Electrolysers

Cathode

where reduction occurs (gain of e)

Anode

where oxidation occurs (loss of e)

Structure of an electrolyser

anode, diffusion layer, catalyst layer, electrolyte, CL, DL, cathode

Describe the reaction in a PEM electrolyser

water in at the anode, anode is oxidising so produces O2 H+ and e, H+ diffuses across to cathode where it forms H2

Describe the reaction in a AEM

water in at the cathode, reducing so water uses e to produce OH- ions and H2, OH- diffuses across electrolyte to anode where it is oxidised to O2 and H2O producing e

Cell potential

E_cell = E_red_cathode - E_ox_anode; >0 = favourable reaction as the reducing species ‘wants’ the e more than the oxidising species can hold onto them

Standard conditions

1 atm, 1M, 298K

How are reduction potentials normally written

oxidised state + ne- → reduced state

Gibbs free energy

deltaG = -nFE_cell, n = no of e transferred, F = faradays constant

What is the mea

membrane electrode assembly = anode, electrolyte , cathode as a thin film: electrodes 8-12um, electrolyte 170-200um

Polymer used in PEMs

perfluorosulfonic acid ionomer Nafion (long side chain) or Aquivion (short side chain), PTFE backbone with fluorinated side chains and an ionically bonded sulphonic acid group at the end SO3H which transports the H+ ions

Equivalent weight

EW = m.M_bb + M_sc, m = backbone repeats, Mbb=backbone unit mass, Msc=side chain mass

What makes a good membrane material

good proton conductor, durable, gas impermeable so H2 and O2 cannot mix

Properties of membrane that affect ionic conductivity

EW and structure of the polymer, hydration number (water vs sulphonic acids), membrane morphology, temp

Catalyst layer

catalyst NPs, carbon support for conductivity, ionomer for proton conductivity, nanopores allow water in and gas out; forms the triple phase boundary of water, e and H+

Catalyst for PEMs

anode: iridium oxide; cathode: platinum on carbon; chosen as best performing with highest activity; Ir and Pt both rare and expensive

Porous transport layer

enable water and H2/O2 to enter/leave the cell; Cathode: carbon paper with 60-150um pore sizes; Anode: Ti due to harsh operating conditions (high potential in acid) needs to be durable, porous, corrosion resistant and conduct e; PLTs often covered with conductive layer to improve e conductivity

Bipolar plates

deliver water to PTLs, conduct e to external circuit, remove gases; often made of steel or nickel

Catalysts for AEMs

cathode: Ni based alloys eg NiMo or Pt on C; Anode: Ni based materials eg Ni foam or NiFe

Anion exchange mechanisms

anions pass through membrane/separator either a diaphragm (alkaline electrolysers) or ionic polymer (AEM); Or ion solvating (electrolyte swells membrane to allow ion conduction)

Are AEMs a cheaper alternative

cheaper Ni catalysts and non-precious metal alloys BUT: performance and durability worse than PEM, less stable, and some new Ni catalysts contain Co

What is overpotential

extra voltage required above theoretical thermodynamic potential to drive a reaction at a given rate; represents energy lost (mainly heat) due to kinetic, mass transport or resistive limits

PTLs for AEMs

Ni based, Ti based are prone to oxidation to TiO2 which is an insulator

Solid oxide electrolyser cells (SOECs)

solid state ceramics operating at 600-800C (higher thermal stress), carries O2- ions

SOEC MEA

multilayer ceramic, want electrodes with high electronic and ionic conductivity and good pore structure for steam transport

Stacks

stack multiple cells in series to increase the voltage