In recent years, there have been numerous articles about hydrogen fuel cells powering everything from mobile phones to cars. It’s easy to see the attraction of these devices – they produce ‘clean’ energy and their waste product is water vapour.
The principles of fuel cell operation have been known for decades; the challenge has been making them efficient and economical enough for mass production. While the automotive industry is the main driver of fuel cell development, the main problem is cost – an 80kW fuel cell is a big, expensive beast. Honda is one of the leaders, with the Clarity being its flagship production car. But it only plans to make 200 such cars between 2008 and 2010.
Fuel cells for notebook computers are still a little way off and are too big to be practical. However, fuel cell based battery chargers for notebooks will be launched soon, possibly as early as Q4 2009. The chargers will use methanol cartridges as the fuel source.
Today, companies are producing fuel cells for more niche applications. German company Smart Fuel Cell (www.sfc.com) claims to be the market leader in mobile energy based on fuel cells for the leisure, industrial and military markets. The company is believed to have sold more than 10,000 polymer electrolyte membrane – sometimes known as proton exchange membrane – (PEM) methanol fuel cells. Its products are rated at between 600 and 1600Whr per day.
One of the primary applications for these products is the leisure market – caravanning, camping and boating. The fuel cell stack is used as the primary power source and charges batteries, which provide load balancing. In 2010, another European company will market a fuel cell based battery charger for portable power tools. Fuel cells are around 50% of the weight of batteries and that’s why designers of military backpacks are also looking at them. In Portugal, cameras in forests enable early fire detection, but the batteries that power the cameras need to be changed every two days. By adding fuel cells to charge the batteries, each site will only need to be visited once per month – a huge saving in operational costs.
US hydrogen fuel cell manufacturer Ballard Power Systems is deploying its products as back up power systems for telecom networks, where their light weight, ease of installation, lack of emissions and low maintenance (an annual filter change) make a compelling financial and environmental case for adopting the technology in place of batteries or diesel generators. And a combination of fuel cells and batteries will probably power the base stations in India’s cellular network – mains power is not always available.
There are various fuel cell technologies, but more than 90% of those being developed are PEM fuel cells which use either hydrogen or methanol as fuel. Hydrogen systems are more efficient, with a current density of up to 2A/cm2, but hydrogen storage takes a lot of space. Methanol systems are more compact, but have typical power density of between 0.2 and 1A/cm2, so there is a big efficiency trade off.
Each cell of a PEM fule cell consists of an anode and a cathode, separated by a PEM. Hydrogen is channelled through slots in so called bipolar plates at one side of the fuel cell. Air flows through similar channels in another plate at the other end of the cell. At the anode, a platinum catalyst causes the hydrogen to split into protons and electrons. The PEM only allows positively charged ions to pass through it to the cathode; negatively charged electrons travel along an external circuit to the cathode and provide an electrical current. Individual fuel cells typically generate around 1V, so the cells are stacked to produce a usable output voltage. Direct methanol fuel cells work in the same way. The methanol molecule (CH3OH) breaks down on the platinum anode electrode to form CO2, protons and electrons.
The protons pass through the membrane in the same way as they do in pure hydrogen fuel cells, whilst the electrons are available to source an electrical current. The CO2 remains dissolved in the fuel solution and is released outside of the stack. However, it’s not much – for every 10Whr delivered, the fuel cell will only create about as much CO2 as that released when a can of fizzy drink is opened.
Bipolar plates are the most significant element, accounting for around 50% of the cost and 70% of the weight of a PEM fuel cell. As they form part of the electrical circuit, they need to be electrically conductive in order to minimise losses. While metal plates would seem the obvious choice, they need expensive passivation to prevent chemical attack by the catalyst. In addition, channels must be etched or milled into the metal to facilitate the flow of gases and water vapour.
Compressed graphite granules held in a resin binder are sometimes used as an alternative material. This eliminates the passivation requirement. As normal resins are insulators, a graphite loading of 90% or more is needed to achieve acceptable conductivity, producing a brittle plate that needs careful handling. The plate must then be etched or milled, just as a metal plate would be; it can take 2 to 3 hours per plate on a conventional CNC machine.
Bac2, based in Southampton, has a solution to that promises to reduce the cost of producing bipolar plates substantially. The company has developed and patented ElectroPhen, a polymer which is at least 1billion times more conductive than other resins. It’s made from commonly available bulk materials and needs only 70% graphite loading achieve the required conductivity. Most importantly, the material can be moulded into plates and cured at room temperature. The plates, which need no further machining or processing, can be made quickly and economically in high volume, so may have a substantial impact on future fuel cell economics.
The fuel cell market is growing, but the early adopters tend to be smaller companies serving niche markets driven by a compelling economic case, rather than an environmental one. As volumes grow and costs fall with improvements in materials and manufacturing techniques, the early promise of fuel cell adoption in high volume automotive and consumer electronics markets will, at last, be realised.
