NX Fuels (dba Carbon Fuels) develops photochemical water splitting technology based on Indium gallium nitride (InGaN) nanowires that can achieve efficient and stable solar water splitting for hydrogen generation.
Challenges: hydrogen fuel
Hydrogen (H₂) is a “simple solution” to address the world’s energy problems and needs. For large scale solar-fuel production, one-step solar water (H₂O) splitting for hydrogen generation is a simple, low-cost method that can use nearly neutral pH water, such as sea water.
In this photochemical water splitting method, the most promising material is InGaN photocatalysts, whose band gap energy can be tuned across nearly the entire solar spectrum and which straddle the water redox potentials under ultraviolet, visible, and near-infrared light irradiation. In addition, InGaN has excellent chemical stability. Consequently, InGaN promises highly efficient and stable water splitting.
However, the efficiency of water splitting with InGaN remains extremely low, primarily due to the unbalanced extraction/collection of charge carriers. When InGaN photocatalysts are in contact with water, fermi-level pinning results in semiconductor surface band bending, which creates an additional energy barrier for charge carrier transport to the photocatalyst-water interface, leading to significantly reduced reaction rate, extremely low efficiency, and corrosion of InGaN. Although the presence of surface band bending is considered advantageous for photoelectrochemical water splitting in which oxidation and reduction reactions occur at separate electrodes, it should be minimized for photochemical water splitting in order to achieve balanced, efficient, and stable redox reactions.
NX Fuels Technology
NX Fuels develops one-step photochemical water splitting technology based on arrays of magnesium (Mg)-doped InGaN nanowires with favorable surface band bending. The Mg dopants reduce hole depletion and an electron accumulation in a near-surface region of the InGaN nanowires, resulting in a more balanced redox reaction for water splitting, thereby enhancing the photocatalytic efficiency and stability of the InGaN nanowires.
Grow InGaN nanowire array
Vertically aligned, catalyst-free multi-stack GaN:Mg/InGaN:Mg nanowire arrays are grown on Si(111) substrates using radio frequency plasma-assisted molecular beam epitaxy (MBE) under nitrogen rich conditions without using any foreign catalyst. Prior to loading into the MBE chamber, the Si(111) substrate is rinsed with acetone and methanol to remove organic contaminants and subsequently with 10% hydrofluoric acid (HF) to remove native oxide. In situ oxide desorption is performed at 770 ºC until the formation of a clean Si(111) reconstructed surface. A thin, monolayer of gallium (Ga) seeding layer is in situ deposited, which promotes the nucleation of nanowires. Thermal effusion cells were used for the Ga, indium (In), and Mg sources. Nitrogen radicals are supplied from a radio frequency plasma source.
The growth parameters include nitrogen flow rate of 1.0 standard cubic centimeters per minute, a forward plasma power of 350W, a Ga beam equivalent pressure (BEP) of 6 × 10⁻⁸ Torr, and an In BEP of 8 × 10⁻⁸ Torr. The Mg effusion cell temperature is optimized to be 200 ºC. The growth temperature for GaN is 750 ºC and the growth temperature for InGaN is in the range of 640 ºC to 680 ºC.
Instead of forming InGaN nanowires directly on Si substrate, a GaN nanowire template is used, resulting in the controlled formation of InGaN nanowires with superior structural and optical properties. In order to minimize non-radiative recombination resulting from misfit dislocations, three segments of InGaN ternary wires are incorporated along the growth direction of GaN nanowire. The InGaN/GaN nanowire segments are doped with divalent Mg²⁺ ions as p-type dopants by controlling the effusion cell temperature of Mg at optimized 200 ºC. The GaN nanowire template is left undoped. As shown in the figure below, the nanowires have an average height of 400-600 nm, a lateral size of 40-100 nm, and an areal density in the range of 1 – 1.5 × 10¹⁰ cm⁻².
Dop InGaN nanowires
The Mg dopant in InGaN nanowires has significant effect on the photocatalytic performances, attributable to the tuning of the near surface band-bending by altering the surface Fermi-level (EFS) position relative to the surface valence band edge (EVS). At a low Mg cell temperature (below 200 ºC), the nanowire surface is barely doped (almost intrinsic) compared to the bulk, as a result of the surface desorption of Mg atoms, which creates a large downward band-bending towards the surface, as depicted on the left side of the figure below. This band-bending is further subject to change when the nanowires are in equilibrium with solution and under photo-excitation. The downward band bending may accelerate proton (H⁺) reduction at the nanowire surface, but it elevates the barrier for hole (h⁺)-diffusion and impedes the hole transport towards the nanowire-water interface.
With increasing Mg incorporation along with increased Mg cell temperature, the surface of GaN nanowires can be transformed to be p-type, due to the dopant segregation effect. This significantly reduces the downward band bending in the near-surface region. Consequently, at optimal doping level with Mg cell temperature of 200 ºC, the transport of photogenerated electrons (e⁻) and holes (h⁺) to the surfaces can be maximized. Due to accelerated hole transport, water oxidation, the limiting step for overall water splitting, is enhanced. This results in a more balanced redox reaction for water splitting, which enhances the photocatalytic efficiency and stability of the nanowires.
It is observed that the photocatalytic performance of the nanowires decreases at high doping level with a higher Mg cell temperature, due to the degradation of nanowire crystalline quality and the resulting enhanced non-radiative recombination. Besides, when Mg concentration is high, nitrogen vacancy-related defects increase. These defects function as n-type dopants and counteract p-type Mg doping.
InGaN nanowires for hydrogen generating
Cocatalysts of rhodium (Rh)/chromium oxide (Cr₂O₃) core-shell and cobalt oxide (Co₃O₄) nanoparticles are photodeposited on Mg doped InGaN nanowire surfaces to promote H₂ and O₂ evolution.
A 4 cm × 4 cm photocatalyst wafer is first stabilized on a Teflon holder. Then the holder is transferred to the 390-ml chamber containing 50 ml of 20 vol% methanol aqueous solution. Then 125 μl of 0.2 mol l⁻¹ sodium hexachlororhodate(III) (Na₃RhCl₆) is added into the methanol aqueous solution. The chamber is covered by a quartz cover and vacuumized. After that, the chamber is irradiated under a 300-W Xe lamp for 10 min. After reaction, 125 μl of 0.2 mol l⁻¹ potassium chromate (K₂CrO₄) is injected into the chamber and the chamber is irradiated for another 10 min.
Similarly, 125 μl of 0.2 mol l⁻¹ cobalt(II) nitrate hexahydrate (Co(NO₃)₂·6H₂O) is also injected into the chamber and then irradiated for 20 min. Finally, the obtained photocatalyst wafer is washed by deionized water and dried at 80 °C in air. It is noted that the deposited metallic cobalt (Co) nanoparticles in photoreduction can be readily oxidized in air, which were finally converted into Co₃O₄ nanoparticles.
The figure below depicts the overall water splitting reaction mechanism of the InGaN nanowire coated with cocatalysts.
The oxidation of water occurs on the more active Co₃O₄ nanoparticle sites to generate oxygen gas (O₂) and protons (H⁺). The produced protons diffuse toward the Rh/Cr₂O₃ core/shell catalyst sites in order to take part in reduction reaction to produce H₂. The Rh core nanoparticles can provide more active sites for water reduction, whereas the Cr₂O₃ shell layer effectively prevents any backward reaction to form water. The Rh/Cr₂O₃ core-shell catalyst also suppresses carrier recombination, thereby enhancing charge extraction.
NX Fuels Patent
- US11484861B2 Methods and systems relating to photochemical water splitting
NX Fuels Products
As shown in the following video, a 4 cm × 4 cm InGaN nanowire photocatalyst wafer submerged in a water-filled chamber operated for hydrogen production under concentrated sunlight.
A 1.1 m × 1.1 m Fresnel lens was used to produce about 16,070 mW cm⁻² of concentrated solar light on an 8 cm × 8 cm region where the photocatalyst wafer was installed. The chamber’s temperature reached 70 ºC, which was directly achieved by harvesting the Sun’s wasted infrared light. The semiconductor was specifically designed to withstand high temperatures without degrading. The results showed that this method has achieved 6.1% efficiency in converting water to hydrogen.
NX Fuels Funding
In October 2022, NX Fuels was granted $60,000 by The U.S. Department of Energy (DOE) for developing its photochemical water splitting technology.
NX Fuels Investors
NX Fuels Founder
Zetian Mi and Saemin Choi are Founders.
NX Fuels CEO
Saemin Choi is CEO.
