Electrochemical cells Sähkökemian peruseet KE Tanja Kallio C213 CH 4.1 – 4.2, 4.7

Electrochemical cells Galvanic cellElectrolysis cell GG < 0> 0 E < 0 CathodeOn the right-hand side On the left-hand side applicationsbatteries, fuel cells, corrosion Electrolysis, analysis methods load i) Electrolyte ii) Electrodes Anode: oxidation reaction Cathode: reduction reaction iii) External circuit e-e- e-e- i) ii) iii)

Experimental set-up two electrode set-up three electrode set-up counter electrode Potentio- stat working electrode

Reference electrode [H + ]=1 M Fe 3+ Fe 2+ H2H2 E cell Pt electrode Au electrode porous membrane Requirements i) High exchange current density = ideally nonpolarizable ii) High concentrations iii) Separated from the system

Common reference electrodes ElectrodesystemreactionE 25 o C vs. NHE E 25 o C vs. SCE Normal hydrogen electrode, NHE Pt|H 2,H + (1 M) ½ H 2  H + + e – Saturated calomel electrode, SCE Hg|Hg 2 Cl 2, KCl (sat’d) Hg 2 Cl 2 (s) + 2e -  2Hg(l) + 2Cl Silver - silver chloride Ag|AgCl, KCl (sat'd) AgCl(s) +e -  Ag(s) + Cl Mercury - mercury oxide Hg|HgO, NaOH (0.1M) HgO(s) + H 2 O + 2e -  Hg(l) + 2OH

Cell diagram [H + ]=1 M Fe 3+ Fe 2+ H2H2 E cell Pt electrode Au electrode porous membrane E cell = E right – E left Equillibrium: i)  G = 0  E cell = 0 ii) i cell = 0; i a = i c Cu | Pt | H +, H 2 || Fe 3+, Fe 2+ | Au | Cu Cathode: Fe 3+ + e –  Fe 2+ Anode: ½ H 2  H + + e – Cell reaction: ½ H 2 (g) + Fe 3+ (aq)  H + (aq) + Fe 2+ (aq)

Cell potential profile E E cathode potential difference anode potentiail difference IR loss

Nernst equation: equilibrium potential of an electrochemical cell Cathode rections: Fe 3+ + e –  Fe 2+ Anode reactions: ½ H 2  H + + e – Cell reaction: ½ H 2 (g) + Fe 3+ (aq)  H + (aq) + Fe 2+ (aq) as E = -  G / (nF) eq (1.6) Nernst equation aA + bB + cC....  dD + eE + f F...

Phase equilibrium in an electrochemical cell (1/3) ½ H 2 (g) + Fe 3+ (aq)  H + (aq) + Fe 2+ (aq) Cu | Pt | H +, H 2 || Fe 3+, Fe 2+ | Au | Cu’  ’ 1) phase  | phase  2) phase  | phase  3) phase  | phase  4) phase  | phase  5) phase  | phase  ’

Phase equilibrium in an electrochemical cell (2/3) eq (2.7) Cu | Pt | H +, H 2 || Fe 3+, Fe 2+ | Au | Cu’  ’

Phase equilibrium in an electrochemical cell (3/3)