Energy conversion from solar to chemical energy is one of the important technologies to store solar energy. The combination of solar cell (SC), which converts solar energy to electricity, and electrochemical cell (EC) for water split to hydrogen and oxygen, which converts electricity to chemical energy, is most expected conversion technique from solar to chemical energy currently. The conversion has not been used widely due to the difficulty of the energy balance matching between a SC and an EC. The simplest electric connection is direct series connection of a SC and an EC. However, it is impossible using typical Si-SC to convert the energy because the required voltage for water splitting EC is usually over 1.5 V whereas the maximum voltage of Si-SC is around 0.65 and 0.70 V. From the operating voltage point of view, In_xGa_1-xP/In_yGa_1-yAs/Ge type three-tandem concentrated photovoltaic solar cell (CPV) is a suitable device because the maximum voltage is over 2.4 V. However, the voltage difference, which is the energy loss for the conversion, is large when the EC is operated by the CPV. One of the simple ways for the voltage matching is to control the number of the devices. This optimization is simple but effective. When the In_xGa_1-xP/In_yGa_1-yAs/Ge type three-tandem CPV and polymer electrolyte electrochemical cell (PEEC) were used, the maximum energy conversion efficiency was almost 15% when the 2-CPVs with 100 times concentration (200 cm² of solar illuminated area) and 3-PEECs were directly connected.

For the direct connection of CPV and PEEC, the lowest energy conversion process was the solar to electricity conversion by CPV. The next was the energy difference between the operating voltage of PEEC and the Gibbs energy of the hydrogen, which is the energy storage amount in hydrogen. The operating voltage includes that water oxidation (oxygen evolution) over potential and water reduction (hydrogen evolution) over potential. The over potential for water oxidation is much larger than that for water reduction, thus, lowering the water oxidation over potential is important issue for the electrochemical water splitting. For this purpose, knowing the water oxidation process is required but it is not well understood currently. This is one of the reasons to study the water oxidation at the interface of n-type GaN surface and an electrolyte.

The n-type GaN is suitable to study the process photoelectrochemically because it is chemically stable. However, even the n-type GaN shows anodic corrosion under photoelectrochemical oxidation condition. Thus, we need to suppress the photoelectrochemical anodic corrosion before the discussion of the water oxidation by n-type GaN. The experimental results showed that the electrolyte was one of the key, that is, the time dependence of photocurrent was relatively stable in NaOH aqueous solution. It was also found that some point defects related to Si-doping played important roles for the anodic oxidation of GaN, since the lower Si-doping GaN showed the more stable photocurrent time dependence. Thus, we evaluated the water oxidation process with using n-type GaN in NaOH aqueous electrolyte. From the photoelectrocemical process comparing with photoluminescence (PL), intermediate hole trap in GaN was found to be related to the water oxidation. The unique characteristic of the hole trap is very long life time of the excited hole. This long life time is believed to be the motive force of the chemical reaction of water oxidation. The NiO loaded n-type GaN shows very stable for the photoelectrochemical water oxidation process. The results of NiO-loading are also discussed.

Speaker(s): Katsushi Fujii,

Location:
Room: Room 603
Bldg: McConnell Engineering Building
3480 rue University
Montreal, Quebec
H3A0E9