sunshine in a bottle

7/26/2019 Sunshine in a Bottle

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Sunshine in a bottle

Mimic the dance between carbon, hydrogen and oxygen, and you can tap

into clean solar energy and ease climate change

byPeter Forbes

Photo by Jessica Holden/Gallery Stock

Peter Forbes

is a science writer whose work has appeared in New

Scientist, TheGuardian, The Times,Scientific Americanand New

Statesman, among others. His latest book, co-authored with Tom Grimsey,

is Nanoscience: Giants of the Infinitesimal (2014). He lives in London.

When I think about the future of renewable energy, I picture the innerworkings of a leaf any leaf. A green plant is a remarkable solar-energy

collector, effortlessly pulling sunlight, water, and carbon dioxide from the

environment, and converting it into stored chemical energy. And the total

amount of energy processed by photosynthesis is enormous. The Sun

bathes the Earth with 173,000 terawatts of solar energy annually. On land

alone, plants convert that energy into more than 100 billion metric tonnes

of biomass. Our global energy use is just 18 terawatts per year, in contrast.

As solar energy proponents are fond of saying: The Sun provides in an hourenough energy to supply the world for a year.
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Humans already have a long tradition of exploiting sunlight trapped by

plants. That is where coal, petroleum and natural gas came from: they are

the fossil remains of ancient biomass, accumulated over many millions of

years. The problem is that burning fossil fuels releases millions of years

worth of carbon dioxide back into the atmosphere all at once. What we

really want to do is replicate the process now, creating new fuel as quickly

as we consume it, with the whole process driven by sunshine. Then we

could bring solar energy to places it has never gone before. We could

provide an unlimited supply of liquid fuels for aircraft and heavy-duty

vehicles such as tractors and trucks, not to mention feedstocks for the

plastics, paints and pharmaceutical industries all with no net carbon

emissions.

The obvious first thought is: why not just let the plants do the work? They

have already mastered the necessary molecular technology. Weve tried

that with biofuels derived from corn, soya, or algaebut if we grow crops

like corn for fuel, were robbing the Peter of food production to pay the Paul

of carbon-neutral energy. We could install algal bioreactors in places where

crops cant be grownbut then the amounts of water, fertiliser, and energy

consumed in processing the fuel are formidable.

We therefore need to tap the suns energy in a novel, synthetic way. Andthat way actually needs to improve on nature, audacious though that

sounds, because the solar energy figures I just mentioned are not quite the

cause for optimism they seem. Natural photosynthesis is only one per cent

efficient. The biomass that became fossil fuels was based on sunlight falling

unhindered on every square centimetre of exposed ground, every second

of every day, for as long as there have been green plants. To make a

meaningful, environmentally sound contribution to the energy supply, we

have to create an industrial process that can make a serious dent in the 36billion tonnes of CO2 emitted annually by human consumption of fossil

fuels, year in and year out. In other words, we need to do what plants do,

but even better.

Although that sounds daunting, the more we know about natural

photosynthesis, the more we can see that, since it has been cobbled

together piecemeal by evolution, rational design ought to be capable of

improving the yield. The essence of the natural process is to split water to

yield hydrogen and to use the hydrogen to remove the oxygen from CO2to


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make hydrocarbons. What nature accomplishesand what we want to do

is to remove some CO2from the atmosphere to create biomass. If our

human nanotechnology can mimic that process, we will use up CO2as

quickly as we produce it. It is almost too elegant that the key ingredient for

addressing climate change could be the substance that is causing the

problem in the first place.

The eventual goal is to obtain the CO2for fuel production from the

atmosphere itself, but there the CO2concentration even at its swollen

level of 400 parts per million is impractically low by current industrial

process standards. At present, waste gases from industrial sources such as

coal- and gas-burning power stations, steelworks and cement factories

constitute the best source of CO2 for fuel generation. They also neatlyencapsulate the appeal of liquid solar fuels, as we could transform

smokestack fumes from polluting industries into the raw material for a new

kind of green energy.

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Fortunately, engineers are not heading into entirely uncharted territory.

Chemical reduction of CO2to make hydrocarbon fuel is already a tried and

tested process. Based on a German invention of 1925, it uses cobalt or iron

catalysts plus energy to make a range of hydrocarbons for fuel, lubricants,

or feedstock. The process has been embraced where economic

circumstances render the extra energy cost acceptable. During the Second

World War, Germany, with no access to oil, used this technology to create

fuel. South Africa today derives about 25 per cent of its fuel by similarmeans.

The German process doesnt achieve the desired environmental goals it

actually increases CO2emissions but it has inspired a promising step

toward true artificial photosynthesis in the hands of George Olah, a

Hungarian-American chemist, now 88 years old. Olahs approach uses

hydrogen produced via renewable electricity in a catalytic process to

reduce CO2to hydrocarbon or alcohol fuels.


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The term reduce has a special meaning in chemistry, and is central both to

the chemistry of life and to the quest for renewable solar fuels. Look around

the countryside on a nice, sunny day and you can see the central chemical

principle of life on Earth. The dense mass of greenery and the blue sky

represents the twin poles of life: oxidation and reduction, or redox. Air in

the sky contains oxygen that liberates energy when it combines with

organic compounds; oxidationis the process that creates fire, and also that

powers your metabolism. The mass of green, on the other hand, is matter

in a chemically reduced state, which is the opposite of what happens in

respiration and combustion. In the presence of oxygen, reduced

compounds can be thought of as having stored energy. Just as oxygen is the

element of oxidation, hydrogen is the element of reduction.

These two elements have been linked in a close dance ever since Earth was

formed, but to complicate matters there is a third partner: carbon. Carbon

can exist in an oxidised state (thats carbon dioxide CO2) or in a reduced

state with hydrogen atoms attached, as in biomass and fuel. All living things

consist of reduced carbon, great long chains and helixes and complicated

clumps of carbon and hydrogen with other key elements attached in

strategic places. Redox reactions the molecular dance between carbon,

hydrogen and oxygen underlie three great mysteries: the origin of life,

how to mitigate global warming, and how to tap the Suns energy without

plants.

The laboratory for Olahs CO2-reducing process is located in Iceland because

of its abundant renewable electricity, generated from that countrys natural

thermal springs. Since 2011, the George Olah Renewable Methanol Plant,

operated near Reykjavik by Carbon Recycling International, has been using

electricity from a thermal power station to split water into water and

hydrogen. A nearby cement works provides a source of waste CO2. Thehydrogen produced by the plant reduces the CO2to methanol. The

methanol (sold by Carbon Recycling International as Vulcanol) can be used

as fuel for vehicles, either straight or mixed with petrol. In July 2015, Carbon

Recycling linked with the UK division of the engineering firm Engie Fabricom

to develop large, standardised CO2-to-methanol plants. Although Icelands

energy situation is unique, George Olah notes that many parts of the world

have access to other forms of cheap renewable electricity (hydropower or

solar-thermal power, for instance) that could drive the plants.


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The Olah process is far from artificial photosynthesis, however. Turning

sunlight directly into useful liquid fuels requires understanding the detailed

electro-chemistry of what goes on in green plants, and then learning how

to beat nature at its own game. The details of the photosynthesis process

are immensely complicated: the water-splitting system in plants, called

photosystem II, has two almost identical halves, each of which has 19

protein subunits that use 35 chlorophyll molecules. But at the most basic

level, scientists understand quite well how plants use sunlight to generate

electricity.

Photosynthesis ultimately depends on the photoelectric effect, explained

by Albert Einstein in 1905, in which photons of light interact with electrons,

knocking them free of their atoms. It is the process behind silicon solarpanels. Normally, when sunlight knocks an electron out of any substance,

the electron jumps straight back in. What the natural photosystems do is to

prevent the electrons recombining by smuggling them down a chemical

pathway from which the electron cannot return. A combination of minerals

magnesium in chlorophyll, manganese and calcium in the water-splitting

photocentreand a surrounding protein matrix constrain the electrons so

they have no choice but to be shuffled away.

If our technical catalytic systems fall short of natures, why not just workwith natural organisms?

The task for artificial photosynthesis researchers is to find an equivalent for

the natural pass-the-electron-parcel chains. A lot of the research has

centred on photosystem II, built around an unusually structured group of

manganese, oxygen and calcium atoms (Mn4O5Ca) known as a cubane,

which is embedded in proteins. The bonds between the atoms of cubane

are distorted by the protein matrix; the resulting strain is what enables its

catalytic (reaction-inducing) activity. This manganese, oxygen and calcium

reaction centre is perhaps the chemical crux of life on Earth. But it turns out

that slavishly copying it might not be the best way to create an artificial

photosynthesis system of our own.

Researchers have tried many, many alternatives to the cubane-based

catalyst in green plants, with only limited success. That slow progress has

inspired a whole other approach: if our technical catalytic systems fall short

of natures, why not just work with natural organisms that already havetheir own alternatives to green-plant photosynthesis? Weve seen the


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drawbacks of using off-natures shelf biomass from corn, soya, or algae, but

could there be a useful halfway point between natural photosynthesis and

a full-blown artificial version? It turn out there is.

There is a group of primitive bacteria the acetogens that can reduceoxides of carbon without photosynthesis. These microbes perform the

special trick of being able to live off the very gases we are concerned with:

oxides of carbon (carbon monoxide and carbon dioxide), along with

hydrogen. They can generate alcohols from these raw materials and, even

better, can do so using a variety of ratios of hydrogen and carbon

monoxide. This flexibility makes them well-suited for industrial use,

because just such mixtures of gases are produced as the polluting waste

products of electricity generation, as well as steel and cementmanufacture.

LanzaTech, a US energy company devoted to producing liquid fuels from

industrial waste gases, is one of the leading proponents of acetogens. These

ancient bacteria are found naturally today around hydrothermal vents in

the deep ocean, where they live on the hot gases that well up from the

ocean floor. LanzaTech is focusing on one specific bacterium,Clostridium

autoethanogenum, to generate ethanol from waste gases, mostly carbon

monoxide and dioxide from steel mills.

Jennifer Holmgren, LanzaTechs CEO, recognises that having a clever idea is

not enough if you are trying to shift the enormous fossil-fuel industry.

Scaling up is the most important thing for any new technology, she says.

If it doesnt scale, it doesnt matter.To that end, the company has created

a demonstration plant at the Baosteel mill in Shanghai, China, and last year

they signed an agreement with the worlds biggest steel-

makers,ArcelorMittal, to build a 87 million fuel-generating plant at their

Ghent steelworks in Belgium. LanzaTech has also signed a deal to supply

Virgin Atlantic with bio-aviation fuel.

This last venture touches on one of Holmgrens key concerns, bringing

carbon reductions to the parts of the energy economy that green electricity

cannot easily reach. If we go to electric vehicles on a large scale, how do

we balance the system? she asks. The system requires production of

fuelsground and aviationand chemical coproducts. If the ground fuels

portion goes off to electric, lets say 30 per cent of ground transport, whathappens to the economics of aviation fuel and chemicals production?


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LanzaTechs approach is an important step toward true artificial

photosynthesis, since it yields biofuels without relying on the usual green

plants, but it is still only a beginning. More far-reaching are the experiments

now underway to develop hybrid fuel-production systemsones that still

exploit energy-harvesting mechanisms found in nature, but that add

synthetic components to make them serve our needs more effectively.

This work has been greatly aided by the remarkable discovery that some

bacteria can live directly off a diet of electricity. Peidong Yang, a Chinese-

born professor of chemistry at the University of California, Berkeley, has

exploited this appetite for electrons by matching the bacteria with

microscopic semiconductors that act as tiny solar cells. The bacteria grab

electrons from the semiconductors and use them to reduce CO2. Its abrilliant synthesis: semiconductors are the most efficient light harvesters,

and biological systems are the best scavengers of CO2.

Methane is not a liquid fuel, but it can readily be converted to one. It can

also be used directly as natural gas to run power plants

Yangs team is currently studying three different systems. In one, the

researchers built a forest of silicon and titanium dioxide nano-wires as the

light harvester, and then cultured the bacterium Sporomusa ovatato grow

over the wires and feed on the electricity. In another system, the

researchers precipitated light-harvesting cadmium sulphide nanoparticles

onto Moorella thermoacetica; the particles enable the previously non-

photosynthetic bacteria to turn light, water and CO2 into acetic acid, which

can readily be transformed into fuels such as butanol, or synthesised into

plastics and pharmaceuticals. It is artificial photosynthesis in a truly

profound way, bringing the photosynthetic ability to an organism that never

had it for billions of years.

The third method is the most conventional, but it also looks like the most

likely one to scale up. Combining an electrochemical cell (driven by

electricity, sunlight, or a combination of the two) with the bacterium

(Methanosarcina barkeri) produces methane with an impressive 10 per

cent solar-to-fuel conversion rate. Methane is not a liquid fuel, but it can

readily be converted to one. It can also be used directly as natural gas to

run power plants. This approach could solve one of renewable energys

most pressing problems. Electricity cannot be easily stored, and both sunand wind are powerful but intermittent energy sources. Solar-generated


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methane can be stored to provide electricity generation when the sun

doesnt shine and the wind doesnt blow.

Unlike natural photosynthesis, all of these artificial systems at present

require concentrated CO2to work. Ideally wed be working with 400 ppm[parts per million] CO2in the atmosphere, but no one knows how to do that

yet, no one, Yang says. There is an upside, though. The current approaches

can be readily coupled with carbon-capture technology to pull CO2 from

smokestack emissions and convert it into fuel. This is the essential element

of a closed carbon cycle that mimics nature, consuming the carbon created

by human industry rather than dumping it into the environment. But that

cycle still ultimately depends on the presence of the polluting industries.

Then again, we do now know how to use CO2 drawn directly from the air,on a laboratory scale at least. In January this year, George Olahs group at

the University of Southern California reported dramatic new work. Olahs

colleague G K Surya Prakash along with the PhD student Jotheeswari

Kothandaraman have developed a combined process that uses a polyamine

(a class of organic molecules essential both to life and to many industrial

chemical processes) to capture carbon dioxidefrom the atmosphere, in

conjunction with a ruthenium-based catalyst to reduce the CO2to

methanol. Ruthenium catalysts have been employed before to reducecarbon dioxide, but making the process work at atmospheric levels of CO2,

in a unified process with the carbon-capturing reaction, is a notable

advance. In tests, up to 79 per cent of the CO2captured from the air was

converted into methanol.

The Olah group have been pursuing their vision of a methanol economy

for many years and, with their experience from the Carbon Recycling plant

in Iceland, they are well-placed to figure out how to make it work in a

commercially viable way. Doing so will involve juggling a bewildering array

of processes and market variables, though. Large-scale capture of

atmospheric CO2would require prodigious quantities of polyamine, which

raises issues of environmental safety. Ruthenium is a rare-earth metal that

has seen considerable volatility in supply and cost. Its current price is

around $42 per ounce, but a decade ago the price was more than $850.

These challenges should not deter us. We have grown used to accepting

that we have to follow wherever the market leads us, which is how fossilfuels have remained so entrenched in the global economy for so long. But


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today there are bigger concerns than short-term market efficiency. We

must have a reliable, secure, long-term, carbon-neutral fuel supply. That is

the cornerstone of our future energy needs, and the other arrangements

will have to be fitted around it.

Carbon is precious. We must learn to recycle it. There should be no waste.

There is no waste in nature

Back in 2008, the photosynthesis expert James Barber of Imperial College

London advocated an Apollo-style programme, comparable in scale and

urgency to the 1960s Moon race, to develop solar fuels. Its taken a while,

but their call is finally being heeded. Once the least known of renewable

energy technologies, solar liquid fuels now have powerful advocates. In

particular, Bill Gates recently organised the Breakthrough Energy Coalition,a group of 28 investors aiming to boost world spending on carbon-free

energy development to $20 billion a year. He also catalysed Mission

Innovation, a 20-major-nation governmental initiative launched at the Paris

climate change conference in December 2015.

Gates has some powerful advantages. He understands both the

technological and financial challenges, and has plenty of financial resources

himself, having pledged $2 billion to the project. His thinking is outlined in

a paper,Energy Innovation: Why we need it and how to get it. The US

government is also getting on board with the new approach. In April 2015,

the Department of Energys Joint Center for Artificial Photosynthesis (JCAP)

announced renewed funding of $75 million and a change of direction, away

from hydrogen production and toward the kind of solar-generated liquid

fuels Ive been describing. With researchers, foundations, major world

governments, and large investors all pulling in the same direction, success

is looking far more probable that it did just a couple years ago although,

as Gates points out, such major technological shifts have typically taken

decades in the past. At the same time, programs like JCAP are puny

compared to the total magnitude of the R&D effort needed.

The costs are high, but the potential payoff is even higher. Holmgren at

LanzaTech lays out a compelling vision: Carbon is precious. This means we

must learn to recycle it. If you can extend its life by reusing it in a fuel, you

will keep that equivalent amount of fossil fuel in the ground. There should

be no waste. There is no waste in nature.
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Redox reactions the dance between carbon, hydrogen, and oxygen

produced the cornucopia of life on Earth. Right now, we are merely running

down those reactions, unwinding millions of years of biochemistry that is

locked away in the planets fossil fuels, and systematically polluting the

atmosphere in the process. We need to understand the redox reactions, so

we can master the biomechanical machinery of photosynthesis and start

building up with it. Success could transform the world economy, and the

global environment. It is a challenge we cannot afford to pass up.

sunshine in a bottle

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