Use of Multi-Functional Flexible Micro-Sensors for in situ Measurement of Temperature, Voltage and Fuel Flow in a Proton Exchange Membrane Fuel Cell
Abstract and Figures
Temperature, voltage and fuel flow distribution all contribute considerably to fuel cell performance. Conventional methods cannot accurately determine parameter changes inside a fuel cell. This investigation developed flexible and multi-functional micro sensors on a 40 μm-thick stainless steel foil substrate by using micro-electro-mechanical systems (MEMS) and embedded them in a proton exchange membrane fuel cell (PEMFC) to measure the temperature, voltage and flow. Users can monitor and control in situ the temperature, voltage and fuel flow distribution in the cell. Thereby, both fuel cell performance and lifetime can be increased.
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Heat is one of the main causes of performance degradation and failure of proton exchange membrane fuel cells (PEMFCs). The ‘hot spot’ and uneven temperature distribution formed when fuel cells operate can result in the severe degradation of membrane and catalyst layers. To better understand these phenomena, this review presents a systematic and comprehensive summary and discussion on the heat transfer within a PEMFC. Notably, (i) the overall introduction of the basic concept and thermal effect; (ii) the analysis of heat transfer mechanisms, including heat generation and energy transport, as well as the effects of multi-scale; (iii) the summary and discussion of the thermal conductivity and contact resistance of porous media; and (iv) the latest developments in numerical calculation and temperature measurement are crucial. In addition, several outlooks are proposed for further advanced research. We consider that this paper will aid readers in deepening their understanding of this field.
Methanol can be used as the hydrogen source for the operation of fuel cells, given its features of safe storage and transportation management, as well as low reforming temperature (250̃300°C). The micro reformer can convert methanol into hydrogen for portable fuel cells, resulting in many advantages. This study integrated micro temperature, flow andpressure sensors using the micro-electro-mechanical systems (MEMS) technology, and embedded integrated multifunction micro sensor in the micro reformer, so as to measure the distributions of temperature, flow and pressure simultaneously for real-time diagnosis. This study also discussed the internal reaction mechanism of micro reformer, and identified the internal reaction of micro reformer correctly for real-time monitoring and adjustment. This study was consisted of two parts: 1) integration of micro sensor: the polyimide (PI)film is used as substrate to develop the micro temperature, flow and pressure sensors; 2)real-time monitoring of micro reformer: the integrated multifunction micro sensor is embedded in the micro reformer for real-time microscopic monitoring of the changes in temperature, flow and pressure.
In recent years, the development of fuel cells has proceeded rapidly, and so the reformation of methanol to produce hydrogen has become a serious problem. Supplying hydrogen from a methanol microreformer to fuel cells is an important topic. The structure of a microflow channel must support the transfer of external heat to the reform reaction, facilitating the diffusion of methanol vapor into the catalyst layer, thereby increasing the rate of transfer of hydrogen. In this investigation, the micro-electromechanical systems (MEMS) technique is utilized to fabricate pressure, temperature, and flow microsensors. Polyimide film (PI) exhibits high temperature resistance and stress corrosion resistance, and is adopted herein as a flexible substrate. Multifunctional microsensors are inserted into methanol microreformers to measure the internal pressure, temperature, and flow in situ, and the signals thus obtained are used to improve their efficiency.
Thermometry plays an extremely important role in engineering and basic sciences. Accurate measurements of temperature are vital for: (a) improving our understanding of various physical phenomena; (b) process control and optimization; (c) determination of material properties; and (d) energy conservation. This review highlights some of the developments that have occurred in thermometry during the period 1984 to 2011. The ensuing discussions focus on the physical principles that have been exploited for the purpose of thermometry. The developments mentioned in this review offer a peek into the past 27 years of thermometry research that has produced several innovations based upon optical refraction, luminescence, fluorescence, thermo-reflectance, color change, radiation, ferro-electricity, acoustic resonance, Raman Effect, and magnetic resonance principles, to name a few. In tune with the emergent interest in micro/nano-scale interdisciplinary technologies, automated high-speed manufacturing processes, and advances in medical engineering, many of the patents reviewed here relate to thermometry for specialized applications. A sizeable majority of these novel arrangements are non-intrusive and non-contact in nature, suitable for remote measurements. Developments on contact-type temperature sensors, that may require invasive techniques and arrangements, are also presented.
Requirement and difficulties of intra-stack measurement are analyzed. The state of the art in intra-stack temperature and humidity measurement is presented. Methodologies in temperature (field) measurement are micro-thermocouple (array), resistive temperature detector (array), infrared image, fiber-bragg gratings and phosphor thermometry; while chromatography, tunable laser absorption spectrum, micro-electronic humidity sensor and fiber-bragg gratings are applied in humidity measurement. Comparison of the measurement technologies shows that micro sensors have good spatial and signal resolution with low cost, while optic fiber bragg gratings benefits in chemical inertness, immunity to electromagnetic interference and multiplexing capability, which has the most brilliant future.
The operating temperature and internal water accumulation influence the fuel cell performance significantly, and various regions inside the fuel cell still have non-uniformity. At present, only external, invasive, theoretical and simulation methods are available. The real information inside the fuel cell cannot be known instantly.
This study used micro-electro-mechanical systems (MEMS) technology to develop a temperature, pressure and flow 3-in-1 micro sensor. First, the PI foil (Polyimide foil, 50 μm thick) was used to develop a 3-in-1 micro sensor, and then the 3-in-1 micro sensor was embedded in the low-temperature fuel cell for real-time microscopic monitoring of internal temperature, pressure and flow of fuel cell.
In terms of temperature inside the fuel cell, as the upstream and midstream formed heat can be carried away by sufficient gas at the inlet, and the downstream reaction is violent, the downstream temperature is higher than upstream and midstream temperatures. In terms of pressure, in long-term observation, the internal pressure fluctuates, the pressure increases momently and reverts at some time points. This is possibly due to the removal of accumulated water from the flow channel. In terms of flow, the flow supplied by flow controller is higher than the upstream and downstream micro flow sensors, and the difference matches the gas leakage value.
A novel test scheme for in situ measurement of temperature within a single polymer electrolyte membrane fuel cell (PEMFC) is proposed, which possesses the following attractive features: measuring interference with the internal environment of the fuel cell is likely reduced to minimum; simultaneous measurements for local temperatures of both sides of the fuel cell are conducted with enough numbers of measurement locations; and the cell temperatures are controlled in relatively careful and stringent strategies. Thermal and electrical behaviors of the cell tested are investigated, including the local and averaged temperatures at the back sides of cathode and anode flow field plates (FFPs), the outlet currents, and their variations with the test time. It is found that both temperatures and outlet currents exhibit complex dynamic behaviors; and the rise of temperature and the non-uniformity of temperature distribution of the back sides of the two FFPs are not negligible.
Temperature, flow and pressure are critical parameters that influence the performance of fuel cells, in terms of potential, current and power density, for example. Hence, the monitoring of non-uniform temperature/flow rate/pressure within a proton exchange membrane fuel cell (PEMFC) is crucial. To prevent degradation of the performance of a PEMFC, the size of sensors is reduced herein to a µm scale using micro-electro-mechanical systems (MEMS). Integrated flexible micro sensors were fabricated and embedded in a PEMFC to measure local pressure, temperature, and flow rate. The temperatures upstream and downstream of the PEMFC were respectively 64.8°C and 64.7°C at RH50% and 0.1 A/cm2, and 62.7°C and 64.3°C at RH100% and 0.1 A/cm2.
An experimental technique for measuring the current density distribution with a resolution smaller than the channel/rib scale
of the flow field in polymer electrolyte fuel cells (PEFCs) is presented. The electron conductors in a plane perpendicular
to the channel direction are considered as two-dimensional resistors. Hence, the current density is obtained from the solution
of Laplace’s equation with the potentials at current collector and reaction layer as boundary conditions. Using ohmic drop
for calculating the local current, detailed knowledge of all resistances involved is of prime importance. In particular, the
contact resistance between the gas diffusion layer (GDL) and flow field rib, as well as GDL bulk conductivity, are strongly
dependent on clamping pressure. They represent a substantial amount of the total ohmic drop and therefore require careful
consideration. The detailed experimental setup as well as the concise procedure for quantitative data evaluation is described.
Finally, the method is applied successfully to a cell operated on pure oxygen and air up to high current densities. The results
show that electrical and ionic resistances seem to govern the current distribution at low current regimes, whereas mass transport
limitations locally hamper the current production at high loads.
In this work, a micro-temperature sensor on a 40μm flexible stainless-steel substrate was fabricated using micro-electro-mechanical systems (MEMS). Embedding a micro-temperature sensor in a proton exchange membrane fuel cell (PEMFC) to monitor temperature will not damage the sensor during the experimental process. This investigation is the first to develop a micro-temperature sensor that can be placed anywhere between the membrane electrode assembly (MEA) and the flow-channel plate inside a PEMFC. The simulated temperature is consistent with the experimentally determined temperature. The performance curve is also consistent with experimental results, revealing the accuracy of the simulation and the effectiveness of monitoring temperature inside a PEMFC.
Recent advances in micro-fuel cells have increased the demand for hydrogen. Therefore, a micro-reformer must be developed. Numerous portable electric devices are extremely small and reformers must therefore be shrunk and combined with micro-fuel cells. The mass production of micro-reformers raises various problems that are yet to be solved, such as the measurement of their internal temperature and flow rate. Such issues influence the efficiency of the micro-reformers. To our knowledge, no investigation has yet properly elucidated the internal operation of micro-reformers. Accordingly, in this work, a flexible micro-temperature sensor, a micro-heater, a micro-flow sensor and the flow field of a stainless steel-based micro-reformer were fabricated by micro-electro-mechanical-systems (MEMS) fabrication technique. The fabrication technique has the advantages of (1) small size, (2) flexible but precise measurement positions, and (3) mass production process.
A quasi-two-dimensional, along-the-channel mass and heat-transfer model for a proton exchange membrane fuel cell (PEFC) is
described and validated against experimental current distribution data. The model is formulated in a
dimensional manner, i.e., local transport phenomena are treated one-dimensional in through-plane direction and coupled in-plane
by convective transport in the gas and coolant channels. Thus, a two-dimensional slice running through the repetitive unit
of a cell from the anode channel via membrane-electrode assembly (MEA) and cathode channel to the coolant channel and from
inlet to outlet is modeled. The aim of the work is to elucidate the influence of operating conditions such as feed gas humidities
and stoichiometric ratios on the along-the-channel current density distribution and to identify the distinct underlying voltage
loss mechanisms. Furthermore, a complicated technical flow field is modeled by a combination of co- and counterflow subdomains
and compared with experimental current densities.
In the first paper of this series, an experimental technique for measuring the current-density distribution with a resolution
better than the sub-millimeter scale of the channel and rib structures in the flow-field plates of polymer electrolyte fuel
cells (PEFCs) was introduced. This method is extended to the determination of local membrane resistance with the same spatial
resolution in the present paper. The combined measurement of current and resistance allows for investigating the interaction
of mass- and charge-transport processes, which determine the local rate distribution across the domain of channels and ribs.
Therewith, the influence of relevant operating parameters such as reactant composition, dew points, and cell compression on
local current generation is investigated. The results show that the distribution of water and oxidant across the channel and
rib are the main reasons for significant current gradients on a scale smaller than a millimeter. Humidity variation mainly
affects the membrane resistance under the channel, while reactant concentration predominantly influences current generation
under the rib-covered cell area.
A stent-type flow sensor is presented for evaluating nasal respiration. To enable the thermal flow sensor to be fixed at inner nasal passage surfaces, it was integrated onto a stent structure, which is normally used as a medical tool. The monolithically integrated sensor was fabricated on a Ti substrate by photolithography and wet etching processes, an advantageous procedure in that both the thermal isolation cavity and the stent structure can be fabricated during the same etching process. The sensor was mounted onto a silicone tube's inner surface by inflating a balloon tube, and then its characteristics were evaluated. A study of the cylindrical stent's mechanical strength under compression conditions showed that elastic deformation occurs when the compression force is 0.08 N or less. The sensor can detect the flow direction at a flow range of 0–2000 ccm. A response time of 260 ms or less was obtained by forming a cavity under the sensor. The sensor characteristics under oscillating flow were studied by using a ventilator, and it was confirmed that the sensor was able to measure the oscillating flow at an output frequency of 2.0 Hz.
Internal temperatures in a proton exchange membrane (PEM) fuel cell govern the ionic conductivities of the polymer electrolyte, influence the reaction rate at the electrodes, and control the water vapor pressure inside the cell. It is vital to fully understand thermal behavior in a PEM fuel cell if performance and durability are to be optimized. The objective of this research was to design, construct, and implement thermal sensors based on the principles of the lifetime-decay method of phosphor thermometry to measure temperatures inside a PEM fuel cell. Five sensors were designed and calibrated with a maximum uncertainty of ±0.6 °C. Using these sensors, surface temperatures were measured on the cathode gas diffusion layer of a 25 cm2 PEM fuel cell. The test results demonstrate the utility of the optical temperature sensor design and provide insight into the thermal behavior found in a PEM fuel cell.
Neutron radiography has proven to be a powerful tool to study and understand the effects of liquid water in an operating fuel cell. In the present work, this experimental method is coupled with locally resolved current and ohmic resistance measurements, giving additional insight into water management and fuel cell performance under a variety of conditions. The effects of varying the inlet humidification level and the current density of the 50 cm2 cell are studied by simultaneously monitoring electrochemical performance with a 10×10 matrix of current sensors, and liquid water volumes are measured using the National Institute of Standards and Technology (NIST) neutron imaging facility. A counter flow, straight channel proton exchange membrane (PEM) fuel cell is used to demonstrate localized performance loss corresponds to water-filled channels that impede gas transport to the catalyst layer, thereby creating an area that has low current density. Furthermore, certain operating conditions causing excess water accumulation in the channels can result in localized proton resistance increase, a result that can only be accurately observed with combined radiography and distributed electrochemical measurements.
The water flooding and two-phase flow of reactants and products in cathode flow channels (0.8 mm in width, 1.0 mm in depth) were studied by means of transparent proton exchange membrane fuel cells. Three transparent proton exchange membrane fuel cells with different flow fields including parallel flow field, interdigitated flow field and cascade flow field were used. The effects of flow field, cell temperature, cathode gas flow rate and operation time on water build-up and cell performance were studied, respectively. Experimental results indicate that the liquid water columns accumulating in the cathode flow channels can reduce the effective electrochemical reaction area; it makes mass transfer limitation resulting in the cell performance loss. The water in flow channels at high temperature is much less than that at low temperature. When the water flooding appears, increasing cathode flow rate can remove excess water and lead to good cell performance. The water and gas transfer can be enhanced and the water removal is easier in the interdigitated channels and cascade channels than in the parallel channels. The cell performances of the fuel cells that installed interdigitated flow field or cascade flow field are better than that installed with parallel flow field. The images of liquid water in the cathode channels at different operating time were recorded. The evolution of liquid water removing out of channels was also recorded by high-speed video.