Fuel Cell 20Q
How does a mass flow controller work? What are the units? Explain STP and give values for each unit.
A mass flow controller automatically controls the flow rate of a gas according to a set flow rate sent as an electric signal, without being affected by use conditions or changes in gas pressure. Mass flow is a dynamic mass per time unit measured in grams per minute (g/min) STP - Standard Temperature and Pressure - is defined by IUPAC (International Union of Pure and Applied Chemistry) as air at 0 oC (273.15 K, 32 oF) and 10^5 pascals (1 bar).
Write down the reactions that occur on each side of the fuel cell and describe the function of the catalyst.
Anode Reaction: H2→2H+ + 2e- Cathode Reaction: 1/2O2 +2H+ +2e-→H2O Catalyst: For the anode provides the activation needed to transform Hydrogen to protons and electrons (oxidizes hydrogen). For the cathode, helps in the formation of water and heat (reduces hydrogen). It facilitates the reaction of hydrogen and oxygen. Increases reactive surface significantly.
Describe the bipolar plates used in a PEM fuel cell in terms of the fabrication methods, material requirements, and flow channel designs.
Bipolar plates (BPs) : uniformly distribute fuel gas and air conduct electrical current from cell to cell remove heat from the active area Separate individual stacks in a fuel cell prevent leakage of gases and coolant. MATERIAL REQUIREMENTS: Make up 80% of total weight and 45% of total cost - must be LIGHTWEIGHT and EASILY/INEXPENSIVELY manufactured Most common material traditionally is non-porous graphite (excellent chemical stability & low resistivity = high electrical output). BUT it has high cost, low mechanical strength, and channels must be machined into it Alternatives being researched: stainless steel (high strength, high chemical stability, low gas permeability), polymer composite plates (lightweight and can be molded into any shape and size) FLOW CHANNEL DESIGNS: The surfaces of the plates typically contain a "flow field," which is a set of channels machined or stamped into the plate to allow gases to flow over the MEA. Additional channels inside each plate may be used to circulate a liquid coolant.
How does a cylinder or in-line gas pressure regulator work? What is the difference between a single stage and two stage gas pressure regulator? When is a two stage pressure regulator preferable?
Cylinder- A standard compressed gas pressure regulator incorporates a gas-loaded or spring-loaded diaphragm mechanism that regulates the opening and closing of a gas discharge orifice. Pressure regulators reduce a supply (or inlet) pressure to a lower outlet pressure and work to maintain this outlet pressure despite fluctuations in the inlet pressure. Inline - The spring valve applies pressure against the opposing pressure in the gas line, enabling the opening of the valve just enough to pressurize the downstream house side according to the pressure level requirement. Single-stage gas pressure regulators reduce cylinder pressure to delivery or outlet pressure in one step. Two-stage gas pressure regulators reduce cylinder pressure in two steps. The single-stage regulator shows little droop with varying flow rates, but a relatively large supply pressure effect. A two-stage pressure regulator is ideal for applications with large variations in the flow rate, significant fluctuations in the inlet pressure, or decreasing inlet pressure such as occurs with gas supplied from a small storage tank or gas cylinder.
What is the Nernst equation and what is it used for in this experiment?
E=E + (RT/2F) ln [PH2,][PO2]1/2/[PH2O] where R is the gas constant (8.314 Joule/mol K), T is the temperature (°K), F is Faraday's constant (96,487 coulombs/mole of e-), E is the reversible potential at non-standard conditions (volts), and PH2, PO2, PH2O are the partial pressures of H2, O2, and H2O, respectively. The effects of temperature and pressure on the reversible potential are calculable using similar thermodynamic relations [5]. In this experiment, The Nernst equation can be used to calculate the reversible potential at non-standard concentrations. With this equation, you can calculate the cell potential for a redox reaction as a function of any concentration, not just the standard state concentrations 1M.
name all the elements comprising the proton exchange membrane material.
Flourine, Hydrogen, Oxygen, carbon, Sulfur
Describe the major components of the fuel cell experiment.
Membrane Electrode Assembly - PT catalyst cathode, Pt catalyst anode, Nafion membrane Fuel -Hydrogen Membrane - Proton exchange membrane
The proton exchange membrane is made of Nafion. What kind of polymer is Nafion? What makes this material unique?
Nafion is a synthetic fluoropolymer. It is the first of a class of synthetic polymers with ionic properties that are called ionomers. These superior conductive properties increase with the level of hydration. Nafion's unique ionic properties are a result of incorporating perfluorovinyl ether groups terminated with sulfonate groups onto a tetrafluoroethylene (PTFE) backbone. Excellent thermal and mechanical properties.
Sketch a typical polarization curve, i.e., a voltage versus current performance curve, obtained from a typical fuel cell experiment and explain the relevance of the shape of the curve in terms of each term in the empirical model.
Picture There are three distinct regions of a fuel cell polarization curve: • At low power densities, the cell potential drops as a result of the activation polarization. • At moderate current densities, the cell potential decreases linearly with current due to ohmic losses. • At high current densities, the cell potential drop departs from the linear relationship with current density as a result of a more pronounced concentration polarization. V=actual voltage graphed E0= max cell voltage blogi is the voltage loss due to activation polarization iR is the loss due to resistance to flow of the current (ohmic polarization) m(exp(ni)) is the transport limitations caused by the reactant being consumed on the surface of the electrode causing a conc gradient
List four types of fuel cells including type of electrolyte and operating temperature range.
Solid Oxide Fuel Cell (solid oxide/ceramic electrolyte and operating temp of 500 - 1000C) Phosphoric Acid Fuel Cell (liquid phosphoric acid electrolyte and temp of 150 - 210C) Alkaline Fuel Cell (aqueous alkaline solution (example KOH) temp of 20 - 90C) Proton Exchange Membrane Fuel Cell (polymeric electrolyte membrane and temp 50 - 150C)
Describe the effects of temperature, oxygen concentration and pressure on the performance of the fuel cell in terms of the polarization curve.
Temperature is one of the most critical performance-changing parameters of Proton Exchange Membrane Fuel Cells (PEMFC). ... We found that increase in temperature increases the performance and efficiency, power production, voltage, leakage current, but decreases mass crossover and durability. Fuel cell performance often improves with increased pressure. However, the need for gas compression and storage may make the system less efficient. Pressurization of the fuel also changes the water management in each cell; therefore, the fuel cell operating conditions must be analyzed from a system perspective. Higher temperature, pressure, and oxygen concentration reduced the total loss.
What are the components of the membrane electrode assembly and how are they assembled?
The MEA is composed of polymer electrolyte membrane (a.k.a the proton exchange membrane or the PEM), the catalyst layers, and the gas diffusion layers, which are also called GDLs or gas diffusion electrodes (GDE). Typically, these components are fabricated individually and then pressed to together at high temperatures and pressures. PICTURE
Why are the anode side humidifier and pre-heater kept at 5 °C higher than the cathode side humidifier and pre-heater? Explain in terms of the conditions within the fuel cell.
The humidification of the anode can be controlled through hydrogen humidification. Managing the water balance at the cathode is difficult because water accumulates due to the reaction and the electro-osmotic drag.
Explain the "ideal potential" of a PEM fuel cell. How does the open circuit potential relate to the ideal potential? What causes the difference?
The ideal potential is calculated from the change in gibbes free energy. Meaning it comes from the total amount of energy available. Delta G=Delta H-TDeltaS. delta H is thermal energy available and -delta s subtracts the energy unavailable due to entropy changes. The open circuit potential is the maximum potential and performance for the fuel cell when no current is flowing. The difference is the open circuit is greater since there is no current flowing.
How does the polymer electrolyte function? Be technical.
The only kind of low temperature fuel cell to use a solid polymer The electrodes are typically made from platinum or platinum alloy supported on carbon with an appropriate amount of ionomer added to bind each electrode and to promote protonic conductivity. Humidified H2 and air enter the system through flow fields machined into graphite plates, flow through porous carbon backings with a bimodal pore distribution, and are distributed uniformly throughout the active area of the respective electrodes, where the electrochemical oxidation and reduction reactions occur
What is the Tafel slope and how is it relevant to this experiment?
The tafel slope shows how efficiently an electrode can produce current in response to change in applied potential. So if the slope (mV/decade) is lower means less overpotential is required to get high current. A = kT/eα k is the Boltzmann's constant T is the absolute temperature e is the elementary change α is the charge transfer coefficient (must be between 0 and 1) The Tafel equation relates electrochemical reaction rate over potential, it is applicable to the two half-reactions that occur on either side of the cathode/anode It is used to calculate the voltage loss due to activation polarization and is one of the parameters solved for in the empirical model
Describe the different water movements to, from and within the polymer electrolyte of a PEM fuel cell and explain the need for water management in the fuel cell membrane in terms of diffusion and electro-osmotic drag.
The water content in the polymer membrane affects the proton conductivity and affects activation overpotentials. If the MEA is not adequately humidified, the protonic conductivity decreases, which means that the cell resistance increases. Electro-osmotic drag happens when hydrogen protons travel through the polymer membrane from the anode to the cathode and carry water molecules with them. The average number of water molecules "dragged" by a single proton is called the electro-osmotic drag coefficient. Back diffusion occurs when the concentration gradient in the cathode drives the diffusion of water through the membrane.
The following empirical model for a fuel cell polarization curve, V=E0-b*log(i)-i*R-m*exp(n*i), has five terms. Give each term a physical meaning.
V=The actual cell voltage (V) Eo is the open circuit potential (V). R is the resistance (Ω-cm2) B is a kinetic parameter (B is often called the Tafel slope) i is the current density (mA/cm2) E, B, A, R, m, and n are fitting parameters. blogi is the voltage loss due to activiation polarization iR is the loss due to resistance to flow of the current m(exp(ni)) is the transport limitations caused by the reactant being consumed on the surface of the electrode causing a conc gradient
What would happen to the performance of the fuel cell if it was run without the humidifiers?
Without external control of the humidity, fuel cells do not operate at optimum condition. They generally have a warm up time lasting many minutes, in which they operate at limited output power. During operation, flooding could occur when there is an excessive amount of moisture built up in the fuel cell when the cell temperature is relatively low and inlet reactant gases are externally humidified. The humid air could condense to water inside the cell. The water then limits the flow of air through the reactant flow conduits and isolates the catalyst surface from the reactant gases and the electrolyte. Air carries oxygen to the active sights in the Membrane Electrode Assembly (MEA). Oxygen flow should be adjusted at the exact stoichiometric ratio otherwise the fuel cell will starve for its reactant fuel, thus the output power efficiency is reduced to unsatisfactory levels.
Describe five ways that can be used commercially to produce hydrogen.
commercial production of hydrogen: natural gas, oil, coal, and electrolysis; which account for 48%, 30%, 18% and 4% of the world's hydrogen production respectively. Renewable liquid reforming- liquid fuels like ethanol are reacted w steam to produce hydrogem Biomass fermentation- biomass is converted to sugar rich feedstock which are fermented to produce hydrogen Electrolysis- splitting water with electricity Photobiological water splitting- microbes consume water in the presence of sunlight and produce hydrogen as a byproduct
