UNIVERSITY OF TORONTO. FINAL EXAM, APRIL 28, hours. EXAMINER D.W. Kirk,

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1 Page 1 of 11 PRINT FIRST NAME LAST NAME STUDENT NUMBER UNIVERSITY OF TORONTO FINAL EXAM, APRIL 28, hours CHE 469F - FUEL CELLS and ELECTROCHEMICAL SYSTEMS Do all questions. The marks add up to 100. EXAMINER D.W. Kirk, Calculator Type 2 - Non-programmable calculators are allowed. No programmable calculators are allowed. No other aids are allowed. ALL WORK IS TO BE DONE ON THESE SHEETS! Use the back of the page if you need more space. Be sure to indicate clearly if your work continues elsewhere. CLEARLY INDICATE YOUR FINAL ANSWERS. G=H-TS ag =vp-st AG=AG' +RTlnQ AG =RTlflK eq AG flfe eii F =96,485 c/mo1. E=E - T1nQ nf In = ogx ah S C1, = C,, = at T R ::: J/(mol K) I atrn= kpa=760mrnhg=1.013bar K=0 C 1m 3 =1000L PV=nRT W=-PAV MolarMass: H = 1.008, 0 = 16.00, N = Na = 23.0, Air = 79%N2 21%07 Ch RT( i 1h =zfd or = ln zf h im Peukert equ.t = C - In.r (anf (anf - 1a exp L RT I = T Butler Volmer general equation ETh= 1.48V for 1-12/02 at 25 C TOTAL MARKS Tafel Equation ij=a+b log i 1=b (log i/io) 303RT Where, a = RT log 10, and b = anf anf Where, iii,,, = limiting current density D = diffusion coefficient Cb= bulk concentration ô = boundary layer thickness 10 = exchange current density 11 = overpotential (= E - E ) ii = number of electrons a4 = anodic transfer coefficient ac= cathodic transfer coefficient n = factor in Peukert equ. Capacitors C = c c A/d (capacitance units F ( farads)) C= QiV (Q coulombs, V volts) E energy stored (J) = 1/2 CV2

2 Page 2 of II Problem 1 [20 marks total] Ethanol (C21-150H) is proposed to be a fuel for an alkaline type fuel cell. Data: AH (kj/mol) C2H50H H H201iq H20gas CO AG (kj/mol) AS (J/mol.K) cp (J/K.mol) a) If oxygen is used for the cathode, what is the maximum voltage that could be generated at 25 C assuming water is in a liquid state? What is the thermoneutral voltage for this cell? Write the anodic '/2 cell reaction. If the cell were operated at 0.5 MPa (ie. anode and cathode gases at 0.5 MPa), what would be the change in theoretical voltage at 25 C.? (an ethanol/water azeotrope has a boiling point of 78.2 C)

3 Page 3 of 11 e) Assuming entropy values are almost constant from 25 C to 60 C what is change in theoretical cell voltage frcm 25 C to 60 C at kp? (Enthalpy values are not constant) f) If the cell is operated with 95.6% ethanol and water for the anode and air as the cathode what would be the theoretical voltage at 25C and kpa?

4 Page 4 of 11 Problem marks totall For an experimental 112/02fue1 cell operating under alkaline conditions, a Tafel slope for the cathode was found to be 125mV/decade and the exchange current density (lo) was extrapolated from the data and was reported to be 0.5 x 10-2 ma/cm2. Severe mass transfer limitations were found at 0.95mAIcm2. The electrolyte (a Nafion membrane) had a conductivity of 5.0 x 102 S/cm and a thickness of 200um. Using the semilog chart provided, plot the activation polarization from to 1.0 ma/cm2 plot the resistance polarization from to 1.0 ma/cm2 On the same chart sketch the expected current voltage profile into the mass transfer region. 1.2 Ii

5 Page 5 of 11 d) The following reaction sequence has been proposed. Experimentally a Tafel slope of V/decade of current has been measured. Which of the following step(s) would provide that Tafel Slope? M e - - M02-11] M02- + H -+ M02H [21 M02H + e - M02H 131 M02H -FH - M M02H2+e-3 MOH + OH- [5J MOH +e - M For a multi step reactior written as a cathodic reaction a=y/v+r3 and a a = (n- y )l v - r where: v = number of times the RIDS occurs in the overall reaction = number of electrons before the RIDS = symmetry factor (default = 0.5) r = I for an electron trarsfer step and r=0 for a chemical step

6 Page 6 of II Problem marks total] How much hydrogen (L at NIP) is needed to produce 1 kwh in a PEM fuel cell stack with 5 cells in series. Each cell is operating at 0.6V? For 0.01 M H2SO4, the molar conductivity = Scm2/mol.. Determine the resistance of a planar cell using 10cm2 electrodes with 0.5 cm separator having a porosity of 60%. c) A NiCd battery rated 1200 mah at 0.1C is to be used with a current draw of 3 amps. Peukert no. = How long will the battery last?. d) From a test using different concentrations of oxygen for the gas fed to a cathode in a PEM cell, the limiting current density was found to depend linearly on the oxygen concentration. The relation was 7.14mA/cm2 per atmosphere of oxygen pressure. Henry's Law for the oxygen solubility in water from air = 1.28 x I 0-3 mol/l.atm (T25 C) The diffusion coefficient of oxygen in water is D= 1.4 x 10-5 cm2/s Determine the diffusion layer thickness Air is 21% oxygen

7 Page 7 of 11 Problem marks total] The positive grid of the lead acid battery has what active material? Describe briefly the function of the GDL Describe the function of the MEA What does the term"flooded" lead acid cell mean? What does the term"valve regulated" lead acid cell mean? 1) What information do you need to determine the energy content of a 1000mAh battery? g) A lithium ion battery is referred to as being an FePO4 type. What does this mean? A lithium ion battery fire can be caused by a single cell shorting. What cell level modification is used to minimize the effect of this event?

8 Page 8 of 11 Qu 4 continued h What is the cause of the memory effect in a NiCd battery? i) Write the reaction that takes place on the cathode of the MCFC fuel cell. If a fuel cell were operating on glucose CH4 as a fuel and oxygen as the oxidant, what is the value of n for the Nernst Equation? Microbial fuel cells have 3 ways of extracting energy from the fuel. What are they?

9 Page 9 of I Problem marks total] a) You have ten 3000F supercapacitors with a 3V rating. What would be the maximum run time for a '/4 HP DC voltage pump which has a cutoff voltage of 4V? Show the supercapacitors layout. 1HP=746W b) A back up power system is required to run a 10 Amp motor at 115V AC Provide the layout (number of cells and connections) to satisfy the motor. Four batteries are available with a rating of 12V 1 50Ah (@0.1 C rate), Peukert No = 1.2 A combined 1:10 transformer and AC/DC inverter/converter is available (80% efficient) for any power requirement. Qu 5 continued on next page

10 Page loofi! c) Direct NH3 Fuel cells operating at 700 C with the characteristics shown below. A custom voltage inverter with transformer 1:10 winding is available for 750W input power rating but is only 85% efficient. Each cell is 20cm2.and operates with pure NH oo > E 9c0 700 IM ( I (A/cm) Determine the maximum power for a single cell Determine the minimum number of cells and configuration needed to operate a 10 Amp motor at 115V AC

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12 Page 1 of 11 PRINT FIRST NAME LAST NAME STUDENT NUMBER UNIVERSITY OF TORONTO FINAL EXAM, APRIL 28, hours CHE 469F - FUEL CELLS and ELECTROCHEMICAL SYSTEMS I. Do all questions. The marks add up to 100. EXAMINER D.W. Kirk, Calculator Type 2 - Non-programmable calculators are allowed. No programmable calculators are allowed. No other aids are allowed. ALL WORK IS TO BE DONE ON THESE SHEETS! Use the back of the page if you need more space. Be sure to indicate clearly if your work continues elsewhere. CLEARLY INDICATE YOUR FINAL ANSWERS. GH-TS 0g = vdp - s3t AG =AGO +RTlnQ AGO =RTI11Keq AG = -flfe , MARKS F=96,485 c/mo!.: E=E -!TlnQ nf lnx = ogx OH DS c = cp, = AT AT T R=8.314J/(molK) 1 atin=101.3 kpa=760 mmhg =1.013bar K=0 C Tm 3 =1000L PV=nRT W=-PAV MolarMass: H = 1.008, 0 = 16.00, N = Na = 23.0,Air = 79%N7 21%07 ihin RT ( = zfd or i In I 1- S zf.peukert equ. t = C In. [ (anf (anf '= ' 1a - exp- 77, RT J RT Butler Volmer genera! equation ETN= 1.48V for 1-12/02 at 25 C 4 5 TOTAL Tafel Equation 77 =a+b log i i=b(log i/i0) Where, a= 2.303RT log :0,and b= 2.303RT an an Where, u1,,1 = limiting current density D = diffusion coefficient Cb= bulk concentration ö = boundary layer thickness io = exchange current density fl = overpotential (= E - E ) n = number of electrons = anodic transfer coefficient ac = cathodic transfer coefficient ii = factor in Peukert equ. Capacitors C = Aid (capacitance units F ( farads)) C= Q/V (Q coulombs, V volts) E energy stored (J) = V2 CV2

13 Page 4 of 11 Problem 2 [20 marks total] For an experimental H2/02fue1 cell operating under alkaline conditions, a Tafel slope for the cathode was found to be 125mV/decade and the exchange current density (lo) was extrapolated from the data and was reported to be 0.5 x 102 ma/cm2. Severe mass transfer limitations were found at 0.95mA/cm2. The electrolyte (a Nafion membrane) had a conductivity of 5.0 x 102 S/cm and a thickness of 200um. Using the semilog chart provided, plot the activation polarization from to 1.0 ma/cm2 plot the resistance polarization from to 1.0 ma/cm2 for a 10cm2 electrode. On the same chart sketch the expected current voltage profile into the mass transfer region IM 0.6 t1 0.2 I I 1 I I I ji i I - II

14 a Page 5 of 11 d) The following reaction sequence has been proposed. Experimentally a Tafel slope of V/decade of current has been measured Which of the following step(s) would provide that Tafel Slope? M+02 +e *M M02 + H M02H 121 M02H + e * M02H 131 M02H +W * M02H2 141 MO2H2+e * MOH + OH- [5] MOH +e M [61 For a multi step reaction written as a cathodic reaction a?iv+r and a a = (n- ' r)i v - r where: v = number of times the RIDS occurs in the overall reaction = number of electrons before the RIDS = symmetry factor (default = 0.5) r = 1 for an electron transfer step and r=o for a chemical step