MES-DEA FC TECHNOLOGY Ing. Gianmario Picciotti, Ing. Roberto Bianchi, Fuel Cell Laboratori, Mes-Dea, via Laveggio, 15 – 6855 Stabio, Switzerland ABSTRACT: PEM MES-DEA Fuel Cell Technology has been developed, since 2002, on the basis of high power density and system smplicity. All the single cell components are made by different graphite material and the system is air cooled. The main features of MES-DEA FC can be summerized as follows: - high power density and specific power - high net efficiency (after auxiliaries) because of the lack of compressors and external humidifier - high simplicity - low operation temperature A very great effort has been profused in light, resistant and low cost material since the beginning of MES-DEA FC activity thus obtaining a good quality / cost ratio. The current range of power is from 500 W up to 6 kW. The 3 kW and 6 kW FC systems are made by two and four 1.5 kW stacks (60 cells) respectively, tha biggest single stack produced. The smaller stacks produced have 500 W (22 cells) and 1 kW (40 cells) power. The single cell nominal voltage is 0.6 Volt, the nominal current is around 45 A and the operation temperature at nominal power is around 60 °C. Due to its characteristics MES-DEA FC system is especially suitable for the propulsion of light two wheels vehicles and portable APU. To demonstrate the feasibility of this applications MES-DEA prepared an H2 fuelled FC powered electrical bike and scooters. The former is driven by a 1 kW FC stack with a top speed of 35 km/h and a range of 25 km without pedalling. The latter can reach the top speed of 50 km/h and a range of 60 kW by means of the 3 kW FC stack mounted on it. Both the lifetime and the start / stop cycles decay were tested and significant improvements have been reached during the last two years. Other important tests are going to be carried out trying to stabilize or improve the behaviour of the FC system under partcular ambient conditions or different load profiles. Once demonstrated the good results of the FC system operation and its applications MES-DEA is now refining the control procedure of the stack operation to increase the general performance, to improve the behavoiur under certain operation conditions and to make it more suitable for the specific applications to which it has been devoted. KEY WORDS: PEMFC, light vehicles, portable APU. The cooling channels are obtained by means of 1. STATE OF THE ART OF MES-DEA FUEL metal corrugated sheets opportunely coated which CELL TECHNOLOGY are placed between the cells to form the all stack. This is closed in its extremities by two glass fiber MES-DEA fuel cell activity began in 2002 and is reinforced plastic end plates, on the inner side of based on PEM membrane, graphite BPPs and air which, two metal foils are placed as current cooling system. collectors. The FC system does not adopt any compressor The single cell has a rectangular shape ( 48 x and any external humidyfier for the conditioning of 172 mm2 ) and is composed by the following items: the reactant gases. Exactly it oparates at nearly ambient pressure and in self-humidyfing - Anodic bipolar plate: conditions. The simplicity, the low volume, the low thin graphite foil with through plane resistivity wheight and the high global efficiency of the around system have been reached by these features. The 0,2 · cm ( at around 12 bar compression ), best applications of MES-DEA FC system are thus mass density around 1,8 g/cm3. mobile and portable. The reactant gases are pure hydrogen and air. - Anodic parallel flow field: The two gas streams inside the stack are in carbon paper gas diffusion media PTFE counter flow. The former is supplied in dead end impregnated. mode at around 0.5 bar overpressure and the latter No channels along the path but only two short is blowed inside the cathodic compartment by ways through to facilitate the hydrogen inlet and means of the suitable radial blower. Other bigger outlet in the anodic compartment of every cell. radial blower are required for cooling the system. It guarantees the in plane gas permeability. Power output Number of Voltage - Anodic GDL: cells range carbon paper soot coated PTFE impregnated (nomi. gas diffusion media. voltage-OCV) No channel structure. It guarantees the through plane gas permeability and preserve the MEA from micro damages. 0.5 kW stack 22 13 – 20.9 V - MEA: very thin membrane electrode assembly with the same Platinum loading on both the electrodes. The active area is 61 cm2. The current density, in the beginning of its life, is ca. 950 1.0 kW stack 40 24 – 38 V mA/cm2. - Cathodic GDL: the same material used for the anodic GDL. 1.5 kW stack 60 36 – 57 V - Cathodic bipolar plate: obtained by compression moulding from a special graphite compound. 3.0 kW stack 120 72 – 114 V The compound is composed of synthetic graphite powder (around 80 %) and resin as binding polymer. The flow field has a channel structure which extends its ribs along the larger dymension of the cell. The through plane resistivity is atround 6.0 kW stack 240 144 – 228 V 0,15 – 0,10 · cm. A metal corrugated sheet is placed between the cells to create the cooling channels. It is a waved metal opportunely coated foil. Every single channel Tab 1. Below summerizes the actual MES-DEA FC is 1,25 mm high, 48 mm long and around 1 mm systems. wide. The cells are stacked in series obtaining three 2. OPERATION CONDITIONS power sizes: 0.5 kW ( 22 cells; nominal voltage: 13.2 V ), 1 kW (40cells; nominal voltage: 24 V ), In nominal conditions the single cell voltage is 1.5 kW ( 60 cells, nominal voltage: 36 V ). Stacks 0,6 Volt and the current is generally between 55 A with power of 3 kW and 6 kW can also be realized and 60 A in the beginning of the life of the stack using two and four 1.5 kW stacks respectively. and after a “conditioning” procedure. This procedure consists of a manual filling of the anodic The FC system has an unregulated voltage compartment with a certain amount of deionized output and it needs a voltage, current or power water when the system is off. This procedure has regulator for the laboratory tests (electronic loads), to be repeated every 10 – 20 minutes of operation and / or a power converter to supply the external time and at the half of each operation interval a load (eg. electric motor). manual short circuit is generally carried out. In some hours (generally 4 – 5 hours) the stack reaches, at nominal voltage, the maximum current (55 – 60 A) from the low initial current (15 - 20 A). After the “conditioning “ operation the stack is “ready” in a way that, in principle, it is able to work properly for the rest of its life if managed in the right manner. A general example of the polarization and power output curve of MES-DEA FC system is shown in Fig. 2 below. It refers to our “standard” 1.5 kW FC stack. Depending on the value of the stack current, Polarisation Curve Fuel Cell Stack DEA 1.5-A60-36 (60 cell stack) the microcontroller calculates the set temperature 1 40 of the FC stack, the stoichiometric reaction air flow 0.9 rate and hence regulates the auxiliaries ( cooling, 0.8 reaction air blower, valves ) in order to operate the 30 FC stack in the optimum operating conditions. Average Single Fuel Cell Power [W] 0.7 Average Single Cell Voltage [V] 0.6 Currently the regular short circuit and purging are 0.5 20 managed as mentioned above, but they could in 0.4 Voltage principle be changed in frequency and duration to 0.3 Power Hydrogen: 0.4-0.5 bar overpressure / dead end 10 optimize still further the operation of the stack. At 0.2 Air: near ambient pressure /stoic 2.5 - 4.5 No humidification of the gases Cell temperature: max. 60 - 63 °C the moment the short circuit is fixed in duration and 0.1 frequency mentioned above and the hydrogen 2 Active area: 61 cm 0 0 0 0.2 0.4 0.6 0.8 Current Density [A/cm2] 1 1.2 1.4 purge has been trasformed in an “intelligent purge” Total Stack values without auxilaries whose duration takes care of the stack current and stack operation conditions. In this way the Fig 1. Polarization and power output curve of hydrogen usage efficiency is increased especially MES-DEA FC standard 1.5 kW system. at low current. As already explained, the FC system management of the PEM humidification is based 3. APPLICATION OF MES-DEA FC on various solutions both in components and TECHNOLOGY operating parameters. The solution of this problem has been reached with the optimization of the flow MES-DEA FC technology was born ( 2002 ) in field structure as well as with suitable GDLs and collaboration with a big italian motorcycles the choice of the respective operating parameters. company ( Aprilia ) which was interested to During the normal operation time the ECU makes develop hydrogen fueled FC powered electrical regular short circuits every 20 s for 50 ms to scooters. Due to its pecularities MES-DEA FC guarantee the good operation of the stack. At the system revealed particularly suitable for this kind of same time, regularly every 20 s for 500 ms, the application and two main project were carried out: ECU ( steering electronics ) commands the the “ Enjoy bike “ and the “SR FC” scooter. opening of the purging valve to remove, in the The electrical bike is powered by a 1 kW FC anodic compartment of the stack, the impurities, system so that it can reach a top speed of around and push out the excess of water from the anode. 35 km/h and a range of around 25 km in the plain The amount of hydrogen emitted in that way is ca. without pedalling. The hydrogen is stored in a 2 l 0.045 Nl. composite cylinder at 300 bar. It mounts the The hydrogen feeding ( dead end mode ) is original DC permanent magnet motor ( the original driven by the main valve at around 0.5 bar electric motor of Aprilia bike ) and its total wheight overpressure and its purity should be at least the is 30 kg. purity classification 4.5. The scooter “SR FC” is powered with a 3 kW FC system reaching the maximum speed of 60 The reaction air is blowed inside the cathodic km/h with a range of around 60 km in the plain. compartment at near ambient pressure and its The hydrogen is stored in a 9 l composite cylinder stoichometric ratio is between 3 and 4.5 in the set at 300 bar. It mounts an electric DC brushless point of operation. The stoic value range depends motor ( by MES ) and its wheight is 120 kg. The obviousley on the stack current output. conditioning of the FC electrical power output is With this conditions the single cell is able to reach carried out by a suitable inverter. a current density of around 800 mA / cm2 of active area at 0.6 V single cell voltage, resulting in a Another interesting application of MES-DEA FC power density of 0,48 W / cm2 or 29 W / cell. In technology was carried out by a Spanish company these conditions the cell temperature must be which made a wheelchair prototype equipped with C around 60 ° and the gross electrical effinciency i s our 500 W FC system to recharge the battery ( around 50 % ( around 43 % after auxiliaries ). The range extender ). Other applications are those in hydrogen consuption is approximately 0,07 kg / which the MES-DEA FC system is used as “ range kWh. extender” in electric vehicles, charging the main battery and / or feeding the auxiliares. The steering electronics measures the stack current , the stack voltage and the two temperatures given by the two temperature sensor located on the outlet reaction air channel. stack could be the right solution for the low power portable electric generators we are going to develop. 4. TEST In order to check and improve the durability, the performance and the range of operating conditions ( eg. ambient temperature, relative humidity ) of the FC system, many tests have been carried out by MES-DEA Fuel Cell laboratory. DURABILITY After having carried out many single cell tests for the right choice of MEA, GDLs and air flow field and having solved the problems strictly connected to production methode of the single cells, MES- DEA FC activity, since 2003, started to assemble many FC stacks prototypes and check their functional characteristics. A first rough duration test was made on a 500 W FC system. Fig 2. Bike application. The test was carried out outside of the company building in costant voltage mode. After around 1100 h the stack current was 5 A less than the nominal current ( 40 A ). We had soon an appreciable improvement with the next duration tests which were carried out with the same type of stack ( 500 W ) and in the same conditions ( outside the building, continous operation in costant voltage mode ). In this case the stack had a decay of the 75 % of the nominal power after 2050 hours. After 2050 hours a current recovery up to more than 40 A ( nominal current ) was made by means of a “reconditioning” procedure thus demonstrating that the total power decay is not completely irreversible. Unfortunately a problem with the electrolyzer occured damaging some cells. A detailed post mortem analys was carried out by our MEA supplier and its results were the following: - In general the MEAs appear to be in good condition. - There are some thickness variation of the membrane, but not enough to suggest that any Fig 3. Scooter application. significant degradation of the ionomer has occured. MES-DEA is also going to apply its FC - There appears to be some local areas in technology to portable APU like those used in which the cathode electrode thickness is thinner camping and boating field. The electric power than expected. This may indicate some degree of output will be between 500 W and 1,5 kW and a cathod carbon corrosion. Maybe the local fuel voltage output of 12 V and 24 V. Furthermore starvation, due to the malfunction of the since some months we are developing a smaller electrolyzer after 2050 h, together with the cell which has about the half active area of the hydrogen consuption, caused an underpressure in “standard” one. In this way the half electric power the anodic compartment so that the air could go is supplied under the same voltage. This small inside the anode during the purging. If this is the Furthermore to check possible mechanical MEA case a cathode carbon corrosion, like in start / stop demages due to the thermical cycles ( changing in cycles, could occur. temparature and humidity ) also load cycles ( full - The debris found in the cathode flow field is load / idle ) tests were carried out. Only after many mainly composed by silica which most likely thousands of load cycles the stack behaviour comes from the silicon rubber used to close the showed appreciable demages. cells. There are also traces of K, Ca and Fe. The Pt presence obviousley comes from the THAW TESTS coating of the membrane and the Al from the sample holder. Thaw tests to see the behaviour in the starting - The flow field side of the cathode GDL contains C phase of the FC system under 0 ° were carried the same elements of the debris described out. The thaw procedure is done using a controlled above, with the addition of Cl and Cu. On the realese of heat in the internal part of the FC stack. MEA side only Si is found. The GDL appears to The FC system is now able to start in around 1 have acted as filter for the other contaminants. minute from – 20 °C. - With the MEA inverted in the cell in a way that the original anode becomes the cathode, much of the lost power is recovered. 4. CONCLUSION - The cathode side of the membrane has a much reduced Pt quantity relative to the anode. Si is Many other tests will be carried out to study all detected on both surfaces. the possible solutions which can improve the performance and / or refine the control of the FC Up to now we are confident to increase our first system. lifetime target of 2’000 hours ( which is already Our efforts are now concentrated on the study satisfyng for light mobile and portable application of the “water balance” inside the stack and its with high power density stacks like ours ) by using relative effect on the reversible decay of the stack in the next future PEM membranes with different Pt power due to the possible “flooding” of the loading and stronger ionomer. electrodes or the “drying up” of the PEM membrane. START/STOP CYCLES Last but not least we are trying to improve the behaviour of the FC stack after a long non Repeated normal starting procedures are operation time by an appropriate self humidifying responsable of the “carbon corrosion” of the procedure. cathode with relative irreversible decay of power Finally the main activity we are going to pursue along the time. Due to this fact another is to improve the FC system itself and make it improvement was carried out to decrease the more suitable for the specific applications to which negative “carbon corrosion effect”. A partial it has been devoted. solution to this problem has been found using a new start up procedure by means of a third valve called “auxiliary” or “direct” valve. In this case we were able to increase more than three times the number of start / stop cycles with the same decay of power. In this case the stack reached the 70 % of the nominal power after about 1500 start / stop cycles. However the dependance on the outside conditions, respect to the laboratory, seems to have significant influence on the stack behaviour. Another solution is already under study to perfectionate our start up and stop procedures thus to decrease the relative decay of the stack power. To see the negative effect of carbon corrosion of the start/stop cycling on the lifetime of the stack, a combined test ( 1/ 2 h FC on and 1 / 2 h FC off ) was carried out. In this case the current decay is faster than in the continous operation duration test mentioned above.