Future Direction of Direction of Direct Alcohol Fuel Cell Dr. S. Basu (Web: paniit.iitd.ac.in/~sbasu/) Department of Chemical Engineering Indian Institute of Technology Delhi New Delhi 110016
Path! Poisoning Operation < 80 0 C Membrane, PEMFC SOFC Cost High Temperature Automobile; Distr Power Thermal Cyclability Redox-cycling DAFC HT Sealants Cost PEM, AEM based Stationary power Micro-fluidic Cost MCFC Portable micro-electronic AFC equip Basu, S. (Ed.) Recent Trends in Fuel Cell Science and Technology, Springer/Anamaya (2007)
Obstacles associated with the use of hydrogen Generation of hydrogen gas High cost Explosion hazard Difficult to storage and distribute Low power output per unit weight of the fuel cell and fuel processor Low energy density: 0.002772 KWh/l at atm. conditions (33 kWh/kg) A. Verma, S. Basu , “Power from hydrogen and fuel cell”, July, 177-181 (2005) Chemical Weekly.
Fuel Methanol � Not a primary fuel � Liquid fuel � Easy to transport and distribute � Toxic � 5 KWh/l (6 KWh/Kg) energy density � 6 electrons per molecule of methanol oxidized � Electrooxidation is easy in alkaline condition Ethanol � Non-conventional liquid fuel - method of production is well established � Easy to transport and distribute � 5.9 KWh/l (7.44 KWh/Kg) energy density � C-C bond cleavage � 12 electrons per molecule of ethanol oxidized at low temperature � Electrooxidation is easy in alkaline condition � Non toxic
Direct Alcohol Fuel Cell Direct Alcohol Direct Alcohol Direct Alcohol Alkaline Fuel Cell PEM Fuel Cell AEM Fuel Cell
Fundamentals of conventional alkaline fuel cell Electrolyte: KOH solution Anode ( Pt/C ) : − − + → + 2 4 4 4 H OH H O e 2 2 Cathode ( Pt/C ) : − → − + + 2 4 4 O H O e OH 2 2 Overall Cell Reaction: + → 2 2 H O H O 2 2 2
ALKALINE FUEL CELL Alkaline electrolyte H 2 + 2OH - � 2H 2 O + 2e - Anode 1/2O 2 + H 2 O + 2e - � 2OH - Cathode Overall H 2 + 1/2O 2 � H 2 O + electrical Energy +heat (CO 3 ) 2- + H 2 O CO 2 +2OH - Poisoning: � Depletion of KOH Myth ! � Poisoning of cathode surface with carbonates Kordesch, K., Cifrain, M., Koscher, G., Hejze, T., and Hacker, V., “A survey of fuel cell systems with circulating electrolytes”, Power Sources Conference 2004, Philadelphia, June 14-17. McLean, G.F., Niet, T., Prince-Richard, S., and Djilali, N., “An assessment of alkaline fuel cell technology”, Int. J. Hydrogen energy, 27, (2002) 507-526 Gülzow, E., and Schulze, M., “Long-term operation of AFC electrodes with CO2 containing gases”, J. Power Sources , 127, (2004) 243- 251
Advantages of alkaline fuel cell � Less electrode poisoning compared to acidic electrolyte Fuel cell forum DA PEM FC � Fuel oxidation in alkaline solution by the non-noble metal catalyst is as active as noble metal catalyst DA AEM FC � Oxygen reduction reaction is DAAFC more favorable in alkaline medium than in acidic medium AFC Verma, A., A. K. Jha , S. Basu “Manganese oxide as a cathode catalyst in flowing alkaline electrolyte direct alcohol or sodium borohydride fuel cell ” J. Power Sources 12. 141 30-34 2005
Anode � Tripkovi ć et al. (1996, 2001) suggested a general mechanism for C 1 -C 4 alcohol electrooxidation in alkaline medium: Alcohol Reaction intermediate Acid (in anionic form) ‘poisoning species’ (CO) CO 2 � Ethanol was most active on Pt surface � Formic acid and acetic acid were the reaction products of methanol and ethanol electrooxidation, respectively � Torresi et al. (2003) reported: * the electrolysis of ethanol on polycrystalline gold in alkaline medium * acetaldehyde and acetic acid were found as a reaction product * no C-C bond cleavage Cathode � Mao et al. (2002) reported two reduction peaks in cyclic voltammogram (CV) correspond to 2 + 2 electron mechanism for MnO 2 � Verma et al. (2005) found only one reduction peak in CV for MnO 2 cathode Tripkovi ć , A.V., Popovi ć , K.Dj., and Lovi ć , J.D., “The influence of oxygen-containing species on the electrooxidation of the C1-C4 alcohols at some platinum single crystal surfaces in alkaline solution”, Electrochim. Acta , 46 (2001) 3163-3173.
Direct Alcohol Alkaline Fuel Cell Schematic diagram Fuel: Methanol / Ethanol Cathode (MnO 2 /C/Ni) 4e - + O 2 + 2H 2 O � 4 OH - Anode (Pt/C or Pt-Ru or Pt-black) Methanol CH 3 OH + 2OH - � HCHO + 2H 2 O + 2e - HCHO + 2OH - � HCOOH + H 2 O + 2e - HCOOH + 2OH - � ? CO 2 + 2H 2 O + 2e - Ethanol C 2 H 5 OH + 2OH - � ? CH 3 CHO + 2H 2 O + 2e - 1. Fuel-electrolyte mixture storage; 2. Exhausted-fuel-electrolyte mixture storage; 3, 4. Peristaltic pump; 5. Load; 6. Anode terminal; 7. Cathode terminal; 8. Air; 9. Anode electrode; 10. Cathode electrode; 11. Fuel and electrolyte mixture; 12. Magnetic stirrer; 13. Anode shield
Photograph of direct alcohol alkaline fuel cell Multimeters Fuel cell Potentiometer Peristaltic pumps Magnetic stirrer Fuel and electrolyte storage tanks Verma, A., and Basu, S ., ‘Direct use of alcohols and sodium boro hydride as fuel in an alkaline fuel cell' J. Power Sources 145, 282-285 (2005)
Effect of KOH concentration 2 M Methanol, 25 o C 2 M Ethanol, 25 o C Pt Black: Anode, MnO 2 : Cathode Pt Black: Anode, MnO 2 : Cathode 1.2 1.2 1 M KOH 1 M KOH 1.0 1.0 3 M KOH 3 M KOH Cell voltage (V) Cell voltage (V) 5 M KOH 5 M KOH 10 M KOH 10 M KOH 0.8 0.8 0.6 0.6 0.4 0.4 0.2 0.2 0.0 0.0 0 50 100 150 200 250 300 350 0 50 100 150 200 250 300 Current density (A/m2) Current density (A/m2) � Verma, A., Jha, A. K., and Basu, S., 2004, Evaluation of an Alkaline Fuel Cell for Multi-fuel System, Proceedings of ASME Conf. on Fuel Cell Sci, Eng and Tech., 14-16 June, 2004, Rochester, US � Verma, A., and Jha, A. K., S. Basu ‘Analyses of Multi-Fuel Alkaline Fuel cell’, Grove Fuel cell Symposium – Fuel cells Science & Technology, Oct. 6-7, 2004 Munich, Germany
Effect of electrode catalyst type and loading Performance curves for anode catalysts in 2 M Ethanol/ 3 M KOH AFC 1.2 180 Pt-Black Power density (W m- 2 ) 150 Pt/C Cell voltage (V) 0.9 Pt/Ru 120 0.6 90 60 0.3 30 0.0 0 0 70 140 210 280 350 Current density (A m- 2 )
Performance of fuel cell for different fuels 0.8 0.6 Cell voltage (V) 0.4 0.2 Ethanol Methanol Sodium borohydride 0.0 0 200 400 600 800 Operation time (hours) A. Verma, A. K. Jha and S. Basu “Evaluation of an alkaline fuel cell for multi- fuel system” ASME J Fuel Cell Science & Technology, 2, 234-237 (2005)
MODEL EQUATION E cell = OCV – ( η ac + η conc + η oh ) Direct methanol alkaline fuel cell ⎛ ⎞ ⎛ ⎞ − − 1 0 . 5 . R T j C C ⎜ ⎟ = − ⎜ ⎟ − − ln M OH .( 1 . 31737 9 . 0336 E E i c ⎜ ⎟ ⎜ ⎟ α cell OH ⎝ ⎠ ⎝ ⎠ n F K + − − + − 2 3 23 . 05735 8 . 107083 2 . 578 0 . 11875 c c T OH OH 1 ⎛ ⎞ RT j ⎜ ⎟ − 0 . 000625 ) ln T ⎜ ) ⎟ ( α − 1 1 ⎝ ⎠ nF j jM O
Model prediction of current-density versus cell-voltage for methanol at different KOH concentrations (t=25 o C, C M =2M) 1.0 Cell Voltage (V) 0.8 1 M KOH (Expt.) 1 M KOH (Model) 3 M KOH (Expt.) 0.6 3 M KOH (Model) 5 M KOH (Expt.) 5 M KOH (Model) 10 M KOH (Expt.) 0.4 10 M KOH (Model) 0.2 0.0 0 50 100 150 200 250 300 Current Density (A/m 2 )
Direct Alcohol PEM Fuel Cell
Schematic of Direct Alcohol Fuel Cell Load Air/H 2 O C 2 H 5 OH/H 2 O/CO 2 Catalyst Layers ANODE CATHODE PEM e - e - H + H 2 O Current collector + Reactant Distributor Current collector + Reactant Distributor C 2 H 5 OH/H 2 O Diffusion Layers Air Anode: (catalyst : Pt / Ru / C ) 12 e - + 12 H + + 2 CO 2 C 2 H 5 OH + 3 H 2 O Cathode: (catalyst : Pt / C ) 3 O 2 + 12 e - + 12 H + 6 H 2 O Overall: C 2 H 5 OH + 3 O 2 3 H 2 O + 2 CO 2
Preparation of Membrane Electrode Assembly (MEA) Pt-Ru/C electrode-catalysts + Nafion Pt-black electrode-catalysts + Nafion ionomer + Activated carbon powder + ionomer + Activated carbon powder + PTFE dispersion PTFE dispersion Mix by ultrasonic agitation Mix by ultrasonic agitation Paint on diffusion layer Paint on diffusion layer Dry diffusion layer with Dry diffusion layer with electrode catalysts electrode catalysts Cathode Sectional and Plan View Anode of MEA Photograph of MEA
Schematic Diagram of Direct Ethanol Fuel Cell
Direct Ethanol Fuel Cell Experimental Setup O 2 cylinder O 2 Humidification column Ethanol + acid storage Peristaltic pump Multimeter Fuel Cell Temperature controller
Comparison of Polarization Curves: Cathode Catalysts Loading Fuel: 0.75 M and 2M ethanol + 0.5 M sulfuric acid Anode Temp. : 90 O C and Cathode Temp. : 60 O C. 1.2 Electrode-catalysts loading: M-9-6,Ethanol:acid(1:20) with 0.75M Ethanol M-9-6,Ethanol:acid (1:30)with 0.75M Ethanol Anode: 1 M-10-1,Ethanol:acid (1:20)with 0.75M Ethanol M-9-6: 1 mg/cm 2 M-10-1,Ethanol:acid (1:30) with 0.75M Ethanol M-10-1: 0.6 mg/cm 2 M-10-1,Ethanol:acid (1:20) with 2M Ethanol Cathode : M-10-1,Ethanol: acid (1:30) with 2M Ethanol 0.8 Cell Voltage (V) M-9-6: 0.5 mg/cm 2 M-10-1: 0.6 mg/cm 2 0.6 0.4 0.2 0 0 5 10 15 20 25 30 35 Current density (mA/cm 2 )
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