Tom Smolinka, Emile Tabu Ojong, and Thomas Lickerta 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]CONTENTSNomenclature 11 Subscripts and Superscripts 12 Abbreviations 12 2.1 Introduction 122.1.1 General Principle 13 2.1.2 Main Cell Components 13 2.1.3General System Layout 14 2.1.4PEM Electrolysis Operation 162.2 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 202.2.3.1Stack Level 20 2.2.3.2 Module Level 212.2.4 Efficiency of the PEM Water Electrolysis Process 22 2.2.4.1 Cell Level 22 2.2.4.2Module Level 232.3Reaction 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 292.4 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 32References 32pi 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 [—] eEfficiency [—] 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 perPeripherypump Pump R Reaction rctReactant rectRectifier revReversible rp reference position stack Electrolysis stack surrSurrounding thThermoneutral theorTheoretical V VoltageAC 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 andpressureWater 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 than200 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.