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On the road to solar fuels and chemicals

27 December 2016

In a new paper in the journal Nature Materials (in an edition focused on materials for sustainable energy), a team from Stanford University and SLAC National Accelerator Laboratory has reviewed milestones in the progress of solid-state photoelectrocatalytic technologies toward delivering solar fuels and chemistry.

Noting the “important advances” in solar fuels research, the review team also noted that the largest scientific and technical milestones are still ahead. Following their review, they listed some of the scientific challenges they see as the most important for the coming years.

As CO2 levels in the atmosphere increase and it becomes clear that this has substantial consequences for our climate, developing alternatives to fossil fuels is more urgent than ever. Fuels are unrivaled for energy storage and we have an enormous infrastructure built to handle fuels for transportation and heating. In addition, the chemical industry and all the products and materials that we rely on in modern society are completely reliant on fossil feedstocks. This motivates the development of sustainable processes to generate fuels and chemical feedstocks from water and CO2 using solar energy; such a process is analogous to photosynthesis in nature and is sometimes referred to as artificial photosynthesis.

There are large scientific and technical challenges involved in making even the simplest fuel, H2, and even more so for carbon-based fuels by means of CO2 reduction. Here we describe some of the recent progress and the obstacles that need to be surmounted to find economically viable solutions. In accordance with the theme of materials for sustainable energy, we focus on solid state devices for the production of solar fuels. There have also been important developments in molecular catalysis for fuels and we refer the interested reader to a recent review.

—Montoya et al.

Various device configuration powered by a renewable energy source perform electrochemical water splitting or CO2 reduction. One such connects a photovoltaic (PV) to a separate electrolyzer with catalysts that drive the necessary conversion reactions (PV/electrolysis). Another possibility is to combine these two pieces into a fully integrated system in which the catalyst is deposited directly on top of the solar absorbers to create a photoelectrochemical (PEC) device. A range of intermediate configurations also exist that allow partial decoupling of the device components and set various bene fi and constraints on the device operation.

PV/electrolysis and PEC devices require photon absorbers that can harvest solar energy efficiently as well as catalysts for water-splitting and fuel production. In addition, PEC devices require effective integration of the photon absorber and the catalyst. Resulting from these requirements, the challenges for solar fuels and chemical production fall into three areas: photon capture, electrocatalysis and integration.

Among the team’s findings and suggestions:

  • Materials discovery remains the biggest challenge for photoabsorber materials. While experimental and computational screening of new materials has begun, there are still many to be simulated and tested. Some insights into structure–property relationships have been derived from the large quantity of available band structure data, but there are likely to be a number of trends that might yet be discovered and leveraged into more effective searches. Stability, particularly in the aqueous environment at the electrode interface, will also be a key concern.

  • While there has recently been “remarkable progress” in developing new, non-precious metal catalysts for hydrogen evolution, there is still significant room for improvement. A larger challenge is to improve the performance of catalysts for the oxygen evolution reaction (OER). Even the best OER catalysts have overpotentials of 0.3 V or higher, suggesting that OER catalysis faces a fundamental obstacle to reducing overpotential losses that is very difficult to break through theory has shown that this overpotential barrier may be explained by scaling relationships between the stability of important intermediates for large classes of catalysts.

    New catalyst design strategies are needed such that the stability of different intermediates and transition states can be varied independently. This likely will require precise engineering of individual active sites and thus new approaches to designing and synthesizing nanostructured materials.

  • Creating solar fuels using CO2 reduction is even more demanding, the authors noted. While reasonably efficient catalysts exist to produce CO from CO2, there are no known catalysts producing hydrocarbons or alcohols with high rates and low overpotentials.

    The current state of the art for hydrocarbon synthesis is still Cu, which requires nearly 1 V of overpotential to produce 10 mA cm−2 of products further reduced than CO. The understanding of CO2 reduction electrocatalysis is still in its infancy. In general, an improved molecular-level understanding of chemical processes at the charged solid–liquid interface, and advanced descriptors (experimental and theoretical) of catalytic activity and selectivity will be necessary to screen for new catalysts.

  • Integrated systems require engineered interfaces between photoabsorber materials and catalysts resulting in the lowest losses.

We are far from having solutions to the production of solar fuels and chemicals, and we cannot rely solely on trial-and-error strategies to solve the challenges ahead. However, by improving fundamental understanding of materials properties relevant to each aspect of the photoelectrocatalytic device that connects the incident photon to the desorbing fuel molecule, we may accelerate the process of making artificial photosynthesis a reality.

—Montoya et al.

Resources

  • Joseph H. Montoya, Linsey C. Seitz, Pongkarn Chakthranont, Aleksandra Vojvodic, Thomas F. Jaramillo & Jens K. Nørskov (2016) “Materials for solar fuels and chemicals” Nature Materials doi: 10.1038/nmat4778

December 27, 2016 in Carbon Capture and Conversion (CCC), Fuels, Materials, Solar, Solar fuels | Permalink | Comments (13)

Comments

Yeah Yeah, we hear about solar gasoline since 12 years and they still need their subsidies for one more decade. STOP subsidizing them now and rely on private subsidies instead. If it's the private sector that give the money, we will have cheap solar gasoline in less than a year.

Energy conservation with move efficient cleaner shared e-mobility, modified construction code to reduce waste, improved HVAC and lights etc could offset the higher cost for cleaner solar fuels.

It will be difficult to convince current fossil fuel $$$$ to switch their investment to future clean solar energy production, storage and distribution. We need more TESLA/Panasonic, Denmark, Norway, California, etc.

We can do solar liquid hydrocarbon fuels, but there is not a huge profit in it and the government is in too much debt to support it.

The good news now is that Solar and Wind (S&W) electricity is coming down on par with the prices of Fossil Fuel (FF) power plants.
However, intermittency and non-dispatchability of S&W render new S&W unable to substitute for new FF power plants, unless e-storage or natural gas backup power plants are also built.

The problem with energy-storage systems is that they will double or triple the cost of Solar and Wind (S&W) electricity when energy-storage is employed in conjunction in order to make S&W reliable 24/7 source. Even worst, battery and pumped hydrostatic storage have very limited capacity that will be depleted after more than one or two days of dark clouds and little wind.

For this reason, Solar and Wind must cost-effectively depend on stand-by natural gas power plants that are already paid for. This will significantly affect the growth potential of S&W.
For example, if a community will need 300 MW of new generation capacity, that community cannot depend on S&W alone, but must build out additional ~250MW of natural gas power plant for use when combined S&W is very weak. This will significantly increase the cost of new S&W capacity and will make S&W much less competitive.

However, if we now shift strategy and employ Solar and Wind (S&W) to make high-value transportation fuel instead, then perhaps the economics may look more compelling.

At 3 cents per kWh and 55 kWh per kg of H2 at the pump, the raw energy cost will be only $1.65. Adding $2 to cover the cost of H2 production facility, distribution cost and operational cost, then 1 kg of H2 at the pump can have a pre-tax-and-profit cost of $3.65 per kg.
Adding profit and tax to this to price this S&W H2 to $6.8 per kg at the pump, and this will still be cost-competitive with gasoline car with 25 mpg and gasoline at $2.5 per gallon, when used in the Honda FCX Clarity at 68 MPGe, when both will have a fuel cost per mile of 10 cents.

Subtracting $3.65 from $6.8 = $3.15, and assuming 50 cents tax per kg of H2, will give a profit of $2.65 per kg of H2. Dividing this $2.65 to $6.2 pre-tax price = 43% profit potential. This is very good, considering profit margin for the major oil companies to be around 23%.

Roger, you neglect the capital cost of the electrolyser. This kills the economics of renewable generated H2. This is particularly the case since you can't run the electrolyser 24/7 because of the intermittent electricity source. On the hand, interest on the capital accumulates all the time.

On the other hand the economics of thermally enhanced H2 production using high temperature nuclear reactors looks good, except for the problem of proving the long term safety of the new class of reactors needed (and their capital cost!).
Nothing is easy.

@msevior,
ITM Power reported Hydrogen cost of £2.69/kg (US$4.13/kg)... after capital amortization, at 70% utilization, with electricity cost of 5c/kWh and 55 kWh per kg.

With the new-low electricity cost of 3 cents per kWh, we can expect a cost reduction of $1.10 per kg, thus only $3.03 per kg.

Within a 10-year capital amortization period, £4.19/kg (US$6.44/kg) with electricity cost of 5c / kWh. With electricity cost down to 3 cents per kWh, the H2 cost will go down to $5.34.
If we assume that the electrolyzer will continue to last for 10 years more after the 10-year amortization period, then the H2 cost will be the average of the $5.34 and the $3.03 = $4.18 per kg. To be cost-competitive with gasoline in the USA, the H2 can be priced at the pump at $6.8 per kg, so that will give a profit margin of about $2 after a $0.50 of fuel tax.

http://www.greencarcongress.com/2013/07/itmpower-20130722.html

Conclusion: Retail price of H2 for FCV at the pump made from the new-low costs of solar and wind electricity will soon be cost-competitive with gasoline on per-mile basis.

Hydrogen production probably the only practical energy for remote wind. Most think the future of wind is connected with hydrogen production. High temperature steam electrolysis improves the economics. I would guess a hybrid generation unit would be most efficient. Maybe variable wind power feeding into a steam power plant for hydrogen production coproduct.

Rooftop solar works well if households did flip to DC power, off the grid. They will still need natural gas connection, but that would be a terrific low emission high efficiency setup at a very nice cost break.

What's nice about wind power hydrogen setup, is the natural gas pipeline already in place. Think of the environmental power biogas that is easily distributed within this pipeline. The benefit this provides to both eliminating natures GW gas pollution and provide an energy source. Same distribution for hydrogen given the natural gas already contains a percentage of hydrogen and one reason the fuel receives a high grade for the environment. You can see the greening of natural gas energy. Same for fuel with ever increasing higher percentage of renewable fuel. The biological solar energy from plant matter that again decreases natures natural GW emissions, converts CO2, and provides an energy source. A triple wammy.

Roger: The assumed utilization rate is 70%. Most European wind sites are lucky to get 25%. The best middle-East solar sites get 30% for their sub-3 cent per KWHr production. So that doubles the cost of the renewable generated H2.

I think the economics would stack up in places like Texas, Quebec and the Pacific-NorthWest where electricity prices are stably low for long periods of time.

The Topic is the current state of solar and wind for the purpose of displacing current mined hydrocarbon feedstocks in the "Petrochemical" industries. The "Need" is to replace the current Carbon-Rich hydrogenated fuels to support the Plastics industries as well, not the diatomic H2 by itself.

The article mentions the CO2 to CO- OER process as being a technical roadblock given the high 1V over potential required with current supporting substrate chemistries/catalysts.

@msevior,
You do have a good point, however, the cost of H2 won't double as you projected. Here's why:

1) The capacity factor of wind in the USA is around 34%. New wind turbines with much bigger blades can produce power with less wind, hence can have capacity factor around 40-50%.

2) Combined solar and wind will raise the capacity factor, especially if half of the solar power can be fed to the grid during peak grid demand during the day, leaving the most of night-time wind for the electrolyzer. So, you would build out twice the solar capacity of the grid daytime demand, and on sunny day, use half of the solar PV power to make H2. On cloudy days, all of the solar PV output can be devoted to the grid. The combination of solar and wind can raise the capacity factor to above 50%, which is quite close to the 70% utilization factor in the cost calculation.

3) The H2-production facility includes power-handler, electrolyzer, purifier, low-pressure storage facility, H2 compressor and high-pressure H2 storage, and necessary piping. Assuming 50% combined wind and solar capacity factor, then the power handler and the electrolyzer will be used only 50% of the time, but the rest of the facility can be sized so that they can be utilized nearly 100% of the time. For example, inexpensive low-pressure H2 storage can store intermittent H2 production to be fed to the H2 purifier and H2 compressor and H2 dispenser that can be sized so that they can be utilized nearly 100% of the time.

When considering all the above factors, the cost of H2 facility will not exceed much of the cost calculation using 70% utilization factor in the H2 cost estimate by ITM Power, even when using RE with combined capacity factor of around 50%.

There are more efficient electrolyzers and more efficient CO2 to CO methods. I said years ago there would be synthetic and bio synthetic fuels. The usual suspect said if it could not do it all forget it. It seems this is a viable method after all.

Unfortunately, early small, badly located, wind turbines had very low (15% to 20%) production factors.

Many well located very large (on land) wind turbines, mounted on very high towers, already have over 50% production factors.

The latter require smaller storage units and are ideal when coupled with (variable production) Hydro plants with large water reservoirs.

H2 made with CAN $0,022/kWh 24/7 clean hydro/wind energy, could sell for about CAN $3.50/Kg.

Off-peak clean e-energy could be negotiated for a lot less (-50%) to produce H2 at about CAN $3.00/Kg.

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