Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Fuel Cell Workshop for Secondary School Teachers Fabrication of a Gas Diffusion Electrode Practical Session Education and Manpower Bureau, HKSAR Department of Chemistry University of Hong Kong Sept 2003 Fabrication of an Alkaline Air Cathode I. Objectives To fabricate a gas diffusion electrode using common materials and apparatus. To verify the performance of the prepared electrode as a cathode in an aluminiumair sea water cell. II. Gas Diffusion Electrode A gas diffusion electrode is needed for sufficient contact and penetration of a gas reactant into the electrode. It is also called “ a breathing electrode” because it allows sufficient transport of a gas reactant, such as oxygen or hydrogen into the electrode. In case of oxygen, the electrode is called “air cathode”. For a gas absorbing electrochemical reaction, the reaction takes place at the three-phase boundary: intersection of gas phase, liquid phase, and solid phase. For example, the oxygen reaction in alkaline, the three phase reaction is catalyst ( s) O2 ( g ) 4e 2H 2 O(l ) 4OH The geometry of the three-phase boundary is a line and has a limited capacity for reaction. In reality, gas molecules can dissolve at the gas-liquid interface and diffusion to the liquid-solid interface for reaction. To reduce the diffusion path and maximize the gas-liquid interface area, a ”thinly wetted layer” is desirable. In addition to the supply of gas, a connected conducting solid phase is required to carry the electrons to or away from the reaction site. A conventional gas diffusion electrode is made of 1) high surface area carbon powder loaded with a catalyst 2) a non-wetting material, PTFE to create pores and hydrophobicity, 3) a binder to hold the loose material together, and 4) a metal mesh or foam, or carbon cloth for the current collection. 1 The electrode may be composed of multiple layers. A reactive “carbon” layer with catalyst supported in active carbon, was pasted in a current collector, nickel foam. The hydrophobic film (Teflon layer) is on the outside surface which becomes the “air side” as shown in figure below. Teflon Layer Air Side Electrolyte Side (water side) Reaction layer: (3 phase boundary) Catalyst on carbon support inside a nickel foam Structure of an air cathode III. Catalyst The air cathode catalysts for oxygen reduction are highly active, non precious electro-catalysts, such as metals, metal oxides and pyrolyzed metal chelates, supported on high surface area carbons. To obtain the maximum performance, different catalysts are employed in different chemical systems, e.g. different pH. Common catalysts include Manganese(KMnO4), Silver(AgNO3), Cobalt Spinel, CoTMPP , Pt, La0.6Ca0.4CoO3. Below are polarization curves of air cathodes with different catalysts. Schematic of catalyst particles absorbed on carbon powder. Cathode Catalysts for Alkaline Fuel Cells For alkaline system, silver nitrate AgNO3 supported on active carbon is a good catalyst. The graph of Polarization voltage vs. Current density is shown. (Catalyst: silver 5M KOH electrolyte. Hg\HgO reference electrode. 60 °C) The air cathode is used in alkaline aluminum air cells with current densities of up to 150 mA/cm. A typical cell voltage is 1.28 V at current density of 100 mA/cm2 in KOH electrolyte. 2 Cathode Catalysts for Saline Medium For saline electrolyte and seawater, manganese catalyst(KMnO4) is preferred. The applications in seawater include Al-air, Zinc-air and Magnesium-air fuel cells. The polarization voltage and current density data is shown in figure below. 12% NaCl electrolyte. Room temperature. If used in an aluminum-air cell with 12% NaCl electrolyte, using aluminum foil as negative electrode, a cell generates 1.07 volts at a current density of 30mA/cm2 IV Experiment An air cathode designed for use in alkaline or saline electrolytes for metal-air/fuel cells will be prepared and tested in an aluminium-air cell. Characteristics of the gas diffusion electrode: Typical Thickness: Average Weight: Safety: 1.5mm 900 grams per square meter (typical) Does not contain mercury, cadmium, lead, lithium, or other dangerous materials. Physical Description: A 2-layer laminate consisting of (a) A current collector of nickel foam with reactive carbon paste (b) A micro-porous PTFE film on the air side. The air cathode is mechanically robust and chemically resistant to a wide range of substances. Instruments/Chemicals 1. 2. 3. 4. 5. 6. 7. Watch glass or Petri dish, 10cm in OD Voltmeter X1 Connector X 2 Mini DC motor with fan blade or LED light Brush or wooden stick Sprayer 25% NaCL solution in H2O (Sea water) 8. Aluminum foil & scissors 9. Carbon powder, Vulcan 72 XC 10. Catalyst (KMnO4) 11. Binder (5%PVA in H2O or paper glue) 12. Teflon emulsion(30% in H2O) 13. Poly propylene Separator or filter paper 3 Procedure (1) -----Paste Preparation (20% KMnO4) Weigh 0.95g of active carbon (XC-72), 0.2g of KMnO4 add together with 10 mL of DI H2O, 5mL 95% ethanol, 10 drops of 5% PVA solution, 5 drops of 1/:3 PTFE suspension, to a plastic Petri dish and stir with a glass rod for several minutes until the slurry become paste like. (2) -----Electrode Fabrication Use a brush or stick to apply the well-mixed paste into the pre-cut nickel foam (4X4cm2 with a lead tab) until both sides are covered with the black paste. Dry in the 100~2000C oven for about 5-8 minutes. Use the sprayer to spray PTFE emulsion(30% in H2O) to ONE SIDE of the electrode until it is wet, dry in the 100~2000C oven for 10 minutes. (3) ----- Assembly of aluminum-air fuel cell and test air cathode’s performance Use scissors to cut 4X4cm2 aluminum foil (with a protruding lead tab) and place on a shallow Petri dish, then cover with a pre-cut PP separator (6X6 cm2)or a filter paper, finally place your air cathode on top of the separator, Teflon side up, (to make a “sandwich” like fuel cell), pour about 5mL 25% NaCL solution onto it (the electrolyte should flood half of the air cathode but NOT to flood the Teflon side. Finally connect each terminal to the motor fan(or a LED ) with a voltmeter, the fan will start turning , record the voltage change against time. ***Note: In order to avoid electrical short-circuit between cathode and anode, the area of air cathode must be less than the area of separator. Air cathode + separator - Al foil Watch glass/ Petri Dish Voltmeter Assembly of an Aluminum-Air Fuel Cell DC motor with fan Fuel Cell system: Air-cathode: catalyst—KMnO4 Anode (Fuel) Aluminium Electrolyte: 25% NaCl in H2O Temperature: room temperature Open circuit voltage(OCV): ~1.05V Location Half Cell reactions Anode Voltage Al + 4 OH-—> Al(OH)4- + 3e -2.35 - Cathode 3/4 O2 + 3/2 H2O + 3e—> 3OH 0.40 Overall Al + 3/2 HO + 3/4 O2 —> Al(OH)3 2.75 V 4 Catalyst Loading Effect 1.5 1.4 Air-Metal Fuel Cell Perfromance: Catalyst percentage effect Anode: Al Foil Cathode catalyst: KMnO4 Electrolyte: 25% NaCL Electrode area: 4X4 cm2 1.3 1.2 1.1 A graph showing the effect of catalyst loading to the performance of air cathode. Cell Voltage(V) 1.0 0.9 0.8 0.7 20% 10% 5% 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0 5 10 15 20 25 30 35 40 Time(min) of discharge Time Time elapsed Cell voltage remark ---------------------------------------------------End of experiment------------------------------------------ 5