Abstract:

In order to economically generate renewable hydrogen fuel from solar
energy using immersed semiconductor-based devices, the U.S. Department of
Energy Fuel Cell Technologies Office has established technical targets of
over 20% solar-to-hydrogen (STH) efficiency with several thousand hours of
stability under operating conditions [1].

We had to employ several key solid-state technological advances to achieve
STH efficiencies exceeding 16% [2]. The first improvement was to increase
the device photocurrent via extending the infrared absorption using a
non-lattice-matched 1.2 eV InGaAs junction, created by the inverted
metamorphic multijunction (IMM) technique developed by NREL’s III-V
photovoltaics group. The second modification was to add a thin n-GaInP2
layer to p-GaInP2 to generate a "buried junction", which increased the
open-circuit voltage of the device by several hundred mV and enabled 14%
STH efficiency. Finally, we increased the top junction photon conversion
efficiency by adding an AlInP "window layer", which is commonly used in
solid-state PV devices to reduce surface recombination. Through the use of
a collimating tube, we measured our devices outdoors under direct solar
illumination and verified over 16% STH conversion efficiency. I will also
identify experimental pitfalls that can influence the accuracy of measured
STH efficiencies, which can be exaggerated for mulitjunction absorbers.


In the second part of this talk I will discuss the challenges encountered
while scaling the IMM III-V absorber areas of from ~0.15 cm2 up to 16 cm2
and incorporating them in a photoreactor capable of generating 3 standard
liters of hydrogen in 8 hours under natural sunlight. To successfully
scale photo-electrochemical water-splitting technologies from bench to
demonstration size requires addressing predictable and unpredictable
complications. Despite using Comsol multiphysics to model our photoreactor
and identify suitable specifications for a prototype, several practical
issues were uncovered during testing that led to multiple iterations of
photoreactor design between the initial and final generation. Several
bottlenecks that ranged from counter electrode composition and orientation
to bubble removal needed redress in order to meet our performance targets.
Ultimately, the demonstration-scale system was able to generate nearly
twice the target volume of hydrogen in an 8-hour outdoor trial.

Bio-Sketch:

Dr. Deutsch has been studying photoelectrochemical (PEC) water splitting

since interning in Dr. John A. Turner’s lab at NREL in 1999 and 2000. He
performed his graduate studies on III-V semiconductor water-splitting
systems under the joint guidance of Dr. Turner and Prof. Carl A. Koval in
the Chemistry Department at the University of Colorado Boulder.

Todd officially joined NREL as a postdoctoral scholar in Dr. Turner’s
group in August 2006 and became a staff scientist two years later. He
works on identifying and characterizing appropriate materials for
generating hydrogen fuel from water using sunlight as the only energy
input. Recently, his work has focused on inverted metamorphic
multijunction III-V semiconductors and corrosion remediation strategies
for high-efficiency water-splitting photoelectrodes. Todd has been honored
as an Outstanding Mentor by the U.S. Department of Energy, Office of
Science nine times in recognition of his work as an advisor to more than
30 students in the Science Undergraduate Laboratory Internship (SULI)
program at NREL.