Sunday 20 March 2011

Is the Hydrogen Economy Dead in the Water?

The idea of a ‘Hydrogen Economy’ was first put forward in the 1970’s and quickly captured the imaginations of scientists and non-scientists alike. The development of a power infrastructure utilising hydrogen gas promised a move away from fossil fuels, depleting and dirty, to clean cars, clean energy and clean living for all. Emitting only water when ‘burned’, the use of hydrogen as a fuel would allow for the development of a carbon free society and due to its high energy density, a more efficient one too. It promised not only to largely reduce CO2 emissions, but also pollution from CO, NOx, SOx, and particulates, all associated with the combustion of fossil fuels. Despite the initial excitement, development has been slow and forty years on, the ‘Hydrogen Economy’ has failed to materialise.

In recent years some steps have been taken in rolling out hydrogen technologies, albeit small ones. The area receiving the most interest has been the automotive industry. In London last year the first permanent hydrogen powered bus was introduced to a commonly used tourist route. This came after successful pilots between 2003 and 2007, and seven more are to be added to the route this year. The buses contain batteries which store energy generated not only from the integrated hydrogen fuel cell, but also from the braking process. Thirty nine of the buses, developed by an energy company based in Canada, also grace the streets of Vancouver and can run for up to 18 hours without needing to refuel.

One of the hydrogen powered buses being used in London

It is America however, that has seen the largest drive implement hydrogen power. Hydrogen was touted as the solution to America’s foreign oil woes. Due to the USA’s abundant supplies of coal and natural gas, from which hydrogen can be derived, a hydrogen energy infrastructure seemed the perfect solution indeed. California Governor Arnold Schwarzenegger promised a ‘Hydrogen Highway’ with 200 filling stations by 2010, connecting all of the major cities in California, yet currently only 30 exist. A further hitch to America’s hydrogen dream came last year when physics Nobel Laureate come Energy Secretary Steven Chu cut all funding for research into hydrogen fuelled vehicles.

With so much potential, what has gone awry with the ‘Hydrogen Economy’? Firstly the production of the material itself is problematic. Although hydrogen is one of the most abundant elements on earth, it doest not exist as molecular hydrogen except for in trace amounts (0.1 ppm) in the outermost atmosphere and can only be separated from other hydrogen-containing compounds. Steam methane reforming (SMR) is currently the most widely used and cheapest method for producing hydrogen, but is estimated that 13.7 kg of CO2 is produced per net kg of H2. Production of hydrogen using SMR also requires an input of energy and so it would be more efficient to simply burn the fossil fuels themselves.

Alternative methods of hydrogen production are being investigated which use different sources to derive hydrogen, most prominently water. Electrolysis currently accounts for approximately 4% of the global production of hydrogen, but current methods do not provide a viable solution. Not only expensive, they also require a large energy input, usually from fossil fuels. Systems which use renewable energy to drive the process offer some improvement. The National Renewable Energy Laboratory (NREL, a laboratory of the U.S. department for energy) are currently developing the wind-to-hydrogen project, which links wind turbines to electrolysers and uses the wind generated electricity to split water.

Photo-catalyzed water-splitting has emerged as a promising technique to efficiently generate hydrogen. It is driven using light in a similar way to photosynthesis and so the process is often termed artificial photosynthesis. In the process a photo-catalyst is excited by photons of visible light with energy equal to or greater than the material’s band gap. This generates electrons and holes which drive the decomposition;

2H2O + 4h+ →  O2 + 4H+
2H2O + 2e- →  H2 + 2OH-

The first example of a photo-catalyst to produce hydrogen was seen in the 1970’s by Japanese researchers A. Fujishima and K. Honda.1 They used a single crystal of TiO2 connected through an electric circuit to a platinum surface, (figure 1a), generating the required electrons and holes to split the water, but the rate of hydrogen production was too low to be economically viable. Further to this, the system produced a mixture of O2 and H2 and so an expensive separation process is required. 

Figure 1 – a) The Fujishima-Honda cell, and b) a photodiode for water splitting 2

In recent years the efficiency of the process has been improved by the use of a photodiode, (figure 1b), which allows separate production of hydrogen and oxygen in the two halves of the cell. The device consists of a metal sheet with a thin film of the photo-catalyst on one side, and a hydrogen-producing catalyst, such as platinum on the other, which sits between two separate water sources. When irradiated with light, oxygen is produced on the photo-catalyst side by holes, whilst hydrogen is produced on the platinum side by electrons which migrate through the metal substrate. Materials other than TiO2 have been investigated as potential photocatalysts, most noteworthy the NaTaO3:La system developed by researchers at the Science University of Tokyo,3 which shows the highest activity and quantum yield to date.

Another stumbling block which has prevented the ‘Hydrogen Economy’ from being adopted is the question of how to store hydrogen. Hydrogen gas occupies large volumes of space at atmospheric pressure, in fact a single gram of hydrogen takes up a whopping 11 litres of space. Due to its low energy density by volume, hydrogen must be either liquefied or pressurised. In its liquid form, it must be stored at cryogenic temperatures and so requires high energy input. Pressurisation also requires energy input and the containers required are weighty and expensive. Alternative storage methods include chemical storage methods such as the use of metal hydrides. Hydrides offer a practical solution for storage and transportation, however there are barriers associated with the high pressure and temperature conditions needed for hydride formation and hydrogen release. Other physical methods of storage are being developed such as the use of carbon nanotubes, nanoporous materials, metal-organic frameworks and polymers but all are costly compared to conventional storage methods for petroleum fuels.

It seems that ultimately what is holding the ‘Hydrogen Economy’ back is not that it is an inferior energy source, or lack of innovation in the field. The reality is that implementing a hydrogen energy infrastructure would be costly not only to governments but also consumers. It arrives to the age old question of conscience vs. cash; just how much are we willing to pay for clean guilt-free energy?

            (1)        Fujishima, A.; Honda, K. Nature 1972, 238, 37.
            (2)        Parkin, I. P.  Education in Chemistry, 2010; 47, 1.
            (3)        Kato, H.; Asakura, K.; Kudo, A. Journal of the American Chemical Society 2003, 125, 3082.

2 comments:

  1. http://habergol.com/journal-of-american-chemical-society/

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  2. http://www.lanl.gov/news/releases/cheaper-hydrogen-fuel-cells.html

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