Berkeley team develops host-guest nanowires for efficient water splitting and solar energy storage
04 February 2016
Although metal oxides that absorb visible light are attractive for use as photoanodes in photoelectrosynthetic cells, their performance is often limited by poor charge carrier transport. Researchers from UC Berkeley and colleagues have now addressed this issue by using separate materials for light absorption and carrier transport.
The team reports on their host-guest system of Ta:TiO2|BiVO4 as a photoanode for use in solar water splitting cells in an open-access paper in the journal ACS Central Science. BiVO4 acts as a visible light-absorber and Ta:TiO2 acts as a high surface area electron conductor. The host–guest nanowire architecture allows for simultaneously high light absorption and carrier collection efficiency for efficient solar water oxidation.
Harnessing energy from sunlight is a means of meeting the large global energy demand in a cost-effective and environmentally benign manner. However, to provide constant and stable power on demand, it is necessary to convert sunlight into an energy storage medium. An example of such a method is the production of hydrogen by photoelectrochemical (PEC) water splitting.
The direct splitting of water can be achieved using a single semiconductor; however, due to the voltage requirement of the water splitting reaction and the associated kinetic overpotentials, only wide-band-gap materials can perform overall water splitting, limiting the efficiency due to insufficient light absorption. To address this issue, a dual-band-gap z-scheme system can be utilized, with a semiconductor photoanode and photocathode to perform the respective oxidation and reduction reactions. This approach allows for the use of lower-band-gap materials that can absorb complementary portions of the solar spectrum and yield higher solar-to-fuel efficiencies. In this integrated system, the charge flux is matched in both light absorbers of the photoelectrochemical cell. Therefore, the overall performance is determined by the limiting component. In most photoelectrosynthetic cells, this limiting component is the semiconductor photoanode.
—Resasco et al.
Many materials that can perform the reaction exist, but most of these candidates suffer from issues, ranging from efficiency to stability and cost. The Berkeley system uses TiO2 nanowires as the host for guest nanoparticles from BiVO4.
BiVO4 is a newly introduced material that is among the best ones for absorbing light and performing the water splitting reaction, but does not carry charge well while TiO2 is stable, cheap and an efficient charge carrier but does not absorb light well.
Together with a unique studded nanowire architecture, the new system works better than either material alone. The authors state their approach can be used to improve the efficiencies of other photoconversion materials.
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| Schematic of the photoanode architecture. The nanowire morphology provides an increased path length for absorption of visible photons by BiVO4, as well as a pathway for efficient electron transfer. The small size of the BiVO4 particles maintains close proximity of the semiconductor liquid junction for holes to carry out the oxygen evolution reaction. Type II band alignment allows electron transfer from BiVO4 to TiO2. Credit: ACS, Resasco et al. Click to enlarge. |
The authors acknowledge funding from the Department of Energy, the National Science Foundation and the University of California, Berkeley.
Resources
Joaquin Resasco, Hao Zhang, Nikolay Kornienko, Nigel Becknell, Hyunbok Lee, Jinghua Guo, Alejandro L. Briseno, and Peidong Yang (2016) “TiO2/BiVO4 Nanowire Heterostructure Photoanodes Based on Type II Band Alignment” ACS Central Science doi: 10.1021/acscentsci.5b00402
Regardless of how transport is powered, whether by BEVs with very large batteries, or FCEV's, or PHEV FCEV's, it is becoming increasingly obvious that much of the power will come from hydrogen or liquid fuels produced from renewables and using electrolysis or direct from solar, as well as wind.
That does not apply so much to the tropics, where sunshine is year round, but to anywhere with a winter.
I quite agree with EP that we should be producing a lot of our power by nuclear, but that is not on the table as in the West at least they are not building it.
So hydrogen and its derivatives are going to play a very big part as an energy vector, regardless of the choice to power transport.
Posted by: Davemart | 04 February 2016 at 06:58 AM
@DM...E-P will not like your comments but regardless of his very strong pro-NPPs attitude, H2 could become a major energy storage/carrier for REs, in the not too distant future.
Of course FCEVs and FC/PHEVs are progressively becoming more interesting as a way to reduce fossil and bio-fuels consumption, reduce GHGs and pollution.
Cost of REs and H2 production/storage may come down faster than batteries' cost and certainly faster than NPPs cost.
Posted by: HarveyD | 04 February 2016 at 08:34 AM
"E-P will not like your comments..."
Don't be cowed by a narcissist bully.
Posted by: SJC | 04 February 2016 at 11:10 AM
I feel sorry for narcissists.
They don't realise how unlucky they are not to be me.....;-)
Posted by: Davemart | 04 February 2016 at 12:25 PM
From my perch on the fence I might not be able to get it right but it makes it easy to tell you when you get it wrong.
So I'll reserve my seat and suggest (from the sidelines) that according to Murphy's Law there is plenty of fodder coming.
There will be more of this:
www.greencarcongress.com/2016/02/20Study finds nanoparticle NMC material used in Li-ion batteries harms key soil bacterium160204-nmc.html#comments
High pressure H2 will present problems when scaled up as we find common to safety standards degrading for various complacency (as in the nuclear power industry), Increasing numbers of lower skilled technicians combined with increasing infrastructure complication, simple numerical scaling leading to greater oportunity.
System installation and maintenance requires large numbers of skilled? semi skilled personnel where system design only needs a small team. It may look good on paper but the reality in the field is always very different.
The old joke goes thus:
A new employee arrives for their first day on the job. The boss says that the floor is dirty and needs sweeping.
The new employee says "but I'm a university graduate here to help save the company from failing owing to new technology challenges."
The boss replies " Oh I'm sorry I didn't realise."
"Here I'll show you how it's done."
Posted by: Arnold | 04 February 2016 at 02:13 PM