application of micro-scale techniques to fuel cell systems design

th

10 Canadian Hydrogen Conference, May 17-20, 2000

APPLICATION OF MICRO-SCALE TECHNIQUES TO FUEL CELL SYSTEMS DESIGN G. McLean, N. Djilali, M. Whale, T. Niet Institute for Integrated Energy Systems, University of Victoria Victoria, B.C., Canada, V8W 3P6 Tel: (250) 721 8931 Fax: (250) 721 6323 E-mail: [email protected], [email protected], [email protected], [email protected]

Abstract The application of microscale principles to devices can lead to orders of magnitude increase in area to volume ratios. In fuel cells, where electrochemical reaction, heat and mass transfer are all surface phenomena, the application of microscale principles could lead to significant breakthroughs in power density, efficiency and cost. In this paper we examine the function of fuel cell components that present opportunities for applying microscale design principles. To appreciate the possibilities and state of understanding of microscale phenomena, an overview of the theoretical foundations is presented, followed by a review of microscale fabrication techniques potentially applicable to manufacturing of fuel cell component. Examples of progress made in microscale fuel cells and in other relevant applications are presented and a the rational for applying microscale principles to fuel cell design is summarized. Benefits include: increased power density, heat and mass transfer and catalyst utilization; enhanced efficiency; reduced system complexity and opportunities for new fuel cell applications. 1 Introduction In most engineered systems, a trend toward miniaturization is pursued to diminish size, weight and complexity while reducing cost and improving performance. In the recent history of fuel cell design, particularly in PEM fuel cells we have witnessed a similar sort of rapid design evolution leading to improved performance. It is reasonable to expect the drive toward size reduction to continue, particularly since the ultimate cost of fuel cell devices will be related primarily to the volume and weight of raw materials [1]. -6

-4

The technologies of micro-scale production with a characteristic length in the range 10 -10 m, (i.e. midway between the length scales of human sized devices (1 m) and the diameter of the hydrogen atom [2] ) lend themselves well to fuel cell processes. Recent advances in our understanding of micro-scale thermo and electrical transport suggest tremendous enhancements in heat and mass transport and electrochemistry could be obtained by engineering fuel cells at this scale. Therefore we anticipate potential order of magnitude improvements in fuel cell performance can be achieved through concerted design efforts focussed on the exploitation of micro-scale phenomena for fuel cell systems design. In this paper we review the state of the art of micro-scale theory and manufacturing practice from the perspective of fuel cell systems design. 2 Fuel Cell Systems The technology of PEM fuel cells has undergone steady and significant refinement over the past ten years to produce fuel cell stacks which are both cost and performance competitive with entrenched technologies, namely internal combustion. A modern, conventional fuel cell stack can be broken into three constituent component groups: • Membrane Electrode Assemblies which fulfill the electrochemical function of the fuel cell • Multi-function plates (Bipolar) which service the MEA's with fuel and oxidant, provide cooling and conductive electronic paths Ancillary components that provide bulk delivery and removal of fuel cell reactants and products.

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A conventional fuel cell stack consists of a repeated interleaved structure of MEA's and multi-function plates to create a bipolar stack. Current is produced across the area of the MEA's and is not concentrated into a restricted cross-section until the current is gathered at the ends of the stack. The MEA's and multifunction plates are clamped together with significant force required to activate mechanical seals, reduce electrical contact resistance between pressure assembled conductive components, and ensure the integrity of the reactant gas flowpaths formed by the interaction of the MEA and multi-function plates. The ancillary functions of gas humidification and stack cooling are generally integrated into the stack, but the overall ancillary systems are plumbed to the stack using conventional pipefittings. Efforts toward higher power density and reduced costs have so far consisted of components optimization. Catalyst loadings are reduced, membranes are thinner, plate-manufacturing costs are lowered but the overall 'plate and frame' structure has so far been retained, with the exception of a few low current density designs targeted at low power applications [3-5]. In fuel cells, the power developed is a function of the active surface area of the electrochemical cells; i.e. the electrochemical reaction is a macroscopically two-dimensional phenomenon. Therefore, unlike internal combustion, the fuel cell offers at least one degree of freedom in its physical dimension that can be arbitrarily chosen for convenience or to satisfy some practical design constraint. In other words, the fuel cell can be made arbitrarily thin. When considering the design of fuel cells there are few lower bounds limiting the performance. 2.1 Practical Constraints dominate in Component Design 2.1.1 Membranes The electrolyte acts as an ionic conductor to transport hydrogen ions from the anode to the cathode. It is an imperfect conductor, producing heat that must be removed. Therefore, it is desirable to make the electrolyte layer as thin as possible to minimize the resistance (which is a simple function of path length or thickness). In principle, one or two atomic layers of electrolyte material could provide the required functionality, provided separation of the reactant gases on either side of the membrane is ensured. In PEM fuel cells the electrolyte is a solid matrix of sulfonic acid groups attached to a fluorocarbon polymer backbone. This polymer electrolyte is self supported and provides high protonic conductivity when fully hydrated. Its availability in this form has enabled the rapid development of the PEM fuel cell, removing previous technical barriers created through the use of liquid electrolytes. However, this form of electrolyte as a solid sheet material also imposes practical limitations on the design of the MEA layer. The reliance of MEA fabrication methods on a solid and reasonably tough membrane sheet forces the designers to consider both the mechanical properties as well as the electrochemical properties of the material. In addition to providing good ionic conductivity the membrane must serve as a structural member onto which gas diffusion electrodes are bonded and must serve as a sealant, preventing gas crossover and interacting directly with mechanical seals clamping the overall stack together. Therefore, the membrane thickness cannot be arbitrarily reduced in conventional designs. This point is emphasized by examining of some of the thinnest membranes that reveal a composite of the membrane supported in the fine mesh of a porous micro-structural member [6]. 2.1.2 Gas Diffusion Electrode In addition to conducting current, the electrodes are the interface through which reactants are diffused from the bulk gas distribution channels to the reaction site, and through which product heat and water are evacuated. Their design and construction is a compromise between often conflicting requirements: evacuation of product water to prevent flooding, transport of water vapour to the membrane to ensure humidification, high conductivity, electrochemical stability, and low contact resistance. Typical gas diffusion electrodes are made of a porous carbon paper, cloth or tissue with a hydrophobic PTFE coating. The manner in which the electrode, catalyst, electrolyte interface is formed is critical. A large three-phase contact zone between reactant (gas), catalyst (solid) and electrolyte (liquid) is necessary to achieve effective electrochemical reaction rates [7]. Consequently porous electrodes with very large surface area/volume ratios are highly desirable.

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2.1.3 Catalyst Layers Catalyst layers are formed by printing, spraying or spreading mixtures of catalyst and carbon with the possibility of non-uniform distribution of the catalyst. The resulting catalyst layers are by no means optimal, with an arbitrary association formed between a catalyst particle, and electronically conducting path and an ionically conducting path. 2.1.4 Multi-function Plates The multi-function plate integrates the functions of current collection, heat conduction, sealing, gas distribution and structural support into a single manufactured component. Graphite has been the material of choice because it is relatively inert in a PEM environment, provides good conduction of heat and electricity, can be made impervious to gases and can be machined in a relatively straightforward manner. Efforts to reduce the thickness of the plate are quickly met with practical limitations, however. Beyond supporting the dimensions of the flow channels, the plates must have sufficient structural integrity to survive the manufacturing process, a requirement in no way related to either flow channel dimensions or to any other specifically fuel cell function. This requirement for structural integrity is further entrenched by the need to rely on relatively large clamping forces to complete the interface between the multifunction plates and the MEA layers. Deformation of MEA's by the compression of the gas diffusion layer into the flow channels places a practical lower limit on flow channel dimensions. Optimal membrane performance requires operation near full water saturation, and dictates operation in o the 70-90 C range typically. Cooling is usually accomplished by forced convection of water at regular intervals within the stack. To simplify manifolding, cooling is implemented in series and a fairly large o temperature differential (~10 C) is accepted within the stack. The high purity graphite bipolar separator plates used in a conventional PEM fuel cell account for a large portion of the cost [8]. This has prompted the search for lower cost, easier to manufacture materials. Metals, such as stainless steel, aluminum and titanium, as well as conductive plastics [9-12]. But so far none of these have been incorporated into mainstream stack development because of lingering life time and contact resistance issues [13]. 2.2 The Design Imperative toward smaller dimensions The limits on dimensions currently encountered in the 'plate & frame' structure are largely imposed by material or manufacturing limitations. They are not fundamental limits. Furthermore, there are numerous design considerations in fuel cells that favour smaller dimensions, causing the move to large active areas to become a tradeoff. In general, as areas become larger it becomes increasingly difficult to maintain a homogeneous environment throughout the stack. Cooling, gas distribution and humidification are all examples of required functions within the fuel cell that could benefit from both reduced fuel cell area and reduced fuel cell stack thickness. 3 Micro Scale Phenomena 3.1 Background Optimal operation of a fuel cell requires effective mass transfer of reactants, high electrochemical activity, and effective transport of product heat and water. All these processes are fundamentally surface phenomena. The dominance of surface effects is precisely one of the characteristics of micro-scale transport. While the surface to volume ratio for a device with characteristic length of 1m is of the order of -1 6 -1 1m , the corresponding ratio for a 1µm device is of order 10 m [2]. The effective exploitation of increases in surface to volume/mass of such magnitude would yield dramatic cost reduction and enhancement in electrochemical, heat and mass transport.

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Micro-scale transport research was spurred in the late 70’s by increasing integrated circuit chip miniaturization and the accompanying need for higher energy dissipation fluxes. The first demonstration of the benefits of microscale was reported by Tuckerman and Pease [14]. Using forced convection through microchannels etched onto the back of a silicon wafer as shown in Figure 1, heat fluxes of 2 almost 800 W/cm were achieved. Combining this with the much larger surface area available for heat transfer, yields an order of magnitude increase in the overall heat dissipation rate in a microchannel heat sink. In the last few years, microscale phenomena and devices have found applications in micro-heat pumps, micro-heat engines, biological reactors, and fluidic control [2], to cite but a few examples.

IC ELEMENTS IC ELEMENTS

FORMING SURFACE HEAT SOURCE

FORMING SURFACE HEAT SOURCE

MICROCHANNEL HEAT SINK

L A

MICROCHANNEL HEAT SINK

COVER PLATE

COVER PLATE A INLET FLOW

OUTLET FLOW

MANIFOLD BLOCK

MANIFOLD BLOCK SECTION A-A

SIDE VIEW

Figure 1: Schematic view of microchannel heat sink (after Phillips [15]) 3.2 Fundamentals of Microscale Transport Phenomena The operation of all energy conversion systems relies on fundamental transport phenomena, whose nature on the macroscale is well understood. Behind the continuum theories, which permit the description of the transfer of energy and mass in such systems, lies a complex and interdependent motion of energy carriers that are constantly interacting. In gases and liquids, diffusion and advection of molecules account for the relevant transport properties. In solids, we must account for electrons, photons (quanta of electromagnetic energy), and phonons (quanta of crystal lattice vibrational energy). For systems whose components have physical dimensions much larger than the space over which these inter-particle interactions occur (or in which the processes occur more slowly than the interaction time), we can formulate an adequate theory by relying on a continuum description of the materials in the limit of local thermodynamic equilibrium. As the size of the systems of interest decreases and the speed of their operation increases, the validity of such an approach is suspect. We must look beyond the assumptions of local thermodynamic equilibrium and continuum to a more generalized theory of transport. Table 1 indicates that microscopic particle theories are necessary when the size of the system is on the order of the relaxation length, lr. This length is the distance over which an excited energy carrier must travel before it reaches a state of equilibrium with the other carriers. As it travels this distance, it suffers multiple collisions each of a length, l, referred to as the mean free path. Length scales Time scales

Wavelength, λ

Collision time Mean free time Relaxation time Diffusion time

Mean free path, l

Relaxation length, lr

Diffusion length

Wave transport Wave transport

Microscopic particle transport theories Macroscopic or Continuum transport

Table 1: Characteristic Time- and Length Scales and corresponding Transport Phenomena of Energy Carriers [16] In situations for which a microscopic particle transport theory is applicable, the Boltzmann transport equation (BTE) constitutes the necessary fundamental tool to describe transport. This equation pertains

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to a statistical distribution function, f(r,p,t), (that varies with time, particle position and momentum) for an ensemble of particles. The BTE relates the temporal and spatial variation in the statistical distribution function to the scattering that results from particle interactions (right hand side of Eq. (1)). Energy and mass transfer due to transport phenomena in a microscopic system can be described, therefore, by solving the BTE for the statistical distribution function, subject to the fundamental scattering mechanisms.

∂f ∂t

+ v ⋅ ∇f + F ⋅

∂f ∂p

 ∂f    ∂t  scat

=

(1)

A body force applied to the particles is accounted for by F, which is usually only present for charged particles such as electrons and ions. That the macroscopic transport laws (Fourier’s law, Ohm’s law, Fick’s law) and all the hydrodynamic equations related to mass, momentum, and energy conservation, as well as the equation for radiative transfer, can be derived from the BTE under macroscopic assumptions is an impressive testament to the BTE’s generality.[17] The challenge in its application to physical situations is to correctly formulate the scattering term for the nature of the carrier interactions. No less difficult a challenge is the development of a means to solve the equation for the statistical distribution function. Under simplifying assumptions, solutions have been obtained which successfully describe many microscale transport phenomena.[18] Most energy devices, including fuel cells, require a flowing fluid for heat and mass transport. The proper understanding of micro-scale fluid mechanics is therefore a prerequisite to understanding and predicting heat and mass transport. The solution of the BTE is impractical for most fluid flow situations of practical interest. In situations where the mean free path l is small compared to the characteristic length scale of the flow L, fluid flow can be approximated as a continuum, provided the flow is not too far from thermodynamic equilibrium. The corresponding hydrodynamic equations, the Navier-Stokes equations (NSE), are derived either from the BTE or directly using conservation of mass and momentum principles. The ratio of the mean free path to the length scale is known as the Knudsen number Kn = l/L. As a guideline, the continuum approach requires Kn ≤ 0.001 [2]. In microscale flows higher Knudsen numbers are typically encountered. As a consequence, for 0.001 ≤ Kn ≤ 0.1, the traditional no-slip boundary condition between a fluid and a solid boundary is no longer valid and a slip velocity (and a corresponding jump condition in the energy equation) has to be prescribed. This and the prescription of an “apparent” viscosity for modelling liquid flows remain challenging problems. 3.3 Application of Microscale Transport A general feature of microscale phenomena is the importance of transport across surfaces, boundaries, and interfaces relative to the bulk transport in a particular material. Evaporation, heat transfer and species transport across the meniscus of a liquid-vapor interface becomes more important than the transport through the liquid layer.[19] Similarly, the nature of the flow of electrons, phonons, and photons through a multi-layer semiconductor structure is dominated by the mismatch in the physical properties between layers rather than the properties of the layers themselves.[20] As a first approach, one would assume that as the dimensions of a particular layer decrease, the overall resistance to transport would also decrease since the bulk material is becoming thinner. Consider, for example, the total thermal resistance of a passivation layer in an electronic device.

Rtotal =

d k

+ Rboundary

(2)

where d is the layer thickness, and k is the material’s thermal conductivity. The first term is related to the bulk thermal conductivity and the second to the thermal resistance at the interface. In the limit we would expect that as the layer thickness decreases, so does the total thermal resistance, which would approach that of the interfacial resistance. This is not necessarily what is observed. Due to the processing of the layers, the microstructure itself varies with thickness, so that while the material may be comprised of the same chemical components, the processing technique dictates its structure and characteristics.[21]

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Because the effect of interfaces, fabrication techniques, temperature, and fundamental material properties can oppose or re-enforce the transport, there is ample room for optimization when designing at the micro-scale. In almost all cases this optimization begins with the modeling of nonequilibrium processes and its application to specific systems. This means that while a first order fuel cell system may be designed by simply adopting micron scale dimensions, the high performance designs we believe are possible will only be realized through the development of a comprehensive understanding of the microscale transport phenomena. What is required, therefore, is a combined research thrust toward development of processes to realize micro-scale fuel cell systems designs with complementary research in the fundamental theory of micro-scale phenomena which are relevant to fuel cell processes. 4 Micro-Scale Fabrication Fuel cells built to exploit micro-scale phenomena would be smaller, make better use of volume and could obtain improved heat and mass transfer. These factors all point to the possibility of significantly improved fuel cell performance when micro-scale phenomena are exploited. However, such benefits can only be realized if the fuel cell devices can be fabricated using available manufacturing techniques. Micro Scale manufacturing for Micro Electro-Mechanical Systems (MEMS) has evolved by borrowing techniques from the integrated circuit manufacturing industry. MEMS technology has been most advanced in the sensor industry, where micro mechanical sensors and actuators are integrated with electronics on single substrates to produce highly integrated sensor packages at very low cost. Micro fabrication defines a series of techniques that can modify a substrate material in an additive or subtractive fashion to convert a thin, generally planar substrate into a complex structure of multiple materials through the interaction of microscopic features [22]. Additive processes allow the deposition of thin layers of materials onto a substrate through vapour deposition, sputtering or screen-printing. Subtractive processes involve selective etching or erosion of regions of the structure through wet or dry processes. An overall MEMS device is constructed through a sequence of manufacturing steps in which additive and subtractive processes are used to evolve a desired structure from some initial substrate. Sacrificial materials are often added at early stages in the manufacturing process to support the creation of membranes and bridges that will ultimately become free standing in the final product when the sacrificial material is removed. The planar dimension of MEMS devices is limited only by the dimensions and structural integrity of the substrate chosen, so in principle quite large footprint devices can be fabricated. The thickness of these substrates can vary, and through deposition techniques the crystal can be built up to an arbitrary thickness. The substrate thickness can be between 20 µm and 500 µm with features defined with dimensions on the order of 1 µm and resolutions well into the sub-micron range [23]. Casual examination of MEMS devices immediately identifies obvious parallels between the general geometrical structure of MEMS and the ideal geometrical structure of the fuel cell. MEMS is based on the surface modification of a thin planar substrate and incorporates many very thin layering processes to create the final product. The fuel cell achieves its electro-chemical functionality through surface interaction of species. MEMS devices, particularly chemical sensors, incorporate channels for gas and fluid flow and in some cases integrate micro pumps and heat exchangers into the MEMS sensor. The fuel cell requires the controlled distribution of reactant gases to the membrane / catalyst / electrode layer. Although a cursory look at MEMS fabrication techniques is attractive, it is impossible to simply reduce the dimensions of a conventional fuel cell design and scale it down to make a MEMS fuel cell. Issues in both materials and manufacturing create a new set of constraints that will ultimately limit the design capabilities for MEMS fuel cells. A few of these are discussed here. Conventional fuel cells derive considerable benefit from the mechanical and electrical properties of the materials used. In MEMS technology the fundamental substrate base onto which thin layers are deposited is traditionally Silicon, although glass, ceramic and some plastic substrates are used [24]. None of these materials has the desirable properties of either the PEM electrolyte or the graphite separator layer found in conventional fuel cells, suggesting that the bulkiest and largest volume component of the MEMS fuel cell would serve no function other than structural support for the active components. This means either that a large volume of the micro structured cell would not contribute to the cell function

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(inefficient) or that considerable manufacturing effort will be required to integrate fuel cell functionality into this otherwise passive structural volume. Deposition of materials onto a formed substrate presents a different challenge due to the relative unselectivity of deposition processes [24]. Deposition of expensive materials onto regions of the structure that will in a later manufacturing step be etched away represents an unnecessary cost and could incur expensive material recovery processes. While this is perhaps a secondary consideration at this early stage of MEMS fuel cell construction it is nonetheless a real issue for practical development of this technology. The only component of a MEMS device that has any mechanical strength is the substrate from which the MEMS structure emerges. The packaging that eventually surrounds the MEMS device works to provide toughness in use, but the substrate must be strong enough to allow the overall device to survive the numerous steps during the manufacturing process. Thus the manufacturing processes tend towards building up a single structure and generally preclude the micro fabrication and alignment of subassemblies. A new approach to fuel cell construction must be developed. Subtractive, etching processes can be thought of as vertical milling operations, using chemical or plasma tools. Very fine (