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PHYS-C6370 - Fundamentals of New Energy Sources Fuel cells and hydrogen Janne Halme University Lecturer, Department of applied physics Main reference: Ryan O’Hayre et al: Fuel Cell Fundamentals. Wiley, 2006. Slides: Peter Lund and Janne Halme 9.11.2016 Why H2 - relevance? Hydrogen is not an energy source but: • a clean energy carrier • produced from all primary energy sources • can be converted to all final energy forms • important industry raw material ”Hydrogen economy”? Why H2 – abundance ? • Hydrogen is abundantly available in compounds, but not in free form; e.g. water, hydrocarbons, alcohols, chemicals ― Covalent bond: H2O, CH4 ― Hydrogen bond: H2O molecules, DNA, proteins ― Hydrid compounds with electropositive elements (H-): MeHx ― 1 m3 of: water= 111 kg H2 , methanol = 100 kg, LH2 =71 kg • Fuels with H2: ― Hydrocarbons: light HCs are gases and heavy 5-12 C-atoms liquids ― Alcohols: OH connects to HCs Hydrogen storage - general • Storage properties of hydrogen – Density 0.08 kg/m3, heating value 120 MJ/kg (33.3 kWh/kg) – Volumetric energy density • STP 2.7 kWh/m3 • If 200 bar 500 kWh/m3; 1000 bar 2700 kWh/m3 • For comparison: gasoline 8,000 kWh/m3, 42 MJ/kg • Ways to store hydrogen – Compressed gas (high p< 1000 bar) – LH2 (cryogenic temperature -252.8°C) – Material based storage (on material surfaces or inside materials) What is a fuel cell ? • A fuel cell is an electrochemical device that converts chemical energy from a fuel into electrical energy without any moving parts • Fuel cells are operationally equivalent to a battery, but the reactants or fuel in a fuel cell can be replaced unlike a standard disposable or rechargeable battery Fuel cell advantages and disadvantages • Advantages: – More efficient than combustion engines – Power and capacity can easily be scaled – No moving parts, silent, no emissions • Disadvantages – Costs – Volumetric power density poor, gravimetric power density better – Fuel (e.g. Hydrogen) – Several operational issues Examples of applications • Mobile, stationary and portable power applications • Power range from mWs to few hundred kWs Chemical energy release – reconfiguring of bonds • • • • Atoms are connected through bonds that lower their total energy Bond is formed energy is released Bond is broken energy is absorbed Net release of energy : energy released > energy absorbed Simple combustion reaction • Basic combustion equation: H2 + ½ O2 H2O + heat • Collision of molecules O2 and H2 bonds break New H2O bonds formed Energy of new configuration lower Heat released • Reconfiguration of bonds involves fast electron transfer; Q: how can we slow the e- transfer from fuel species to oxidant species ? A: separate reactants so that electron reconfiguration is much slower Physical principle of a fuel cell • In a fuel cell, electrons are forced to move through an external circuit before completing the reaction (i.e. reconfiguring the bonds) • How ? An electrolyte is employed to allow ions (e.g. H+, O2-) but not electrons (e-) to flow • Electrolyte = ionic conductor • A simple fuel cells has two electrodes (for both half reactions) and an electrolyte • An ionically permeable membrane may be used to keep the gases separate Gas separation Basic operation of FC • Reaction area determines the current (electricity) production large areas lead to large current maximize surface-to-volume thin and porous structures • Anode = oxidation reaction (electrons liberated) • Cathode = reduction reaction (electrons consumed) • Good gas access necessary; oxidant (air) and reactant (fuel) separated by the electrolyte Major steps in a fuel cell 1. 2. 3. 4. Flow field plates (channels, groves) distribute the reactants over the electrodes Fast electrochemical reactions result in high current; catalysts needed; kinetics is a limiting factor Charge balance requires ion transport (by hopping), slow and losses thin electrolyte preferred Product removal. Similar to 1) Fuel cell types • Fuel cells are distinguished based on the electrolyte used • All have same underlying operation principle, but operate at different temperatures, use different materials, differ in performance, etc. • Most important fuel cells are: Polymer electrolyte membrane fuel cell (PEMFC) and solid oxide fuel cell (SOFC). SOFC Fuel cell general characteristics Peter Lund 2013 PEM Fuel Cell Anode: Cathode: Overall: • The most popular fuel cell type used in mobile, stationary and portable applications (1 mW-100 kW); low temperature operation (<<100 oC) • Solid electrolyte (polymer) that requires water to make H+ conductive; slow electrochemistry on the cathode (air) requiring a Pt-catalyst • Mass and heat flow management important Peter Lund 2013 Fuel cell components (example: PEMFC) System Stack Cell Fuel cell components (example: PEMFC) Membrane electrode assembly (MEA) Reaction sites at the electrodes (threephase boundary) MEA SOFC Fuel Cell Anode: Cathode: Overall: • SOFC= high temperature fuel cells (>700 oC); ceramic electrolyte with T-dependent ion conductivity • High T flexible to fuels, no catalysts • High T material problems, slow response • Well suitable for co-generation Fuel cell performance • Fuel cell performance is described by the current-voltage (I-V) curve • Normalized : current density mA/cm2, power density W/cm2 (kW/L, W/kg)) • Ideal thermodynamical voltage versus real voltage with loss mechanisms – Activation losses (electrochemical reactions, kinetics) – Ohmic losses (ionic and electronic conduction) – Concentration losses ( mass transport) Thermodynamic open circuit voltage Linking Gibbs energy to electric work • A change in Gibbs free energy is – G= U – TS + pV dG = dU – TdS – SdT + p dV + V dp – dU = TdS – dW = T dS – p dV - dWelec ; insert above dG=-S dT + V dp – dWelec – Assume constant pressure and temperature dG= – dWelec Welec = - ∆g rxn (per mole basis, kJ/mol) (maximum amount of electric work that can be extracted from a chemical reaction/fuel at constant conditions) Linking Gibbs energy and work to voltage • What is electric work Welec ? – If charge is q and potential difference is E, then qE is the work needed to move the charge, i.e. Welec = qE = nFE , if the charge carries n electrons; F=Faraday’s constant – From Welec = - ∆g rxn follows that ∆g= - nFE – E = -∆g /(nF); n is the number of electrons involved in the reaction • Example H2+ ½O2 H2O – ∆g rxn = -237 kJ/mol (STP) E = -∆g /(nF) = 237 kJ/mol /( 2 mol e-/mol × 96400 C/mol) = 1.23 V • Example CH4+ 4O2 2 H2O + CO2 – ∆g rxn = -763 kJ/mol (STP) E = -∆g /(nF) = 763 kJ/mol /( 8 mol e-/mol × 96400 C/mol) = 0.99 V Efficiency of a fuel cell • Heat engine limited by the Carnot efficiency – ηI= energy produced/∆H – ηII=1-T2/T1 – In practice η= 30-60% (1st law) (2nd law) • In a fuel cell, ∆H turned into ∆G (=electricity) – Max ηI = ∆G/∆H (1st law) – ∆G/∆H ≈ 80-95% (T=25 C) • Example H2+ ½O2 H2O – ∆g rxn = -237 kJ/mol (STP) – ∆h rxn(HHV) = -286 kJ/mol (STP) ηthermo= -237/-286 = 83% Enthalphy = ∆H Chemical energy Heat Mechanical energy Electricity Practical fuel cell efficiency • Ideal Efficiency = Useful energy ÷ Total energy (=work/enthalpy) • Ideal efficiency ηthermo = ∆g/∆h • Main loss mechanisms: Voltage loss and fuel utilization losses – ηvoltage = V/Ethermo – ηfuel = (i/nF)/υfuel = 1/λ λ= stochiometric factor (λ>1 overstochiometric) υ=injection rate of fuel (mol/sec) i = current generated (A) • Practical efficiency: – ηreal = ∆g/∆h × V/E × 1/λ Activation losses Activation losses are caused by slow electrode kinetics • • Electrochemical reactions (≠ chemical reactions) involves charge transfer between an electrode and a chemical species; Heterogeneous reactions − − • surface-limited reaction taking place at the electrode/electrolyte interface E.g. electrons cannot exist inside the electrolyte, nor H+ inside electrodes The rate of the electrochemical reactions determine the current i Electrode kinetics are limited by an activation barrier Reaction rate is finite • • Current is limited due to the activation barrier ∆G which is due to the minimum energy path of hydrogen H2 2H++eProbability to cross the barrier depends exponentially on the barrier height. Net current across the interface is the sum of forward and reverse reaction currents • At equilibrium j1 = j2= j0; • With η away from equil. • Net current = j1-j2 Butler-Volmer equation • Accounting for surface concentrations (rate limitations) (R=reactancts, P=products) No concentration limitations: With concentration limitations: Fuel cell kinetics depends on fuel cell type (1) • Hydrogen oxidation is often fast, oxygen reduction slow Kinetics can be improved by catalyst and electrode design • Triple phase boundary (TPB): electrolyte, gas, catalyst in contact • Pt most important catalyst (PEM), high-T SOFC nickel, ceramic cat. • Electrodes 100-400 µm, catalyst layers 10-50 µm Charge transport causes ohmic losses Effect of ohmic losses on fuel cell performance R= ∑ Ri Mass transport constraints in a fuel cell Diffusion of the reactants • Electrochemical reactions consume reactants on the electrode-electrolyte surface surface concentration of reactants drops and reaction products increase – cR*< cR0 and cP*> cP0 (0=concentrations in flow channels) • Two consequences: 1) Nernstian losses 2) Reaction rate (activation) losses (Butler-Volmer eq) Flow channel design • Flow channel patterns: – Parellel flow – Serpentine (series) flow – Interdigitated flow Fuel cell performance summary Fuel cell microgeneration • Distributed combined heat and power plants (1-100 kW) ALSTOM-Ballard P2B (212kW PEMFC ηe = 34 %) • Buildings a good application • Different fuels: biogas, bioalcohols, natural gas, hydrogen Siemens-Westinghouse (100kW SOFC ηe = 45 %) (Lähde: Laurikko,VTT) Residential Use PEMFC Cogeneration System: Matsushita Electric (Collaboration with OSAKA GAS) Sanyo Electric (JGA Project) Fuel cells in transport