## Stanford team increases power of corrosion-resistant solar cells; advance for solar fuels

##### 30 November 2015

Researchers at Stanford, with colleagues at University College Cork in Ireland, have shown how to increase the power of corrosion-resistant solar cells, setting a record for solar energy output under water. Instead of pumping electricity into the grid, the power these cells produce would be used in the production of solar fuels.

This new work, published in Nature Materials, was led by Stanford materials scientist Paul McIntyre, whose lab has been a pioneer in the field of artificial photosynthesis. Artificial photosynthesis proposes using the energy from specialized solar cells to combine water with captured carbon dioxide to produce industrial fuels.

The results reported in this paper are significant because they represent not only an advance in performance of silicon artificial photosynthesis cells, but also establish the design rules needed to achieve high performance for a wide array of different semiconductors, corrosion protection layers and catalysts.

—Paul McIntyre

Until now, artificial photosynthesis has faced two challenges: ordinary silicon solar cells corrode under water, and even corrosion-proof solar cells had been unable to capture enough sunlight under water to drive the envisioned chemical reactions.

Four years ago, McIntyre's lab made solar cells resistant to corrosion in water. In the new paper, working with doctoral student Andrew Scheuermann, the researchers have shown how to increase the power of corrosion-resistant solar cells, setting a record for solar energy output under water.

Metal oxide protection layers for photoanodes may enable the development of large-scale solar fuel and solar chemical synthesis, but the poor photovoltages often reported so far will severely limit their performance. Here we report a novel observation of photovoltage loss associated with a charge extraction barrier imposed by the protection layer, and, by eliminating it, achieve photovoltages as high as 630 mV, the maximum reported so far for water-splitting silicon photoanodes.

—Scheuermann et al.

The vision is to funnel greenhouse gases from smokestacks or the atmosphere into giant, transparent chemical tanks. Solar cells inside the tanks would spur chemical reactions to turn the greenhouse gases and water into solar fuels.

We have now achieved the corrosion resistance and the energy output required for viable systems. Within five years, we will have complete artificial photosynthesis systems that convert greenhouse gases into fuel.

—Andrew Scheuermann

Years of work have gone into developing solar cells that could operate in water permeated by corrosive greenhouse gases. McIntyre's lab solved the corrosion problem in 2011, by coating the electrodes with a protective layer of transparent titanium dioxide.

However, the first-generation, corrosion-proof cells still couldn’t extract enough voltage from the sunlight as it filtered though the water.

Scheuermann has shown how to make the corrosion-resistant solar cells more powerful by adding a layer of charged silicon between the titanium oxide and the basic silicon cell.

The resulting device consists of several layers with different electronic functions. The active silicon layer rests at the bottom, absorbing sunlight and exciting electrons. Above that active layer sits the new silicon dioxide booster, which increases the voltage. On top of the booster the transparent titanium dioxide seals the system and prevents corrosion, and also serves as a conductor.

These three layers are coated with iridium, which serves as the catalyst that allows CO2 and H2O molecules to meet. The electricity conducted from below breaks the chemical bond on these two molecules and recombines the elements to produce pure oxygen and methane (CH4).

Beyond this specific application, the engineers also provided design principles to help the photovoltaic industry and scientific community build energy-efficient, corrosion-protected solar cells for other purposes. Here they collaborated with Paul Hurley, co-author on the paper and senior research scientist at the Tyndall National Institute in Cork, Ireland.

Resources

• Andrew G. Scheuermann, John P. Lawrence, Kyle W. Kemp, T. Ito, Adrian Walsh, Christopher E. D. Chidsey, Paul K. Hurley & Paul C. McIntyre (2015) “Design principles for maximizing photovoltage in metal-oxide-protected water-splitting photoanodes” Nature Materials doi: 10.1038/nmat4451

Anyone who thinks that the only, the sole, the inevitable way that we will power transport in the future is by BEV cars with large batteries has their eyes closed tight and their fingers in their ears.

It is about as inevitable as the triumph of the proletariat.

I'll be an enthusiastic supporter of Hydrogen or other synfuels when they can compete on price, safety, availability and "well to wheel" emissions.

But it will be a long road from a proof of principle artificial leaf to a nationwide and global shift to production, storage, distribution, retailing and transportation use of these fuels.

Like so many technology innovations, it's a race for market share. When $30k, 200 mile BEVs are in production in 2017-2018, hydrogen will be a very long shot indeed. Liquid synfuels may have a role, but absent a carbon tax or other limiting mechanism for fossil fuels, it all comes down to price. I'm an enthusiastic supporter of battery electric cars, but the costs of batteries are nowhere near low enough, nor are energy densities increasing fast enough to make them competitive to ICE cars ex subsidy. And FUD on the 'dangers' of hydrogen comes from the same place that FUD on excessive danger of batteries did. Replacing 200 million vehicles at a 1% rate will leave us with 1% on the road. We need a low carbon bridge, this could be one of those. Combustion pollution is another problem, so reform those renewable fuels on the vehicle then use fuel cells. @SJC: Combustion pollution (VW notwithstanding) has been largely mitigated for gasoline-powered cars (CH4-powered cars are even cleaner). The energy density and quick refueling capability of liquid fuels makes them hard to beat for vehicular transport. Green synfuels could be a great solution if created cheaply from low/no-carbon sources such a sunlight or nuclear power. Look at Mexico City with few controls, their air is improving but how about the lungs of people who have lived there for decades? Many cities in the world could use cleaner air. Renewable gasoline and diesel made from biomass is lower carbon. There is lots of CO in gasified biomass, some solar hydrogen would make even more fuel. Nick Lyons, "Green synfuels could be a great solution if created cheaply from low/no-carbon sources such a sunlight or nuclear power." If those low/no carbon sources fed directly into a battery to drive a vehicle efficiency and cleanliness would be best. As alternative vehicles, FCEVs have a huge (range and quick refill) advantage over PHEVs and BEVs, during cold winter days. Reported in the Automobile Section of today's local paper: 1. Volt e-range was up to 94 Km (max) in summer time. 2. Volt e-range was UNDER 40 Km on cold winter days. Similar performance reduction probably apply to most PHEVs and BEVs. Batteries will have to be "winterized" to perform much better on very cold winter days before they can claim to be adequate for extended range BEVs. Nobody seems to recognize the two huge holes in this so-called "miracle" scheme: 1. It relies on iridium, an expensive and RARE platinum-group metal. It would be ironic if a push for this made it economic to refine spent nuclear fuel (so-called "high-level waste") for iridium, because that's the only source that's actually growing. 2. It relies on the effluent of fossil-fuel combustion as its source of material; it is a once-through scheme from fossil in the ground to atmospheric dump. The lack of insight here continues to disappoint me. SJC> Look at Mexico City Not to mention the California Central Valley, where more people die each year from air pollution than from auto accidents. It has not yet achieved "Bladerunner" proportions, but drive through The Valley late in the afternoon as the sun sets and you can see the air you're breathing. It's not too much to say that the sunsets are practically post-apocalyptic. How bad is it? The county air district will give you an extra$3,000 to drive a zero-emissions vehicle.

I have been through the valley in late December, I asked a station attendant what was up with the air quality, she indicated it was always that way. They get bay area pollution, another reason for cars that don't pollute as much.

California Central Valley sounds like a great place to set up a solar powered atmospheric scrubber as the land form naturally concentrates and holds airborne pollution from spreading. aka trees.
Maybe the cities feeding the pollution should pass laws requiring every person to plant enough trees each year (in the valley) to offset their share of the carbon footprint created by humans living there.

The focus of the article is on artificial leaves with a solar cell under water. I still don't understand why this is a good idea. Why not do the obvious: separate the solar cells from the electrolysis cell? The business case for solar requires long lasting solar cells (e.g. 30 years). That may be possible with cells in air. It seems unlikely in a water environment. And the cost of windows, water seals, and gas seals spread out over large area solar arrays seems like a big problem. Also, why should the optimum current density be comparable to the optimum solar density? With a "leaf" under water one has to have an electrolysis system that has about the same area as the solar array. It seems more likely that one can optimize the system by using normal solar arrays to produce electricity, use high efficiency power electronics to get the correct voltage and current, and then optimize the electrolysis cell separately.

EP:

If this were the only possible pathway and iridium the only possible catalyst, and combination with CO2 the only possible use of the outputted hydrogen, then your objections might have some force.

Since on this blog a huge array of pathways have been outlined then your narrow critique is rather disappointing.

Harvey:

Just so.

A car with a ~30kw or so range extender will do hugely better in any cool climate, and the need for a huge battery is obviated.

The development of this and similar pathways to produce hydrogen and its derivative compounds in no way detracts from the ability to use batteries.

Having this sort of technology to hand simply means that batteries can be used more effectively.

I cannot understand the mentality which feel the need to decry advances which may reduce the supposed universality of the battery solution.

Fuel cells, hydrogen, synthetic gasoline and so on combine superbly with batteries, and they complement each other rather than being a detriment.

TC
Air scrubbers are after the fact, don't put the pollution into the air in the first place.
C9
I agree, efficient solar and electrolysis would do well. The oxygen can be used for biomass gasification then the hydrogen can be combined with CO to make more renewable fuel.

Unless the carbon being fed to the fossil plants came from the air scrubbers in the first place, the system as a whole is still making a mess.

You could make this work with nuclear power.  Fission heat replaces most of the hydrocarbon combustion.  You run air scrubbers with e.g. zeolite sorbents to grab CO2 from the air.  You use off-peak nuclear heat to regenerate the sorbent and extract CO2.  You use this CO2 to generate the remaining hydrocarbons you still need.  If you want to, you can pack away CO2 in reservoirs like deep saline aquifers (if you have a place to dump the water) and store it against need.  Having big reservoirs of CO2 as feedstock or for emergency climate adjustments would be a handy thing to have.

If hydrocarbon fuels cost $5/gge but electricity was cheap, the vehicle market would probably optimize toward PHEVs for commuting and long-range fast-charging BEVs ala Tesla for the rest. Small sustainer engines mostly providing cabin heat would be the preferred option for cold-weather zones; if you could heat the cabin and keep the battery warm without impact on range, the EV would even be good in the far north. What this article missed is the JCAP project across the bay at berkley labs. The joint center for artificial photosynthesis is then 2nd 5 year research grant from DOE to make a working prototype based on these submergible photocells to catalyze H20 and CO2 into hydrocarbon fuels. The titanium dioxide coating is readily used in the prototype. Despite rhetoric on the usage of the artificial photosynthesis, this is carryover technology till the world completely switches to electrical transportation ( BEV, FCEV, MagLev etc..) MIT researcher Daniel Nocera (now at Harvard) was hot on the trail of artificial leaves as far back as 2008 and raised a fair amount of money ($18m) for his company Sun Catalytix.

The company eventually pivoted and pursued batteries. It was acquired by Lockheed Martin in 2014.

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