Electrolysis

1
Electrolysis
General RD Needs
Renewable Energy H2O
Intermittent Base load Geothermal

• Commercial Large and Small plants
• alkaline systems and PEM
• RDD Components
• PEM and high temperature electrolysis to increase
  efficiency
• High cost today
• Engineered systems for lower cost
• Engineered electrolysis/renewable
  energyproduction systems
• Materials issues, more at high temperature

Hydrogen
2
High(er) Temperature Electrolysis
Only makes sense if.

• The heat is free and it takes a minimum amount of
  energy to move it were you want to use it.
• Ability to use materials with better properties.
• Catalysts properties are greatly improved
  enhanced kinetics, lower overvoltages.
• Membranes with higher conductivity.
• Lower cost electrodes with longer lifetimes.

3
Reversible PEM Fuel Cell Electrolyzer and Fuel
Cell Combined

• Concept is great, but the materials issues are
  different.
• Carbon is OK as a catalyst support for a fuel
  cell, but cannot be used on the oxygen evolving
  side of the of the electolyzer (CO2).
• For the best efficiency, the catalysts are
  different Pt vs. Ir and Ru.
• Electrodes are different C or SS vs. Ti
• Likely to always be a higher cost, lower
  efficiency option.

4
Photolysis - Water Splitting with visible light
RD Needs
Renewable Energy H2O
Longer term RD Issues Materials fundamental
understanding algal/bacterial/photoand chemical
systems, hydrogen containment, engineering
researchfor photoreactors. Maintain awareness of
economicsand life cycle
Hydrogen
5
Band Edges of p- and n-TypeSemiconductors
Immersed in Aqueous Electrolytes to Form Liquid
Junctions
E
conduction
p-type


O
2H
O
2e
2OH
H
2
2

2
E
valence
E
conduction


H
H
O
2h
2H
1/2 O
n-type
2
2

2
E
valence
6
Technical Challenges (the big three)Material
Characteristics for Photoelectrochemical Hydrogen
Production
Electron Energy

• Material Durability semiconductor must be
  stable in aqueous solution
• Efficiency the band gap (Eg) must be at least
  1.6-1.7 eV, but not over 2.2 eV
• Energetics the band edges must straddle H2O
  redox potentials (Grand Challenge)

H2O/H2
E
g
1.23 eV

1.6-1.7 eV
Counter Electrode
H2O/O2
p-type Semiconductor
All must be satisfied simultaneously
i
7
Bandedge Energetic ConsiderationsOxides
T. Bak, J. Nowotny, M. Rekas, C.C. Sorrell,
International Journal of Hydrogen Energy 27
(2002) 991 1022
8
Gallium Indium Phosphide/Electrolyte System
Understanding semiconductor/electrolyte junctions
(-)
Eg 1.83 eV
E
CB


Energy
Band edges are 0.2-0.4 V too negative
p-GaInP
2
H2O/H2


Band edges are pH sensitive
E
F
E

VB
()


H2O/O2
9
Technical ChallengesEnergetics

• Grand Challenge
• Understanding the interface and its influence on
  the energetics of the semiconductor bandedges and
  on the electron/hole charge transfer processes.
• The Semiconductor/Electrolyte Interphase
• In contrast to metal electrodes, semiconductor
  electrodes in contact with liquid electrolytes
  have fixed energies where the charge carriers
  enter the solution. So even though a
  semiconductor electrode may generate sufficient
  energy to effect an electrochemical reaction, the
  energetic position of the band edges may prevent
  it from doing so. For spontaneous water
  splitting, the oxygen and hydrogen reactions must
  lie between the valence and conduction band
  edges, and this is almost never the case.

10
Technical Challenges (Cont.)

• Catalysts
• Oxygen (most important -- highest energy loss).
• Hydrogen
• Transparency might be necessary
• Non-precious metal (lower current density!)
• Band edge engineering
• Semiconductor hybrid designs
• Low cost system designs featuring passive controls

11
High-Throughput Discovery of New and Optimized
Metal Oxide Photocatalysts Eric W. McFarland
(PI), Tom Jaramillo, Sung-Hyeon Baeck, Alan
KleinmanDept. of Chemical Engineering,
University of California, Santa Barbara
Tungsten-Molybdenum Mixed Oxides