Electrosynthesis

  • Some materials is best synthesised by electrosynthesis, and for many it is the only known way:
    • Electrowinning from ore
    • Exclusive: Alkalimetals, earthalkalimetals (Li, Na, K, Mg, Ca, Sr, Ba, Ra, Al, Ta + F\(_2\), Cl\(_2\))
    • Competitive: Cr, Mn, Co, Ni, Cu, Ag, Au, Zn, Cd, Ga, In, Ti

Hall-Hèrault process for Al

  • Al\(_2\)O\(_3\) dissolved in melt of cryolite, Na\(_3\)AlF\(_6\) (+CaF\(_2\)):
    • \(\mathrm{Al}_{2} \mathrm{O}_{3}+8 \mathrm{~F}^{-}=2 \mathrm{AlF}_{4}^{-}+3 \mathrm{O}^{2-}\)
  • \(\operatorname{AlF}_{4}^{-}(f u s)+3 \mathrm{e}^{-}(A l) \rightarrow \operatorname{Al}(\ell)+4 F^{-}(f u s)\)
  • \(2 \mathrm{O}^{2-}(f u s)+\mathrm{C}(s) \rightarrow 4 \mathrm{e}^{-}(C)+\mathrm{CO}_{2}(g)\)
  • \(2 \mathrm{Al}_{2} \mathrm{O}_{3}(s)+3 \mathrm{C}(s) \rightarrow 4 \mathrm{Al}(\ell)+3 \mathrm{CO}_{2}(g)\)
  • (fus) = smelte / fused
  • Explaining this figure is typical for exam

Chloralkali industry

  • NaCl(aq) + electricity
  • Products: Cl\(_2\), H\(_2\), OH\(^-\)
  • OH\(^-\)(NaOH) recycled
  • Role of membrane: Not mixing NaOH + Cl\(_2\)
  • Many further products
    • OCl\(^-\), ClO\(_3^-\), ClO\(_4^-\)
  • Anode: \(\quad 2 \mathrm{Cl}^{-}(a q) \rightarrow 2 \mathrm{e}^{-}+\mathrm{Cl}_{2}(g)\)
  • Cathode: \(\quad 2 \mathrm{H}_{2} \mathrm{O}(\ell)+2 \mathrm{e}^{-} \rightarrow \mathrm{H}_{2}(g)+2 \mathrm{OH}^{-}(a q)\)

Organic electrosynthetic chemistry

  • Relatively new field
  • Nylon: Hydrogen step on Cd electrode; acrylonitride diaponitrile
  • \(2 \mathrm{CH}_{2} \mathrm{CHCN}+2 \mathrm{H}_{3} \mathrm{O}^{+}+2 \mathrm{e}^{-} \rightarrow \mathrm{NCCH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CN}+2 \mathrm{H}_{2} \mathrm{O}\)

Electrolysis of water

  • Cathode(negatrode)
  • Electrolysers:
    • Alkaline(OH\(^-\)) and PEM(H\(_3\)O\(^+\))
    • NEL (Norway) one of the Worlds leading suppliers
      • Originated from Norsk Hydros electrolysers for ammonia production
    • Medium sized to large alkaline electrolysers
    • Small to medium sized PEM electrolysers also exist.

Steam electrolysis

  • Norway has experties in solid-state high-temp (ceramic) electrolytes
  • Enables electrolysis of steam rather than liquid
  • Thermodynamically more beneficial if steam and/or heat is free and available
  • Main features of PEM, proton ceramic, and solid oxide electrolysers:

CO2 + H2O co-electrolysis

By Oxygen ion membrane, it can become CO + H\(_2\) + O\(_2\)

Electrodialysis

  • NaCl solution inflow splin in denunded and concentrated outflows:

Summary

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