Two inexpensive commercial materials store solar energy as electrons and release hydrogen fuel on demand in darkness, requiring no external power and setting a new performance record.
(Nanowerk Spotlight) Imagine a truck arriving at a factory in northern Norway during the polar night. It carries no compressed gas cylinders, no cryogenic tanks, just containers of liquid. Inside the factory, a technician adds a small amount of catalyst powder. Hydrogen gas begins bubbling off immediately, ready to feed a fuel cell or industrial process. The energy driving this reaction was captured from sunlight far away, in a region where sun is plentiful but demand is not.
This scenario remains speculative. No one has yet demonstrated that such a system can retain its stored energy over the weeks required for long-distance transport. But new research suggests the underlying chemistry may now be within reach.
Hydrogen burns cleanly, producing only water. As a fuel and industrial feedstock, it promises to decarbonize sectors from heavy transport to steelmaking. Yet two problems have stalled its wider adoption. Producing hydrogen from sunlight requires continuous illumination, and clouds and nightfall shut the process down. Storing or transporting hydrogen gas demands either compression to 700 times atmospheric pressure or liquefaction at −253 °C, introducing safety risks and infrastructure costs.
Nature long ago solved a related challenge. In photosynthesis, plants capture sunlight and store its energy in chemical intermediates. These intermediates later power reactions that synthesize sugars without further illumination. Scientists have pursued artificial versions of this two-step process, but most laboratory demonstrations of “dark photocatalysis” require elaborate molecular architectures, expensive custom catalysts, or external electrical inputs.
Polyoxometalates offer a promising workaround. These clusters of metal and oxygen atoms, typically built from tungsten or molybdenum, can reversibly accept and release multiple electrons, behaving like molecular batteries. Earlier research showed polyoxometalates could assist hydrogen production when coupled with electrochemistry. But every system using commercially available polyoxometalates still required an applied current to drive the reaction in darkness.
A paper published in Advanced Materials (“Solar Energy Storage in Polyoxometalate for On‐Demand Hydrogen Transportation and Evolution”) breaks this limitation. Researchers based primarily at Lanzhou University in China report a system that stores solar energy as electrons and releases them to produce hydrogen in complete darkness, with no external power. The work achieves a record hydrogen evolution rate among all dark photocatalytic systems reported to date, using only off-the-shelf materials.
The schematic diagram of a dark photocatalytic system composed of g-C₃N₄ and W₁₂. (Image: Reproduced with permission by Wiley-VCH Verlag) (click on image to enlarge)
The team combined graphitic carbon nitride (g-C₃N₄), a semiconductor photocatalyst, with ammonium metatungstate (W₁₂), a commercial polyoxometalate. The experiments took place in an aqueous solution containing 10 vol% methanol, which serves as a sacrificial reagent. The methanol consumes photogenerated holes, allowing electrons to accumulate in the polyoxometalate rather than recombining. This means the system does not split pure water; it requires an added hole scavenger to function.
When light hits the carbon nitride, it generates electron-hole pairs. Electrons transfer to neighboring W₁₂ clusters and remain stored after illumination ends. The solution shifts from pale yellow to deep blue as tungsten atoms accept electrons and drop from the +6 to +5 oxidation state.
Two factors explain why this pairing works. Under acidic conditions, amine groups on the carbon nitride surface pick up protons, giving the material a positive charge. W₁₂ clusters carry negative charges. Opposite polarities drive the components into tight electrostatic contact, positioning the polyoxometalate where it can efficiently intercept photogenerated electrons.
Energetics reinforce the partnership. The conduction band of g-C₃N₄ and the reduction potential of W₁₂ are well matched, with a relatively small driving force that enables efficient, spontaneous electron transfer. Among six polyoxometalates tested, W₁₂ showed the most favorable alignment with the carbon nitride band structure.
Releasing the stored energy proves simple. The researchers introduced a platinum-on-carbon co-catalyst into the darkened solution. Platinum provides active sites where protons combine with electrons to form hydrogen gas. Light harvesting, energy storage, and fuel generation thus occur in separate steps, at different times and potentially different locations.
After one hour of illumination with 420 nm LED light at 100 mW cm⁻², the system yielded 13.5 µmol of hydrogen during the subsequent dark phase. The maximum dark hydrogen evolution rate reached 3220 µmol g⁻¹ h⁻¹, higher than any previously reported dark photocatalytic system. Doubling both W₁₂ concentration and illumination time doubled hydrogen output, confirming that electron storage capacity sets the ceiling.
Outdoor tests confirmed practical viability. Under natural sunlight, the system delivered 954 µmol g⁻¹ h⁻¹ in darkness. This represents the first sunlight-driven dark photocatalytic hydrogen production without applied electrical bias.
Multiple techniques validated the mechanism. Photoluminescence measurements showed W₁₂ extends charge carrier lifetimes by giving electrons an escape route from recombination. X-ray photoelectron spectroscopy detected reduced tungsten binding energies under illumination, confirming electron uptake. Electron paramagnetic resonance identified W⁵⁺ species forming only when light struck the composite.
Durability held up over laboratory timescales. W₁₂ retained full function through five consecutive charge-discharge cycles, and UV-vis spectroscopy confirmed the polyoxometalate remained stable in solution over 12 hours. The carbon nitride showed no structural degradation.
Both materials are inexpensive and require no specialized synthesis. If future work can demonstrate that the charged suspension retains its stored electrons over longer periods, the approach could enable solar energy to be captured in sun-rich regions and transported as a stable liquid to locations where sunlight is scarce. At the destination, adding catalyst would release hydrogen on demand, sidestepping the hazards of high-pressure gas and the expense of cryogenic infrastructure.
The research demonstrates that simple electrostatic assembly of commercial components can store solar energy and convert it to hydrogen in complete darkness without external power. Whether this chemistry can scale to practical energy transport remains to be seen, but the fundamental barrier has been crossed.
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