Transcript Hydrogen Gas Generation by Splitting Aqueous Water Using n

Hydrogen Gas Generation by
Splitting Aqueous Water Using nType GaN Photoelectrode with
Anodic Oxidation
Katsushi Fujii et al., Japanese Journal of
Applied Physics, Vol 44, No. 18, 2005,
pp 543-545
Presentation by Greg Brown (9-24-07)
Table of Contents
• Background on redox reactions
• Semiconductor/electrolyte interfaces
• Promise of using GaN
• Experimental setup
• Results
• Summary
Redox reactions can be shown using band
diagrams
Fermi level is halfway between reduced and
oxidized states.
2H2O + 2e- -> 2OH- + H2
O2 + 2 H2O -> 4OH- + 4H +
Splitting of states due to orientation of
solution. Polar water molecules can rearrange
and change the energy of the oxidized
molecule.
Putting electrolyte in contact with a
n-type semiconductor
If Ef > Ef(redox) electrons will leave
semiconductor and cause band
bending
Electric field in the depletion region
causes excited electron-hole pairs
(EHPs) to be separated
GaN should be a good
material to use in
photoelectrochemical cells
N-type GaN should cause
band bending
Before this paper, no
evidence of hydrogen
generation by GaN
J.W. Ager III, W. Walukiewicz, K.M. Yu, W. Shan, J.Denlinger, and J. Wu, Group III-nitride
materials for high efficiency photoelectrochemical cells, Mater. Res. Soc. Symp. Pric. Vol
884E, 2005.
Sample used in Experiment
Grown by Metalorganic Vapor Phase Epitaxy
E(AgCl/Ag) = 0.212V vs SHE
Light Illuminated area =
contact area with electrolyte
= 10mm diameter
Light source = 150W Xe-lamp,
tested with both AM0 and
AM1.5 filters.
Electrolyte = mol/L KOH (ph =
14)
Ohmic contact = Ti
(10nm)/Au (50nm)
Confirmed GaN band edges
using electrochemical
impedance spectroscopy
No Hydrogen gas generation
when bias = 0V
Need roughly 1mA/cm^2 to
observe hydrogen generation
Need bias of 1V to get
generation
Photocurrent between -0.8 to
0V due to “affected by the
charge accumulation the
semiconductor/electrolyte
interface due to the slow charge
transfer process”
After 300min at 1V bias, gas
collected = 94% Hydrogen, 6%
Nitrogen, trace Oxygen
First observation of hydrogen
generation
Using Faraday’s law and the
equation
Hydrogen volume is calculated
Difference attributed to hydrogen
being dissolved in electrolyte
GaN was etched by anodic oxidation
Lost 1.98mg after 300min. Charge
required is 6.8C, half the current
measured. Nitrogen gas collected
attributed to this process.
Summary
Hydrogen generation from GaN as photoelectrode observed
when applied bias = 1.0 V
Problems are weak photocurrent when no bias applied and
etching of GaN during process