SunHydrogen (formerly known as HyperSolar) 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 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.
- Semiconductor absorbers
At least one type of light-absorbing semiconductor material is present in the p-n junction or Schottky junction of the PAH. The PAH may contain two or more p-n junctions or Schottky junctions connected in series to generate a high photovoltage for water splitting.
Suitable p-type semiconductor materials include p-doped silicon (Si), tin(II) sulfide (SnS), zinc sulfide (ZnS), cadmium sulfide (CdS), cadmium selenide (CdSe), cadmium telluride (CdTe), copper(I) sulfide (Cu₂S), tungsten sulfide (WS₂), CuₓO, Cu₂ZnSnS₄ (CZTS), CuInₓGa₁₋ₓSe₂ (CIGS), gallium nitride (GaN), indium phosphide (InP), and silicon carbide (SiC).
Suitable n-type semiconductor materials include n-doped Si, indium(III) sulfide (In₂S₃), CdS, CdSe, CdTe, Cu₂S, WS₂, CuₓO, Cu₂ZnSnS₄,CuInₓGa₁₋ₓSe₂, GaN, InP, and SiC.
- Protective coating layer
A protective layer is applied to semiconductors to protect them from corrosion. The protective coating is transparent and uses suitable electrically insulating material, such as aluminum fluoride (AlF₃), aluminum oxide (Al₂O₃), silica (SiO₂), zirconium oxide (ZrO₂), titanium(IV) fluoride (TiF₄), zinc oxide (ZnO), and titanium dioxide (TiO₂).
The cathode adjacent to the semiconductor acts as an electrocatalyst, efficiently transferring electrons from the cathode bulk to promote the hydrogen evolution reaction. Suitable cathode materials include platinum group metals such as Au and Pt, transition metals, transition metal oxides (e.g. NiO), metal carbides (e.g. WC), metal sulfides (e.g. MoS₂), and electrically conducting carbon containing materials such as graphite, graphene, and carbon nanotubes.
The cathode has a selectively permeable barrier which allows only protons and molecular hydrogen to pass through. The barrier prevents the reduction of other species in the electrolyte that would decrease the efficiency of the PAH. The selectively permeable barrier materials include chromium (III) oxide (Cr₂O₃), Nafion membranes (made from sulfonated tetrafluoroethylene-based fluoropolymer-copolymers), hydrogen permeable polymers such as acrylics, and metal oxide hydrogen ion/molecule conductors comprised of mixtures of metal oxides such as WCrOₓ, and WZrCeOₓ.
The anode on the opposite side acts as an electrocatalyst to promote the oxygen evolution reaction. The anode materials include metals, metal oxides, their mixtures from metals such as Ru, Ag, V, W, Fe, Ni, Pt, Pd, Ir, Cr, Mn, Cu, Ti, metal sulfides (e.g., MoS₂), and electrically conducting carbon containing materials such as graphite, graphene, and carbon nanotubes.
The working mechanism of PAH is depicted in the figure below.
The semiconductors generate electrons (e⁻) and holes (h⁺) when they absorb light. Electrons transport to cathode electrocatalyst, which efficiently transfers electrons from the cathode bulk to reduce protons in the electrolyte and produce hydrogen gas, whereas holes transport to anode electrocatalyst, which efficiently transfers holes from the anode bulk to oxidize water and produce oxygen gas.
To improve the energy conversion efficiency of PAH, multiple interfaces within PAH are specifically designed.
The interface between the semiconductor and the protective structure is designed to minimize electron/hole recombination. For example, a thin layer of AlF₃ formed by atomic layer deposition serves as the interface layer to minimize surface electronic trap states.
The interface between the cathode (or anode) and the semiconductor is designed to minimize the resistance and charge recombination between the semiconductor and the electrode. For example, the interface between n-type silicon and the platinum cathode could be hydrogen terminated silicon, Si-H, prepared by treating the Si in dilute buffered HF solution with a layer of Ti to serve as an ohmic contact, i.e. n-Si/H/Ti/Pt.
The figure below depicts a PAH device with a honeycomb-shaped protective structure, such as porous anodic aluminum oxide (AAO) membrane. The pores of AAO membrane are filled with autonomous, isolating semiconductor absorbers (50-1,000 nm in length and 20-200 nm in diameter).
When dipped in electrolytes, the semiconductor is protected from corrosion by the pore wall. Anode and cathode electrocatalysts cap the top and bottom of the semiconductors to further reduce corrosion by separating the semiconductors from the electrolyte. The selectively permeable membrane on the cathode allows protons and hydrogen gas to transport but blocks the other species from reacting, thereby increasing the Faradaic efficiency for hydrogen production.
The figure below illustrates the PAH device fabrication process.
The process begins by forming an electrically conducting catalyst layer (such as Au) on one of the sides of the AAO membrane by physical vapor deposition, followed by the subsequent steps:
- Step 1: Atomic layer deposition is used to coat the surface of the AAO membrane with a protective layer of AlF₃;
- Step 2: Electrical deposition fills the pores with at least one semiconductor junction;
- Step 3: A second electrode material is deposited onto the exposed surface of the semiconductor material (Whether the second electrode material functions as a cathode or anode depends on the type of semiconductor material that is adjacent to the second electrode material.);
- Step 4: A selectively permeable membrane is applied to the cathode-facing surface;
- Step 5: At least a portion of the AAO is etched to form a porous structure with a larger surface area and greater transparency.
The figure below depicts SunHydrogen’s PAH system, which integrates PAH devices with a proton exchange membrane to produce hydrogen gas.
The system has a separator composed of a proton exchange membrane integrated with an array of PAH devices. With the separator, the housings form two chambers for housing electrolyte and gaseous products. Each chamber has an electrolyte inlet and gas outlet. The chamber’s face is transparent so that PAH devices can absorb light.
PAH devices absorb sunlight to perform water splitting. On one side of the separator, a hydrogen evolution reaction produces hydrogen gas, while on the other side, an associated oxygen evolution reaction generates oxygen gas. The proton exchange membrane advantageously prevents recombination of the co-products, reduces ion transfer ohmic losses associated with the redox reactions, and prevents the product gasses from mixing and forming a dangerous mixture with high explosion hazard.
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In 2019, SunHydrogen unveiled a prototype of the first generation and demonstrated 1,000 hours of continuous hydrogen production.
In 2022, SunHydrogen’s unveiled its most advanced prototype, which combines the catalysts, semiconductors, and membrane integration assembly. The prototype makes efficient use of sunlight to maximize hydrogen production during the day while consuming minimal water.
SunHydrogen is registered under the ticker OTCPINK:HYSR.
SunHydrogen is funded by TRITON FUNDS LLC.
Tim Young is CEO.