There is mounting concern over the sustainability of global energy supplies.  Among the key drivers are: (i) climate change, ocean surface acidification and air pollution, which imply the need to control and reduce anthropogenic emissions of greenhouse gases, especially emissions from the combustion of fossil fuels in transportation and thermal power stations; (ii) the diminishing reserves of oil and natural gas; (iii) the need for energy security adapted to each country, such as decreasing the dependence on fossil-fuel imports from regions where there is political or economic instability; (iv) the expected growth in world population with the ever-increasing aspiration for an improved standard-of-living for all, especially in developing and poor nations.

Hydrogen is being promoted world-wide as a panacea for energy problems in that it may eventually replace, or at least greatly reduce, the reliance on fossil fuels.  Although the most abundant element in the universe "the stuff from which stars are made" hydrogen does not occur freely on earth, but is predominantly found in combination with oxygen as water and with carbon as fossil fuels.  Chemical, thermal or electrical energy has to be expended to extract hydrogen from these sources.  Hydrogen is therefore not a not a new form of primary energy, but a vector (or carrier) for storing and transporting energy from any one of a myriad of sources to where it may be utilized.  In this respect, it is analogous to electricity, which is also a secondary form of energy.  Hydrogen and electricity are complementary:  electricity is used for a multitude of applications for which hydrogen is not suitable, whereas hydrogen, unlike electricity, has the attributes of being both a fuel and an energy store.  These two energy vectors are, in principle, inter-convertible; electricity may be used to generate hydrogen by the electrolysis of water, while hydrogen may be converted to electricity by means of a fuel cell.

Specifically, hydrogen has the following key attributes:

it can be derived from fossil and non-fossil sources (renewable or nuclear energy)
it can serve as an alternative fuel for internal combustion engines
it is ideal for use in fuel cells for transportation and for distributed energy supply
it is oxidised cleanly to water with no emissions of greenhouse gases; when obtained from water using renewables, the fuel cycle is closed and no pollutants are released in the overall process.

The proposal to use hydrogen as a sustainable medium of energy has become known as the Hydrogen Economy.  The combined use of hydrogen and electricity as the preferred clean energy vectors is known as hydricity.  It is a long term vision, but many see the transition to a hydrogen economy as a worthwhile goal in itself.  During this transitional phase, hydrogen is produced from fossil fuels, preparing the way for an age when hydrogen will be manufactured from renewable energy sources and used as a storage medium and as a super-clean fuel.  Not unexpectedly, the building of a Hydrogen Economy presents great scientific and technological challenges in production, delivery, storage, conversion, and end-use.  In addition, there are many policy, regulatory, economic, financial, investment, environmental and safety questions to be addressed.

from "Transitions"  Andrew Dicks and David Rand,  CSIRO Publishing 2008.