In year 2004 June, British Petroleum provided an intriguing statistics on global oil reserves estimated in the company’s yearly review. According to the data at hand, company projects that the world’s oil reserve is just enough to last the world 53.3 years at the current production rates. It’s of no doubt that energy is one of the major challenges that mankind will face in next few decades to come. Not only our main source of energy; petroleum, is depleting and its energy producing reaction; combustion, generates carbon dioxide and carbon monoxide (incomplete combustion) usually with toxic gases like NOxs and SO2. These gases are commonly associated for air pollution and greenhouse effect.
Opportunities and Challenges
Over the years Hydrogen energy is given the utmost attention as the prospective fuel source of the future. This is not only because Hydrogen has highest energy density by weight (141.80 MJ/Kg compared to 44.80 MJ/Kg in diesel) among the available fuels but also its combustion generates only water as by product. But the problem is almost all of the world hydrogen production is coming as byproducts of hydrocarbon purification. Hence, an alternative method to produce Hydrogen is of paramount importance if we are to fulfil tomorrow’s energy demand.
Fortunately, the largest reserve of Hydrogen is not in short supply in the world. It is water; life-giving combination of two Hydrogen atoms covalently bonded in to a central Oxygen atom. If we can somehow split the bonds we can get a Hydrogen gas molecule from each water molecule. However, it turns out that splitting of the water molecule would require more energy than the energy we can produce by burning Hydrogen that was made. This means, energetically at least, it is a losing game. However, if we can use free source of energy available to us to drive this water splitting reaction, the table would turn. This is exactly what scientists are trying to achieve by a technique called photocatalytic water splitting.
In photocatalytic water splitting, special type of nanoparticles called photocatalysts absorb light energy to produce an electron and a hole in its structure. These species migrate in to the surface of the water splitting catalyst and attack water molecules and split the chemical bond. Among other techniques, photocatalytic water splitting has emerged as the most practical and less complicated methodology. Hydrogen generation with photocatalytic nanoparticles however, was mainly limited to catalysts such as doped TiO2. However, conventional water splitting catalysts only can absorb light in the UV wavelengths and also the energies of the hole species produced didn’t have enough oxidation power to drive the hydrogen generation reaction. This has significantly affected the efficiency of these products to be used as the practical methods of making hydrogen from water.
Game changing catalyst
A team at MIT, led by famous chemist Daniel Nocera, invented a superb water splitting catalyst that made the grim prospects of practical hydrogen generation whole lot brighter again. The material team has invented is mostly composed of pure silicone; the material most solar cells are made of. It also contains cobalt, nickel, zinc and molybdenum which are abundantly found in the earth.
One side of the silicone sheet is coated with nano sized cobalt based catalyst which drives the oxygen generating reaction of the cell. The other side is coated with a layer of an alloy made from nickel-molybdenum-zinc which drives the all-important hydrogen generating reaction.
The team calls the invention, “artificial leaf” meaning that as natural leaves absorb energy from the sun and water from the environment to produce sugars, artificial leaf also takes energy and the water from the environment to produce energy form that we are very fond of; Hydrogen.
The water splitting catalyst can be viewed as a silicone solar cell with different catalytic materials bonded on to the two sides of the sheet. It does not require any wiring or any control circuits to operate. Unlike conventional solar cell that converts solar energy to electrical energy, artificial leaf converts solar energy directly to the chemical energy that get stored in the Hydrogen gas. We can use Hydrogen gas to run internal combustion engines to run our cars or to generate electricity with a hydrogen fuel cell. The cell is so advance in terms of efficiency that when exposed to sunlight, oxygen and hydrogen bubbles from two sides of the catalysts are formed almost instantly. The team usually keep the two sides separated through a physical barrier and collect each of the gasses to store them separately to use them later.
The team has started a company called Sun Catalytix, which specialized in photocatalytic water splitting technologies using sunlight for futuristic applications such as home hydrogen production.