Electrochemical cells

1. Electrochemical cells Sähkökemianperuseet KE-31.4100 Tanja Kallio tanja.kallio@aalto.fi C213 CH 4.1 – 4.2, 4.7

2. Electrochemical cells • i) Electrolyte • ii) Electrodes • Anode: oxidation reaction • Cathode: reduction reaction • iii) External circuit iii) load e- e- i) ii) ii)

3. Experimentalset-up Potentio-stat working electrode counter electrode twoelectrodeset-up threeelectrodeset-up

4. Ecell Reference electrode Requirements i) High exchange current density = ideally nonpolarizable ii) High concentrations iii) Separated from the system H2 Au electrode Ptelectrode Fe2+ Fe3+ [H+]=1 M porousmembrane

5. Common reference electrodes

6. Ecell Cell diagram Cathode: Fe3+ + e– Fe2+ Anode: ½ H2 H+ + e– Cellreaction: ½ H2(g) + Fe3+(aq)  H+(aq) + Fe2+(aq) H2 Cu | Pt | H+, H2 || Fe3+, Fe2+ | Au | Cu Au electrode Ptelectrode Ecell= Eright– Eleft Equillibrium: i) DG = 0  Ecell = 0 ii) icell= 0; ia = ic Fe2+ Fe3+ [H+]=1 M porousmembrane

7. E anodepotentiaildifference E IR loss cathodepotentialdifference Cell potential profile

8. Nernst equation: equilibrium potential of an electrochemical cell Cathoderections: Fe3++ e– Fe2+ Anodereactions: ½ H2H++ e– Cellreaction: ½ H2(g) + Fe3+(aq) H+(aq) + Fe2+(aq) Nernst equation as E = - DG / (nF) eq (1.6) aA+ bB + cC.... dD + eE + fF...

9. Phase equilibrium in an electrochemical cell (1/3) ½ H2(g) + Fe3+(aq)  H+(aq) + Fe2+(aq) Cu| Pt | H+, H2 || Fe3+, Fe2+ | Au | Cu’ a b s e g a’ 1) phasea| phaseb 5) phaseg | phasea’ 3) phases| phasee 4) phasee| phaseg 2) phaseb| phases

10. Phase equilibrium in an electrochemical cell (2/3) eq (2.7) Cu| Pt | H+, H2 || Fe3+, Fe2+ | Au | Cu’ a b s e g a’

11. Phase equilibrium in an electrochemical cell (3/3)