Design,Fabrication and Preliminary Study of a Mini Power Source with a Planar Six‐cell PEM Unitised Regenerative Fuel Cell Stack

A novel micro planar fuel cell power supplier, in which a six‐cell PEM unitised regenerative fuel cell (URFC) stack is used as the power generator, was designed and fabricated. Six membrane electrode assemblies were prepared and integrated on one piece of membrane by spraying catalyst slurry on both sides of the membrane. Each cell was made by sandwiching a membrane electrode assembly (MEA) between two graphite monopolar plates and six cell units were mechanically fixed in two organic glass endplates. When the stack was operated in an electrolysis mode, hydrogen was generated from the splitting of water and stored using a hydrogen storage alloy; conversely, when the stack was operated in fuel cell mode, hydrogen was supplied by the hydrogen storage alloy and oxygen was supplied from air by self‐breathing of the cathode. At room temperature and standard atmospheric pressure, the open‐circuit voltage (OCV) of the system reached 4.9 V, the system could be discharged at a constant current density of 20 mA cm^–2 for about 40 min, and the work voltage was ∼2.9 V. The system showed good stability for 10 charge–discharge cycles.     

Performances of micro-tubular solid oxide cell with novel asymmetric porous hydrogen electrode

Asymmetric-porous hollow-fiber has been fabricated by a phase-inversion process and employed as the hydrogen electrode for micro-tubular solid oxide cell (MT-SOC). The microstructure and electrochemical properties of MT-SOC were investigated in detail. The asymmetric-porous hydrogen electrode possesses unique two layer finger-like porous micro-structure with a thin functional layer and a thick fuel delivery layer. When the MT-SOC was operated in fuel cell mode, maximum power densities of 0.54, 0.71 and 1.25 W/cm² were obtained at 800, 850 and 900 °C, respectively. On the other hand, when the MT-SOC was operated in electrolysis mode at 900 °C with an applied voltage of 1.3 V, current densities of 0.68 A/cm² and 2.57 A/cm² were obtained at 30 vol.% and 80 vol.% absolute humidity (AH), respectively. These results indicate that novel-microstructured MT-SOC can be effectively fabricated towards high performance fuel cell and electrolysis cell.     

Development of a Direct Alcohol Alkaline Fuel Cell Stack

Direct alcohol alkaline fuel cells (DAAFC) are one of the potential fuel cell types in the category of low temperature fuel cells, which could become an energy source for portable electronic equipment in future. In the present study, a simple DAAFC stack has been developed and studied to evaluate the maximum performance for a given fuel (methanol or ethanol) and electrolyte (KOH) at various concentrations and temperatures. The open circuit voltage of the stack of four cells was nearly 4.0 V. A particular combination, 2 M fuel (methanol or ethanol) and 3 M KOH, results in maximum power density of the stack. The maximum power density obtained from the DAAFC stack (25 °C) was 50 mW cm^–2 at 20 mA cm^–2 for methanol and 17 mA cm^–2 for ethanol. The stack power density corroborated with that obtained from a single cell, indicating there was no further loss in the stack.     

A direct methanol fuel cell using acid-doped polybenzimidazole as polymer electrolyte

A direct methanol/oxygen solid polymer electrolyte fuel cell was demonstrated. This fuel cell employed a 4 mg cm^–2 Pt-Ru alloy electrode as an anode, a 4 mg cm^–2 Pt black electrode as a cathode and an acid-doped polybenzimidazole membrane as the solid polymer electrolyte. The fuel cell is designed to operate at elevated temperature (200°C) to enhance the reaction kinetics and depress the electrode poisoning, and reduce the methanol crossover. This fuel cell demonstrated a maximum power density about 0.1 W cm^–2 in the current density range of 275–500 mA cm^–2 at 200°C with atmospheric pressure feed of methanol/water mixture and oxygen. Generally, increasing operating temperature and water/methanol mole ratio improves cell performance mainly due to the decrease of the methanol crossover. Using air instead of the pure oxygen results in approximately 120 mV voltage loss within the current density range of 200–400 mA cm^–2 .     

Effects of organic acid catalysts on the hydrogen generation from NaBH4

Sodium borohydride has received much attention from fuel cell developers due to its high hydrogen storage capacity. In this study, organic acid solutions such as malic, citric, acetic acids were successfully utilized to accelerate and control hydrogen generation from stabilized sodium borohydride solutions. The generated hydrogen by malic acid was then continuously supplied to a PEMFC single cell. A power density of 168 mW cm⁻² was achieved with a hydrogen flow rate of 0.050 L min⁻¹ that was generated by adding 10 wt% aqueous malic acid to the stabilized sodium borohydride solution at an air flow rate of 0.11 L min⁻¹ without humidification. Further increase of power density to 366 mW cm⁻² is practicable by maintaining a precise hydrogen flow rate of 0.3 L min⁻¹. The current study focuses on the development of an instant hydrogen generation method for micro fuel cell applications. We successfully demonstrated that fast and direct generation of hydrogen could be achieved from stabilized borohydride using inexpensive organic acid solutions rather than expensive metal catalysts and a PEMFC single cell could be operated by generated hydrogen.     

An investigation of scale-up on the response of the direct methanol fuel cell under variable load conditions

The electrical response of the direct methanol fuel cell, based on solid polymer electrolyte, to variable load is reported. The dynamic power response of the direct methanol fuel cell is of importance particularly when the cell is used for transportation applications. The study reports the dynamic characteristics of a small-scale cell (active area 9 cm²), a large-scale cell (active area 272 cm²), and a three-cell stack. The effect of operating conditions (i.e., flow rate, cathode pressure and solution concentration) on the voltage response is described and the effect of a change of scale is discussed.     

Development and operation of a 150 W air-feed direct methanol fuel cell stack

A five-cell 150 W air-feed direct methanol fuel cell (DMFC) stack was demonstrated. The DMFC cells employed Nafion 117^® as a solid polymer electrolyte membrane and high surface area carbon supported Pt-Ru and Pt catalysts for methanol electrooxidation and oxygen reduction, respectively. Stainless steel-based stack housing and bipolar plates were utilized. Electrodes with a 225 cm² geometrical area were manufactured by a doctor-blade technique. An average power density of about 140 mW cm^–2 was obtained at 110 °C in the presence of 1 M methanol and 3 atm air feed. A small area graphite single cell (5 cm²) based on the same membrane electrode assembly (MEA) gave a power density of 180 mW cm^–2 under similar operating conditions. This difference is ascribed to the larger internal resistance of the stack and to non-homogeneous reactant distribution. A small loss of performance was observed at high current densities after one month of discontinuous stack operation.     

DMFC at low air flow operation: Study of parasitic hydrogen generation

In this paper, the effect of hydrogen generation in direct methanol fuel cells (DMFC) is described. Under certain operating conditions hydrogen generation occurs in DMFC causing an additional methanol consumption and a decrease of the cell voltage.For the present experiments a segmented cell with an active area of 244 cm² is used. The cell has 196 segments which are regularly distributed on the whole area. By this experimental setup hydrogen generation was found in regions with insufficient air supply. Hydrogen generation was analyzed by systematically applying different air flow rates and detecting the local current densities. The theory for hydrogen generation is confirmed by the results obtained from the segmented cell. A correlation between open circuit voltage (OCV), air flow rate and hydrogen generation was observed. Furthermore, half-cell measurements with different methanol concentrations were performed and used for analyzing the processes during hydrogen generation.The work clearly indicates the importance of sufficient cathode air supply for DMFC. Starved cathode areas not only do not contribute to the overall current generation but in addition reduce the power and efficiency by the parasitic generation of hydrogen.     

In suit Investigation of Anode Support on Cell Performance Reduced under Various Temperatures for Planar Solid Oxide Fuel Cells

The performance of anode support of Ni‐YSZ reduced from room temperature (T_R) to working temperature (T_w) and at T_w in anode‐supported planar solid oxide fuel cell was investigated quantitatively in situ. A 2 μm thick Pt voltage probe was embedded at the interface between the anode support and the function anode in the cell. Results showed that the power densities of the stack that was reduced from T_R to T_w (stack 1) and stack reduced at T_w (stack 2) were 0.343 W cm⁻² and 0.583 W cm⁻² with the corresponding fuel utilization of 36.28% and 63.87%, respectively, under the operating voltage of 0.8 V. The degradation rate of stack 1 was 7.76 times more than that of stack 2 when the stack was discharged under a constant current of 0.476 Acm⁻² for 100 h. Ni particles agglomerated in the anode support of the cell inside stack 1, whereas Ni particles in the anode support of the cell inside stack 2 were evenly distributed. The performance of stack 1 was poor mainly because of the increasing ohmic and polarization resistances caused by Ni agglomeration and decreasing porosity of the anode support.     

Durability of ABPBI‐based MEAs for High Temperature PEMFCs at Different Operating Conditions

High temperature PEMFCs based on phosphoric acid‐doped ABPBI membranes have been prepared and characterised. At 160 °C and ambient pressure fuel cell power densities of 300 mW cm^–2 (with hydrogen and air as reactants) and 180 mW cm^–2 (with simulated diesel reformate/air) have been achieved. The durability of these membrane electrode assemblies (MEAs) in the hydrogen/air mode of operation at different working conditions has been measured electrochemically and has been correlated to the cell resistivity, the phosphoric acid loss rate and the catalyst particle size. Under stationary conditions, a voltage loss of only –25 μV h^–1 at a current density of 200 mA cm^–2 has been deduced from a 1,000 h test. Under dynamic load changes or during start–stop cycling the degradation rate was significantly higher. Leaching of phosphoric acid from the cell was found to be very small and is not the main reason for the performance loss. Instead an important increase in the catalyst particle size was observed to occur during two long‐term experiments. At high gas flows of hydrogen and air ABPBI‐based MEAs can be operated at temperatures below 100 °C for several hours without a significant irreversible loss of cell performance and with only very little acid leaching.     

Experimental results on the direct electrochemical oxidation of methanol in PEM fuel cells

The cell performance of direct methanol fuel cells (DMFC) is 0.5 V at 0.5 A cm^–2 under high pressure oxygen operation (3 bar abs.) at 110 °C. However, high oxygen pressure operation at high temperatures is only useful in special market niches. Therefore, our work has now focused on air operation of a DMFC under low pressure (up to 1.5 bar abs.). At present, a power density of more than 100 mW cm^–2 can be achieved at 0.5 V on air operation at 110 °C. These measurements were carried out in single cells with an electrode area of 3 cm² and the air stoichiometry only amounted to 10. The effects of methanol concentration and temperature on the anode performance were studied by pseudo half cell measurements and the results are presented together with their impact on the cell voltage. A cell design with an electrode area of 550 cm², which is appropriate for assembling a DMFC stack, was tested. A three-celled stack based on this design revealed nearly the same power densities as in the small experimental cells at low air excess pressure and the voltage–current curves for the three cells were almost identical. At 110 °C a power output of 165 W at a stack voltage of 1.5 V can be obtained in the air mode.     

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.     

Miniature Direct Electron Transfer Based Enzymatic Fuel Cell Operating in Human Sweat and Saliva

We present data on operation of a miniature membrane‐less, direct electron transfer based enzymatic fuel cell in human sweat and saliva. The enzymatic fuel cell was fabricated following our previous reports on miniature biofuel cells, utilizing gold nanoparticle modified gold microwires with immobilized cellobiose dehydrogenase and bilirubin oxidase. The following average characteristics of miniature glucose/oxygen biodevices operating in human sweat and saliva, respectively, were registered: 580 and 560 mV open‐circuit voltage, 0.26 and 0.1 μW cm^–2 power density at a cell voltage of 0.5 V, with up to ten times higher power output at 0.2 V. When saliva collected after meal ingestion was used, roughly a two‐fold increase in power output was obtained, with a further two‐fold increase by addition of 500 μM glucose. Likewise, the power generated in sweat at 0.5 V increased two‐fold by addition of 500 μM glucose.     

A new technique for two-dimensional current distribution measurements in electrochemical cells

A new technique has been developed using a magnetic loop array to measure current distribution in electrochemical cells. The main advantage of this approach is the combination of high spatial and time resolution and stack integration with an easily handled measurement carried out independently of the cell operation. A polymer electrolyte fuel cell (PEFC) of technically relevant dimensions (about 600 cm² electrode area) with 5 × 8 current sensors has been constructed and operated, thus confirming the feasibility of the measurement technique.     

Effect of pre-calcined ceramic powders at different temperatures on Ni-YSZ anode-supported SOFC cell/stack by low pressure injection molding

Recently, powder injection molding (PIM) has been exploited in the field of solid oxide fuel cells (SOFCs), especially for fabricating anode supports. The current study employs low pressure injection molding (LPIM) to manufacture near net shape, porous, tubular NiO-yttria stabilized zirconia (YSZ) anode supports for anode-supported SOFCs. The study investigates the effects of pre-calcining temperature of the ceramic powder on the microstructure, porosity and electrochemical performance of the cells in detail. Archimedes tests reveal that the porosity of an unreduced NiO-YSZ anode with 900 °C pre-calcined powder reaches a high of 25.9%, approaching the optimal value of 26%. Meanwhile, the anode prepared under this condition possesses more porous and homogeneous microstructures. At 800 °C, with humidified hydrogen as fuel and ambient air as the oxidant, the single cell with 900 °C pre-calcined powder delivers a maximum power density of 671 mW cm⁻² while the cell with raw powder, 555 mW cm⁻², and the cell with 1000 °C pre-calcined powder, 648 mW cm⁻². A four-cell stack is assembled by connecting four single cells in series. The stack could provide a maximum output power of 4.6 W and an open circuit voltage of 3.2 V when fuelled with humidified hydrogen at 800 °C.     

Cell/stack electrochemical performance and stability of cone-shaped tubular SOFCs for direct operation with methane

Cone-shaped tubular anode-supported solid oxide fuel cells (SOFCs) and two-cell-stack based on NiO-YSZ traditional anodes direct utilization methane as fuel were successfully developed in this study. The single cell exhibited maximum power densities of 1.255 W cm⁻² for hydrogen and 1.099 W cm⁻² for methane at 800 °C, respectively. A stability test of the single cell was performed with different constant current densities at 700 °C in methane. The results indicated that the single cell can be operated stable at high current density in methane. And EDX results showed that there is no measurable coking effect of operation in methane at relatively high current density.A two-cell-stack based on the above-mentioned SOFCs was fabricated and tested by direct utilization of methane. Its typical electrochemical performance was investigated. The two-cell-stack provided a maximum power output of about 3.5 W (350 mW cm⁻² calculated using effective cathode area) by directly using methane at 800 °C. The stack experienced 20 h durability testing. The results demonstrated that the stack was kept at around 1 V (J = 0.05 A cm⁻²) at 700 °C. The stack presented basically stably during the whole test, and the performance of the stack is acceptable for application.     

A high-performance fuel electrode-supported tubular protonic ceramic electrochemical cell

Protonic ceramic electrochemical cells (PCECs) have received extensive attention for encouraging energy conversion and storage efficiencies. One of the key obstacles hindering the development of PCECs is the time-consuming and costly fabrication processes. In this study, we have fabricated fuel electrode-supported tubular PCECs via a facile and well-controlled phase inversion methhod combined with a dip-coating process. When operated in a fuel cell (FC) mode, single cells exhibit excellent peak power densities of 0.48, 0.91, 1.50, and 2.62 W cm^-2 at 550–700 ℃, respectively. While in electrolysis cell (EC) mode, these tubular cells achieve high electrolysis current densities of − 0.61, − 1.59, − 3.03, and − 4.67 A cm^-2 with reasonable Faradaic efficiencies at 550–700 ℃ and an applied voltage of 1.3 V, respectively. Additionally, cells can be stably operated at 0.5 A cm^-2 for 120 h in FC mode and − 0.5 A cm^-2 for 110 h in EC mode at 650 °C.     

The Effects of Operating Conditions on the Performance of a Solid Oxide Steam Electrolyser: A Model‐Based Study

To support the development of hydrogen production by high temperature electrolysis using solid oxide electrolysis cells (SOECs), the effects of operating conditions on the performance of the SOECs were investigated using a one‐dimensional model of a cathode‐supported planar SOEC stack. Among all the operating parameters, temperature is the most influential factor on the performance of an SOEC, in terms of both cell voltage and operation mode (i.e. endothermic, thermoneutral and exothermic). Current density is another influential factor, in terms of both cell voltage and operation mode. For the conditions used in this study it is recommended that the SOEC be operated at 1,073 K and with an average current density of 10,000 A m^–2, as this results in the stack operating at almost constant temperature along the cell length. Both the steam molar fraction at the inlet and the steam utilisation factor have little influence on the cell voltage of the SOEC but their influence on the temperature distribution cannot be neglected. Changes in the operating parameters of the SOEC can result in a transition between endothermic and exothermic operation modes, calling for careful temperature control. The introduction of air into the anode stream appears to be a promising approach to ensure small temperature variations along the cell.     

Electrochemical impedance spectra of solid-oxide fuel cells and polymer membrane fuel cells

Electrochemical impedance spectroscopy (EIS) is a very useful method for the characterization of fuel cells. The anode and cathode transfer functions have been determined independently without a reference electrode using symmetric gas supply of hydrogen or oxygen on both electrodes of the fuel cell at open circuit potential (OCP). EIS are given for both polymer electrolyte fuel cells (PEFC) and solid oxide fuel cells (SOFC) at current densities up to 0.76 A cm⁻² (PEFC) and 0.22 A cm⁻² (SOFC). With increasing current density the PEFC-impedance decreases significantly in the low frequency range reaching a minimum at 0.4 A cm⁻². At even higher current densities an increasing contribution of water diffusion is observed: the cell impedance increases again. From EIS of SOFC a finite diffusion behavior is observed even at OCP, depending on water partial pressure of the anodic gas supply. This additional element reflects the influence of water partial pressure on the cell potential. The simulation of the measured EIS with an equivalent circuit enables the calculation of the individual voltage losses in the fuel cell.     

Effect of operational potential on performance decay rate in a phosphoric acid fuel cell

The effect of operational potential on the cell voltage decay rate in a phosphoric acid fuel cell was estimated using the calculation of the platinum dissolution rate. The voltage loss, due to deterioration in activation polarization, was used in calculating the cell voltage decay rate. The voltage loss due to activation polarization was estimated using the relationship between the activation polarization and the platinum surface area. The change in the Pt surface area, arising from the dissolution of platinum, was obtained under accelerated open circuit voltage (o.c.v.) conditions. The study was focused on typical voltage decay rates from 0.1 to 10 mV per 1000 h. It was found that the cell voltage decay became significant even at low temperature during long-term operation (at o.c.v.) and that the cell had to be operated below 840 mV (iR-free) at 200 mA cm^–2) 200°C for a decay rate of 1 mV per 1000 h. From the present estimation, operational conditions such as temperature, cathode potential, and holding time at a given potential, can be roughly determined for a given decay rate. It was concluded that the voltage loss due to platinum dissolution may be negligible at a rated power operation.