Energy Engineering - Electrochemical Energy Conversion and Storage

Full exam

EECS – Prof. A. Casalegno – 5 th September 20 2 2 1 Standard EECS exam [duration 1 h 40 min] To upload (maximum 5 single - side A4 pages – PDF file name MUST BEGIN with person code) Basic Part ( 4.5 points per question, minimum score 12) 1. Write an example of electrochemical reactions occurring in a Lithium - ion battery and the typical operating voltage range 2. Draw the electrical potential in the double layer of a PEMFC for the OR R in equilibrium 3. Depict the polarization curve of a n alkaline electroly z er with reasonable value s Exercise A Li - ion battery with nominal capacity of 1 5 Ah discharg es as in the figure . Assuming that : 1. Tafel kinetics is valid for both electrodes 2. ion and mass transport effects in electrodes are negligible 3. activities of reactants in discharge (or charge ) are close to the value of SoC ( or 1 - SoC) 4. electrolyte resistance is equal to 10 m Ω 5. transfer coefficients of both electrodes during discharge are equal to 0.5 Calculate the voltage valu e at C - rate B ( approximat ed to a natural number) and SoC= 2 0% . Interm e diate Part ( 3 points per question ) 1. Write Butler - Volmer and Tafel equations and r epresent them on the same graph 2. Represent the impedance spectrum of a PEMFC operating at low current and indicate where information on membrane and catalyst layer ion conductivity can be obtained . !"#0%∆%2.75 V4%20%100%50%3.55 V3.06 V2.92 V 3 V 2.85 V C-rate=0.5 EECS – Prof. A. Casalegno – 5 th September 20 2 2 2 Exercise A PEM fuel cell operates at 8 0°C with pure H 2 and air at 0. 3 MPa , the portion of the cell at cathode inlet, where O 2 molar fraction in the cathode channel is equal to 0. 2 , suppl ies at 0. 7 V a current density equal to 2 A/cm 2 . Assuming that : 1. anode behaves as an ideal H 2 electrode ( ! ! + # ! =0) 2. ion and mass transport effect s in catalyst layers are negligible 3. membrane ion conductivity and thickness are respectively 0.07 S/ c m and 15 μ m 4. transfer coefficient of ORR is equal to 0.5 5. diffusion layer diffusivity at 0. 1 MPa is equal to 0.0 3 cm 2 /s and its thickness is 1 2 0 μ m Demonstrate that the current density at cathode outlet, where O 2 molar fraction in the cathode channel is equal to 0. 12 , is lower than 1 . 5 A/cm 2 . Difficult Part ( 1.5 point s ) Exercise A Li - ion battery with nominal capacity of 10 Ah operates at ambient pressure and temperature. Assuming that: 1. BV kinetics is valid for both electrodes 2. transfer coefficients of both electrodes during discharge are equal to 0.5 3. ion and mass transport effects in electrodes are negligible 4. ! " # = 3 . 9 ) 5. * $% " ≅ 1 6. * '( = - '( ) = [ ( 0 . 9 − 0 . 05 ) ∗ 567 + 0 . 05 ] ) 7. 9 ∗ : " = 150 ; 8. electrolyte resistance is equal to 5 mΩ Calculate the positive electrode potential at Crate=2 and SoC=50% . Laboratory Part (1.5 points) 1. Depict accurately on the Nyquist plot two impedance spectra of PEMFC doubling the operating current. Then explain if there is a constraint in voltage oscillation amplitude to perform EIS and, in case, the typical value for PE MFC.