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CACHE Modules on Energy in the Curriculum Fuel Cells Module X (Draft): H2 Production by Natural Gas Assisted Steam Electrolysis Module Author: Michael D. Gross Module Affiliation: Bucknell University Course: Membrane Separations Text Reference: Geankoplis 4th ed., Chapter 13 Concept Illustrated: Problem Motivation: Fuel cells are a promising alternative energy conversion technology. Hydrogen is currently the preferred fuel for fuel cells. The generation of pure hydrogen is important, particularly for low temperature fuel cells such as PEMFC, because the Pt catalyst is sensitive to impurities. One way to generate hydrogen is by electrolysis, a process that uses electricity to decompose water into hydrogen and oxygen. One could accomplish this by running a fuel cell backwards as discussed in Module x. Another route would be to supply methane at the anode and water at the cathode. This method, called Natural Gas Assisted Steam Electrolysis (NGASE), produces hydrogen without supplying electricity from an external source. In this configuration, the electrolyzer operates as a fuel cell and the electricity is generated by the electrochemical reaction. The NGASE reactions are: Anode: Cathode: Overall: Electron Flow (Current) - - e e CH4 H2O H2 O2- CO2 H2O CH4 CH4 O H2O CO2 CH4 CH4 H2O CH4 + 4O-2 CO2 + 2 H2O + 8 e4 H2O + 8 e- 4 O-2+ 4 H2 CH4 + 2 H2O 4 H2 + CO2 O2O 2- H2 H Air 2O In Anode Gas Chamber Cathode Gas Chamber Fuel Cell H2O H & CO 2 2& H22O Out H2 Anode Cathode Electrolyte Figure 1: Reactions within NGASE Draft 1 Cell Voltage CH H24 In H2 H2 2- Electric Load H2 Air Out Figure 2: Flow Diagram for NGASE -1- September 18, 2008 For each mole of methane consumed, 4 moles of H2 are generated and 8 moles of electrons are passed through the external circuit. To convert electron flow (moles of electrons/s) to electrical current (coulombs/s or amps), one would use Faraday’s constant: F 96,485 coulombs / mole of electrons. The rate of hydrogen production is directly related to the current as follows: i nF where dN H2 or dt dN H2 i dt nF i current density n amperes coulombs cm 2 s cm 2 2 moles of e 1 mole of H 2 coulombs mole of e moles of H 2 cm 2 s F 96,485 dN H2 t Figure 3 shows the relationship between current density, i, and electrolyzer voltage. There are several things to note here. 0.3 Voltage, V EOCV = 0.25 V 0.2 0.1 0.0 0.0 0.1 0.2 0.3 Current Density, A•cm-2 Figure 3. A performance curve for NGASE. Draft 1 -2- September 18, 2008 The theoretical maximum voltage of this electrolyzer is 0.25 V. This is called the “open circuit voltage” EOCV. Any drop in voltage from EOCV is termed “overvoltage.” It is desired to minimize the overvoltage so that the electrolyzer can operate as efficiently as possible. The hydrogen reaction rate is directly proportional to the current, since for each hydrogen molecule that reacts, two electrons are formed. For operating voltages (Eop) 0 ≤ Eop ≤ EOCV, hydrogen is produced without an external power source. This corresponds to a possible current range of 0.25 A·cm2 to 0 A·cm2. At current densities between 0 A/cm2 and 0.25 A/cm2, there is a linear fall in voltage as the current density increases. Assuming fast kinetics, this voltage drop is caused by a resistance to oxygen ion flow across the electrolyte membrane. In this case the operating voltage can be described as Eop = EOCV – iRA, where i is current density, R is resistance, and A is cross sectional area of the electrolyzer. In physics and electrical engineering, this effect is referred to as Ohm’s law. The maximum rate of hydrogen generation corresponds to the maximum current, 0.25 A/cm2 and Eop=0, which is called short circuit. It is always desired to operate at short circuit since this is the highest and most efficient rate of H2 generation. While the NGASE method uses an electrochemical device for separation of water into hydrogen and oxygen, it draws many parallels to typical membrane separation processes across a dense membrane. Our primary interest is the rate of H2 production, however, O2is the species transported across the membrane and so the flux across the membrane is written with respect to O2-. For every one mole of H2 produced, 2 moles of e- are collected and one mole of O2- is produced at the cathode. The flux equations for an electrolyzer and a separation with a dense membrane are compared below. Draft 1 -3- September 18, 2008 cio Ni ciL L Figure 4. Concentration profile for a membrane process Table 1. Flux equations for a membrane process and NGASE. Equations Units mol O 2 cm s 2- Membrane Separation D N i i c iO ciL L N O2 i nF i i V E OCV E op E OCV E op V RA RA L E OCV N O2 mol O 2 - C mol O 2 - mol e- 2 2 C cm s cm s mol e V i RA NGASE cm 2 mol O 2 s cm cm 3 C C cm 2 cm 2 s s C 1 1 V V cm 2 s cm cm cm 2 E op L L nF E OCV mol O 2 - V mol O 2 - mol e- 2 2 C cm s cm mol e E op N O 2 - flux i current density E OCV open circuit vo ltage E op operating voltage RA area specific resistance O 2 - conductivi ty (S/cm or 1/ cm) L thickness of the membrane D diffusivit y ciO and ciL concentrat ions at the membrane interfaces Draft 1 -4- September 18, 2008 The only difference between the flux equations for the electrolyzer and membrane separation is that the electrolyzer takes into account electrical units. For the electrolyzer, voltage directly corresponds to the concentration of oxygen at each membrane interface via the Nernst equation. The concentration of oxygen is typically expressed as a partial pressure of oxygen, PO2. For more information regarding the Nernst equation, refer to Module 9, under Thermodynamics. Draft 1 -5- September 18, 2008 Example Problem A company is developing a new car powered by a fuel cell system that runs on H2. You have been asked to consider generating the H2 by NGASE. The H2 tank to be used is 10 liters in volume and a fill-up requires a pressure of 500 psi. a. The electrolyzer should be designed for maximum performance. Plot the maximum rate of H2 generation as a function of electrolyte membrane thickness (1 μm to 1 mm) given the following data. What is the ideal membrane thickness? EOCV = 0.25 V σ = 0.02 S/cm (S = 1/Ω) b. Given the following data, how long would it take to fill the tank? Assume an ideal gas and 25ºC. Electrolyzer stack consists of 100 cells in series Each cell has an area of 10 cm2 Electrolyte membrane thickness = 10 μm Example Problem Solution Part a Step 1. Calculate the flux of O2-, NO2-, across the electrolyte membrane. N O2 L nF E OCV E op The conductivity, σ, and open circuit voltage, EOCV, were given in the problem statement. n 2 mol e mol O 2 F 96485 C mol e - The maximum rate of H2 generation will occur at the maximum current density. The maximum current density corresponds to Eop = 0 V. For an electrolyte membrane thickness of 10 μm: Draft 1 -6- September 18, 2008 N O2 L nF E 1 cm 0.25 V - 0 V E op 2 mol e C 10 10 4 cm 96485 1 mol O 2mol e 0.02 OCV N O 2 2.59 10 5 mol O 2cm 2 s The oxygen ion flux as a function of electrolyte membrane thickness is summarized in the following table: l (cm) 1.00E-04 1.00E-03 1.00E-02 1.00E-01 NO22.59E-04 2.59E-05 2.59E-06 2.59E-07 Step 2: Calculate the rate of hydrogen generation with the oxygen flux. Every one mole of H2O reacted, generates one mole of H2 and one mole of O2-. Therefore, the rate of H2 generation is equal to the flux of oxygen ions. 1.E-03 dN H 2 mol H 2 dt cm 2 s 1.E-04 l (cm) 1.00E-04 1.00E-03 1.00E-02 1.00E-01 1.E-05 dNH2/dt 2.59E-04 2.59E-05 2.59E-06 2.59E-07 1.E-06 1.E-07 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 l (cm) Ideally, the smallest electrolyte membrane thickness possible would be used. However, in real systems the electrolyte is typically 10 μm thick or more. Membranes thinner than 10 μm can leak, causing the reactants on either side of the membrane to mix. Once reactants from either side of the electrolyte mix, the concentration of oxygen is equal on both sides and EOCV = 0. Part b From part a, we know for a 10 μm thick electrolyte membrane Draft 1 -7- September 18, 2008 dN H 2 dt 2.59E - 05 mol H 2 cm 2 s Total area of electrolyzer system: # of cells in stack area of one cell 100 cells 10 cm 2 cell 1000 cm 2 The moles of H2 required can be calculated with the ideal gas law, PV=nRT. 1 atm 34 atm P 500 psi 14.696 psi n 34 atm 10 L 13.9 mol H 2 L atm 0.08206 298 K mol K fill up time Draft 1 13.9 mol H 2 n 540 sec onds ~ 9 min dN H 2 mol H 2 2 1000 cm 2.59 E 04 area cm 2 s dt -8- September 18, 2008 Home Problem The example problem demonstrated how the flux across an electrolyte membrane changes with thickness. This problem addresses how the flux changes with temperature. The conductivity of an electrolyte membrane as a function of temperature (in Kelvin) is as follows: T 3.6 10 e T 5 -8104 RT A company is developing a new truck powered by a fuel cell system that runs on H2, and you have been asked to consider generating the H 2 by NGASE. The H2 tank to be used is 20 liters in volume and a fill-up requires a pressure of 750 psi. a. The electrolyzer should be designed for maximum performance. Plot the maximum rate of H2 generation as a function of electrolyzer temperature (500ºC to 1000ºC) given the following data. What is the ideal operating temperature of the electrolyzer? Electrolyte membrane thickness = 10 μm EOCV = 0.25 V b. Given the following data, how long would it take to fill the tank? Assume an ideal gas and a tank temperature of 25ºC. Electrolyzer stack consists of 50 cells in series Each cell has an area of 5 cm2 Electrolyzer T = 800ºC Draft 1 -9- September 18, 2008