JCAP team reports first complete “artificial leaf”; >10% solar-to-hydrogen conversion efficiency
28 August 2015
Researchers at the Joint Center for Artificial Photosynthesis (JCAP) report the development of the first complete, efficient, safe, integrated solar-driven system—an “artificial leaf”—for splitting water to produce hydrogen. JCAP is a US Department of Energy (DOE) Energy Innovation Hub established at Caltech and its partnering institutions in 2010.
The new system has three main components: two electrodes—one photoanode and one photocathode—and a membrane. The photoanode uses sunlight to oxidize water molecules, generating protons and electrons as well as oxygen gas. The photocathode recombines the protons and electrons to form hydrogen gas. A key part of the JCAP design is the plastic membrane, which keeps the oxygen and hydrogen gases separate. If the two gases are allowed to mix and are accidentally ignited, an explosion can occur; the membrane lets the hydrogen fuel be separately collected under pressure and safely pushed into a pipeline.
One possible existing approach to solar-driven hydrogen production involves connecting photovoltaic (PV) panels, modules or cells physically and electrically in series with an electrolyzer (E), the researchers noted. Peak system efficiencies of 12.6% and 24.6%, respectively, could be obtained by use of an electrolyzer in conjunction with a high-efficiency (21%) Si PV module or a high-efficiency (41%) III-V triple junction PV operated under optical concentration.
Such systems have been demonstrated at commercial, laboratory and research scales, the team said. However, at the commercial level, the high balance of systems cost and low capacity factor results in high levelized hydrogen costs relative to hydrogen produced by steam reforming or grid electrolysis using fossil or low-carbon electricity. Thus, a lower-cost “artificial leaf” technology could offer significant advantages as an alternative.
In a paper published in the RSC journal Energy & Environmental Science, the researchers reported that their photoelectrosynthetic cell (GaAs/GaInP2/TiO2/Ni photoanode connected to a Ni-Mo coated counterelectrode) exhibited a solar-to-hydrogen conversion efficiency (ηSTH) of 10.5% under 1 sun illumination, with stable performance for > 40 h of continuous operation at an efficiency of ηSTH >10%.
An intrinsically safe solar-hydrogen prototype system (1 cm2) built with the device exhibited a hydrogen production rate of 0.81 μL s-1 and a solar-to-hydrogen conversion efficiency of 8.6% under 1 sun illumination, with minimal product gas crossover while allowing for beneficial collection of separate streams of H2 and O2.
This result was a stretch project milestone for the entire five years of JCAP as a whole, and not only have we achieved this goal, we also achieved it on time and on budget. This new system shatters all of the combined safety, performance, and stability records for artificial leaf technology by factors of 5 to 10 or more.
Our work shows that it is indeed possible to produce fuels from sunlight safely and efficiently in an integrated system with inexpensive components. Of course, we still have work to do to extend the lifetime of the system and to develop methods for cost-effectively manufacturing full systems, both of which are in progress.—Nate Lewis, Caltech George L. Argyros Professor and professor of chemistry, and the JCAP scientific director
Semiconductors such as silicon or gallium arsenide absorb light efficiently and are therefore used in solar panels. However, these materials also oxidize (or rust) on the surface when exposed to water, so cannot be used to directly generate fuel. A major advance that allowed the integrated system to be developed was previous work in Lewis’s laboratory, which showed that adding a nanometers-thick layer of titanium dioxide (TiO2) onto the electrodes could prevent them from corroding while still allowing light and electrons to pass through. The new complete solar fuel generation system uses such a 62.5-nanometer-thick TiO2 layer to prevent corrosion and improve the stability of a gallium arsenide–based photoelectrode.
Another key advance is the use of active, inexpensive catalysts for fuel production. The photoanode requires a catalyst to drive the essential water-splitting reaction. Rare and expensive metals such as platinum can serve as effective catalysts, but in its work the team discovered that it could create a much cheaper, active catalyst by adding a 2-nanometer-thick layer of nickel to the surface of the TiO2. This catalyst is among the most active known catalysts for splitting water molecules into oxygen, protons, and electrons and is a key to the high efficiency displayed by the device.
The photoanode was grown onto a photocathode, which also contains a highly active, inexpensive, nickel-molybdenum catalyst, to create a fully integrated single material that serves as a complete solar-driven water-splitting system.
A critical component that contributes to the efficiency and safety of the new system is the special plastic membrane that separates the gases and prevents the possibility of an explosion, while still allowing the ions to flow seamlessly to complete the electrical circuit in the cell. All of the components are stable under the same conditions and work together to produce a high-performance, fully integrated system.
|A fully integrated photoelectrochemical device performing unassisted solar water splitting for the production of hydrogen fuel. Credit: Erik Verlage and Chengxiang Xiang/Caltech|
Funding was provided by the Office of Science at the US Department of Energy, and the Gordon and Betty Moore Foundation.
Verlage, Erik and Hu, Shu and Liu, Rui and Jones, Ryan J. R. and Sun, Ke and Xiang, Chengxiang and Lewis, Nathan and Atwater, Harry A., Jr. (2015) “A Monolithically Integrated, Intrinsically Safe, 10% Efficient, Solar-Driven Water-Splitting System Based on Active, Stable Earth-Abundant Electrocatalysts in Conjunction with Tandem III-V Light Absorbers Protected by Amorphous TiO2 Films.” Energy and Environmental Science doi: 10.1039/C5EE01786F.
This method by an entirely different route using conventional solar cells and electrolysis also reckons that 10% efficiency is possible with existing technology solar - hydrogen:
It hardly seems a sustainable position that hydrogen and fuel cells are fossil fuels by another name, as has been so often claimed.
When it is time to railroad.....
Posted by: Davemart | 28 August 2015 at 10:18 AM
@Dave, it depends where the h2 comes from. If it is reformed from natural gas, it is fossil hydrogen; if it comes from this or electrolysis, it is "renewable" hydrogen.
Then we have the problem of what to do with it ...
Do we pipe in away as H2, do we pipe it into the natural gas network as the Germans do; do we use it to crack long chain hydrocarbons into shorter ones ...
Also, is there anything useful (and economic) that can be done with pure Oxygen ?
10% H2 reforming is pretty good, however, so well done guys.
Posted by: mahonj | 28 August 2015 at 02:51 PM
If you can produce the mass amount of hydrogen needed for the millions of projected FCVs using only sunlight and water, I'll buy in yesterday; but, don't try using this as propaganda to sell FCVs that are fueled by reformed hydrocarbons that are compressed...that then becomes a lie by association; which is what Toyota is doing.
Posted by: Lad | 28 August 2015 at 03:09 PM
Digest biomass to make methane, create methanol from the methane then reform methanol on vehicle to hydrogen for PEM fuel cells.
Posted by: SJC | 28 August 2015 at 06:00 PM
Am I the only one that has a big "Tweets" box that is blocking my ability to read much of the content here? This has been going one for many days now with my iPad Air.
Posted by: TheOne | 29 August 2015 at 12:04 AM
I have Twitter blocked in /etc/hosts, so no problem. Does your browser support the Greasemonkey plugin? If so, you can resize, hide or delete the box.
Hydrogen is difficult to store, but using hydrogen to make liquid fuels implies losses (including upstream in the CO2-supply chain). Six of one, half-dozen of the other.
Posted by: Engineer-Poet | 29 August 2015 at 10:30 AM
This seam to be efficient right now, so what I want they do is to start to produce synthetic gasoline with this method right now for my car, a dodge neon 4 cylinder 2 liter 5 speed manual. This method is non-polluting and sustainable for all the time. Build a synthetic fuel factory in the desert near sea water and start production right now. Im sick of paying high price for my gas, gas prices will collapse with this new unlimited factory. We can make unlimited quantity of gas with this almost free hydrogen and co2 capture in normal atmosphere.
On top of that we can sell this hydrogen for normal ice cars. All it take is a normal low cost gas tank and a second pressurized hydrogen tank and we mix these 2 fuel in the intake manifold and we save gas, dramatically reduce pollution and speed up the burning rate for added efficiency and we can retard the ignition timing to reduce drag and increase efficiency further more.
We can install these new machinery directly at gas stations so no need to stock hydrogen and transport it long distance.
Posted by: gorr | 29 August 2015 at 11:03 AM
It is not Toyota who are lying, which is an extraordinary claim to make.
That accusation could more justly be levelled at those who have been claiming that hydrogen and the use of fuel cells is fossil fuel burn by another name.
This method aside, in every jurisdiction that I am aware of where fuel cell stations are being build, it is legislated that at least 33% is from renewables, including biomass and so on.
Posted by: Davemart | 29 August 2015 at 11:10 AM
EP said that hydrogen is difficult to store, which is fair enough, at least in comparison with liquid fuels.
In Germany and some other areas storage in massive quantities in salt caverns is possible though, and the round trip efficiency is very high.
The natural gas pipeline network itself can hold substantial volumes of hydrogen mixed in which can be extracted at point of use.
The issue may be circumvented in any case, with one of the methods for using waste CO2 to add to solar hydrogen producing synthetic gasoline, which certainly can be stored.
When it comes to using it, excellent progress is being made on intermediate temperature solid oxide fuel cells, which would use it just fine and still without combustion engine emissions.
Posted by: Davemart | 29 August 2015 at 11:16 AM
I continue to be puzzled by the logic of these attempts at artificial leaves. In this case, they have made a photovoltaic system (GaAs/GaInP2) and then incorporated catalysts for producing H2 and O2. How can this be more cost effective than using a quality photovoltaic cell to produce electricity and then run the electricity into a quality electrolyzer to produce H2? (The electrolyzer can either use precious metal catalysts or one of the new "earth abundant" material catalysts). The economics of photovoltaic electricity require very long life systems (people often suggest a 30 year useful life). How can one hope to make a long life system that incorporates the photovoltaic cell into a system with windows, water, and catalysts? The authors made a cell that lasts for 40 hours. And how can this combined system be less costly than separate photovoltaic modules and electrolyzers? With the artificial leaf, each module can be ruined by poisoning from impurities in the water, or corrosion of the semiconductor material, or a leak that lets the H2 escape. With a separate system, the relatively expensive photovoltaic part is independent. Electrolyzer modules can be replaced at relatively little cost, leaving the photovoltaic modules to continue to function.
Posted by: Cautious9 | 29 August 2015 at 12:51 PM
Concentrated solar PV at 30+% efficiency used in 80+% efficient electrolysis produces 24+% efficient hydrogen. You get lots of oxygen which can be used to gasify biomass to produce lots of bio synthetic hydrocarbon liquid fuels to use in the 200 million engine vehicles in the U.S.
Posted by: SJC | 29 August 2015 at 01:49 PM
I believe the thinking is that it is preferable if possible to get as near as possible to the point where the solar energy is captured to make the hydrogen.
One of the approaches which is looking hopeful is Hypersolar, which is now at 1.4 volts, having got there pretty quickly, and needing 1.5 volts they reckon for viability:
Another is the hydrogen peroxide route, which I believe is suitable for large scale plants:
'Producing hydrogen from sunlight is challenging. The best-known procedure is photoelectrolysis: a photovoltaic cell captures the energy from the photons and the potential difference is used to split water. The minimum energy needed is 1.23eV, but the direct water-splitting reaction proceeds by a complex, four-electron pathway, so a voltage of around 2V is required to achieve a respectable reaction rate. This can call for three or even four solar cells, which increases the expense.
A simpler option is photocatalysis, in which a powdered light-absorbent catalyst is simply suspended in the water, absorbing photons and catalysing the splitting process. This is, in principle, much cheaper and easier to industrialise. However, many photocatalysts are less than 0.1% efficient, require expensive materials, such as gold, or stop working quickly, often because side reactions, such as hydrogen peroxide production, poison the catalyst.
Researchers at Soochow University in China led by Zhenhui Kang decided to maximise hydrogen peroxide production. They designed a composite catalyst containing cheap, earth-abundant C3N4 in a specially-designed composite containing carbon nanodots. The C3N4 photocatalyst splits water into hydrogen and hydrogen peroxide, which would normally stick to the surface of the catalyst poisoning it. However, the carbon nanodots act as a chemical catalyst that decomposes the hydrogen peroxide into water and oxygen. The nanodots also allow the catalyst to absorb more light.
The new catalyst has a solar-to-hydrogen conversion efficiency of 2%. The best water-splitting photocatalyst to date is nanocrystalline cobalt oxide, which has a conversion efficiency of around 5%.2 However, this began to lose its activity within 1 hour. The current photocatalyst, however, showed no degradation after 200 days. The researchers calculate that if they optimised their photocatalyst so it had a 5% conversion rate this would lower the cost of hydrogen production to $2.30/kg (£1.50/kg) – well below the US Department of Energy’s target of $4/kg. ‘Even at this stage, the number we get is only about $6,’ says co-author Yeshayahu Lifshitz, now at Technion – Israel Institute of Technology.'
At this stage of the game I am a firm believer in highly targeted research - make the wall the target, throw pretty basic ( and cheap ) research at it, and see what sticks! ;-)
When it is early days, I don't think it pays to try to be too clever or selective.
The rate of progress is astonishing, and it won't be long before some of the contenders stretch away ahead of the field.
The route you suggest is already at perhaps 10% efficiency, which is not too dusty, although further progress on durability and the use of earth abundant materials is needed.
Posted by: Davemart | 29 August 2015 at 02:02 PM
However, unless your area has beds of salt or equivalent material (e.g. potash), you're not going to be able to store H2 that way. Also, it takes about 3x as much volume of H2 as methane to store the same energy.
This is one of the more pernicious bits of pro-H2 propaganda. The hydrogen injected into natural-gas lines is effectively unrecoverable as hydrogen. Entropy makes it inherently difficult and costly to pull all the H2 out of a mixed stream, and carrying it along requires a stream of (carbon-spewing) methane just to move it. The energy content is puny. Take the Hythane mix which is used to accelerate the combustion of methane to make it a good spark-ignition fuel. It is 20% H2 by volume, but only 7% of the energy content is from hydrogen. A pipeline network which is limited to 7% or so "clean, carbon free" content is a miserable failure by any objective measure.
The only reason to make gasoline is to run the existing fleet of motor vehicles, which will have turned over by half in ten years. It makes far more sense to stop at methanol, which is much more easily reformed back to CO + H2 syngas and is a much higher-octane motor fuel.
Excellent question. It suggests that the "artificial leaf" is mostly useful as Green propaganda. However, photochemically-active electrodes which convert photons directly to chemical energy may be cheaper than plain electrochemistry.
The reaction of hydrogen peroxide to O2 and water releases a considerable amount of energy, suggesting that whatever reaction creates H2O2 instead of O2 has quite a bit of room for optimization.
Posted by: Engineer-Poet | 29 August 2015 at 08:40 PM
I always have a lot of respect for your views, but my remarks were based on the systems being tested in Hawaii, Germany and the UK, which as far as I know have not yet been classed as a flunk.
Here is how they plan to extract the hydrogen:
'PSA technology is based on the different properties of two the gases (methane and hydrogen) under pressure. The methane will stick to a catalyst bed, while the hydrogen will pass through and can be taken off at the station. Release the pressure, and the methane is returned to the system and returned to consumers.
PSA technology is well-established, and TGC is evaluating systems from several potential suppliers, Kissel said.'
I am in no way qualified to undertake an engineering valuation of the technology, but those who are seem happy with it at the moment.
While here in the UK it is the planned delivery method:
Posted by: Davemart | 30 August 2015 at 11:05 AM
As a followup to my previous post, I am not questioning the value of research on solar production of H2. But I am questioning the rationale for an artificial leaf that is really just a photovoltaic cell with electrolysis catalysts plated on part of the cell. I do not think that this can possibly compete with separate photovoltaic and electrolysis modules, where each can be optimized. I will be amazed if a combined "leaf" can have the durability, efficiency, optimized current density, serviceability, etc. of a system with separate photovoltaic and electrolysis modules.
In the world of electric vehicles, the power electronics can efficiently charge batteries with a wide variety of cell working voltage. So I don't see any real value in trying to optimize the photovoltaic cell voltage to match the best H2 evolution potential. I think these electronics will be a small fraction of the cost of a solar to H2 system.
Some work on solar to H2 production involves more simple approaches in which an active catalyst powder in water absorbs sunlight and produces H2 and O2. If one can find a powder or a set of redox systems that are durable in water, absorb a reasonable fraction of the sunlight, and are reasonably efficient at producing H2, then this could be a real winner. A wide variety of research is following this path, but so far none of it has the kind of efficiency and H2 production rate to be economically viable.
Posted by: Cautious9 | 30 August 2015 at 11:26 AM
PSA separation of H2 from CO and CO2 is actually part of the SMR (steam methane reformer) technology for industrial production of H2. Should work fine for CH4 and H2 separation.
Gaseous O2 is used in massive quantities for everything from medical applications to steel making.
Posted by: CJY | 30 August 2015 at 12:06 PM
PSA separation requires absorbing the (much larger) volume of methane and then pumping it back up to its normal pressure for re-shipment. This is exactly the sort of energy cost I was talking about.
Posted by: Engineer-Poet | 30 August 2015 at 03:41 PM
'For a 10% concentration and 80% recovery factor, the estimated cost of hydrogen extraction by PSA from a 300 psi pipeline is $3.3–$8.3/kg hydrogen extracted, for a range of recovery rates of 1,000–100 kg/day. For a 20% concentration and 80% recovery factor, the extraction cost is $2.0–$7.4/kg hydrogen extracted, for the same range of recovery rates. These additional supply chain costs are high relative to a competitive hydrogen cost goal of $2–$4/kg for FCEV markets (Ruth and Joseck 2011). However, if hydrogen is extracted at a pressure-reduction facility, the high cost of recompressing the natural gas to the original natural gas pipeline pressure can be avoided.
The resulting estimated extraction cost for a 10% concentration and 80% recovery factor is $0.3–$1.3/kg, with the range resulting from economies of scale for a system size or recovery rate of 1,000–100 kg/day (see Figure 18). These costs per kilogram are reduced by approximately 10% if the hydrogen concentration is increased to 20%. PSA extraction could therefore become a relatively small cost component of the total delivered cost of hydrogen if the extraction is done at a pressure-reduction facility. With major pressure reduction stations often located near large urban areas, downstream extraction could prove to be an economical delivery option. It has been
estimated that there are 11,200–14,800 metering and pressure regulating stations with inlet pressures greater than 300 psig in the United States (see section 3.1), and 34,600–56,700 stations with inlet pressures between 100 and 300 psig. Approximately 23%–25% of the stations with inlet pressures greater than 300 psig are contained within vaults, which is typical for stations located near urban or suburban areas. Therefore, it is likely that several thousand high-pressure city gate stations are located in close proximity to large U.S. urban areas where natural gas is
transferred from transmission lines to distribution lines, and many of these may be candidates for hydrogen extraction.'
(nrel, ibid, pg xi)
Posted by: Davemart | 31 August 2015 at 01:00 AM
'It suggests that the "artificial leaf" is mostly useful as Green propaganda.'
I understand that most of the companies engaged in synthetic photosynthesis are university spin offs etc, which do not have the resources to set up a business for propaganda purposes.
They may have picked the wrong technology, but seem hardly likely to be invested so heavily in it just for propaganda.
Posted by: Davemart | 31 August 2015 at 01:04 AM
Note that you have to have such extraction stations at ALL the pressure-reduction points downstream of the injection point, or the expensive hydrogen is simply lost. This raises the question of what you do with hydrogen in excess of local demand. Do you need a new compressor station and a new pipeline to return it to the main line downstream of the break point?
The standard heat of formation of liquid water is -285.8 kJ/mol, so a kilogram of H2 has about 135 kBTU HHV in Imperial units. This hydrogen probably costs several dollars per kg. That's multiples of the retail price of natural gas where I am, which runs about USD0.60 per 100kBTU; those 10% losses to methane consumers are going to be expen$ive.
Last, having to use the ΔP across a pressure-reduction station to operate PSA equipment loses the option for energy recovery using expansion engines.
An examination of who provides the grant money to get this work started might be fruitful.
Posted by: Engineer-Poet | 31 August 2015 at 07:47 AM
'Note that you have to have such extraction stations at ALL the pressure-reduction points downstream of the injection point,'
Posted by: Davemart | 31 August 2015 at 07:54 AM
'An examination of who provides the grant money to get this work started might be fruitful.'
Pretty harsh and pretty circular reasoning.
Either grant money or venture capital has been behind almost all new technologies, and of course both those which worked out and those which didn't.
That includes very large sums for nuclear power which both you and I favour as well as solar in all its forms, so not much fancying a particular technology and singling that out as being promoted solely for propaganda purposes seems tendentious.
The sums so far put into solar photosynthesis have been pretty tiny anyway, and considering that good progress has been made.
Posted by: Davemart | 31 August 2015 at 08:05 AM
Because otherwise you're losing a very costly fuel to be wasted as a poor substitute for methane. Losing 60% of your stream, 50% through un-filtered taps and another 10% through leakage past PSA systems, raises your cost per delivered kg by 150%.
Yes, but the imagery of the "artificial leaf" is very powerful and appealing to certain groups despite the impracticality of combining all the functions in a single collector. Could there be people who WANT attention to be focused on the impractical, to starve other things of oxygen?
Posted by: Engineer-Poet | 01 September 2015 at 01:35 PM
I have no idea where you have got your figures for losses from, so no constructive response is possible.
I've linked the information I have used.
Posted by: Davemart | 05 September 2015 at 01:38 PM
I got them from you. 'For a 10% concentration and 80% recovery factor....'
Posted by: Engineer-Poet | 07 September 2015 at 06:38 PM