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Researchers develop technique to create new tailored molecule with high density of active catalytic sites; potential low-cost alternative to platinum for splitting water

10 February 2012

Chang-MoS-Th
Using a molybdenite complex and the PY5Me2 ligand, Berkeley Lab researchers synthesized a molecule that mimics catalytically active triangular molybdenum disulfide edge-sites. The result is an entire layer of catalytically active material. Molybdenum atoms are shown as green, sulfur as yellow. Credit: Berkeley Lab. Click to enlarge.

Researchers with the US Department of Energy’s Lawrence Berkeley National Laboratory have developed a technique for creating a new molecule that structurally and chemically replicates the active part of the widely used industrial catalyst molybdenite. This technique holds promise for the creation of catalytic materials with high densities of active sites that can serve as effective low-cost alternatives to platinum for generating hydrogen gas from water that is acidic.

Molybdenite (molybdenum disulfide, MoS2) is the crystalline sulfide of molybdenum and the principal mineral from which molybdenum metal is extracted. Molybdenite is one of the most widely used catalysts in industry today as the standard for hydrodesulfurization (HDS) of petroleum and natural gas. However, recent studies have shown that in its nanoparticle form, molybdenite also holds promise for catalyzing the electrochemical and photochemical generation of hydrogen from water.

Christopher Chang and Jeffrey Long, chemists who hold joint appointments with Berkeley Lab and the University of California (UC) Berkeley, led a research team that synthesized a molecule to mimic the triangle-shaped molybdenum disulfide units along the edges of molybdenite crystals, which is where almost all of the catalytic activity takes place.

As is the case with many inorganic solids, the catalytic activity of MoS2 is localized to rare surface sites, whereas the bulk material is relatively inert. High-resolution scanning tunneling microscopy studies and theoretical calculations performed on nano-particulate MoS2 structures that form under sulfiding conditions implicate the formation of disulfide linkages or triangular MoS2 units along the fully sulfided catalytically active edges of the layered structure. However, the precise molecular structures and modes of action of these sites remain elusive. Because of the bulk material’s layered structure, which favors the growth of plate-like crystals, a single crystal with a large edge dimension is extremely challenging to prepare.

Here, we report the synthesis of a well-defined molecular analog of the proposed MoS2 edge structure, a side-on bound MoIV-disulfide complex. Electrochemical reduction of this molecule leads to the catalytic generation of hydrogen from acidic organic media as well as from acidic water, lending support to the proposed active site morphology in the more active heterogeneous catalyst.

—Karunadasa et al.

Since the bulk of molybdenite crystalline material is relatively inert from a catalytic standpoint, molecular analogs of the catalytically active edge sites could be used to make new materials that are much more efficient and cost-effective catalysts.

Chang and Long are the corresponding authors of a paper in the journal Science describing this research; other authors are Hemamala Karunadasa, Elizabeth Montalvo, Yujie Sun and Marcin Majda.

Using molecular chemistry, we’ve been able to capture the functional essence of molybdenite and synthesize the smallest possible unit of its proposed catalytic active site. It should now be possible to design new catalysts that have a high density of active sites so we get the same catalytic activity with much less material.

—Christopher Chang

Inorganic solids, such as molybdenite, are an important class of catalysts that often derive their activity from sparse active edge sites, which are structurally distinct from the inactive bulk of the molecular solid. We’ve demonstrated that it is possible to create catalytically active molecular analogs of these sites that are tailored for a specific purpose. This represents a conceptual path forward to improving future catalytic materials.

—Jeffrey Long

Preparing molybdenite with a high density of functional triangular molybdenum disulfide edges in a predictable manner is extremely challenging, the authors note. Chang, Long and their research team met this challenge using a pentapyridyl ligand known as PY5Me2 to create a molybdenum disulfide molecule that, while not found in nature, is stable and structurally identical to the proposed triangular edge sites of molybdenite. It was shown that these synthesized molecules can form a layer of material that is analogous to constructing a sulfide edge of molybdenite. The synthesized molecules performed robustly in evolving hydrogen from water, even using crudely filtered California seawater.

The electronic structure of the molecular analog can be adjusted through ligand modifications, Long says, suggesting that the material’s activity, stability and required over-potential for proton reduction can be tailored to improve its performance.

In 2010, Chang and Long and Hemamala Karunadasa, who is the lead author on this new Science paper, used the PY5Me2 ligand to create a molybdenum-oxo complex that can effectively and efficiently catalyze the generation of hydrogen from neutral buffered water or even sea water. Molybdenite complexes synthesized from this new molecular analog can just as effectively and efficiently catalyze hydrogen gas from acidic water.

The ability to prepare, characterize, and evaluate molecular analogs of the active components of inorganic solids has broad implications for the design and optimization of functional metal sites, not the least of which is control over the density of these units. For example, recent electronic structure calculations conducted on nanoparticulate MoS2 indicate that only a quarter of the edge sites are used for hydrogen production. Increasing the number of active edge sites per unit volume by tailoring progressively smaller nano-structures or changing the electronics of the system to increase the enthalpy of hydrogen adsorption is a major challenge in inorganic materials and nanoscience. We present an alternative strategy using discrete molecular units, which in principle can be tailored to give a high density of catalytically active metal sites without the rest of the inactive bulk material.

—Karunadasa et al.

This research was supported by the DOE Office of Science, in part through the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub.

Resources

  • Hemamala I. Karunadasa, Elizabeth Montalvo, Yujie Sun, Marcin Majda, Jeffrey R. Long, and Christopher J. Chang (2012) A Molecular MoS2 Edge Site Mimic for Catalytic Hydrogen Generation. Science 335 (6069), 698-702. doi: 10.1126/science.1215868

  • Hemamala I. Karunadasa, Christopher J. Chang and Jeffrey R. Long (2010) A molecular molybdenum-oxo catalyst for generating hydrogen from water. Nature 464, 1329–1333 doi: 10.1038/nature08969

February 10, 2012 in Catalysts, Hydrogen Production | Permalink | Comments (26) | TrackBack (0)

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This is one efficient method to do hydrogen gas for fuelcell cars from water. It cost next to nothing, do not pollute and replace gasoline and diesel for cars, trucks, tractor-trailer trucks, airplanes, ships, motorcycles, everything. Also it do not pollute. Im interrested to buy a fuelcell car or just a converted hydrogen ice car but with this water electrolyzer put inside the car and the hydrogen is made while driving, we just have to put water inside the electrolyzer once in a while and drive without fuel cost nor pollution.

You really do not understand physics.

@A_D: Would that it were so.

@A_D:

Back to inventing perpetual motion machines.

Wouldn't be simpler to design and mass produce domestic NG to Hydrogen converters + low cost compressor. Eventually, this type of converter could be light and small enough for on-board installation, specially for large vehicles such and trucks, buses and locomotives.

You do not have the quick fill advantage. With a fueling station, you can put in 5 kg of H2 in a matter of minutes.

That presumes that you have the filling station, and the H2 generation network to supply it... at a competitive price. Those are some serious gotchas.

oh yeah...well there IS that! :) I was just going the the flow of conversation, if you want to make H2 out of NG at the fueling station then fine. They will have to develop the adsorbant to lower storage pressure, get the price down and reliability up, but heck, once they get over those hurdles it could be smooth sailing.

Although the article says many times that this is cheaper than existing platinum catalytic, I had to look up how efficient platinum is. Turns out to be 50% to 80%, not bad at all. Still compared to a battery, the hydrogen cycle would by 80% (water to hydrogen) * 90% (compression/decompression) * 70% (fuel cell) = 50% vs about 90% for lithium battery. This means the fuel would be only about 80% more expensive. Much better than I expected.

Are drivers willing to pay a 1.8 times premium for fast hydrogen fill-up vs DC quick charge, and at the same time give up the low-cost convenience of charging at home?

I don't think so.

PEM fuel cells are around 50% efficient in cars and electrolysis is around 70% nominally. SO 7. x .9 x .5 makes about 31% or so.

@Roy H,
Please don't forget to calculate the depreciation cost of the Lithium battery for each discharging cycle and add that to the total cost of battery electricity. For example, for LiFePO4 battery costing ~$400/kWh capable of 3000 cycles, each kWh would cost $400/3,000= 13 cents of battery life depreciation + 10 to 15 cents of grid electricity cost = 23-28 cents per kWh of battery electricity PLUS the premium fee charged for the use of Quick Charge station, which costs a lot to build and must be amortized.

With H2 storage tank, the depreciation cost of the tank is less than 1/10th of the depreciation cost of battery per kWh. So, even if H2 at the filling station would cost, per kWh, double the retail cost of grid electricity, the total cost to the car owners would come out comparable!

@SJC,
Latest electrolyzers are capable of 80% efficiency, while latest FCV's are capable of 65% efficiency,so the round-trip efficiency would be in the order of 52% for H2-FCV.

For bulk energy storage of excess solar energy obtained in the springs or falls to be used in winters, batteries would simply be too expensive and bulky. H2 is a more efficient and less expensive method of bulk energy storage from one season to the the next.

In the winters, when both heat and electricity will be needed, the efficiency of H2 utilization can approach 100% when used for heating or when waste heat from electrical generation will be used for home heating or cabin heating of cars or buses.

For buses, trucks, or personal vehicles that must run all day hence needing quick refill, H2 will be more advantageous. Quick charging for large battery packs is out of the question, due to the tremendous current requirement that would brown out the whole grid!

3 things, Roger:

  1. The $400/kWh figure will be considerably lower by 2015, when FC cars are supposed to be hitting the market.
  2. The 3000-cycle value is also likely to increase, as crystalline materials are replaced by new ones based on things like graphene.
  3. The cost of electricity is likely to be considerably less than 10¢/kWh for overnight charging, and DSM/V2G service may even return revenue to the vehicle owner.

Assuming it is 50% efficient to get electricity out of the wall and then get it back out of the fuel cell, it is still about 80% efficient to get it in and out of a lithium cell. Batteries are more like $10,000 while fuel cells are more like $100,000 now.

Neither looks at the efficiency of getting electricity to the wall. Coal plants are perhaps 40% efficient and there are transmission losses. So you have 40% X 50% for 20% and then you need to go through the power control section at 90% and motors at 90% for about 15% mine to wheel efficiency.

I have done the analysis before of one therm of natural gas at a power plant making it all the way down and through the batteries to turn the wheels of a car on an EV versus putting that therm of natural gas in a car now and running it. While the battery car wins by a bit the fuel cell car with electrolysis less so.

Look at a therm of natural gas at a power plant producing electricity at 40% efficiency, even with no transmission losses you have supposedly 80% electrolysis efficiency and supposedly 65% PEM fuel cell efficiency (both optimistic) for 20% from the therm to getting the electricity out of the fuel cell.

Now you have the controller at 90% and the motor at 90% (both optimistic) for 80% to turn the wheel. So you have about 15% efficiency from that therm of natural gas at the power plant to turning the wheel of the car. Battery cars come out at about 25% and internal combustion of the natural gas in a car comes out to about 20%.

If these numbers are even close, then you have 25% with a BEV, 20% with an ICE and 15% with an FCV using electrolysis. Round numbers that do not take into account wear, depreciation or other infrastructure factors, but you can see there are few silver bullets.

As a final analysis, let's say you can reform natural gas to H2 at the fueling station with 70% efficiency and you can compress and run a fuel cell in a car at 60% (optimistic) then you have 40% from therm to electricity in the car.

Now you have the 80% combined of controller/motor to get the mechanical energy to the road for 30% total therm to ground. Even though these numbers are rounded you end up with:

30% reforming natural gas for a fuel cell
25% power plant to BEV
20% natural gas in an ICE car
15% power plant to electrolysis to fuel cell to the road

If these are even close to the real numbers, then reforming natural gas for H2 at the fueling station wins. But now you have a 10,000 psi tank that is big, heavy and has a limited life span. Or you have a big, heavy tank with adsorbant at lower pressure. Or you turn the natural gas into methanol and reform it on the car (my favorite).

@E-P and SJC,
Agree that the cost of battery will come down, and the cost of renewable energy will come down too, eventually to be cheaper than fossil-fuel energy. This is so because fossil fuel is a finite resource, while renewable energy is infinite, in relative term. At that point in the future, then, the H2 economy will make sense, as a means of bulk energy storage for the intermittent renewable energy sources. Battery can store renewable energy for a few hours to a few days, while H2 can store energy for days, weeks to months. Right now, H2 cannot yet compete cost-wise with natural gas.

That's why the research as reported in this article is very important, to make H2 as practical as soon as possible. The world cannot afford to wait!

I would like to see us get rid of combustion for cars, so that is why I favor bio methanol for fuel cells. Methanol can be used in ICE cars as M100 or mixed with synthetic gasoline made from DME. The pump would have SG100 and M100 and you can blend at the pump what ever your vehicle requires, no "blend wall".

Once we get enough M100 ICE cars then the need for synthetic gasoline (SG100) reduces. M100 can be put in fuel cell car tanks and reformed into H2 with bio CO2 being released without combustion and NOx. There is a good possible future, but as long as short sighted greed and selfishness buys its own future, we will be at a loss.

I think people have the efficiencies wrong here. NG-fired CCGT plants hit 60% (LHV basis), the best simple-cycle GTs hit the mid-40's. Grid transmission averages 93% efficient, Li-ion batteries 95%. The best chargers announced claim 99% efficiency. This gives a net BEV efficiency from pipeline to motor of ~39% for NGGT, 52% for CCGT.

If reforming NG to H2 is 70% efficient and the FC is 70% efficient, you're already down to 49% without considering compression overhead or leakage losses. The BEV is not just neck-and-neck with H2FCV at worst, it avoids a $trillion in new infrastructure.

@E-P,
Thank you for your update of the efficiency numbers.
Reforming NG to H2 is only a transitional step to the ultimate use of H2 as storage medium for renewable energy.

A $trillion invested in renewable energy and H2 infrastructure will create a lot of jobs and will create a big economic stimulus to the local economies everywhere and in all regions. In contrast, some fossil fuels are imported, and even if obtained domestically, only benefit a certain region.
With renewable energy and H2 infrastructure, we will continually reap the benefit from cleaner environment and avoidance of climatic catastrophy.

A $trillion invested in renewable energy and H2 infrastructure will create a lot of jobs and will create a big economic stimulus to the local economies everywhere and in all regions.
Spending money just to get back to where we are now is not a stimulus. That's already got a name: the "broken windows fallacy".

Make the numbers accurate to the third decimal point if you like, but the FVC and EV numbers don't lie. Once you calculate the TOTAL loses from well to wheel, there are no silver bullets in the picture.

Then people will change the game and say we should do nuclear power, but then there IS that pesky waste issue for oh, say thousands of years. It is good to get the whole picture and not keep changing the rules to suit the preconceived desired outcome.

There's the little fact that the USA has roughly 100 times as much energy available as uranium IN INVENTORY than the entire proven reserves of natural gas. The most efficient product of wind, solar and nuclear power is the same: electricity. The FCV is fighting uphill compared to the BEV.

@E-P,

>>"Spending money just to get back to where we are now is not a stimulus. That's already got a name: the "broken windows fallacy".

Changing over to mostly-renewable energy economy from the mostly-fossil-fuel economy of today is a very big advancement for humanity. The economy will be far more stable without the occasional energy crises like we have been having. Energy prices will be rock stable once the switch over will occur and that will help businesses plan well for the future and start hiring people.


>>"The FCV is fighting uphill compared to the BEV

Not really, FCV's can be big vehicles including HDV's, including ocean liners, while BEV's can be smaller vehicles that will be largely LDV. Both will easily co-exist in the future renewable energy economy.

Even if you throw nuclear into the picture, nuclear energy plants will be operated at near full-rated capacity to recoup the high investment cost. This will means that excess nuclear electricity produced in the springs and falls will still have to be stored as H2 for use in the winters.

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