Artificial Photosynthesis: Nanoscale Reactors Making Fuel from Water and Sunlight
Hydrogen fuel, with its potential as a clean energy source, has been the focus of extensive research. Not only can hydrogen be burned cleanly, with only water as its byproduct, but it’s also an incredible fuel. Hydrogen's energy density surpasses that of gas or diesel by almost threefold, meaning that you could drive three times as far in a hydrogen-powered vehicle as in a car using the same amount of gas. Recognized automakers like Toyota and Hyundai have already ventured into the market with hydrogen-powered vehicles. Yet, their widespread commercialization remains hampered by the current costly and fossil fuel-dependent hydrogen production methods.
The Ensembles of Photosynthetic Nanoreactors (EPN) Energy Frontier Research Center (EFRC) has developed an ingenious strategy to address both the cost concern and the fossil fuel dependence. Taking a cue from nature, scientists at EPN are building devices that rely on similar mechanisms that underpin plant photosynthesis. EPN devices harness the power of sunlight, utilizing it to extract hydrogen from one of the cleanest and most abundant resources on Earth—water. To achieve this extraction from water, EPN scientists are building nanoreactors, miniscule machines that absorb the sun’s energy and use it to make fuel. A schematic of these nanoreactors is shown in Figure 1. These reactors are 100,000 times smaller than a grain of rice! Despite their diminutive size, each reactor can convert water into hydrogen fuel without making byproducts apart from oxygen. A single chemical process can produce billions of these nanoreactors, working collectively to produce substantial amounts of hydrogen.
Nanoreactors, innovative devices for chemical reactions at a minuscule scale, consist of three key components. First, a light absorber functions like solar panels, capturing sunlight and converting it into usable energy. Next is a selective coating, akin to a gatekeeper, which precisely controls the movement of chemicals and energy in and out of the reactor. The third component is a catalyst, a special molecule that's like a matchmaker in chemical reactions, speeding them up without being consumed. Each component is crucial for the nanoreactors' operation, comparable to different yet essential parts of a plant. For example, the light absorber can be thought of as the chloroplasts, the selective coating as the endoplasmic reticulum and Golgi apparatus, which regulate material flow within cells, and the catalyst as enzymes that facilitate biochemical reactions. The tiny size of these nanoreactors is key to their effectiveness. By designing billions of these tiny reactors instead of one large material, we significantly increase the surface area available for reactions. Imagine a cluster of tiny beads or grains: each bead represents a nanoreactor, offering a vast surface area in a compact space. In nanoreactors, most of the reactive material is thus exposed on the surface, readily available to absorb light and produce fuel. Furthermore, these nanoparticles possess unique properties not found in their larger counterparts. These properties enable them to absorb and use light energy more efficiently, much like how a finely tuned instrument can play a note more precisely than a larger, less refined one. This efficiency is crucial for maximizing the effectiveness of the nanoreactors in producing fuel from light.
Similar to how leaves in plants absorb sunlight, the light-absorbing nanocrystal in nanoreactors converts light energy into excited electrons, akin to electricity. These nanocrystals are remarkably efficient due to their high “absorption cross-section.” Despite their small size, they can absorb a comparable amount of solar energy as larger surfaces, a feature that contributed to their recognition with the 2023 Nobel Prize in Chemistry. Each reactor is also equipped with a selective coating, much like a gatekeeper. This coating allows only water to enter the reactor, while ensuring that only hydrogen fuel and oxygen can exit. In this way, the coatings serve a dual role, like the protective bark and water absorbing roots of a plant, safeguarding the reactors interior while managing the essential flow of materials. At the heart of each reactor is its catalyst, the key component that drives the transformation of water into hydrogen fuel, which involves extracting hydrogen (H2) from water (H2O). The catalyst requires energy to initiate this conversion, which it receives from the light-absorbing nanocrystal. If there is sunlight for the absorber to capture, the reactor can generate clean power. The catalyst then utilizes this energy to process water, filtered through the selective coatings, into clean hydrogen fuel.
In its early stages, EPN has focused on studying partial reactors to assess their efficiency. Using several techniques, including advanced microscopes, they have managed to study factors limiting the efficiency of the reactors.1 This guides future nanoreactor design. Moving forward, EPN scientists will work to understand every aspect of the reactor—the light absorber, the selective coatings, and the catalysts—including exploring their synergy. This concerted effort will deepen our understanding of these materials' fundamental properties and behavior and will help move us toward making clean hydrogen fuel at an economic scale.
(1) Zutter, B.; Chen, Z.; Barrera, L.; Gaieck, W.; Lapp, A. S.; Watanabe, K.; Kudo, A.; Esposito, D. V.; Bala Chandran, R.; Ardo, S.; Talin, A. A. Single-Particle Measurements Reveal the Origin of Low Solar-to-Hydrogen Efficiency of Rh-Doped SrTiO3 Photocatalysts. ACS Nano 2023, 17 (10), 9405–9414. https://doi.org/10.1021/acsnano.3c01448.
- Analysis of the photocarrier concentration within this research was supported as part of Ensembles of Photosynthetic Nanoreactors (EPN), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science under Award Number DE-SC0023431