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Stanford solar tandem cell shows promise for efficient solar-driven water-splitting to produce hydrogen

Researchers at Stanford University, with colleagues in China, have developed a tandem solar cell consisting of an approximately 700-nm-thick nanoporous Mo-doped bismuth vanadate (BiVO4) (Mo:BiVO4) layer on an engineered Si nanocone substrate. The nanocone/Mo:BiVO4 assembly is in turn combined with a solar cell made of perovskite.

When placed in water, the device immediately began splitting water at a solar-to-hydrogen conversion efficiency of 6.2%—matching the theoretical maximum rate for a bismuth vanadate cell. Although the efficiency demonstrated was only 6.2%, the tandem device has room for significant improvement in the future, said Stanford Professor Yi Cui, a principal investigator at the Stanford Institute for Materials and Energy Sciences and senior author of an open access paper describing the work published in Scientific Advances.

… nanocone structures have been considered as one of the highly promising candidates for high-efficiency thin-film photovoltaics. However, few studies have explored nanocone-based PEC [photoelectrochemical] devices, particularly with porous photoactive layers on nanocone structures. In PEC cells, the photoactive layers deposited on the nanocone conductive substrates may not only enhance the light absorption of the photoactive material but may also maintain efficient charge separation and provide a large contact surface area at the electrode/electrolyte interface to promote the surface water oxidation process.

We report here a facile strategy for the deposition of an approximately 700-nm-thick nanoporous Mo-doped BiVO4 (Mo:BiVO4) layer on an engineered cone-shaped nanostructure and demonstrate that the unique photoanode achieves a remarkable water-splitting photocurrent at low applied voltage with the best-reported STH conversion efficiency to date. Our study presents the first successful case for realizing a thick nanoporous photoabsorption layer with highly efficient charge separation through the engineered cone-shaped nanostructure and solves the urgent issue concerning the incompatibility of light absorption capability with carrier transport length. The strategy of depositing photoactive materials on the engineered light-trapping architectures offers a new photoelectrode architecture for high-performance PEC water-splitting cells.

—Qiu et al.

Schematic illustration of the fabrication process of the conductive nanocone substrate and electron microscope images of Mo:BiVO4 on the nanocone substrate. (A) First, the glass/Si substrate was deposited with one close-packed monolayer of SiO2 to produce a mask for etching and then the shrinking stage was used to adjust the diameter and spacing of the SiO2 nanoparticles through a selective and isotropic RIE process. Second, a Si nanocone array structure was generated via Cl2-based RIE of the Si substrate. The subsequent third process is to oxidize Si nanocone arrays at high temperature in air. The fourth step is to prepare conductive nanocone substrates by coating one layer of Pt and another functional layer of SnO2. The final step is to deposit the nanoporous BiVO4 photoactive layer through a sol-gel process.

(B) Scanning electron microscope (SEM) images (60° tilting) of the final SiOx/Pt/SnO2 nanocone arrays.

(C) Cross-sectional SEM images of Mo:BiVO4 on the SiOx/Pt/SnO2 nanocone substrate. Some exposed nanocones were also marked in the figure. Scale bars, 500 nm. Qiu et al. Click to enlarge.

Bismuth vanadate is an inexpensive compound that absorbs sunlight and generates modest amounts of electricity—but is a poor conductor of electricity. To carry a current, a solar cell made of bismuth vanadate must be sliced very thin, 200 nanometers or less, making it virtually transparent. As a result, visible light that could be used to generate electricity simply passes through the cell.

To capture the sunlight before it escapes, Cui’s team used the silicon nanocones, each about 600 nanometers tall.

Nanocone structures have shown a promising light-trapping capability over a broad range of wavelengths. Each cone is optimally shaped to capture sunlight that would otherwise pass through the thin solar cell.

—Prof. Cui

To suppress photocorrosion under illumination, the researchers deposited an active OER catalyst of Fe(Ni)OOH on the Mo:BiVO4-absorbed layer using a facile two-step electrochemical deposition technique.

The photoanode delivered a “remarkable” photocurrent density of 5.82 ± 0.36 mA cm−2 at 1.23 V versus RHE. In tandem with a single perovskite solar cell, the photoanode produced a photocurrent of 5.01 mA cm−2, corresponding to the theoretical STH efficiency of 6.2%.

The tandem solar cell continued generating hydrogen for more than 10 hours, an indication of good stability, said Cui.


  • Yongcai Qiu, Wei Liu, Wei Chen2, Wei Chen, Guangmin Zhou, Po-Chun Hsu, Rufan Zhang, Zheng Liang, Shoushan Fan, Yuegang Zhang, and Yi Cui (2016) “Efficient solar-driven water splitting by nanocone BiVO4-perovskite tandem cells” Science Advances Vol. 2, no. 6, e1501764 doi: 10.1126/sciadv.1501764



Im still dreaming that someday someone will sell me solar gasoline at a better price than actual costly petroleum gasoline. I WANT to pay less for my gasoline, is it clear now ?


Its becoming clear now to any but those who wilfully refuse to see that hydrogen from renewables is rapidly becoming a reality, with all the consequences that has for fuel cells versus BEVs.

Sure, 6% or so is a lot lower than the 20% from solar pv, but unlike that it is storable and transportable, which is a different ball game.

When the renewable, price competitive, end-to-end H2 delivery system is a commercial reality, you can gloat, Davemart.

Until then, another day, another scientific paper.

I applaud the scientific development. Bravo! But by the time any commercially competitive H2 generation, distribution and dispensing system is ready for deployment (which will take another decade at least) there will be millions of EVs sold annually. Continental fast charging networks will be in place. And the need for liquid fuels for passenger vehicles will be in serious doubt.

That becomes more evident with every passing year.


ECI is basically correct. This is just a neat scientific development adding to our overall knowledge which is a good thing. However, it is very unlikely that it will result in low cost hydrogen from renewable energy.


Im still dreaming that someday someone will sell me solar gasoline

For me the great attraction of BEVs over FCEVs is I will NOT need someone to sell me fuel to keep my car running. I could generate the energy myself from solar panels on my roof and pay less than less for my "gasoline." Is it clear now?

I'm not saying hydrogen wont have its uses just that I wont be used.


Believe me, if you can't afford hydrogen made from 20% efficient cells feeding a separate 70% efficient electrolyzer, you won't be able to afford it from a 6.2%-efficient "artificial leaf".

Hydrogen has gone from being a way to patch over the massive unreliability of solar and wind power and become a religion.



You assume 6.2% efficient is the maximum possible, when that is simply an early figure.

You assume that it is efficiency which finally arbitrates, when it is cost, with only an indirect relation to efficiency.

So I can see no logic in your claims.


I'm assuming that a Rube Goldberg system that does two completely different and hard-to-combine functions in one unit is going to sacrifice other things, such as cost, efficiency or lifespan.  So far nobody's shown me to be wrong.  Why on earth WOULD you try mixing these things up, when you can get 25% PV * 70% electrolyzer for 17.5% throughput already... and have a lot more flexibility besides?  The "artificial leaf" can't charge a battery or run a heat pump.



This is one of a host of pathways to hydrogen, and only one of them has to work out well.


The thing that gets me about ideas like this is the tandem solar cell is placed in water - and works when light shines on it. So to produce lots of H2 you need a location with lots of sun and water. A desert with a lake?


If you could put them out in the middle of the ocean you might have something.  I still don't see why the two functions need to be combined in the same chip; it doesn't seem to gain anything, and subjects the PV part to chemical attack that could just as easily be isolated from it.


Exactly, there's no gain. And plenty of losses. For example, water attenuates light so you need to keep the cells close to the surface. Water evaporates under strong light so you need to contain it and the container is also going to block some light while adding to the cost.


Also, lets consider sun angle. Out in the middle of the ocean (or any open source of water) light is coming in from above but if the sun is not directly overhead you get reflection and refraction and an increase in attenuation.


Here is a large floating solar array in the UK:


Yes, but those are conventional PV panels sitting above the water, on mounts that allow tilting towards the sun.

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