Micro-fuel Cells Research Articles

Pt/Y0.16Zr0.84O1.92/Pt thin film solid oxide fuel cells: Electrode microstructure and stability considerations

Symmetric, free standing thin film micro-solid oxide fuel cells of Pt/Y0.16Zr0.84O1.92/Pt structure are studied in this report. The role of parameters such as electrolyte thickness and electrode porosity on fuel cell power output was investigated. A peak power density of 1037mWcm−2 was exhibited with an optimized structure consisting of nano-porous Pt electrodes and 100nm thick Y0.16Zr0.84O1.92 with an open circuit voltage of 0.968V at 500°C using H2 as fuel and standing air as the oxidant. A twelve hour test of these devices indicates that overall performance shows extreme sensitivity to microstructural changes in the pure metallic electrodes. Results presented herein enable mechanistic routes to performance optimization, provide device stability data and are relevant to advancing micro-fuel cells for portable energy.

A million-channel CO-PrOx microreactor on a fingertip for fuel cell application

A silicon micromonolith containing ca. 40000 regular channels of 3.3μm in diameter per square millimeter has been successfully functionalized with an Au/TiO2 catalyst for CO preferential oxidation (CO-PrOx) in the presence of hydrogen. The functionalization of the silicon microchannels has been accomplished by growing a SiO2 layer on the channel walls, followed by exchange with a titanium alkoxyde precursor and decomposition into TiO2 and, finally, by anchoring carbosilanethiol dendron protected pre-formed Au nanoparticles. Catalytically active centers at the Au–TiO2 interface have been obtained by thermal activation. With this method, an excellent homogeneity and adherence of the catalytic layer over the microchannels of the silicon micromonolith has been obtained, resulting in geometric exposed surface area values of about 4×105m2/m3. The functionalized silicon micromonolith has been tested for CO-PrOx at 363–433K and λ=2 under H2/CO=0–20 (molar), and the results have been compared with those obtained over a conventional cordierite monolith with 400cpsi loaded with the same catalyst. The performance of the silicon micromonolith, which converts ca. 1NmL of CO per minute and mL of microreactor at 398K under H2/CO ∼20, is two orders of magnitude higher than that of conventional monolithic structures, suggesting that silicon micromonoliths could be particularly effective for hydrogen purification in low-temperature microfuel cells for portable applications.

Microfuel cells: Modern state and future development (Review)

Literature sources of the last 15 years were analyzed and the results were considered for the studies carried out in one of the directions of hydrogen power engineering of the most current interest: development of portable low-power fuel cells (microfuel cells, MFCs). MFCs with high power density and high efficiency are used as a basis for a new generation of power sources for various stand-alone electronic devices. The latest silicon micro- and nanotechnologies are considered, same as the technologies for obtaining nanostructured catalysts and problems regarding development of MFCs with the power of 0.5–20 W, methods of catalytic layers application in MFCs, choice of fuel, and methods of supplying it into MFCs, and MFC water managemen and temperature control. Special attention was paid to borohydride MFCs, combined (hybrid) systems, and MFCs with a mixed reagent. The results of electrochemical tests of MFC layouts are presented.

Designing Microfuel Cells for Portable Electronics

The main driving force in the developments of consumer electronics, such as cell phones and laptop computers, is longer run times and more functionality. In this quest for higher energy densities, battery characteristics improve at a constant pace. Fuel cells seem to be the next big technology breakthrough improving energy density with a factor of 3–10 compared with current lithium-ion batteries. In particular, the direct methanol fuel cell (DMFC) is an interesting opportunity because of the high energy capacity of methanol and the handling of the fuel making “charging” easy, safe, and fast. To get information on the different aspects that determine the boundaries of the DMFC power source, a power source for an MP3 player, the Samsung YP-Z5F, is designed. This design is based on a DMFC plus battery (DMFC hybrid) and utilizes standard available components [Flipsen, 2007, “Design Challenges for a Fuel Cell Powered MP3 Player,” International Power Sources Symposium, Bath, Great Britain]. The design of a DMFC hybrid power source in a conventional way (standard practice engineering) will not result in a smaller power source for this particular application. The design has a power and energy densities of lower than the currently available lithium-polymer battery, mainly because of the low fuel-efficiency of the cell at low temperatures, the use of commercially available but still too bulky components, and a large amount of dead space (≈34%). There are three ways to increase the power and energy densities of the system. First is by increasing the fuel-efficiency of the cells membrane. Second is by scaling down the system components to the right proportions and third is by improving the systems architecture diminishing empty space. This paper presents the design of a DMFC hybrid with scaled-down components. A literature study is done on the efficiency improvements of DMFC cells. The results are presented in a computer aided design (CAD) model and evaluated, comparing the “improved design” with “standard practice” design and the current lithium-polymer battery. The energy density of the redesigned fuel cell system is still low compared with the used lithium-polymer battery but an improvement to the preliminary design.

Towards a compact SU-8 micro-direct methanol fuel cell

This paper presents an all-polymer micro-direct methanol fuel cell (microDMFC) fabricated with SU-8 photoresist. The present development exploits the capability of SU-8 components to bond to each other by a hot-pressing process and obtain a compact device. The device is formed by a membrane electrode assembly (MEA) sandwiched between two current collectors. The MEA consists of a porous SU-8 membrane filled with a proton exchange polymer and covered by a thin layer of carbon-based electrodes with a low catalyst loading (1.0 mg cm −2). The current collectors consist of two metalized SU-8 plates provided with a grid of through-holes that allow delivering the reactants to the MEA by diffusion. Fuel cell characterization was performed by measuring the polarization curves under different methanol concentrations and temperatures. The components were first tested using an external casing. A maximum power density of 4.15 mW cm −2 was measured with this assembly working with a 4 M methanol concentration and at a temperature of 40 °C. The components were then bonded to obtain a compact micro-direct methanol fuel cell that yielded a power density of 0.65 mW cm −2 under the same conditions. Despite this decrease in power density after bonding, the drastic reduction of the device dimensions resulted in an increase of more than 50 times the previous volumetric power density. The results obtained validate this novel approach to an all-polymer micro-fuel cell.

Effect of Flow Fields and Scan Rates on Performance of Micro-DMFC

A micro-DMFC is a promising power source to substitute the lithium battery and used in the portable electronic devices. In this study, the effect of four types of flow fields of bipolar plates in a micro-direct methanol fuel cells (micro-DMFCs) on their performance were experimentally investigated. MEMS technology was applied in fabrication of micro-channel for the micro-DMFCs. The effect of the type of flow field design on the performance of micro-DMFCs was electrochemically evaluated for the four types of flow fields. The micro-DMFCs with double-channel serpentine presented the highest maximum power density and the micro-fuel cells with mixed multi-channel serpentine with narrow channels had the lowest maximum power density.

수치해석을 통한 PEMFC 채널의 설계와 SU-8을 이용한 채널 제작 및 성능 평가

연료전지는 환경문제와 천연자원 고갈을 해결할 수 있기 때문에 가장 주목받고 있는 새로운 동력원이다. 본 논문에서는 마이크로 연료전지의 효율을 높이기 위해 다양한 형상에 대해 마이크로 채널에서 수소가스 유동을 수치해석 하였다. 이미 개발된 채널과 새롭게 고안된 채널, 모두 여섯 개의 다른 형상을 가진 채널이 같은 유동 특성과 경계조건으로 수치해석을 수행하였다. 수치해석 결과를 통해 유속, 유동분포 유동균일성 등이 채널형상에 따라 영향을 많이 받는 것을 알았다. 이에 수소 가스 유동을 고려한 채널형상 최적화를 통해 연료전지의 성능을 향상시키고자 하였다. 수치해석을 통한 최적화된 채널형상을 사용한 실험결과 평균보다 높은 전류밀도를 나타내었다. 그러나 내구성이 낮았다.

Adoption of Micro-Fuel Cells

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A flexible direct methanol micro-fuel cell based on a metalized, photosensitive polymer film

This paper presents and investigates a concept for a flexible direct methanol micro-fuel cell (FDMMFC) based on the microstructuring of a Cr/Au metalized, thin polymer film of photosensitive SU-8. The inscribed microchannels in the electrodes are 200 μm × 200 μm in crosssection and spanning an active fuel cell area of 10 mm × 10 mm with a Pt-black catalyst on the cathode side of the membrane electrode assembly (MEA) and a Pt–Ru alloy catalyst on the anode side. Subsequently, the paper focuses on a thorough electrical characterization of the FDMMFC, under the employment of a variable resistor simulating an electrical load as well as a classical galvanostatical measurement technique. The fuel cell is also tested while operating in a bent, non-flat configuration. An extensive parameter study revealed an optimal and long-term stable operating condition for the fuel cell employing for both electrodes a serpentine flow field and a volume flow rate of 0.14 ml min −1 of a 1 M methanol solution at the anode side with a gas volume flow rate of 8 ml min −1 of humidified O 2 at the cathode side yielding a power density of 19.0 mW cm 2 at 75 mA cm 2 at a temperature of 60 °C. Furthermore, a flow-visualization of the two-phase flow occurring at the anode side has been performed by utilizing fluorescence microscopy. The strong influence of the two-phase flow on the performance of a fuel cell at high current densities becomes apparent in correlating the observed flow patterns with the corresponding current density of the polarization curve. The paper also investigates the functionality of the present FDMMFC under different bent conditions. The tests showed an insignificant drop of the electrical performance under bending due to an inhomogeneous contact resistance.

Manufacturing of Bipolar Plates for Micro-fuel Cells

Manufacturing of Bipolar Plates for Micro-fuel Cells

Microfabricated microfluidic fuel cells

We demonstrate high performance microfuel cells (μFC) operating at room temperature. The smallest μFC has a reaction surface of 0.11 cm 2 and has an output power density equal to 22.9 mW cm −2. Methanol and air are supplied using microchannels etched into silicon wafers using microfabrication techniques which can accurately determine the μFC surface and the microchannel dimensions. The insertion of a novel hydrophilic fibrous layer into the anode diffusion layer stack produces 9.25 mW cm −2 for an input fuel flow rate of 550 nL min −1. The benefits of size-scaling and architecture optimization in μFC are demonstrated. Our observations and conclusions are by no means unique to methanol μFC but could be applied to other microfluidic liquid fuel μFC based on, e.g. microbial fuel cells, bio-ethanol and glucose solution.

Fuel cell micro-CHP techno-economics: Part 1 – model concept and formulation

This article presents the concept and mathematical treatment for a techno-economic modelling framework designed to enable exploration of fuel cell micro combined heat and power (micro-CHP) system design and control. The aim is to provide a tool that can help to focus research and development attention on the system characteristics critical for commercial success of these technologies, present cost targets for developers, and to ensure policy makers provide appropriate instruments to support commercialisation. The model is distinctive in that it applies mixed integer unit commitment formulation to link design and control decisions for micro-CHP, and explicitly characterises stack degradation in a techno-economic framework. It is structured to provide depiction of the fuel cell stack and balance-of-plant, supplementary thermal-only system (e.g. tail gas burner), thermal energy storage, and electrical power storage. Technically, the fuel cell stack is characterised by steady-state thermal and electrical efficiencies for full and part-load operation, its nameplate capacity, minimum operating set-point, and stack degradation via performance loss rate proportional to power density and thermal cycling rate. The dynamics of operation are emulated via ramp limits, minimum up-time and minimum down-time constraints, and start-up and shutdown costs and energy consumptions. The primary performance evaluation metric adopted is the maximum additional capital cost a rational investor would pay for the fuel cell micro-CHP system over and above what they would pay for a competing conventional heating system. The companion article (Part 2) applies the developed model to consider the impact of stack degradation on economic and environmental performance.

An enhanced microfluidic control system for improving power density of a hydride-based micro fuel cell

Microfuel cells (MFCs) can potentially power emerging technologies that require power sources in the microliter size range. The recent development of a microfluidic mechanism for self-regulated generation of hydrogen has enabled fabrication of MFCs orders of magnitude smaller than previously possible. In this study, we report an order of magnitude enhancement in the power density of a microliter-scale fuel cell incorporating a new microfluidic design. The microfluidic mechanism is part of an on-board hydrogen generator that uses a reaction between a metal hydride, LiAlH 4, and water vapor to generate hydrogen. The hydrogen generated exits the hydride reactor through a porous silicon wall to reach a Nafion-based membrane electrode assembly (MEA). The microfluidic design increased the water vapor release rate to the hydride reactor by one order of magnitude over a previous design. A 9 μL device incorporating the enhanced microfluidic design delivered a power density of 92 W L −1. Details of a parametric study conducted to improve the water vapor release rate of the microfluidic mechanism and performance analysis of the integrated device are presented in this paper.

Photon-assisted synthesis of ultra-thin yttria-doped zirconia membranes: Structure, variable temperature conductivity and micro-fuel cell devices

We report on the synthesis and functional properties of nanoscale (∼50 nm) dense Y-doped zirconia (YDZ) electrolyte thin films by photon-assisted oxidation of Zr–Y precursor alloy thin films. Crystalline zirconia films with grain size of ∼5 nm were successfully grown at room temperature by oxidation under ultra-violet (UV) photon irradiation. Microstructure of the films was characterized by transmission electron microscopy. The electrochemical conductivity of UV grown YDZ electrolytes was investigated over a broad range of temperatures using Pt electrodes as a function of yttria doping concentration. The slightly lower electrical conductivity in UV grown films at intermediate temperature range (400–550 °C) is consistent with previous reports on oxygen defect annihilation under photo-excitation. Micro-fuel cells utilizing such ultra-thin YDZ membranes yielded ∼12 mW cm −2 power density at 550 °C. The results are of potential relevance in advancing low temperature ultra-thin oxide membrane synthesis for energy applications.

Performance analysis for the part-load operation of a solid oxide fuel cell–micro gas turbine hybrid system

Abstract In this paper, the performance evaluation of a solid oxide fuel cell (SOFC)–micro gas turbine (MGT) hybrid power generation system under the part-load operation was studied numerically. The present analysis code includes distributed parameters model of the cell stack module. The conversions of chemical species for electrochemical process and fuel reformation process are considered. Besides the temperature distributions of the working fluids and each solid part of cell module by accounting heat generation and heat transfers, are taken into calculation. Including all of them, comprehensive energy balance in the cell stack module is calculated. The variable MGT rotational speed operation scheme is adopted for the part-load operation. It will be made evident that the power generation efficiency of the hybrid system decreases together with the power output. The major reason for the performance degradation is the operating temperature reduction in the SOFC module, which is caused by decreasing the fuel supply and the heat generation in the cells. This reduction is also connected to the air flow rate supplement. The variable MGT rotational speed control requires flexible air flow regulations to maintain the SOFC operating temperature. It will lead to high efficient operation of the hybrid system.

LaSrCoFeO Offers Alternate Cathode Material to Pt for Micro-Fuel Cells

LaSrCoFeO Offers Alternate Cathode Material to Pt for Micro-Fuel Cells

Integration of silicon micro-hole arrays as a gas diffusion layer in a micro-fuel cell

Abstract This work examines silicon micro-hole arrays (Si-MHA) as a gas diffusion layer (GDL) in a micro-fuel cell that was fabricated using micro-electro-mechanical systems (MEMS) fabrication technique. Pt was deposited on the surface of the Si-MHA, to increase the conductivity of the micro-fuel cell. The Si-MHA with three micro-holes, replaces the traditional GDL, and the performance of the micro-proton exchange membrane fuel cell was discussed. Wet etching was performed on a 500 μm-thick layer of silicon to yield fuel channels with a depth of 450 μm and a width of 200 μm. The Si-MHA formed by deep reactive ion etching (DRIE) in the fabricated structure had diameters of 10 μm, 30 μm and 50 μm; the thickness of the structure was 50 μm. The experiment yielded results for 10 μm, 30 μm and 50 μm micro-hole arrays, for various fuel flow rates and under various operating conditions. The maximum power density was approximately 8.13 mW/cm2 with H2/O2 gas flow rates of 30/30 ml/min and 10 μm micro-hole arrays at 20 °C.

Design and Manufacturing of Mesoscale Tilting Pad Gas Bearings for 100–200 W Class PowerMEMS Applications

This paper introduces a design and manufacturing of mesoscale flexure pivot tilting pad gas bearing with a diameter of 5 mm and a length of 1–2.5 mm for PowerMEMS (micro electromechanical systems for power generation) applications with power ranges of 100–200 W. Potential applications include power source for unmanned air vehicles, small robots, microgas turbines to be harnessed by very small solid oxide fuel cells, microblowers/compressors for microfuel cells, etc. The design studies involve scaling analysis, time-domain orbit simulations for stability analyses, and frequency-domain modal analyses for prediction of rotor-bearing natural frequencies. Scaling analysis indicates that direct miniaturization of macroscale tilting pad gas bearing can result in a large bearing number, which may render the rotor-bearing system unstable. However, the scaling analysis provides the baseline design from which the final design can be derived considering manufacturing issue. The generalized modal analysis using impedance contours predict damped natural frequencies close to those from orbit simulations, providing high fidelity to the developed numerical methods. It was predicted that the designed mesoscale tilting pad gas bearings would show very stable operation up to a maximum simulated speed of 1,000,000 rpm. The designed mesoscale tilting pad gas bearings were manufactured using X-ray lithography and electroplating.

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

Abstract Micro-proton exchange membrane fuel cells are considered to be the next generation power sources for micro-scale power applications, but onboard hydrogen storage and generation with high energy density at the small scale is still a technical barrier. This paper introduces a hydrogen generation method based on an onboard hydride fuel and a byproduct water recovery mechanism for micro-hydrogen PEM fuel cells. The water recovery is carried out by water diffusion from the more humid cathode side to the less humid anode side through the proton exchange membrane. The micro-fuel cells based on this water recovery method were constructed and tested. The results demonstrate that the relative humidity has a significant affect on the fuel cell performance as well as the opening area on the cover layer, the type of hydrides, and the thickness of the Nafion membrane also can affect the fuel cell performance. A 10 mm 3 prototype water recovery micro-fuel cell has been built and tested, and the device has produced a maximum power density of 104 W L −1 and a maximum energy density of 313 W h L −1 .

A simulated auto‐thermal membrane reformer process for a PEM fuel cell micro cogeneration unit

AbstractThere are several methods of producing hydrogen‐rich gas from fossil resources such as natural gas or naphtha, for example, steam reforming, partial oxidation and auto‐thermal reforming. In this paper, an integrated ATR membrane reactor system was simulated. The effect of operating parameters on the product distribution, fuel cell hydrogen utilization and the net electric efficiency of the overall system were discussed. The overall system was integrated with a 1‐kWe PEM fuel cell. The ASPEN‐HYSIS 3.2 software has been utilized for the simulations and calculations of the fuel processing reactions. Natural gas fuel has been used as feedstock and applied to the simulated flow‐sheet model. It was desired to produce hydrogen‐rich gas with a low CO formation using an autothermal membrane reformer. A very low CO content with higher content of hydrogen was provided by the membrane reformer, eliminating the use of the conventional preferential oxidation (PrOx) reactor. Different combinations of TATR, S/C, O2/C ratios and UH2 have been parametrically studied. Fuel processing efficiency and net electrical efficiency of all selected operating conditions have been calculated as well. Results indicate that the system parameters are very critical for the appropriate operation of the residential cogeneration system with ATR membrane unit. Copyright © 2009 Curtin University of Technology and John Wiley & Sons, Ltd.