Hydrogen as fuel

GREEN EARTH

Hemamala Karunadasa, Christopher Chang and Jeffrey Long of the University of California and the Lawrence Berkley National Laboratory at Berkley, California report in the journal Nature that they have developed an effective, synthetic catalyst that can help generate large volumes of hydrogen gas from water, including sea water. Getting hydrogen economically would be a source of energy without carbon dioxide emission.

There are many instances of false ‘clean’ energy. Airports and railways run battery operated transport at airports or railway stations and announce that they are ‘environment friendly’. But the lead-acid batteries they use, apart from being expensive, need to be charged, and this uses electricity. While generation of electricity itself is partially efficient, the conversion to battery power and then to run transport is wasteful – and it would have been greatly more environment friendly to have directly used a diesel or petrol engine. Truly environment friendly energy would be the kind where the process is not  dependent on conventional energy production.

Hydrogen as fuel
There are different ways of using hydrogen as fuel. The hydrogen atom consists of one heavy, positively charged particle, the proton, as its nucleus and one light, negatively charged particle, the electron, in orbit around the nucleus. For stability, hydrogen tends to give up its lone electron to another atom, which could gain stability by having one more electron. The oxygen atom can do with two more electrons. Two hydrogen atoms thus combine with one oxygen atom to yield H2O, or water.
Hydrogen has less energy when it is in combination, as water, or other chemical pairings. But when, with the use of energy, hydrogen is plucked out of this low energy condition, the free hydrogen has more energy, usually the energy used to set it free from combination. This free hydrogen can be stored and the energy can be tapped, either as heat, by burning in oxygen, to produce water (vapour), or in other chemical reactions, or even in an arrangement that creates electricity directly.

This arrangement is called a fuel cell. If an electric current is passed though water, the water splits into hydrogen and oxygen. As electric current is a flow of electrons, the electric force is able to pull apart the hydrogen atoms which have lost an electron to oxygen, and recreate free, gaseous hydrogen by supplying the missing electron.
In a reverse way, if hydrogen is pumped into the medium between two metallic surfaces, the hydrogen begins to combine with oxygen or other substances in the medium and create an electric charge on the metallic surfaces. When the surfaces are connected, a current flows between them, which is electricity generated by the supply and consumption of hydrogen.

The other way of using hydrogen is simply to burn it in oxygen, which creates heat, to run turbines, etc, to create electricity, just like burning coal or natural gas. The difference is that the product of burning hydrogen is only water vapour and the environment is not polluted with carbon dioxide. The oxygen used up, also, is equal to the oxygen that must have been released along with the production of the hydrogen, which even things up.

Producing hydrogen
The usual way to get hydrogen is by passing electricity through water. In this process, the cost, in terms of pollution in producing the electricity used to generate hydrogen is much more than the clean energy that can come from the hydrogen. There are cheaper methods to produce hydrogen, like using steam and coke or by chemical reactions, but finally the arithmetic is the same, that we spend more than we gain.

An answer may lie in the method used by natural systems. There are micro-organisms that need hydrogen to power their metabolism. These organisms employ chemical intermediaries, called catalysts, which enable processes that produce hydrogen in difficult conditions. The catalysts, called hydrogenase enzymes, containing metals like nickel or iron, have been observed in algae and bacteria. These organisms extract hydrogen from suphates or other chemical sources. The enzymes speed up the reaction where the hydrogen molecule gives up two electrons and splits into two positively charged atoms. The normal form of hydrogen is the molecule, where two atoms share their single electrons, to mimic a more stable condition of having two electrons each. But in the presence of metals, like platinum, the molecule can give up the electrons and split into two positively charged atoms. And the reaction is reversible, which means the two positively charged atoms can join and form a hydrogen molecule, if they meet a source of electrons.

It is this separation of the hydrogen atom from a state of combination to the free state and recombination, which enables a hydrogen atom to get free from compounds and then to meet with another hydrogen atom and start life afresh as a hydrogen molecule.  Biological methods use light energy or ambient heat to set hydrogen free, and mimicking the processes is an attractive quest. But the trouble with most biological processes and the catalysts to make them efficient is that the substances involved are unstable outside bacterial or cellular confines and are not suitable for large scale hydrogen generation.

Promising discovery
The development reported by Professor Jeffrey Long and others is of a synthetic, complex molecule, which combines a metal-oxygen bond, to be effective in setting hydrogen atoms free, but avoids the pitfalls. There are other metal-sulphur complexes, which can generate hydrogen ions in organic solvents. There are also metal complexes that work well but they also need organic acids or additives or solvents. Precious metals too show high efficacy, but their high cost rules them out. 

The discovery is promising and may lead to processes that could generate hydrogen from water with the help of sunlight – of great relevance to sustainable energy cycles.