How the operative conditions influence the current distribution in a PEMFC: an experimental study through a high-resolution segmented cell Giosuè Giacoppo1*, Orazio Barbera1, Nicola Briguglio1, Paulina Pianko-Oprych2 and Francesco Cipitì1 1National Research Council of Italy Institute for Advanced Energy Technologies, Italy 2West Pomeranian University of Technology, Szczecin, Institute of Chemical Engineering and Environmental Protection Processes

Agenda Introduction Experimental Procedure Test Results Summary

Motivation Understand the current distribution in a Pem fuel cell when operated at different cathodic flowrate. Complement the polarization curve measurements with a localized resolved measurement of the current to evaluate the performance of the fuel cell. Use the obaitained current maps as an experimental database for validating numerical Computational Fluid Dynamics modelling.

Experimental procedure: Test bench Test station (Greenlight ® G8310) with: Hydrogen leakage sensor 3 gases for anode mixture (H2, CO, CO2) 1 gas for purging (N2) 1 gas for cathode (Air/O2) Humidification dry – 100% Fully programmable software Hyware II

Experimental procedure: Segmented cell Segmented cell: Current Scan 50 (S++ Simulations ® :16x16 = 256 current sensors and 8x8 = 64 Temperature sensors) Current sensors Temp sensors current scan shunt Simple methode for current density distribution measurement in fuel cell stacks. Sensor plate of type current scan shunt for a 50cm² cell The devices of type current scan shunt allow: + detailed analysis of flow fields + detailed analysis of materials + optimization of fuel cells + fault diagnostics in fuel cells + long-term tests + resolution depends on design + current range depends on design + standard devices are available + special designs on request + mass production possible With the current scan shunt, the current density distribution in fuel cells can be measured. Application The device of type current scan shunt shown above has a resolution of 10 x 10 measurement cells and an active area of 50cm². Because of the simple internal structure it is suitable for mass production. So the current scan shunt can also be applied in series production of fuel cells. It can be used for example for control applications. The picture below shows the sensor plate. current scan shunt sensor plate The sensor plate shown above has a thickness of 0.7mm. Because the sensor plates are very thin they can be laminated together with a thicker part where arbitrary channals can be milled into the surface. So if it is wished by the customer the senor plate can be used as a flow field and the gas diffusion layer can be placed directly on the gold plated sensor plate. The sensor plate can be placed at an arbitrary position in the stack. Because of the simple internal structure the sensor plates are suitable for mass production. The next picture shows a typical current distribution. Measurement data Measurement Principle The devices of current scan shunt series use a low ohmic shunt resistor for current measurement. We produce two different designs. The first one is made out of material with a very low temperature coefficient. In this case the measurement is temperature independent and no further calibration is necessary. This is an efficient and easy way for measurement.

Experimental procedure: Flow field Sensors distribution along the serpentine flow field Reactants flow path Counter-current along the flow field Co-current trhough the GDL Graphite plate: thickness: 2mm size: 90 x 90 mm

Experimental procedure: Test protocol Matrix of the experiments MEA Specs Each experiment is repeated 3 times Operative conditions were changed after each experiment An equilibration time is set after each condition.

Test results: Effect of the air flow rate High humidity Intermediate humidity low humidity Cathodic stoichiometry has strong impact on the fuel cell performance at each humidification conditions Intermediate and low humidity showed similar trend.

Test results: High humidity – low current High currents are located at gas inlets. Saturation conditions can be reached even at low current. Imax decreases with the air stoich. increasing High current are located ad gas inlet where high reactants partial pressure and high Saturation condition can be reached even at low current… in this conditions cells is very sensitive to cathode stoichiometry because its capacity to remove excess water. To have an idea on the current uniformity over the cell area, we can consider the Imax as an indicator how far is the local current from the average current. We can observe the current is reducig with air stoichiometry increase. This implies the current is more uniformly distributed all over the cell surface. As previuos enphasized the local distribution can be different even with same polarisation value Imax= 0.076 Imax= 0.071 Imax= 0.069

Test results: High humidity – high current Low current areas detected at the cathode outlet flooding condition may occur there. Higher stoichiometry mitigates oxygen starvation due to water accumulation Imax= 0.310 Imax= 0.298 Imax= 0.295 The cell operates at over-staurated conditions than water is accumulated and trasported to the outlet. In this conditions flooding occurs in the channels (as others have demonstrated). This is reflected in the current lowering at the cathode outlet. Moreover low cathode stoichiometry can be insufficient to deplete the excess water, than an increased air stoichiometry is beneficial for water removal. In this case also, the current density is improved at outlet an the uniformity also.

Test results: Intermediate humidity – low current Imax reduces with cathode stoichiometry increase. Increasing the air stoich leads high pressure gradient across the membrane that can enhance the water back diffusion High air stoich distribute currents evenly Water across the membrane is transported through back diffusion – and electro osmotic drag. At low current the back diffusion predominates over electro osmotic drag. For this reason increasing the air stoichiometry enhance the pressure gradient across the membrane and then a more uniform current distribution is reached. L’acqua prodotta al catodo quindi un lieve scostamento dalle condizioni di equilibrio puo’ causare formazione di acqua liquida al catodo l’incremento della stechiometria catodica fa amuentare il DP anodo/catodo e quindi il trasporto di acqua per osmosi attraverso la membrana. In questo modo la membrana è uniformemente idratata consentendo di raggiungere una maggiore uniformità di utilizzo dell’elettrodo. Imax= 0.073 Imax= 0.071 Imax= 0.069

Test results: Intermediate humidity – high current Flooding can occur at cathode outlet due to «reduced» air-flow for water removal. A more uniform utilization of the active area is achieved with higher air stoich because water removal improvement Due to fully humidified air at cathode some channels may be flooded at cathode outlet Imax= 0.244 Imax= 0.230 Imax= 0.243

Test results: low humidity – low current Current is almost uniform all over the cell surface. Increasing the air stoichiometry improve the current uniformity. Current is almost uniform all over the cell surface. It means the water distribution inside the cell is balanced In this conditions cells perfom better in terms of uniformty than other conditions. Imax= 0.072 Imax= 0.072 Imax= 0.067

Test results: low humidity - high currents Imax remains almost constant with the stoichiometry Increasing the air stoich. high current areas are less than other cases. low current areas reduce with air stoich increase. Imax= 0.257 Imax= 0.251 Imax= 0.257

Summary Performance of a single cell was studied along with the air flow rate and the gas inlet humidity. Current maps can give more insight the fuel cell operation can help to find optimal operative conditions to use the electrode as uniformly as possible. The higher is the cathode flow, the more is the current uniform at all studied conditions. Segmented cell helps to identiy regions with low current potentially due to bad water balance in the cell. Current maps can be used as reference for CFD model validation. Temperature distribution will be furhter investigated. Current maps can give more insight…

Thank you ! Current maps can give more insight…