ABSTRACT

Tom Smolinka, Emile Tabu Ojong, and Thomas Lickert

a Tafel slope [mV dec−1] a{i} (Molar) Activity of substance i [mol l−1] A Surface or area [m2] b Tafel constant [A m−2] ci Molar concentration of substance i [mol m−3] Ea Activation energy [kJ mol−1] F Faraday constant [96,485 C mol−1] ΔG Molar change in Gibbs free energy [kJ mol−1] hi Molar enthalpy of substance i [kJ mol−1] H Molar enthalpy change [kJ mol−1] D Hi Enthalpy flow of substance i [kJ s−1]

ΔHV Enthalpy of evaporation [kJ mol−1] HHV Higher heating value [kJ mol−1] i Current density [A m-²] io Exchange current density [A cm−2] I Electrical current [A] kReaction rate constant [L mol−1 s−1] l Membrane thickness [m] LHV Lower heating value [kJ mol−1] m Mass [g] MMolecular mass [g mol−1] n Amount of substance [mol] ni Molar flow rate of substance i [mol s−1] p Pressure [MPa]

CONTENTS

Nomenclature ........................................................................................................................................................................... 11 Subscripts and Superscripts ................................................................................................................................................... 12 Abbreviations ........................................................................................................................................................................... 12 2.1 Introduction ..................................................................................................................................................................... 12

2.1.1 General Principle ................................................................................................................................................ 13 2.1.2 Main Cell Components ...................................................................................................................................... 13 2.1.3General System Layout ...................................................................................................................................... 14 2.1.4PEM Electrolysis Operation .............................................................................................................................. 16

2.2 Thermodynamics ............................................................................................................................................................ 17 2.2.1 Heat of Reactions and Nernst Equation .......................................................................................................... 17 2.2.2 Faraday’s Law ...................................................................................................................................................... 20 2.2.3Mole and Energy Balances ................................................................................................................................ 20

2.2.3.1Stack Level............................................................................................................................................. 20 2.2.3.2 Module Level ........................................................................................................................................ 21

2.2.4 Efficiency of the PEM Water Electrolysis Process .......................................................................................... 22 2.2.4.1 Cell Level ............................................................................................................................................... 22 2.2.4.2Module Level ........................................................................................................................................ 23

2.3Reaction Kinetics ............................................................................................................................................................ 23 2.3.1 Kinetic Losses inside a PEM Electrolysis Cell ................................................................................................ 23 2.3.2 Faradaic Losses ................................................................................................................................................... 24 2.3.3 Non-Faradaic Losses .......................................................................................................................................... 28 2.3.4Polarization Curves ............................................................................................................................................ 29 2.3.5Measures to Improve Electrolysis Cell Performance ..................................................................................... 29

2.4 Key Performance Indicators .......................................................................................................................................... 30 2.4.1 Production Capacity, Power, and Gas Quality ............................................................................................... 30 2.4.2 Efficiency, Lifetime, and Degradation ............................................................................................................. 31 2.4.3Investment and Hydrogen Production Cost ................................................................................................... 32

References .................................................................................................................................................................................. 32

pi Partial pressure of substance i [MPa] P Electrical power [W] QElectrical charge [C] Q Heat flow [kJ s−1]

r Reaction rate per unit area [mol s−1 cm−2] R Universal gas constant [8.314 kJ kmol−1 K−1] Ri Ohmic resistance of cell component i [Ω] S Electrochemical active site [—] ΔSMolar change in entropy [kJ K−1 mol−1] t Time interval [s] TTemperature [K] ∆TTemperature difference [K] VVoltage [V] z Charge number [—] α Heat transfer coefficient [W m−2 K−1] β Symmetry factor [—] εEfficiency [—] ηOverpotential [V] λFactor for the membrane hydration [–] νi Stoichiometric factor of substance i [–] σ Conductivity [S m−1]

0 Standard state for temperature and pressure (1 atm, 298.15 K)

AC Alternating current actActivation adsAdsorbed an Anode b benchmark BoP Balance of plant bubBubbles cath Cathode cell Cell comprCompressor DC Direct current diff Diffusion enEnergy f Formation (g) Gaseous state he Heat exchanger iArbitrary species I Current (l) Liquid state loss Thermal losses to the surrounding memMembrane modModule ohmOhmic op Operating condition pdtProduct perPeriphery

pump Pump R Reaction rctReactant rectRectifier revReversible rp reference position stack Electrolysis stack surrSurrounding thThermoneutral theorTheoretical V Voltage

AC Alternating current ASR Area-specific resistance BoPBalance of plant BPPBipolar plate CCCurrent collector DC Direct current ELElectrolysis GDEGas diffusion electrode HER Hydrogen evolution reaction HHVHigher heating value HP High pressure HTHigh temperature KPIKey performance indicator LHVLower heating value LPLow pressure LT Low temperature MEAMembrane electrode assembly OCVOpen cell voltage OEROxygen evolution reaction PEMProton exchange membrane or polymer elec-

trolyte membrane PFSAPerfluorinated sulfonic acid PGMPlatinum group metals PSAPressure swing adsorption SPESolid polymer electrolyte STP Standard conditions for temperature and

pressure

Water electrolysis is an electrochemical process in which electricity is applied to split water into hydrogen and oxygen. It represents one of the simplest approaches to produce hydrogen and oxygen in a zero-pollution process and has already been known for more than

200  years (Kreuter and Hofmann 1998). In particular, alkaline water electrolyzers have been in use for more than 100 years in industrial applications (LeRoy 1983), but due to its several advantages, the proton exchange membrane (PEM) electrolyzer has become an emerging technology with a growing market share; see Chapter 1.