satisfactory ionic exchange capacity (1.01 mmol g⁻¹), thermal stability, mechanical property, ionic conductivity (0.0145 S cm⁻¹) and methanol permeability (9.59 × 10⁻¹² m² s⁻¹), suggesting a great potential for application in direct alcohol fuel cells. The open circuit voltage of air-breathing ADAFC using plasma-grafted alkaline anion-exchange membrane is 0.796 V with 1 M EtOH solution at ambient temperature.     

High performance direct liquid fuel cells powered by xylose or glucose

In this work, we report the first abiotic, direct liquid fuel cells powered by the monosaccharide xylose using both a fully alkaline fuel cell (with anion exchange membrane) and a split pH fuel cell (with cation exchange membrane). We also report that the same fuel cells can be used with the monosaccharide glucose to produce much higher maximum power density than previously reported for abiotic, direct glucose fuel cells. This first alkaline direct xylose fuel cell (DXFC) produces a maximum power density of 57 mW cm⁻² at optimum conditions, while the split pH DXFC produces a maximum power density of 160 mW cm⁻². Our significantly improved alkaline direct glucose fuel cell (DGFC) produces 90 mW cm⁻² at optimum conditions, while the split pH DGFC produces 189 mW cm⁻². In addition to being high performing, these sugar molecules are naturally abundant, renewable, and known to convert to valuable products such as gluconic acid, glucaric acid, and xylonic acid during electrochemical oxidation. Other fuel cell and electrochemical cell data is also reported herein to understand the role of pH and fuel concentrations on behavior toward the electrochemical oxidation of these sugar molecules in alkaline media.     

Performance characteristics of air-breathing anion-exchange membrane direct ethanol fuel cells

An air-breathing direct ethanol fuel cell (DEFC) with an anion-exchange membrane (AEM) and Pt-free electrodes is designed and investigated. Particular attention is paid to studying the performance characteristics of the air-breathing AEM DEFC. Experimental results reveal that this air-breathing AEM DEFC yields a peak power density as high as 38 mW cm⁻² at room temperature, which is comparable to the conventional Pt-based proton exchange membrane direct methanol fuel cells (PEM DMFCs). The overshoot/undershoot behaviors of both the cell voltage and cell temperature are avoided in the air-breathing AEM DEFC due to the use of ethanol-tolerant cathode catalyst. It is also found that the cathode water flooding behavior occurs in this air-breathing AEM DEFC, thus lowering the cell performance.     

Structural optimization of the direct methanol fuel cell passively fed with a high-concentration methanol solution

In a high-concentration direct methanol fuel cell (HC-DMFC), the methanol crossover is typically decreased to an acceptable level by two main mechanisms: high methanol transport resistance between the anode reservoir and the membrane electrode assembly (MEA), and high water back flow from the cathode to the anode. Based on the semi-passive HC-DMFC fabricated in this work, the effects of methanol barrier layer (MBL) thickness and electrolyte membrane thickness on cell performance, methanol and water crossover, and fuel efficiency have been studied. The results showed that a thicker MBL could significantly decrease the methanol and water crossover by increasing the mass transport resistance between the anode reservoir and the MEA, while a thinner Nafion^® membrane could also significantly decrease the methanol and water crossover by enhancing the water back flow from the cathode through the electrolyte membrane to the anode. Using Nafion^® 212 as the electrolyte membrane, and a 6.4 mm porous PTFE plate as the MBL, a semi-passive HC-DMFC operating at 70 °C produced the maximum power density of 115.8 mW cm⁻² when 20 M methanol solution was fed as the fuel.     

Effect of pressure for direct fuel cells using DME-based fuels

The performance of direct fuel cells using dimethyl ether(DME)-based fuels is presented at a relatively low temperature of 80 °C. DME is supplied to the fuel cells either by gas phase or aqueous phase for the operation of direct fuel cells. In order to keep DME in liquid phase during operation, fuel cells were operated at higher pressure up to 5 bar. For further increase of the power density from direct DME fuel cells, DME was mixed with methanol solution and fed into the fuel cells by the vapor pressure of DME itself without a liquid pump. In this study, we have obtained the highest power density of 210 mW cm⁻² at a temperature of 80 °C when the fuel cell is operated with the mixed fuel with 2 M methanol solution under 4 bar.     

A novel direct ethanol fuel cell with high power density

A new type of direct ethanol fuel cell (DEFC) that is composed of an alkaline anode and an acid cathode separated with a charger conducting membrane is developed. Theoretically it is shown that the voltage of this novel fuel cell is 2.52 V, while, experimentally it has been demonstrated that this fuel cell can yield an open-circuit voltage (OCV) of 1.60 V and a peak power density of 240 mW cm⁻² at 60 °C, which represent the highest performance of DEFCs that has so far been reported in the open literature.     

Effect of carbon black concentration in carbon fiber paper on the performance of low-temperature proton exchange membrane fuel cells

This study uses fuel cell gas diffusion layers (GDLs) made from carbon fiber paper containing carbon black in proton exchange membrane fuel cells (PEMFCs) in order to determine the relationship between carbon black content and fuel cell performance. The connection between fuel cell performance and the carbon black content of the carbon fiber paper is discussed, and the effects of carbon black on the carbon fiber paper's thickness, density, and surface resistivity are investigated. When a carbon fiber paper GDL contains 10 wt% phenolic resin and 2% carbon black, and reaction area was 25 cm² and operating temperature 40 °C, tests show that a carbon electrode fuel cell could achieve 1026.4 mA cm⁻² and maximum power of 612.8 mW cm⁻² under a 0.5 V load.     

Single passive direct methanol fuel cell supplied with pure methanol

A new single passive direct methanol fuel cell (DMFC) supplied with pure methanol is designed, assembled and tested using a pervaporation membrane (PM) to control the methanol transport. The effect of the PM size on the fuel cell performances and the constant current discharge of the fuel cell with one-fueling are studied. The results show that the fuel cell with PM 9 cm² can yield a maximum power density of about 21 mW cm⁻², and a stable performances at a discharge current of 100 mA can last about 45 h. Compared with DMFC supplied with 3 M methanol solution, the energy density provided by this new DMFC has increased about 6 times.     

A direct borohydride fuel cell based on poly(vinyl alcohol)/hydroxyapatite composite polymer electrolyte membrane

A new poly(vinyl alcohol)/hydroxyapatite (PVA/HAP) composite polymer membrane was synthesized using a solution casting method. Alkaline direct borohydride fuel cells (DBFCs), consisting of an air cathode based on MnO₂/C inks on Ni-foam, anodes based on PtRu black and Au catalysts on Ni-foam, and the PVA/HAP composite polymer membrane, were assembled and investigated for the first time. It was demonstrated that the alkaline direct borohydride fuel cell comprised of this low-cost PVA/HAP composite polymer membrane showed good electrochemical performance. As a result, the maximum power density of the alkaline DBFC based on the PtRu anode (45 mW cm⁻²) proved higher than that of the DBFC based on the Au anode (33 mW cm⁻²) in a 4 M KOH + 1 M KBH₄ solution at ambient conditions. This novel PVA/HAP composite polymer electrolyte membrane with high ionic conductivity at the order of 10⁻² S cm⁻¹ has great potential for alkaline DBFC applications.     

Ultra-low catalyst loading cathode electrode for anion-exchange membrane fuel cells

A new cathode architecture for anion-exchange membrane fuel cells (AEMFCs) is proposed and fabricated by direct deposition of palladium (Pd) particles onto the surface of the micro-porous layer (MPL) that is interfaced with a backing layer. The MPL is composed of carbon nanotubes while the backing layer is made of a carbon paper. The sputter-deposited electrode with a worm-like shape not only extends the electrochemical active surface area, but also facilitates the oxygen transport. This new cathode, albeit with a Pd loading as low as 0.035 mg cm⁻², enables the peak power density of an AEM direct ethanol fuel cell to be as high as 88 mW cm⁻² (at 60 °C), which is even higher than that using a conventional cathode with a 15-times higher Pd loading. The significance of the present work lies in the fact that the new sputter-deposited electrode is more suitable for fuel-electrolyte-fed fuel cells than the conventional electrode designed for proton-exchange membrane fuel cells (PEMFCs).     

Polymer electrolytes based on sulfonated polysulfone for direct methanol fuel cells

This paper reports the development and characterization of sulfonated polysulfone (SPSf) polymer electrolytes for direct methanol fuel cells. The synthesis of sulfonated polysulfone was performed by a post sulfonation method using trimethyl silyl chlorosulfonate as a mild sulfonating agent. Bare polysulfone membranes were prepared with two different sulfonation levels (60%, SPSf-60 and 70%, SPSf-70), whereas, a composite membrane of SPSf-60 was prepared with 5 wt% silica filler. These membranes were investigated in direct methanol fuel cells (DMFCs) operating at low (30–40 °C) and high temperatures (100–120 °C). DMFC power densities were about 140 mW cm⁻² at 100 °C with the bare SPSf-60 membrane and 180 mW cm⁻² at 120 °C with the SPSf-60-SiO2 composite membrane. The best performance achieved at ambient temperature using a membrane with high degree of sulfonation (70%, SPSf-70) was 20 mW cm⁻² at atmospheric pressure. This makes the polysulfone-based DMFC suitable for application in portable devices.