Energy Engineering - Electrochemical Energy Conversion and Storage

Full exam

EECS – Prof. A. Casalegno – 4th February 20 22 1 To upload: https://forms.office.com/r/brPrSW5qu8 (maximum 5 single -side A4 pages) Basic Part (4.5 points per question , minimum score 12) 1. Write the Nernstian equilibrium potential in generic form for an electrochemical electrode 2. Draw the electrical potential in the double layer of a PEMFC for the HO R in two cases: equilibrium and operation 3. Depict the dis charge curve of a Lithium -ion batt ery at high current with reasonable values Exercise An automotive PEMFC stack , composed of 400 cells, supplies 10 0 kW at full load (1.8 A/cm 2). Assuming that 3% of H 2 is wasted and its performance follows the reported polarization curve , e stimate the mass of H 2 that is necessary to operate the stack at an average power of 25 kW for 800 Km at an average vehicle speed of 1 00 km/h. Interm ediate Part (3 points for each theory question and 3.5 for the exercise ) 1. Depict the three losses related to electrodes and electrolyte as a function of current density during operation of an alkaline electrolyzer . 2. Depict three polarization curve s of a PEMFC operating at 80°C : 1) O2 feeding at 1 atm and RH= 10 0% ; 2) air feeding at 3 atm and RH= 20% ; 3) air feeding at 3 atm and RH= 80% . ∆������ 0 ������ 0.8 6 V 0.65 V 0.3 4 A cm -2 1.8 A cm -2 EECS – Prof. A. Casalegno – 4th February 20 22 2 Exercise A Li -ion battery with nominal capacity of 20 Ah operates at ambient pressure and temperature, supplying 3.8 V during discharge at SoC= 80% and C - rate= 2; requiring 4 V during charge at SoC= 20% and C -rate=2 . Assuming that: 1. Tafel kinetics is valid for both electrodes 2. Tafel slope b for both electrodes during charge and discharge is equal to 0.12 V 3. ion and mass transport effects in electrodes are negligible 4. activ ities of reactants in discharge and charg e are close to the value SoC and (1-SoC) respectively 5. elec trolyte resistance is eq ual t o 3 mΩ Calculate if 3 batteries are sufficient to operate the cycle in the figure with an overall efficiency higher than 85%, choos ing symmetrical variation of SoC , then depict the SoC variation in time. Difficult Part (1.5 point s) Exercise A PEM FC operates at 70°C with pure H 2 and air at 0.2 MPa , in the figure polarizations curves are reported before and after a long period of operation , as well as impedance at 1. 6 A cm -2 before operation . Assuming: 1. anode behaves ideal ly (������−+ ������−=0) 2. ion and mass transport effects in catalyst layers are negl ected 3. O2 molar content in the channel is equal to 0. 13 4. GDL thickness is 200 μm and O 2 diffusivity at 0.2 MPa is equal to 0.0 4 cm 2/s 5. high frequency resistance remains constant Identify the most probable origin of degradation , estimate the variation of the affected physical parameters and depict on the Nyquist plot the impedance at 1. 6 A cm -2 after degradation. 0 ∆������ 0.8 8 V 0.6 5 V 0.83 V 0.59 V 1.6 A cm -2 0.1 A cm -2 ������ ������������ (������) −������������ (������) 0.1 ������ ������������2 −0.02 ������ 0.15 ������ ������������ 2 charge dis charge