Researchers develop free-standing nanowire mesh for direct solar water-splitting to produce H2; new design for “artificial leaf”
03 December 2014
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| The mesh with BiVO4 nanowire photoanode for water oxidation and Rh-SrTiO3 nanowire photocathode for water reduction produces hydrogen gas without an electron mediator. Credit: ACS, Liu et al. Click to enlarge. |
Researchers from UC Berkeley, Lawrence Berkeley National Laboratory and Nanyang Technological University, Singapore have developed a new technology for direct solar water-splitting—i.e., an “artificial leaf” to produce hydrogen—based on a nanowire mesh that lends itself to large-scale, low-cost production. A paper describing their work is published in the journal ACS Nano.
In the design, semiconductor photocatalysts are synthesized as one-dimensional nanowires, which are assembled into a free-standing, paper-like mesh using a vacuum filtration process from the paper industry. When immersed in water with visible light irradiation (λ ≥ 400 nm), the mesh produces hydrogen gas. Although boosting efficiency remains a challenge, their approach—unlike other artificial leaf systems—is free-standing and doesn’t require any additional wires or other external devices that would add to the environmental footprint.
To develop an efficient solar water splitting system, it is necessary: (1) to develop semiconductor materials which absorb in the visible region of solar spectrum; (2) to design architectures for effective capture and conversion of sunlight, at the same time, allowing easy transport of protons and gas products; (3) to develop robust ion-conducting membranes, which are impermeable to the gas products; and (4) to integrate each individual component into a complete and functioning system.
In the present study, we developed a new architecture for direct solar water-splitting. In this design, semiconductor photocatalysts were synthesized as one-dimensional nanowires, which were assembled into free-standing, paper-like mesh for solar water-splitting. The large aspect ratio of semiconductor nanowires allows for the formation of intertwined and porous nanowire networks. The porous structure of nanowire mesh networks can benefit photochemical reactions by decoupling directions for light absorption and charge carrier extraction as well as providing a large area of catalytic surfaces. Furthermore, the porous structure can also facilitate proton transport and gas evolution.
—Liu et al.
As a proof-of-concept, they used BiVO4 and Rh-SrTiO3 nanowires for overall water-splitting. The BiVO4 nanowires act as a photoanode for water oxidation and the Rh-SrTiO3 nanowires work as a photocathode for water reduction.
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| Photoelectrochemical overall water splitting over linked Rh-SrTiO3 and BiVO4 photoelectrodes without applying any external bias under visible light irradiation. Dashed line: half amount of electrons which had passed through the external circuit of linked photoelectrochemical cell; (■) hydrogen evolution rate and (●) oxygen evolution rate. Insets show photocurrent versus time of externally short-circuited Rh-SrTiO3 and BiVO4 photoelectrodes (left) and schematic of a Rh-SrTiO3 and BiVO4 photoelectrolysis cell system for overall solar-driven water splitting (right). Credit: ACS, Liu et al. Click to enlarge. |
In their experiments, they first made two photoelectrodes of BiVO4 and Rh-SrTiO3, then loaded the co-catalysts (CoOx for BiVO4 and 1 wt % Ru for Rh-SrTiO3) on the surface.
The team selected Ru as the co-catalyst instead of Pt because Ru is an effective co-catalyst for hydrogen evolution that does not enhance back-reaction for water formation from evolved H2 and O2.
They assembled two types of nanowire mesh films, including mixed Ru/Rh-SrTiO3 and BiVO4 nanowire mesh film and bilayer Ru/Rh-SrTiO3 and BiVO4 nanowire mesh film. Prior to photoelectrochemical testing, the nanowire mesh films were annealed at 500–800 °C in argon to promote good contact between the nanowires.
Under testing, the resulting cells split water into H2 and O2 in a stoichiometric ratio without using any electron mediator.
The total evolved H2 and O2 was ∼4.5 μmol, which corresponds to an overall solar-to-fuel conversion efficiency of 0.0017%. Photoactivity depended on the relative amount of Ru/Rh-SrTiO3 to BiVO4. The highest photoactivity was obtained using mixed nanowire mesh film assembled from equal amounts of Ru/Rh-SrTiO3 and BiVO4 nanowires.
The study was supported by the US Department of Energy’s (DOE) Office of Basic Energy Sciences and the Singapore-Berkeley Research Initiative for Sustainable Energy (SinBeRISE).
Resources
Bin Liu, Cheng-Hao Wu, Jianwei Miao, and Peidong Yang (2014) “All Inorganic Semiconductor Nanowire Mesh for Direct Solar Water Splitting” ACS Nano 8 (11), 11739-11744 doi: 10.1021/nn5051954
Still gotta compress the hydrogen to 10,000 psi which takes additional energy. If you think fuel explosions and fuel fires are dangerous now, wait until you install a 10,000 psi hydrogen tank in every car.
Posted by: Lad | 03 December 2014 at 11:06 AM
@Lad:
Presumably you were one of those who was also too terrified of battery fires in electric cars to use them, and certainly would not dream of plugging them in to charge them in the rain as you would certainly be electrocuted.
The science behind that is just as sound as what you are advancing here.
Posted by: Davemart | 03 December 2014 at 01:10 PM
The efficiency would have to be multiplied many times before it becomes a reality?
Posted by: HarveyD | 03 December 2014 at 02:31 PM
@Harvey:
Some of the other techniques for solar to hydrogen are way, way more efficient.
It is a case of getting them all together, efficiency, durability and so on in one package.
When and if is not clear yet, but there is swift progress being made on many fronts.
Posted by: Davemart | 03 December 2014 at 03:07 PM
>>The total evolved H2 and O2 was ∼4.5 μmol, which corresponds to an overall solar-to-fuel conversion efficiency of 0.0017%
This is the key result. Efficiency has to be better than 20% or else solar panels are simply better.
Good research, though. If the H2 leaves become as efficient as sugar cane at 8% conversion efficiency we might start to talk.
Posted by: Jus7tme | 03 December 2014 at 03:28 PM
There is something wrong in that article. Hydrogen when use in big quantity and when use as soon as it is produced is way efficient as a fuel. In the future we can build an efficient hydrogen infrastructure where hydrogen is distributed in small hose to every factories and houses. This system is in direct competition with batteries and windmills and solar panels installed and connected everywhere. To date we don't know which system is cheaper and more efficient.
The problem is producing energy without petrol, coal, foods, land, wood, everything that is polluting costly and in limited quantity and not renewable.
This is just a study and it seam to not be the holy grail unfortunately. But I have more hope in hydrogen technology than in battery for the long term. If we invent or discover a way to produce hydrogen chemically with catalysts efficiently then it will probably be a better method than batteries but anyway today batteries and hydrogen are costlier and less efficient than petrol and recently petrol prices are getting lower but someday the awakening of industrialize peoples could be dramatic if petrol flow suddently stop.
Posted by: gorr | 04 December 2014 at 08:52 AM
The latest record for a four layer solar cell is 46% efficiency, you have to concentrate but with a cold mirror you get heat and electricity. A high temperature electrolyzer can reach 80% efficiency, so you are close to 40% with sun.
Posted by: SJC | 04 December 2014 at 12:33 PM
With all this hydrogen tech taking off I wonder if storing hydrogen in a green liquid form such as the University of Glasgow professor did is the answer? He stored it into a liquid-based inorganic fuel. A redox mediator might be the answer. No compression and its supposed to be easily retrievable hydrogen.
Posted by: D | 05 December 2014 at 11:46 AM
I suspect this may be sensitive to water impurities.
Posted by: dursun | 17 December 2014 at 05:38 PM