H9. Fuel Cells I. INTRODUCTION. II. OBJECTIVE OF THE EXPERIMENT. III. THEORY. III.1. Electrolysis of Water

Transcription

1 H9. Fuel Cells I. INTRODUCTION. Chemical pollution is a serious problem that demands the attention of the scientific community in the early 21 st century. The consequences of pollution are numerous: heating of the atmosphere due to greenhouse gases, modification of the ozone layer, acid rain, health of the urban population, One of the main actors of this pollution is motorized transportation. The fight against car pollution requires the development of electrical vehicles, as long as the source used for producing electricity is clean. To fulfill the needs of people, we need a new generation of vehicles whose performances at least match the current combustion engine cars, most notably in autonomy (500 km) and charge time (a few minutes). These requirements aren t met by current electrical vehicles, that run on classical batteries (lead or cadmium nickel). The only way to overcome the limitation of the charging time is to use some sort of fuel and a device capable of converting the fuel into electric energy. This device is called a fuel cell, and is actually nothing but controlled oxidation of hydrogen by air, producing electricity and water simultaneously. The fuel cell was invented in 1839 by an English pastor, Sir William Grove, and its first spectacular application were inhabited spacecrafts (Gemini, Apollo and Space Shuttle). For car applications, the type of fuel cell that is used is an acid battery with a polymer electrolyte PEMFC (Proton Exchange Membrane Fuel Cell) with a working temperature of ~ 70 C but already supplies a significant amount of power at room temperature. Connecting 350 cells of 250 cm 2 supplies a power of 35 kw (~47 hp) and if the system is connected to super-capacitors, it could supply an extra 15 to 20 kw for a few minutes. II. OBJECTIVE OF THE EXPERIMENT. We shall study hydrogen fuel cells, both with a liquid electrolyte (KOH) and with a solid polymer PEM (proton exchange membrane) electrolyte. We will then describe the performance (maximal power, efficiency, etc.) of each fuel cell. Finally, we will study the production of hydrogen through electrolysis, and describe the performance of the full process (production of hydrogen and fuel cell). III. THEORY III.1. Electrolysis of Water Electrolysis is the process of chemical decomposition of a substance (liquid or solid) using an electric current. For instance, to electrolyze water, we can use a system made up of two

2 EPFL-TRAVAUX PRATIQUES DE PHYSIQUE H9-2 tubes, connected at their bases, where the electrodes are located. This is called the Hofmann apparatus (fig. 1). Chemical reactions at the electrodes Fig. 1: Hofmann apparatus, used for electrolysis. Cathode: 2 H 2 O + 2 e! -> H OH - Anode: 2 OH - -> 1/2 O 2 + H 2 O + 2 e Faraday s law: The mass m of matter freed from the electrodes is proportional to the electrical charge Q that has passed through the electrolyte, of atomic mass M a, and is inversely proportional to "ν", the number of valences broken in the electrolyte (number of electrons) M = 1 F Q Mat ν F = Faraday constant = C per atom-gram Since the number of moles n = M, we can write: F = Q Mat n ν where n is determined from the ideal gas law: PV = nrt III.2. Fuel cell The fuel cell is an apparatus allowing the inverse reaction of the electrolysis: a controlled oxidation of hydrogen, producing electricity and water simultaneously. A fuel cell is made of two electrodes (cathode and anode) separated by an electrolyte (fig. 2) that can be liquid (caustic potash KOH, phosphoric acid) or solid (conducting polymer, doped ceramic e.g. ZrO 2 doped with Y 2 O 3 ). The electrodes are generally made of a porous metal, allowing a rather fast diffusion of the gas, all the while stopping the inverse diffusion of the electrolyte if it s liquid. The adsorption

3 EPFL-TRAVAUX PRATIQUES DE PHYSIQUE H9-3 of the reacting gas should preferentially be done to the combustible product, in order to preserve the catalyst. If we look at the periodic table, we notice that elements such as Ni, Ag, Pt, Pd, are electronegative with respect to hydrogen and electropositive with respect to oxygen. They can therefore be used for both electrodes. (cathode and anode), since they are capable of ionizing both gases. Liquid Electrolyte Fuel Cell (KOH) Fig. 2: Oxyhydric gas fuel cell (KOH electrolyte) Chemical reactions at the electrodes: cathode: 1/2 O 2 + H 2 O + 2 e! -> 2 OH - the OH - ions move from the cathode to the anode through the electrolyte (KOH) anode: H OH - -> 2 H 2 O + 2 e! electrons freed at the anode go back to the cathode through the outer circuit. Solid PEM Electrolyte Fuel Cell (Proton Exchange Membrane): Chemical reactions at the electrodes: anode: H 2 -> 2 H e! the H + ions move from the anode to the cathode cathode: 1/2 O H e! -> H 2 O

4 EPFL-TRAVAUX PRATIQUES DE PHYSIQUE H9-4 IV. SUGGESTED EXPERIMENTS A Hofmann apparatus is available for electrolyzing water (fig. 1). The glass tubes are filled with water and a small amount of sulfuric acid, to increase the ionic conductivity of the water, for an easier start of the process. Also available is one plastic electrolyser cell that allows production of hydrogen under pressure. Oxygen is produced at the anode side and bubbles up through the water in the reservoir. At the cathode side, the hydrogen gas produced comes out of the cell via a gas exit on the side of the cell. The hydrogen is directed by a tube to the phase separator where the liquid water present in the hydrogen gas is removed. Three hydrogen fuel cells with a solid electrolyte (PEM) are available. Each consists of a plastic housing and functions by air-breathing : the oxygen present in the surrounding air can access the fuel cell through the slits on the cathode side of the cell while the hydrogen is provided by the electrolyser. It is important that the slits remain unobstructed while the cell functions. A porous material is placed inside the kit for the homogeneous diffusion of hydrogen over the whole surface area of the membrane electrode assembly. 1. Production of hydrogen and oxygen through electrolysis. Verify Faraday s law, i.e. determine Faraday s constant. (The voltage applied to the electrodes should not exceed 10 to 15 V). 2. Use the plastic electrolyser. Starting up the electrolyser cell: Fill the water reservoir with distilled water Connect the power supply to the positive and negative poles of the electrolyser cell (red terminal of the power supply to the positive pole of the electrolyser on the bottom left hand corner of the cell and black terminal of the power supply to the negative pole of the electrolyser in the centre of the cell) Turn on the power supply and set a current of 3A maximum. Check that oxygen gas is being produced, observation of gas bubbling up through the distilled water in the reservoir. Fill the bubbler with distilled water to the 100ml graduation mark and close tightly with the bottle top equipped with gas tubes provided. Connect the hydrogen gas exit tube to the bubbler entry tube at the bottom of the manometer and make sure that hydrogen gas is being produced. Verify that all gas tubes are well connected Allow hydrogen to flow for a few minutes so as to purge all air present in the bubbler Check the distilled water level in the reservoir regularly Starting the fuel cell: Connect the tube with the valve to the fuel cell gas exit. Tubes with an exterior diameter of 4mm are used for the gas line. Hydrogen produced by the electrolyser is passed through the phase separator (bubbler). Connect the gas exit of the bubbler (tube not in water) to the fuel cell entry; the valve located at the fuel cell exit should be open. The hydrogen pressure in the gas line is shown on the manometer situated on top of the bubbler. Purge the fuel cell with hydrogen gas for a few minutes (at least 5).

5 EPFL-TRAVAUX PRATIQUES DE PHYSIQUE H9-5 Connect the fuel cell to a charge resistance according to figure 3a. Make sure that the charge is at least 1MΩ. Verify that the test bench displays an open circuit voltage between 0.85 and 1V and that zero current passes through the fuel cell. Close the valve at the fuel cell gas exit. The fuel cell works in dead end mode. The optimum pressure is 0.5bar. To activate the cell, it is recommended to increase the hydrogen pressure by applying a current of 3A to the electrolyser and running the fuel cell at a lower current. The greater the difference between the electrolyser current and the fuel cell current, the faster the increase in gas pressure (ATTENTION Pmax = 1bar). Then the fuel cell is connected in a short circuit (to produce maximum current). Repeating several times the sequence of purging, pressure increase and short-circuiting the cell can increase its performance. 3. Plot the fuel cell s current and the power versus voltage (cf. fig. 3b) and determine the optimal efficiency conditions, i.e. the maximal power. 4. In the optimal conditions, determine the fuel cell s efficiency (from the formation energy of water Q=286 kj/mol) as well as the global efficiency of the whole process (electrolysis of water and production of electricity). For the latter, the results will be more informative if a stationary state is reached between the gas production and the fuel cell consumption. 5. Use in the same way the other two fuel cells, plot the cell s current and the power versus voltage and calculate the efficiency of the cells. Discuss the effect of the membrane thickness and the concentration of the catalyzer. Before stopping the electrolyser, it is recommended to disconnect the charge. The fuel cell can undergo serious damage if it is forced to supply a current with an insufficient hydrogen supply. 4 P I (A) 2 1 I P (W) U (V) Fig. 3 : a) Electric setup of the fuel cell s charge resistance. b) Current vs. Voltage and Power vs. Voltage of the fuel cell