SunHydrogen (formerly known as HyperSolar), an American cleantech company founded in 2009, develops photocatalytic water splitting technology that generates hydrogen gas using sunlight, semiconductor junctions, and any source of water, including seawater and wastewater.
Challenges: hydrogen fuel
Hydrogen (H₂) is a “simple solution” to address the world's energy problems and needs. Green hydrogen can be produced via electrolysis of water (H₂O) by using electricity converted from renewable energy sources, such as solar energy. Scientists have discovered that photocatalysts directly absorb sunlight and split water into hydrogen gas. Photocatalytic devices are Schottky or p-n junction-type photochemical diodes.
The Schottky-type photochemical diode is formed by the junction of a semiconductor and a metal immersed in an electrolyte. The semiconductor absorbs light and generates electrons (e⁻) and holes (h⁺). As depicted in the figure below, for n-type semiconductor (left), electrons move across the ohmic contact to the metallic layer, where they are injected through the metal/electrolyte interface into the electrolyte to drive a hydrogen evolution reaction (reduction reaction). Holes are injected through the semiconductor/electrolyte interface into the electrolyte to promote an oxygen (O₂) evolution reaction (oxidation reaction). The charge flows are opposite in p-type semiconductors (right).
In the p-n junction type photochemical diode, as depicted in the figure below, p-type and n-type semiconductors are stacked via an optimal metal layer. Both semiconductors form ohmic contacts with the metal. In p-type semiconductor, photogenerated electrons are injected through the semiconductor/electrolyte interface into the electrolyte to promote an hydrogen evolution reaction, while photogenerated holes move across the ohmic contact to the metallic layer, where they recombine with photogenerated electrons from the n-type semiconductor. The photogenerated holes in the n-type semiconductor are injected through the semiconductor/electrolyte interface into the electrolyte to promote an oxygen evolution reaction.
The commercialization of photochemical diodes for hydrogen production remains challenging. In addition to photochemical diodes’ low energy conversion efficiency, the semiconductors themselves react with substances in the electrolyte, leading to corrosion and deactivation of the semiconductor photochemical diodes.
SunHydrogen Technology
SunHydrogen develops more stable and efficient photochemical diodes, also known as photoelectrosynthetically active heterostructures (PAHs), for hydrogen production. SunHydrogen develops methods for fabricating PAHs devices that are capable of producing hydrogen on a commercial scale.
Photoelectrosynthetically active heterostructures (PAHs)
The structure of SunHydrogen’s PAH is depicted in the figure below.