There are so many papers on water splitting using TiO2 based photocatalysts. However, the yields of hydrogen production are very low. I think some compositions other than the trends need to be synthesised and tested...I would like to have your opinion on this and would like to welcome your views.
There are a lot of materials that are being looked at as many commenters above have indicated. I think many of these are interesting for scientific reasons and I'm all for studying them. However it is my opinion that many of the scientifically interesting materials are not interesting from a pragmatic point of view. That is, various properties including potential negative environmental impact (e.g. Cd-based) and more importantly limited efficiencies and economic considerations mean that they will be unlikely to impact the world on a large scale (not to say they won't find a niche application here or there.) If one wants to change the world on a large scale via this technology it is important to remember that we already know how to make H2 from sunlight at a reasonable efficiency if not cost. PV driven electrolyzers will do the job. So when thinking of new chemistries to explore we must ask ourselves is our goal to gain scientific insight that may one day be applied to make improvements (i.e. change the world indirectly), or do we expect our work to have more direct impact. If it is the latter, then one should be convinced that the material, or more generally the technology, they have in mind has some possibility of being better than known and fieldable options.
To more directly answer your question - yes it is time to consider more and better options.
Agree, but there are numerous reports on other catalyst like CdS and Other non-oxides and non TiO2 based oxides which are better performer than TIO2
@ Uttam, yes but more work in literature can found on TiO2....as it is cheap, non-toxic, and stable than other materials...
Hello everybody.
I agree and personally think that Ti-based perovskites for example should be much considered as the flexibility of the structure playing with A-site codopant can offer a lot of susbtitutions at the B site and modify the optical o catalytic properties.
CdS will have a much wider absorption spectrum, but is liable to instability. Eg = 2.4 eV against 3.1 eV for titania
Type-II hetero-junction have better photocatalytic hydrogen production efficiency than TiO2 alone. Further An important class of metal oxide semiconductors posses layered structure and it has drawn significant attention for their use as photocatalyst.The inter layer spacing of these layered metal oxide semiconductors provide accessibility (proximity) to reactants (H+ and/or H2O) to the reaction sites and also allows the possibilities to intercalate foreign cations such that their properties can further be tuned. The incorporation of alkaline metal cations in such layered structures lead to higher degree of the hydration of the interlayer, providing reactants (H2O or H+) access to reaction sites or in other words, access to the excitons.
Agree, and as mentioned by Gilles Gauthier, perovskites should receive special attention, since these materials provide suitable properties for hydrogen production. In general, materials with higher reduction potential than titania are of interest for water splitting.
There are many new materials under investigation. New semiconductor materials are being developed with bandgaps matching the potentials of water splitting. The bandgaps can be altered by doping and surface modification. Also there are composite materials under investigation that include light absorbers coupled with co-catalysts where the co-catalyst is either for oxygen evolution or hydrogen evolution. The oxygen evolution and hydrogen evolution catalysts themselves are under development.
Materials like CdS are not good candidates for obvious environmental impact. The trick is to separate the electron-hole pair created by TiO2. The low efficiency overall is mostly the result of recombination instead of low photo-efficiency.
There are a lot of materials that are being looked at as many commenters above have indicated. I think many of these are interesting for scientific reasons and I'm all for studying them. However it is my opinion that many of the scientifically interesting materials are not interesting from a pragmatic point of view. That is, various properties including potential negative environmental impact (e.g. Cd-based) and more importantly limited efficiencies and economic considerations mean that they will be unlikely to impact the world on a large scale (not to say they won't find a niche application here or there.) If one wants to change the world on a large scale via this technology it is important to remember that we already know how to make H2 from sunlight at a reasonable efficiency if not cost. PV driven electrolyzers will do the job. So when thinking of new chemistries to explore we must ask ourselves is our goal to gain scientific insight that may one day be applied to make improvements (i.e. change the world indirectly), or do we expect our work to have more direct impact. If it is the latter, then one should be convinced that the material, or more generally the technology, they have in mind has some possibility of being better than known and fieldable options.
To more directly answer your question - yes it is time to consider more and better options.
Dear all, I too agree with all of you and your views on the alternative photocatalysts. Recently various types of photocatalysts are under preparation however they suffer due to the lower photocatalytic activity. I agree with James, we need to think new chemistry and combination of different catalytic process for water splitting.
Does any one have the idea about the highest hydrogen production using any catalyst ?
I fully agree with James and most of the other comments. In spite of decades of work on TiO2 and other catalysts, a good efficiency has been not reported so far. The problems are numerous and have been already quoted. Even the use of sacrificial agents has not been succesful. Although not impossible, the goal seems to be almost utopic if we don't tackle this issue from a new perspective. To conclude with a constructive idea, perhaps nanomaterials could perform better...Does anyone know if this field has been explored so far?
Presuming you mean photocatalytic or photoelectrochemical water splitting from sunlight reaching earth, the first material consideration must be the band gap, which dictates how much of the solar spectrum can be absorbed. This criteria quickly rules out TiO2, since with a band gap of ~3 eV, TiO2 can only absorb wavelengths up to about 400 nm, a tiny fraction of the available sunlight.
Promising results have been reported for water splitting using composite photocatalysts but I am not too sure about the yields. The biggest challenge so far has been the tailoring of such materials e.g. rGO/CdS or CNT/TiO2. production of single sheets of highly ordered (non-defective) graphene is a massive challenge whereas it is key to ensuring enhanced photocatalytic activity of the resulted nanocomposite. Personally i believe there is still a lot that can be explored using titania/titania based composite photocatalysts. Let us explore other options but not shut the give up on what is already on the table
Graphene will not solve the problem of photocatalysis as I have seen in some paper even graphene based composite photocatalyst e.g. rGO-CdS has low solar to hydrogen conversion efficiency than their semiconductor hetero-junction analogues like MoS2-CdS.
Solar hydrogen production by water splitting with a conversion
efficiency of 18%, International Journal of Hydrogen Energy 32 (2007) 3248 – 3252.. this is the highest hydrogen production catalyst. But the catalyst is very expensive.
You are right, Kamala, and since 2007 no improvement has been reported. We are missing something...
Kamala, I am a little confused by your reference (Peharz et al., International Journal of Hydrogen Energy 32 (2007) 3248). It looks like the authors just paired up a solar cell and an electrolyzer (in a very elegant way) and the efficiency is the product of efficiencies from the separate components. There is no photo-catalyst involved.
Perhaps a photo-catalyst could be introduced to decrease the activation energy for the water splitting process, but this would require the electrolyzer to be exposed to solar irradiation. In this case, wouldn't it make more sense to just expand the solar cell array, which has pretty impressive efficiency of nearly 20%? I suppose it could be economical to boost the efficiency of the electrolyzer stage by a few percent using a photo-catalyst, but that's more of an industrial consideration.
I do agree with R. J. Rayade. Some researches are ongoing on Fe-oxide and its modification.
Kevin, while you are essentialy right, the solar cell is also based in semiconductor technology, electron-hole separation and so on, with the advantage that it all happens in a dry environment with no side deleterious reactions usually encountered in aqueous media.
Besides, the addition of a photo-catalyst into the water on a PEM electrolyzer array and its exposure to sunlight would not be easy, if working at all...
As you stated there so many papers were puplished. But still there are rooms to enhance the efficiency of TiO2, for example i saw one paper which is puplished on Sci with 24 percent achieved using disorder engineered black TiO2."chen, X; Liu, L;Yu, p.Y; Mao,S.S. increasing solar absorption for photocatalysis with black hydrogenated titanium dioxide nanocrystals. Science 2011, 331, 746-750". So in my opinion it is better to further explore using TiO2 and beside try to find other candidate. Multidirectional way to achieve, efficient, stable, low cost, environmental friendly..
Hydrogen + heat = very partial reduction of the TiO2 surface,giving blue-black coloration and some measure of electronic conductivity. Anyway the reference is a 2011 paper, and very little heard of it since.
Dinsefa, thank you for providing such an interesting reference, but it has to be noticed that 24% was the maximum energy conversion efficiency reported in the presence of 50% methanol as sacrificial agent using Pt loaded doped nanocristalline TiO2; therefore far away of any feasible industrial application...As Augustin Mcevoy says, very little of it read afterwards.
Though TiO2 absorb less then 5% of light photons from solar spectrum, it shows excellent chemical stability compare to other metal-oxide or Cd-based systems. Most of the reports shows short term performance. Better we need to concentrate on photo anode/cathode stability if we choose other materials compare to TiO2.
Secondly, PV driven photo catalyst is the interesting topic. Need to check the efficiency and compare with bulk electrolysis.
---Silicon based semiconductors, please consult:
Wireless Solar Water Splitting Using Silicon-Based Semiconductors and
Earth-Abundant Catalysts
Steven Y. Reece, Jonathan A. Hamel, Kimberly Sung, Thomas D. Jarvi, Arthur J. Esswein, Joep J. H. Pijpers, Daniel G. Nocera
SCIENCE VOL 334 2011, 645
---porphyrin derivatives deposited on TiO2 (many reports)
---metal oxide semiconductors (Rh-Cr2O3 core-shell particles on oxynitride semiconductor particles)
Here you have some interesting results using TiS2:
http://onlinelibrary.wiley.com/doi/10.1002/anie.200701626/abstract
answer is a definite YES. There must be materials out there that can do a better job than TiO2 for water splitting, if by the latter is meant formation of Hydrogen and Oxygen simultaneously in the proper ratio in the ABSENCE of anyy sacrificial electron donor or acceptor.
Tantalum nitrides also shows better hydrogen production than TiO2. It is best catalyst alternative for TiO2.
See also
---CdS modified NaNbO3 core–shell nanorods
---Niobium compounds in association with different dyes.
Tungsten oxide may be a good choice due to stable chemical property under light illumination. Isn't it?
Eugenia, Are CdS modified NaNbO3 core–shell nanorods stable during the process?
Fengmei, Tungstene oxide is certainly stable, but as far as I know it does not perform better than TiO2...
Yeah, the photo-catalytic performance of tungsten oxide should be considered. Some methods, such as doping, composition with other semiconductors, maybe useful. These have been reported in many papers .
@ Prof Juan Casado
Yes. CdS modified NaNbO3 core shell-nano rods are stable towards photocatalytic hydrogen generation for 8 hours with continuous irradiation. We have also pelletized the powder and studied its stability by Chrono -amperometry for 5 hours. we have published the first part of the paper in RSC Advances. The results of hydrogen production are due for publication...
@ kamala kanta nanda
Yes, modifed CdS may be better than TiO2....which light (UV or vis) is used for hydrogen production?
Thank you, Fengmei, but could you provide any reference showing a better performance of tungstene oxide vs. titanium dioxide?
Kamala, please let us know these new results when published...
The answer regarding stability was already given, so...
I can add:
double-perovskites with two distinct sites in the crystal structure, one for barium, and another for a lanthanide element (eg holmium or gadolinium), but the most efficient was praseodymium.
I thank that, the TiO2 as catalyst for waste water treatment have full of applications, not investigated, for example, it is a good alternative for developing countries , who have sunlight in abundance , and it is a chip application for water treatment.
Yes. TiO2 is one of well investigated materials for water purification. We have several publications in this direction. Compared to H2 generation water purification needs a more reliable and safe material. TiO2 nano particles and anowires are good choices. For energy application we should consider other choices since TiO2 can only harvest UV band of solar energy. Till now most doping researches are proved are tedious and less economic. This discussion can help us to focus some potential systems for more promising H2 yielding.
TiO2 is the first and very basic semiconductor oxide studied for PEC applications, both in the case of water splitting (Honda et al) DSSCs (Gratzel et al). Though the band edge positions of TiO2 are favorable for both water oxidation and hydrogen evolution, TiO2 has a wide band gap, which necessitates the need for UV light excitation. A number of solid state semiconductors and inorganic oxides have been proposed for water splitting, and the field is growing stronger by the day. A comprehensive review on this topic and the newer materials being designed can be found in this exhaustive compilation- (Photoelectrochemical Water Splitting : Materials, Processes and Architectures Editor(s): Hans-Joachim Lewerenz, Laurie Peter, RSC publishing). Another good book is- (Photoelectrochemical hydrogen Production by Gratzel, Springer)
It has been proven that % STH greater than 10% cannot be achieved by a single light absorber material. Thus the need for multiple light absorbers/ hybrid catalysts is the research direction that we are heading in present day. It is essential that we achieve zero bias (wireless) water splitting with cheap earth abundant materials under visible light. Only such an achievement could enable any real benefit to people on the whole. Present day research on high efficiency PEC water splitting places its focus on two areas.
1. Combination of molecular catalyst and earth abundant semiconductor (Artificial Photosynthesis/Artificial leaf). A wide variety of molecular catalysts are known, and being studied upon. This direction was chiefly initiated by the Nocera Group at MIT.
2. Use of multiple light absorbers and Tandem cells. This has been pioneered by the Gratzel group at EPFL.
In addition to these, a number of inorganic WOCs have been developed, which facilitate water splitting reactions at relatively high efficiency..
You may want to refer to this manual for a comprehensive overview of experimental procedures and protocols for productive research on PEC water splitting. (Photoelectrochemical Water Splitting
Standards, Experimental Methods, and Protocols, Springer)
@Rajesh Jagannath Tayade,
It is better to use the light from a solar simulator. Other wise visible light from a xenon lamp can be used but it may overestimate the solar to hydrogen conversion efficiency as it is not calibrated with AM 1.5.
TiO2 is gold standard for waste water treatment.Only thing is that it should be modified with metal nanoparticles which is visible active. We have published recently a Enhancement in the photocatalytic activity of Ag loaded N-doped TiO2
nanocomposite under sunlight in J Mater Sci: Mater Electron
DOI 10.1007/s10854-014-2001-4. Kindly refer this paper for the better photocatalyst in place of TiO2
In the last period of time, due to approach to biomimetic systems, porphyrins are more intensivelly used to split water into its elements, with increased yields and low costs.
I will name here some cationic cobalt-porphyrin systems as photosensitizers.
Dear all, water split for H2 production using photocatalyst is not efficient, better to use electrolyzer powered by PV. if The aim is producing H2 then better to use water shift reaction using emission sources from industrial plant converting CO in H2 using water and environmentally friend low cost catalyst
@kamala kanta Nanda : i think solar simulator may not be appropriate for scaling up ... catalyst other than TiO2 may be costly, at the same timeonly TiO2 is not highly efficient in solar light...hope there may be some progress on combination of techniques which may be useful forenvironmental friendly production of hydrogen at lower cost .
@Rajesh Jagannath Tayade, You are right sir. Fe2O3 and BiVO4 would be the best choice if we can solve their enrgetic criteria for water splitting.....
@ K.M. Garadkar: yes you are correct... but still no practical use has been at higher level of water purification in practice.....
@ Eugenia Fagadar-Cosma: bimetallic system must be stable....then only it can be used in continuous process...they should not have loss of structure or the activity after some time....then only they may useful...
@Juan Casado,
Dear Sir
The hydrogen production results for CdS-NaNbO3 are not published yet. Some optimization and modification is going on...
Dr. Paolo Tripodi - I agree with your statement re. PV electrolysis, however I am curious what sources of CO you might be referring to?
Paolo Tripodi: I agree with Jim Miller, where would you get the CO - perhaps you are thinking of reducing CO2 to CO which also requires a catalyst or photocatalyst. And by the way when one thinks of water splitting, it is NECESSARY to demonstrate that BOTH H2 and O2 are produced in the expected ratio of 2 to 1, too often forgotten by far too many people.
Dear colleagues James and Nick,
In all industrial process where carbonious compounds are used as a fuel, in particular I'm involved recently in the cogeneration process using wood gassification, CO is the fuel and there is a great amount of CO in the emission gases, mainly saturated with water, I mentioned the water gas shift reaction CO+H2O CO2+H2.
Dear Paolo Tripodi,
off-course it is possible if our target is only to get H2, with out considering environmental impact (here CO2 is a by product and to add pollutant gas to the environment) , in my opinion it should be a major point that we give more emphasis, in addition, cost and large scale production.
Dear Dinsefa, CO2 is not a pollutant is a GHG, there are hundreds of more dangerous GHG. As a paradox we need CO2 in the air for respiration, our lungs work on CO2 concentration!!!
Paolo, yes, CO2 may be considered not a pollutant as it is natural and it is needed in photosynthesis, but we do not need it for respiration.
Rajesh, this is a discussion about water splitting, not about wastewater purification. Please let's avoid mixing up different topics.
@ Juan Casado: Yes, I agree with Juan, but Paolo has suggested water gas shift reaction which is also connected with hydrogen production and interesting. As TiO2 mainly used for wastewater treatment so mainly time it get mixed up with hydrogen production. I hope still we can continue this discussion on water spilling and discuss about new techniques, catalyst and their possibility to use for hydrogen production. I would like to thank all the researcher for expressing their view and suggestions.
Yes, water gas shift reaction produces hydrogen, but wastewater treatment usually does not.
Yes, considering the complex physical-chemical processes involved in the water splitting under sun light, it is the time for more complex systems. Pure TiO2 will not satisfy the full requirements. Dopping is an otpin that already has been researched with improved properties but still the full light spectra is not available. It's time for composite materials! But this is a big task because it is required to have also controlled compatibility between component materials!
It is possible used other material but TiO2 the best . Have to couple it with onother to make it as composite , Cds, cdte, inSb, in, cu, Au, etc.... u can get high efficiency after synthesis.
@ Ali Kamel Mohsin, Yes, but coupling is not stable and there is leaching of metal ion or reduction in activity with respect to time....
Right, Rajesh.
Ali, Could you provide any literature example of high efficiency using "composite" materials?
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