Micro-Fuel Cells A Kundu, K Karan, and BA Peppley, Queen’s RMC Fuel Cell Research Centre, Kingston, ON, Canada Y Sahai, The Ohio State University, Columbus, OH, USA & 2009 Elsevier B.V. All rights reserved.
Introduction Micro fuel cells are being developed as an alternative to rechargeable batteries including those based on lithium ions. The current battery systems are not suitable for the high-power and long life span portable devices due to their limited specific energy, although significant research and development is ongoing to overcome such limitations. In order to compete with conventional batteries, many properties and variables should be considered in the development process of micro fuel cells. These include energy, size, weight, operation time, transient behavior, and cost as the key properties and energy density, specific energy, cartridge for fuel, change of power density with time, specific cost, and life cycle cost as the associated key variables. The interaction between these key variables and properties is illustrated in Figure 1. Fuel storage and fuel cell stack are large contributors to system volume. Larger amounts of fuel storage increase the runtime of the system before requiring refilling; however, an increased stack increases the total power and/or efficiency of the system, as micro fuel cells run more efficiently at low power densities. Therefore, a larger fuel cell stack operating at lower power densities offers a higher efficiency, decreasing the fuel consumption. Consequently, the size of the fuel storage and the fuel cell stack should be optimized. Microelectromechanical systems (MEMSs) technology has been developed for environmental and internal sensors, machining of silicon and metal derivatives, optical and biomedical systems, and microfluidics. Microelectromechanical systems fabrication processes can be used for miniaturization and economical mass production of small fuel cells. The main advantages of
the application of MEMS processes in micro fuel cells are the electromechanical integration of the fuel cell structure with high precision, repeatability, and productivity required and is expected to achieve simple and mass-producible fuel cells with uniform specifications, similar to manufacturing of integrated circuits (ICs). Microelectromechanical systems technology is able to provide the following improvements in the fuel cell: a significant reduction of the amount of catalyst and a higher power output due to the controlled microstructure of three-phase boundaries; lower contact resistance at the layer interface and controlled gaspermeable structure due to electromechanically integrated fabrication; and flexible connection design for multiple in-plane cells. Most conventional cell stacks of proton-exchange membrane fuel cells (PEMFCs) consist of separators with gas channels, carbon paper sheets, carbon particles, and catalysts supported by carbon particles. In these stacks, the reactant molecules must be transported from the gas channels to the reaction sites on the catalyst particles. The product (carbon dioxide and/or water) must be transported from the reaction sites to the gas channel on the same side they are generated or to the gas channel on the other side after being transported through the electrolyte. It is generally thought that the main mechanism of transport of these molecules in the electrode pores is via diffusion although convective transport can play an important role based on flow field design. By using MEMS technology, these diffusion paths can be reduced by preparing fine channels in which the materials pass not by diffusion but by bulk flow. This may considerably increase the performance of the fuel cell stack. Insofar as the material of construction is concerned, MEMS-based micro fuel cells
Energy (energy density)
Energy density (Wh L−1)
Specific energy (W kg−1)
Cartridge for fuel
Change of power density with time (W cm−3 s)
Specific cost ($ W−1)
Life cycle cost ($)
Figure 1 The interaction between key variables and properties of micro fuel cells.
Fuel Cells – Exploratory Fuel Cells | Micro-Fuel Cells
can be fabricated from silicon, stainless steel, and polymer. In the case of onboard hydrogen production for fueling PEMFC, it is possible to integrate an MEMSbased fuel reformer, catalytic combustor for heating source in reforming reaction, and a reactor for purification of the reformate gas. It is also possible to heat the microreformer in a controlled manner by platinum lines deposited on the rear side of the silicon wafer, which makes the total system attractive in terms of elimination of the catalytic combustor.
Micro Fuel Cells on a Silicon Wafer Using Microelectromechanical Systems Technology Silicon is the most common material encountered in MEMS-based fuel cells. All components of fuel cells are monolithically integrated by using bonding technologies including anodic and eutectic bonding, which are already used in MEMS devices such as pressure sensors. Localized bonding performed at a low temperature and a solid state electrolyte could minimize the bonding area and maximize the active area available in the fuel cell. As a result, components such as gaskets, fasteners, and end plates can be eliminated. Furthermore, by integrating the auxiliary devices such as micropumps, valves, connectors, controllers, and electronics onto a common silicon substrate, small fuel cell systems with high power density could be realized. The electronics are fabricated using IC process sequences (e.g., CMOS, bipolar, or BICMOS processes), whereas the micromechanical components are fabricated using compatible ‘micromachining’ processes that selectively etch away parts of the silicon wafer or add new structural layers to form mechanical and electromechanical devices. There are at least seven MEMS fabrication technologies falling into three classes: bulk micromachining, surface micromachining, and high-aspect-ratio micromachining (HARM). Bulk micromachining is a subtractive process that involves the selective removal of the wafer substrate material to form the MEMS structure, which can include cantilevers, holes, grooves, and membranes. Surface micromachining is an additive process that involves depositing combinations of thin structural and sacrificial layers, wherein the sacrificial layers are subsequently removed to form raised structures that can include gears, comb fingers, cantilevers, and membranes. HARM includes deep ultraviolet or Xray lithography techniques collectively known as LIGA (from the German Lithographie, Galvanoformung, and Abformung, meaning lithography, electroplating, and molding). LIGA makes it possible to create microcomponents out of polymers, metals, and ceramic materials using micromachined molds.
Figure 2 shows a typical preparation process of a thin-film planar micro fuel cell stack using the silicon process technology. First, a silicon nitride (Si3N4) film (or SiO2) is deposited and patterned on a p-type silicon substrate. Then, a porous silicon layer is formed in the opened area of silicon nitride windows by anodizing in a hydrogen fluoride –ethanol solution. The porous silicon is thermally oxidized at 900 1C for electrical insulation and the deep reactive ion etching (DRIE) process is applied for making cells. platinum/titanium catalytic electrodes are formed on the porous silicon dioxide film by evaporation and are separated using a lift-off process. The micro gas channels for supplying fuel and air to the electrodes are formed on the surface of a Pyrexs glass sheet. The chromium film in the nanometer scale is deposited on a glass sheet and is patterned in the shape of the gas channels. This will act as the current collector. It is also possible to sputter Ti/Pt/Au composite metal layers, which can act as current collectors. The glass sheet is etched by a wet process for making channels. The feed holes of fuel and air are opened by sandblasting from the backside or DRIE. The glass sheet with micro (1) Si3N4 formation and patterning Si3N4 (2) Anodization and thermal oxidization
(4) Catalytic electrode formation
(5) Glass process and anodic bonding
(6) PEM attachment/coating PEM
Glass (7) Packaging and wiring
Figure 2 Fabrication process for a planar stack of micro fuel cells. DRIE, deep reactive ion etching; PEM, proton-exchange membrane.
Fuel Cells – Exploratory Fuel Cells | Micro-Fuel Cells
gas channels is bonded to the silicon substrate using anodic bonding method. The catalyst can be electrodeposited on the surface of the current collector. The polymer electrolyte film is sandwiched between the silicon substrates and hot pressed at 80 1C. The other method is to micropattern the membrane–electrode assembly (MEA) according to the structure of the cell followed by sandwiching between two silicon substrates. Microelectromechanical systems technology for micro fuel cells has been much improved since porous silicon films were introduced. It is possible to support the MEA with a porous silicon layer instead of the porous silicon dioxide layer. Despite many advantages, fuel cells based on a silicon substrate will not match the raw-material costs of traditional carbon-based fuel cell components, but if packaging integration and on-chip control become the cost drivers for the system, the silicon-based fuel cell power source will offer advantages over the more traditional carbon-based embodiments. Micro-Proton-Exchange Membrane Fuel Cell Initially, the development of MEMS-based micro fuel cells on a silicon substrate was limited to single-cell fabrication. As the power output from a single cell is not sufficient for practical use, multicells on a single silicon wafer with planar stacks were designed. Figure 3 shows a six-cell micro proton-exchange membrane fuel cell with planar configuration. The configuration claims a peak power of 0.9 W at 250 mA cm 2 for the stack and an average power density of 104 mW cm 2 for each cell.
microchannels in a microreformer can be filled with catalyst powder forming a micro packed-bed reactor or coated with the catalyst material forming a wall-coated microchannel reactor. The walls of the microchannels can be coated with catalytic material by a number of methods including sputtering, evaporation, slurry washcoating, and sol–gel coating. The pictures of MEMS-based microreformers from different groups are presented in Figure 4. Micro Direct Methanol Fuel Cells An MEMS-based micro direct methanol fuel cell (DMFC) is advantageous owing to methanol’s higher energy density and its simpler operation and safer handling. Liquid fuel usage also eliminates the need for a complicated fuel reforming system and a bulky balance of plant (BoP), which simplifies the system considerably. It only needs one liquid pump and one methanol sensor in BoP as an air-breathing DMFC is generally used. Several works have been conducted to investigate the feasibility of MEMS technology in a micro-DMFC system. Figure 5 shows one such complete system of a micro-DMFC (with airflow in the cathode) where different components like a silicon membrane micropump for pumping the collected water back to the anode and passive liquid–gas separator for carbon dioxide removal are integrated onto the same silicon wafer. In this way, interconnections are eliminated. Water flooding in the cathode has been avoided by using a silicon back plane at
Microreformer with Micro Fuel Cell Silicon-based microreformers are very attractive because of their very small size and advantages with respect to the improvement in heat and mass transfer and hydrodynamic flow resulting in higher conversion. The
Polymer packaging (a) Fuel/off gas
Figure 3 A six-cell stack of micro-proton-exchange membrane fuel cell (PEMFC).
Figure 4 Different configurations of microreformers.
Fuel Cells – Exploratory Fuel Cells | Micro-Fuel Cells 3% Methanol CO2 separator
Silicon wafer with integrated separator, pump, and CMOS
Air inlet 100% Methanol
Porous backing layers
CMOS control circuit
Figure 5 A micro-direct methanol fuel cell (DMFC) system.
the cathode and applying a localized hydrophobic coating to passively guide the water to flow to the desired collecting point. The collected water at the cathode can be pumped back to the anode. This allows avoiding sizable water storage. To eliminate the liquid pump for fueling methanol, fully passive type of MEMS-based micro-DMFC has been introduced. The fuel feeding takes place by diffusion of the methanol molecules from a reservoir. The maximum power density obtained from the system is around 10 mW cm 2. The drawback of this system is the requirement to maintain a consistent performance in all orientations and fluctuations, in the performance due to change of the flow of the reactant by diffusion with change in the surrounding atmosphere. There is also an MEMS-based micro fuel cell powered by ethanol and formic acid. In the case of ethanolfueled micro fuel cell, the electrodes are fabricated using macroporous silicon technology. The pores developed act both as microcapillaries/wicking structures and as a built-in fuel reservoir, reducing the size of the fuel cell. The pore sizes dictate the pumping/priming pressure in the fuel cell. The porous silicon electrode thus eliminates the need for an active external fuel pump. The porous silicon electrode is fabricated using photoelectrochemical
porous silicon etching technique. It is performed in a photon-assisted electrochemical etch rig. The wafer is anodically biased and the etching is done using 2% (v/v) HF solution and controlled current density. The backside of the wafer is exposed to light from a 250-W quartz– tungsten halogen lamp. This light produces the electron– hole pairs that drive the silicon dissolution (etching) process. The micro fuel cell developed in this process exhibited a maximum power density of 8.1 mW cm 2 at room temperature with minimal dependence on cell orientation using platinum and Nafions 115 as catalyst and membrane, respectively, and oxygen flow in the cathode. The system is orientation independent, because the capillary force generated by the pores in the porous silicon has the ability of ‘wicking’ the fuel toward the electrode irrespective of the orientation of the cell, resulting in a uniform and regulated supply of the fuel. A silicon-based micro fuel cell has also been tested with formic acid as the fuel. The maximum power density is 17 mW cm 2 at room temperature with forced oxygen, where the catalyst is electrodeposited platinum black on silicon and Nafion is the membrane. The performance is increased up to 28 mW cm 2 with the electrodeposited palladium-containing catalyst at the anode.
Fuel Cells – Exploratory Fuel Cells | Micro-Fuel Cells
Micro Fuel Cell on a Metal Substrate Using Microelectromechanical Systems Technology Microelectromechanical system-based substrates have been employed to counter the problems associated with silicon wafers being brittle causing easy breakage during fuel cell assembly. A titanium substrate with microflow channels and a stainless-steel bipolar plate or the employed micropatterning technologies for both current collector and flow fields on the stainless-steel substrate, which have been utilized to reduce costs significantly, are used in MEMSbased micro fuel cell and have higher mechanical strength than silicon wafers.
Micro Fuel Cell on Polymer Materials Using Microelectromechanical Systems Technology Two types of polymer materials (i.e., polydimethylsiloxane (PDMS) and poly(methyl methacrylate) (PMMA)) are also used for MEMS-based micro fuel cells. In the case of PMMA material, the flow fields are made with a carbon dioxide laser. A gold layer is sputtered over the substrate surface to act as both the current collector and corrosion protection layer. The reported power density from this type of PEMFC is 315 mW cm 2 at 0.35 V with Pt/C and Nafion 1135 as catalyst and membrane, respectively. Polydimethylsiloxane-based micro fuel cells are fabricated by employing micromolding with a dry-etched silicon master (photolithography replica molding technique). The PDMS spin-coated on micromachined silicon is then cured and peeled off from the master. The process is simple and not expensive. The reported power density is very low (0.8 mW cm 2). The cell can be bonded by corona discharge, which is very simple but expensive. Another polymer-based material, printed circuit board (PCB), is employed for the development of a micro fuel cell. The advantages of this technology are the design flexibility, potentially higher power densities, and ease of device integration. The current-collecting pattern and the circuit for the accessories on the backside can be easily made and this makes the PCB material more attractive. The power density obtained with this type of micro-PEMFC is in the range of 700–800 mW cm 2 with hydrogen and oxygen flow and Pt/C and Nafion 112 as catalyst and membrane, respectively. The photosensitive polymers, such as AZ1512 and SU8, are used for simplifying the process steps and enhancing the cell performance by friendly adhesion characteristics with membrane in the development of a micro-DMFC. The good adhesion in the polymer-topolymer interface of an all-polymer fuel cell may reduce
the contact resistance and prevent fuel leakage. The microchannels are fabricated with a photosensitive polymer using ultraviolet (UV) photolithography process. The current collector layer is deposited on the polymer chip by sputtering of platinum. A maximum power density of 8 mW cm 2 is obtained.
Design of Micro Fuel Cell For successful integration of fuel cells into electrical appliances, the dimension of the fuel cell must be in accordance with the existing geometries of the device. The possible cavities of most devices consist of a rather flat geometry. Thus, in some cases, fuel cells with a conventional stack design might be difficult to integrate. A flat design of fuel cells accommodates these needs. As the fuel cell in this packaging concept serves as part of the housing, the volume needed for the planar-designed fuel cell as a power source is optimized. The ideal planar design consists of an open cathode side to allow passive, full self-breathing operation of the fuel cell. Thus, a planar fuel cell could be integrated into the back of a notebook computer or a cell phone, so that the side-byside cells with a monopolar plate may be best in portable electronics. There are two kinds of side-by-side stack: (1) planar flip-flop configuration and (2) planar banded configuration. The devices and manifolding for recycling excess hydrogen can be removed to reduce the volume for hydrogen delivery, and a dead-ended hydrogen supply is used. The dead-ended arrangement for hydrogen supply is achieved by maintaining a relatively constant hydrogen pressure at the anode gas diffusion layer (GDL) interface. Constant gas pressure can be achieved with a high-pressure hydrogen supply and a pressure-regulating valve, or through desorption of hydrogen from a metal hydride. As the anode outlet is blocked, water accumulation in the anode channel and gas diffusion media may hinder hydrogen transport, resulting in the starvation of fuel for the anode. There are many ways to remove water from large fuel cells in an optimized way in order to protect the membrane from dehydration as well as the anode and cathode from flooding. As micro fuel cells are air-breathing fuel cells, those methods are not applicable for removing water, and evaporation is the only means by which liquid water can be removed. The development of an electroosmotic pump may be helpful for active removal of liquid water from open air-breathing cathode and anode. The advantages of using an electroosmotic pump are the compatibility with the size of the pump in comparison with MEMS-based micro fuel cell and low consumption of power. The other important requirements for the design of MEMS-based micro fuel cells are uniform distribution of reactants to each cell in a stack and inside each cell, maintenance of required
Fuel Cells – Exploratory Fuel Cells | Micro-Fuel Cells
temperature in each cell, minimum resistive losses (choice of materials, configuration, and uniform contact pressure), no leakage of reactant gases (between the cells or to the ambient), mechanical sturdiness (internal pressure including thermal expansion, external forces during handling and operation, including shocks and vibration), and clamping/sealing of the stack.
See also: Applications – Portable: Micro Hybrid Power System: Fuel Cells/Capacitors; Portable Devices: Fuel Cells; Fuel Cells – Direct Alcohol Fuel Cells: Direct Ethanol Fuel Cells: Catalysts; Direct Methanol: Overview; Overview.
Further Reading Concluding Remarks Microelectromechanical system-based micro fuel cells are described for different substrates namely silicon, stainless steel, and polymer. Among the numerous solutions developed, the basic structure of the micro fuel cell remains the same: a thin-film planar stack with a commercial ionomer, most often Nafion, the reported layers being micromachined (microchannels or porous media) for gas/liquid management and coated with gold for current collecting. The performance may vary from a few tenths of a mW cm 2 up to several hundreds of mW cm 2. It is not the materials selection that really matters to achieve high performances but mostly the way the fluids are managed, the types of fuels used, and the stack. Silicon remains the most employed material, but stainless steel and polymers have shown interesting possibilities for future commercial developments.
Nomenclature Abbreviations and Acronyms BICMOS BoP CMOS DAFC DFAFC DMFC DRIE GDL HARM IC LIGA
MEA MEMS PCB PDMS PEM PEMFC PMMA UV
Bipolar complementary metal–oxide– semiconductor balance of plant Complementary metal–oxide– semiconductor direct alcohol fuel cell direct formic acid fuel cell direct methanol fuel cell deep reactive ion etching gas diffusion layer high-aspect-ratio micromachining integrated circuit Lithographie, Galvanoformung, and Abformung (lithography, electroplating, and molding) membrane electrode assembly microelectromechanical systems printed circuit board polydimethylsiloxane proton-exchange membrane proton-exchange membrane fuel cell poly(methyl methacrylate) ultraviolet
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