Energy Engineering - Electrochemical Energy Conversion and Storage

Full exam

EECS – Prof. A. Casalegno – 17th June 202 2 1 Standard EECS exam [duration 1 h 40 min ] Students who passed TiP reduced number of questions marked with (TiP ) [duration 1 h 10 min] To upload https://forms.office.com/r/548eLjrfzX (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 the Nernstian equilibrium potential for an electrochemical cell based on H 2/O 2 as a function of chemical activities 2. Write the Nernst -Plank equation for mass transport in a generic electrolyte 3. (TiP ) Depict the charge curve of a Lithium -ion battery with reasonable values Exercise (TiP ) A PEMFC stack with 500 cells supplies at full load 120 kW at 335 V (LHV=120 MJ/kg). Assuming that 3% of H 2 is wasted and its performance follows the reported polarization curve, estimate the efficiencies at full power ( 335 V) and low power ( 390 V) . Interm ediate Part (3 points for each question ) 1. Is the electrode overpotential positive or negative when ORR occurs? Demonstrate analyzing ������̃ 2. (TiP ) Show how voltage limits in batteries could reduce the available capacity during both discharge and charge at high C -rate ∆������ 0 ������ 0.78 V 0.6 7 V 0.3 A cm -2 1.8 A cm -2 EECS – Prof. A. Casalegno – 17th June 202 2 2 Exercise (TiP ) A Li -ion battery with nominal capacity of 5 Ah operates at ambient pressure and temperature, supplying 3. 6 V during discharge at SoC=80% and C -rate=2 and requiring 4.0 5 V during charge at SoC= 30 % and C -rate=1. Assuming that: 1. Tafel kinetics is valid for both electrodes 2. ion and mass transport effects in electrodes are negligible 3. chemical activities of reactants are close to the value of SoC 4. electrolyte resistance is equal to 0.02 Ω 5. transfer coefficients of both electrodes during discharge are equal to 0.5 Cal culate a meaningful efficiency starting from SoC=0.8 operating with the following 4 step profile: i. discharging at C -rate=2 for 12 min ii. discharging at C -rate=1 for 6 min iii. charging at C -rate=1 for 3 0 min iv. charging at C -rate= 0.1 for 30 min Difficult Part (1.5 point s) Exercise (TiP ) A PEM FC operates at 70°C with pure H 2 and air at 0.2 5 MPa , in the figure polarizations curves are reported before and after real operation including start -up/shut -down . Assuming: 1. anode behaves ideally ( ������−+ ������−=0) 2. ion and mass transport effects in catalyst layers are negl igible 3. transfer coefficient of ORR is equal to 0.5 4. O2 activity in the channel is equal to 0.2 5 5. GDL thickness is 15 0 μm and O 2 diffusivity at 0.1 MPa is equal to 0.06 cm 2/s 6. high frequency resistance remains constant , equal to 0.1 ������ ������������2 Identify the most probable origin of degradation and which physical parameters are affected, estimate their variation. Draw on the Nyquist plot the electrochemical impedance of the cell at 0. 1 A/cm 2 before and after aging , indicating the most relevant values. Laboratory Part (1.5 points) 1. (TiP ) Draw the effect on the performance of a PEMFC operating at fixed middle current, roughly quantifying the impact on voltage and on its stability , varying: a. the air stoichiometry from 1.6 to 5 b. the H 2 stoichiometry from 1.3 to 4 ∆������ 0.88 V 0.65 V 0.85 V 0.5 5 V 1.8 A cm -2 0.1 A cm -2 ������