WO2013085378A1

WO2013085378A1 - A fuel cell device

Info

Publication number: WO2013085378A1
Application number: PCT/MY2012/000227
Authority: WO
Inventors: Siti Kartom KAMARUDIN; Wan Ramli Wan Daud; Umi Azmah HASRAN; Norhafiz HASHIM
Original assignee: Universiti Kebangsaan Malaysia (Ukm)
Priority date: 2011-12-10
Filing date: 2012-08-10
Publication date: 2013-06-13
Also published as: MY172738A

Abstract

A fuel cell device is provided, wherein the device includes a membrane electrode assembly between a plurality of plates, methanol solution in fluid connection with the plurality of plates, wherein one anode plate positionable between 2 MEAs and 2 cathode plates, wherein the methanol solution is introduced into the anode plate, such that the device maintains steady voltage levels over temperature.

Description

A FUEL CELL DEVICE

FIELD OF INVENTION The present invention relates to a fuel cell device. BACKGROUND OF INVENTION

Tool kits are an important resource for academic and industrial research and can be used to practice and understand various disciplines. Moreover, tool kits reflect current methodological analyses that are relevant to the field of interest. To increase awareness and critical thinking on the choice of hand tools, training in hand ergonomics is necessary. One of the most effective and efficient strategies of instruction is the tool kit. Tool kits contain a concise compilation of information on current and occasionally controversial topics that facilitate consistent and reliable information transfer from professionals to their intended audience(s). Tool kits allow individuals to become experts in their field, and were created to assist developers in adopting new technologies. Furthermore, tool kits provide a detailed overview on available technologies and allow the user to make informed decisions and smoothly navigate the planning process. Numerous papers such as Breiger, R.L. A tool kit for practice theory. Poetics 27(2000) 91 -115 and Garmer, K. S., L, Forsberg, A. A hand-ergonomics training kit: development and evaluation of a package to support improved awareness and critical thinking. Applied Ergonomics 33 (2002): 39-49 have been published on the applications of DMFC tool kits for portable equipment; however, research on the development of DMFC tool kits has not yet been conducted.

In most active DMFCs, fuel is provided to the cell by a pump and the oxidant is supplied by a gas compressor. However, these auxiliary devices increase the complexity of the fuel cell system and decrease the maximum energy and power density due to parasitic power losses. Thus, the addition of auxiliary equipment limits the application of active DMFC systems to portable devices. To achieve a more compact and simple fuel cell system and to reduce parasitic energy losses from ancillary devices, a portable passive DMFC system was developed, where both the fuel and the oxidant are supplied passively. The fuel is transported by diffusion from an accessible reservoir, while the oxidant is supplied under ambient conditions. To commercialize DMFCs, a compact device that can operate for extended periods of time must be developed. Therefore there is a need for a simple solution that operates under ambient conditions and low methanol concentrations.

SUMMARY OF INVENTION

Accordingly, there is provided a fuel cell device, wherein the device includes a membrane electrode assembly between a plurality of plates, methanol solution in fluid connection with the plurality of plates, wherein one anode plate positionable between 2 MEAs and 2 cathode plates, wherein the methanol solution is introduced into the anode plate, such that the device maintains steady voltage levels over temperature.

The present invention consists of several novel features and a combination of parts hereinafter fully described and illustrated in the accompanying description and drawings, it being understood that various changes in the details may be made without departing from the scope of the invention or sacrificing any of the advantages of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be fully understood from the detailed description given herein below and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, wherein:

Figure 1 shows a schematic diagram of a fuel cell device in the preferred embodiment of the invention;

Figure 2 shows a schematic diagram of a second configuration of a fuel cell device as a planar configuration in the preferred embodiment of the invention; Figure 3 shows a graphical representation of performance of single MEA in single cell with different catalysts loading 50% Pt/Ru on anode and 50% Pt on the cathode side;

Figure 4 shows a graphical representation of results from the 4 unit MEAs in passive planar fuel cell stack with different catalyst loading;

Figure 5 shows a graphical representation of performance of 4 unit MEAs combination of single cells with catalyst loading 6.0 mg cm-2 of 50% Pt/Ru on the anode side and 4.0 mg cm-2 of 50% Pt on the cathode side; and

Figure 6 shows a graphical representation of long term power changes for different types of fuel cell Tool Kits.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention relates to a fuel cell device. Hereinafter, this specification will describe the present invention according to the preferred embodiment of the present invention. However, it is to be understood that limiting the description to the preferred embodiment of the invention is merely to facilitate discussion of the present invention and it is envisioned that those skilled in the art may devise various modifications and equivalents without departing from the scope of the appended claims.

The following detailed description of the preferred embodiment will now be described in accordance with the attached drawings, either individually or in combination.

A fuel cell device is described herein. The device includes a membrane electrode assembly between a plurality of plates, methanol solution in fluid connection with the plurality of plates, wherein one anode plate positionable between 2 MEAs and 2 cathode plates, wherein the methanol solution is introduced into the anode plate, such that the device maintains steady voltage levels over temperature. The plates used in this embodiment of the invention are Perspex plates. The fuel cell device may be applied in 3 different configurations such as a combination of 4 single cell, a fuel cell stack and a planar design.

All experiments of the fuel cell device were conducted under ambient conditions (21~23°C, 1 atm). Using CNC (Computer Numerical Control) and a laser cutter machine, a passive planar DMFC stack containing four MEAs was designed and fabricated at the Fuel Cell Institute of UKM. To produce DMFC stacks, the MEAs were placed between three Perspex plates . where one anode plate was positioned between two MEAs and two cathode plates. The Perspex plate at the anode was 8 mm, and both plates at the cathode were 5 mm. A solution of methanol was introduced into the cell at the anode; thus, a thicker plate allowed greater methanol storage. While both plates at the cathode possessed a hole for air diffusion, one plate also possessed two channels to store the methanol solution. One channel was connected to the 2 MEAs, and 2.0 mL of 4 M methanol was added to this channel. The results of several studies revealed that the optimal concentration of methanol in stacked passive DMFCs is 4 M. The total volume of the methanol solution used in planar DMFC stacks was 4 mL. Figures 1 and 2 illustrates the configuration of the anode and the cathode in the three type of configurations used in this.

Oxygen is diffused into the cathode under ambient conditions without any need for an external device such as a pump or a fan, and the methanol solution was stored in the reservoir at the anode. The solution of methanol diffused into the anode due to a concentration gradient between the reservoir and the anode. A piece of stainless steel mesh was used as the current collector. Stainless steel possesses excellent conductivity and mechanical properties, which is important for electrical resistance. To prevent methanol in the fuel reservoir from leaking, two gaskets made from silicone rubber were placed at the anode. The high compressibility of silicon rubber provided good contact between the edge of the membrane and the current collector. The performance characteristics of the cells were measured using an electronic load (3315D). Moreover, the suitability of planar DMFC stacks in portable equipment was investigated by monitoring the power output over time with a Fluke 189 True RMS Multimeter. MEA selection

To achieve high and consistent voltage in planar DMFC stacks, MEAs with superior individual performance were selected. The voltage profile of the stack was affected by several factors including the individual performance of MEAs, reactant distribution, temperature profile and the accumulation of products in the electrodes, including C02 and water. A non-uniform voltage profile based on the performance of individual MEAs in the stack may cause a rapid decline in cell performance. Moreover, changes in the performance of individual MEAs may be caused by local corrosion, which can affect the durability of the cell. Thus, to achieve optimal results, a stack of individual MEAs with a uniform voltage profile was produced.

Individual MEAs were tested in a single cell. In this study, three sets of electrodes with different catalyst loadings were synthesized and evaluated. Specifically, set A contained 5.0 mg cm-2 of 50% Pt/Ru at the anode and 3.0 mg cm-2 of 50% Pt at the cathode, while set B contained 4.0 mg cm-2 of 50% Pt/Ru at the anode and 2.0 mg cm-2 of 50% Pt at the cathode. Alternatively, in set C, 6.0 mg cm-2 of 50% Pt/Ru and 4.0 mg cm-2 of 50% Pt were used at the anode and cathode, respectively. Overall, 4 unit single cells with different catalyst loadings were tested. The performance of each set of electrodes in a single cell is show in Figure 3. Four electrodes with a similar maximum power density were used to produce a planar DMFC stack and were tested in a single cell. As shown in Figure 3 (c), set C displayed superior results, achieving a maximum power density of approximately 3.2-3.8 mW cm-2, while set A (Figure 3 (a)) produced a maximum power density of approximately 2.3-2.6 mW cm-2. The lowest maximum power density was 1.8- 2.0 mW cm-2, and was observed in set B (Figure 3 (b)). Differences in the maximum power density of each set may have been caused by errors in the manual casting method. After identifying the optimal combination of single cells, the MEA was assembled in the planar DMFC stack.

Planar DMFC stack

After the performance of each MEA was identified, a four unit MEA was fabricated in a planar DMFC stack, and its performance was evaluated. The stack was built with three Perspex plates, where one plate was placed at the anode and two plates were used at the cathode. Each stack was monitored for approximately 8 hours under open circuit voltage (OCV) conditions, and the performance of the cell was subsequently evaluated. In this study, the current collector was connected in series to the MEAs of the DMFC stack. Cell performance was significantly improved with a series connection, compared to a single cell or a parallel connection. Ohmic loss resulting from the high current of a parallel connection led to low cell performance. Thus, the single cell performance of four individual cells was evaluated with 1.5 mL of methanol solution and the results were compared to those of four MEA units connected in series in a planar DMFC stack. Figure 4(a) shows the open circuit voltage (OCV) curve of a planar DMFC stack under passive feed conditions. The OCV results of a passive DMFC system were different from those obtained in an active DMFC system. Specifically, in a passive DMFC system, the OCV varied over time, while a fairly stable voltage was observed in an active DMFC system. The time dependence of OCV is a direct result of the concentration of methanol in the reservoir at the anode. Specifically, as a solution of methanol is injected into the reservoir of a passive DMFC cell, the OCV begins to change over time. The results indicated that the OCV pattern of planar DMFC stacks was identical in set A and C. In both cases, the voltage decreased initially and then increased until a maximum value was observed. Lastly, the voltage in set A and C decreased slowly over time. In set C, the voltage decreased from 1.8 to 1.6 V and then increased gradually until a maximum voltage of 2.5 V was obtained. After 7 hours, the voltage began to decline. Variations in the OCV were caused by the oxidation of methanol in the stack and changes in cell temperature. Voltage decreased in the early stages of the experiment due to the initiation of methanol oxidation. A subsequent increase in voltage was caused by an increase in cell temperature, which accelerated the rate of methanol oxidation and oxygen reduction, leading to improved cell performance. The temperature of the cell eventually stabilized, resulting in a constant voltage. The voltage decreased at later stages of operation due to an increase in methanol crossover and an accumulation of water at the cathode. Furthermore, the cell temperature under steady state conditions was higher at later stage of operation because the concentration of methanol at the anode and the rate of methanol crossover decreased over time. In set C, the voltage initially decreased from 1.2 to 1.1 V, increased gradually to a voltage of 1.9 V and subsequently decreased after 7 hours. Alternatively, the OCV pattern for set B was slightly different. For instance, the voltage decreased initially, increased slowly over time and remained stable for 8 hours. The voltage of set B did not decline within 8 hours due to less methanol crossover. Despite the differences in OCV patterns, the voltage at equilibrium in each set was similar. In these experiments, the reservoir at the anode was filled with water for the first 5 minutes. To obtain a faster reaction, a solution of methanol was injected into the anode to wet the surface of the MEA.

After 8 hour of OCV testing, performance tests were conducted to identify the maximum power density of each stack and to obtain polarization curves, which provide the relationship between voltage and current density in the stack. As shown in Figure 4 (b), the maximum power density of set A, B and C were 3.6, 3.4 and 4.8 mW cm-2, respectively. The combination of electrodes in set C provided a higher maximum power density due to higher catalysts loadings. Moreover, each MEA gave slightly higher maximum power densities in single cell tests. A higher catalyst loading increase the active surface area and mass transport resistance of methanol. In this study, all catalysts loadings were higher at the anode (Pt/Ru 50%) than the cathode (Pt 50%) to prevent methanol crossover, which may occur due to high production activity. The combined single cell performance of the four individual cells in set C was evaluated. In these experiments, 1.5 ml. of methanol solution was used in each single cell test, and the results were compared to those obtained from four MEA units connected in series. The catalysts loading at the anode and cathode for each MEA in set C were 6.0 mg cm-2 of 50% Pt/Ru 50 and 4.0 mg cm-2 of 50% Pt, respectively. As shown in Figure 5, the maximum power density obtained from the combination of four individual cells was 3.7 mW cm-2 at a current density of 3.0 mA cm-2, which was lower than the peak power density of a planar DMFC stack containing the MEAs of set C connected in series (4.8 mW cm-2 at 5.5 mA cm-2). Although the total volume of 4 M methanol in the combination of four individual cells was 6.0 mL more than the total volume of methanol in the planar DMFC stack (4.0 mL), the performance of a planar DMFC stack with a series connection was superior to the combination of four individual cells. A series connection enhanced cell performance by decreasing the size of the system, increasing the temperature of the cell and reducing the power loss from individual cells. Moreover, the poor performance of a system based on the combination of four individual cells may have been caused by poor electrical contact between current collectors and electrodes. Based on the aforementioned results, a planar DMFC stack with a series connection was used to evaluate the potential of DMFCs in portable applications.

Performance of passive DMFC stacks to tool kits

Passive DMFC systems were suitable for portable applications. The applicability of DMFCs to portable equipment was analyzed by monitoring long-term performance characteristics, including the duration of power stability. Power stability indicated that the cell could produce a significant and constant amount of power over a specific period of time. The durability of DMFCs was related to cell lifetime and was defined as the ability of a cell to resist a permanent change in performance over time. The results indicated that a high catalyst loading provided high durability and low methanol and water crossover. After a certain amount of time, the output of the cell decreased because the methanol inside the fuel tank had become completely oxidized.

DMFC tool kits were applied to portable devices including, but not restricted to, a calculator, a mini torch-light, a stopwatch and a clock. All of these devices can be operated with a lithium battery. Due to their high maximum power density, the MEAs of set C were connected in series and were applied as a planar DMFC stack for portable devices. The equipment was tested with a Fluke 189 True RMS Multimeter. In each test, the power requirements of each device was different and varied over time; however, to operate the equipment, a voltage greater than 1.5 V was applied and the current was varied depending on the specific needs of the device. As shown in Figure 6, different amounts of power were required for different pieces of equipment. For instance, the stopwatch required a power supply of approximately 0.03-0.06 mW, while the calculator required 0.1-0.25 mW. Alternatively, a larger amount of power was necessary to operate the mini torch-light and the clock, where approximately 30-50 mW and 4-6 mW of power were used, respectively. The results of each device were different; however, after operation for an extended period of time, the power supplied to all of the equipment decreased due to the complete oxidation of methanol inside the DMFC tank. As shown in Figure 6 (a), (c) and (d), the power requirements of the calculator, stopwatch and clock were not identical and varied over time. Specifically, the requirements of the calculator were variable because calculator usage increased the demand for power. The calculator could be used for approximately 400 minutes before all of the methanol in the cell was completely oxidized. A similar trend in power usage was observed with the stopwatch. Furthermore, the power use graph for the clock displayed fluctuations due to ringing. Ringing occurred at 500 and 900 minutes, where the power supplied to the clock decreased during ringing and increased during normal operations. The clock could be used for 60 hours with a DMFC tool kit. As shown in Figure 6 (b), the mini torch-light required more power than the other devices, and the power supplied by the cell increased over time. The power supply to the torch increased to 50 mW, and then decreased over time. As the power supply increased, the brightness of the torch-light also increased. At approximately 300 minutes, the torch-light became dim. Results of planar DMFC stacks with a series connection revealed that the maximum power density was 4.8 mW cm-2 at 5.5 mA cm-2 under ambient conditions. The catalysts loadings at the anode and cathode were 6.0 mg cm-2 of 50% Pt/Ru and 4.0 mg cm-2 of 50% Pt (set C), respectively. DMFC stacks with a series connection showed a superior performance compared to a combination of single cells. Thus, planar DMFC stacks were evaluated in portable equipment by monitoring the long-term performance of cells. Specifically, the power stability and the duration of power stability of the cells were assessed. Finally, DMFC tool kits were successfully applied to a calculator, mini torchlight, stopwatch and clock. The results indicated that planar DMFC stacks based on four MEAs connected in series can operate the same equipment as a lithium battery. Thus, DMFCs have the potential for commercialization in the near future.

Due to the success of Li-ion batteries, there has been recent interest in the use of DMFCs to power portable equipment for commercial applications. DMFCs that produce a direct- current may soon replace Li-ion batteries. The advantages to DMFCs include high energy density compared to other liquid fuels, low emissions, good electrochemical activity and ease in handling. The methanol used to supply DMFCs can be generated from various sources such as natural gas, coal or biomass. A fundamental limitation to DMFC technology is methanol crossover, which is the transport of methanol from the anode to the cathode by diffusion and electro-osmosis. In methanol crossover, methanol reacts directly with oxygen, and does not produce a current. Thus, methanol can have a poisoning effect on cathode catalysts, which reduces DMFC performance. To minimize methanol crossover, a dilute methanol solution is often used as a fuel. This invention is adapted for use with fuel cell devices. The disclosed invention is suitable, but not restricted to, for use in portable equipment which requires monitoring long-term performance of cells.

Claims

1. A fuel cell device, wherein the device includes:

a membrane electrode assembly between a plurality of plates;

methanol solution in fluid connection with the plurality of plates,

wherein one anode plate positionable between 2 MEAs and 2 cathode plates, wherein the methanol solution is introduced into the anode plate, such that the device maintains steady voltage levels over temperature.

2. The device as claimed in claim 1 , wherein the plates are Perspex plates.

3. The device as claimed in claim 1 , wherein both plates at the cathode possessed a hole for air diffusion, one plate also possessed two channels to store the methanol solution.

4. The device as claimed in claim 1 , wherein two gaskets made from silicone rubber were placed at the anode

5. The device as claimed in claim 1 , wherein the fuel cell device is a stackable device.

6. The device as claimed in claim 1 , wherein the fuel cell device is a planar device.

7. The device as claimed in claim 1 , wherein the fuel cell device is a combination of 4 single cells.

8. The device as claimed in claim 1 , wherein the fuel cell device is applicable to portable devices including a calculator, a mini torch-light, a stopwatch and a clock.