Chemical Eng. Dept. Seminar Series

“Photoelectrochemical Water Splitting on III-V-Nitride Semiconductor Electrodes”

Todd G. Deutsch, National Renewable Energy Laboratory, Golden, CO.

ABSTRACT

Photoelectrochemical (PEC) hydrogen production has been considered the holy grail of electrochemistry for several decades.  Despite significant efforts in engineering known materials as well as new material discovery, no system currently has all the characteristics required to achieve a viable PEC device. There are three primary material requirements  that must be satisfied for a PEC device to work.  First, the band gap of the material must be wide enough to generate carriers of sufficient potential to decompose water (>1.6eV) while still absorbing visible light at a reasonable efficiency (<2.2eV).  Second, the material’s band edge energies must encompass the hydrogen and oxygen redox potentials at the semiconductor/solution interface.  Finally, the material must be stable for thousands of hours under operating conditions in aqueous solution.

Single crystal GaPN (2%N) epilayers were grown by metal organic chemical vapor deposition on n and p-type silicon substrates.  The band gap of the GaPN material was found to be 2.0 eV by photocurrent spectroscopy.  For the band edges to be aligned and allow water splitting, the flatband potential must be more positive than the oxidation potential of water.  The sample set grown on n-Si substrates has band edges that are too negative to allow direct water splitting.  The same epilayer composition grown on p-Si has flatband potentials that indicate photoelectrolysis should be possible across the whole pH spectrum. 

The potential boost that aligns the bands comes from a buried p/n Si junction that absorbs photons having energies below the band gap of the epilayer and above the band gap of Si.  This buried absorber is formed by the diffusion of phosphorous, an n-type dopant into the p-Si substrate during growth.  The result is a monolithic tandem device that has two light-absorbing layers.  By absorbing otherwise unused photons, sufficient photopotential is generated to allow water splitting with no applied bias. Cathodic photocurrent at short circuit conditions, accompanied by bubble evolution at the working and counter electrodes, confirms these materials are capable of PEC water splitting. 

Sample durability was assessed by operating electrodes as photocathodes for 24- hours.  Afterwards electrode surface etching was evaluated by stylus profilometry.  Gallium concentration in the durability solutions was also determined by ICP-MS.  Preliminary results indicate that the III-V nitride epilayer material is far more durable than GaInP2/GaAs tandem materials that are capable of highly efficient water splitting.  However, these novel GaPN/p-Si materials are still susceptible to corrosion. 

About Speaker: Dr. Todd G. Deutsch is a senior scientist at the hydrogen technologies & systems center at the National Renewable Energy Laboratory in Golden, Colorado. Dr. Deutsch received his PhD from University of Colorado in 2006 and worked with Dr. John A. Turner for his dissertation work.