HYDROGEN AND FUEL CELLS. Experiment 1

Transcription

1 HYDROGEN AND FUEL CELLS Experiment ENG Power Distribution and Storage Equipment Electrolyser and fuel cell apparatus De-ionised water Dummy load 2 x Ammeter 2 x Voltmeter Electrolyser power supply Time : 3 hours This equipment can be permanently damaged by mis-use. As the operator, YOU are responsible for reading all operational information and using the equipment safely. If you are unsure, ask your demonstrator. This equipment produces Hydrogen, which is of course EXPLOSIVE. The amount contained in the equipment is only small, but you should still exercise caution to ensure that no naked flames or electrical discharges are present near this gas. Preliminary Answer questions 1 and 2. Introduction Fuel Cells PEM Fuel cells work by uniting hydrogen and oxygen to form water. In the process energy is recovered in the form of electrical current. The hydrogen (H2) entering at the anode is split into two electrons (e - ) and two hydrogen ions (H + ), or protons as they of course can be described. This is achieved by the catalyst, in our case platinum. H 2 2H + + 2e The PE membrane only allows protons to enter, not electrons or gas. The electrons must flow around the cell, via the load, to get to the cathode. At the cathode the protons combine with oxygen, and the electrons coming from the load, to form water. 1 2 O 2 + 2H + + 2e H 2 O 1

2 Figure 1. A PEM fuel cell. Or you can look at a video: Electrolysers A PEM electrolyser is effectively a fuel cell in reverse. Electrons will be stripped from the water, releasing oxygen, and the remaining hydrogen protons will flow through the membrane. On the other side of the cell the electrons will be returned to the hydrogen protons thus forming hydrogen. We therefore have oxygen and hydrogen gas released from each side of the cell. Figure 2. A PEM electrolyser. 2

3 3314ENG Power Distribution and Storage The Equipment Electrical Energy Electrical Energy H2 Water Water Storage Electrolyser O2 Fuel Cell Figure 3. The electrolyser and fuel block diagram. In the experiment today we will be generating hydrogen and oxygen in a PEM electrolyser and storing them in a water column. The two gases will then be used by a fuel cell to generate electrical power. Figure 3 shows this arrangement. Q1 What will be the ratio of hydrogen to oxygen produced in the electrolyser? Q2 Calculate the energy contained in 20ml of hydrogen. Show your working and references. The Experiment Figure 4. The electrolyser and fuel cell apparatus. 3

4 1 Hydrogen and Oxygen Generation Ensure you know which unit is the electrolyser and which is the fuel cell! Do not run the electrolyser without a water supply. Only ever power the electrolyser from the electrolyser power unit (2V max). (a)ensure the electrolyser power unit is switched off. (b)check the electrolyser cell hoses are connected up to the storage cylinders as shown in figure 4. (c)fill the cylinders with de-ionised water up to the top level mark. (d)open both hose clamps to allow the water to fall to the bottom of the cylinder. (e)leave the hose clamps open. (f)connect the electrolyser power unit to the power supply via an ammeter. (g)connect a voltmeter across the electrolyser power unit. (h)with the electrolyser voltage set to 2V switch on the power. (i)when the current into the cell has settled, close the hose clamps and start timing the gas production until 20ml of hydrogen has been produced. You should take current readings every 5 seconds during this period. (j)repeat (h) and (i) but with the electrolyser voltage set to 1.8V and 1.6V. Calculate the power and energy used in the cell for all three trials. Calculate, and graph, the production rate of hydrogen (in litres/m) for each power setting. Calculate your efficacy of production of hydrogen in litres/wh and the efficiency (Wh/Wh) at each power setting. 2 The PEM Fuel Cell Do not allow water to run into the fuel cell. Never connect the fuel cell to a power source. (a)run the electrolyser for long enough to build a supply of gas. (b)connect a picoscope across the output of the fuel cell. (c)connect the dummy load unit, set to 5 Ohms and switch set to off, to the output of the fuel cell. 4

5 (d)start the picoscope sampling slowly and switch on the load. Record the cell voltage until it stabilises. Save this trace and data set. (e)record the stabilised fuel cell voltage at this load setting. (f)repeat parts (c) - (e) with the dummy load set to 10 and 15 Ohms. (g)run the cell at the load setting that delivers maximum power and measure how long it takes to use up 1ml of hydrogen. Plot the power output of the cell, versus time, during the startup phase of this fuel cell, for all three power settings. Calculate the stabilised current and power output of this cell at all three power settings. Calculate the efficacy of this cell in Wh/litreH2, and the efficiency (Wh/Wh), at all three power settings. Discussion Your report should discuss the performance of both the electrolyser and the fuel, including any limitations these small scale lab units may possess. 5