The power of hydrogen

Materials World magazine
,
2 Jun 2015

Anna Ploszajski reports on a group of PhD students who are setting the record straight on hydrogen.

As the most abundant element in the universe, and with proven routes to harnessing its energy cleanly, hydrogen could become a key player in the future renewable energy landscape. It promises to free us from the geopolitically unstable and planet-warming fossil fuels, yet the general public remains suspicious of and uninformed about hydrogen’s exciting possibilities. 

At the heart of UCell’s system lies a polymer electrolyte membrane (PEM) hydrogen fuel cell. Although a great many different types of fuel cell exist, PEM fuel cells are the most convenient for portable applications, since they have a high power density and relatively low operating temperature. The PEM hydrogen fuel cell converts the chemical energy of hydrogen into electricity without the need for combustion, and the only waste products at the point of use are heat and water vapour. 

It achieves this with simple electrochemistry. Hydrogen molecules are separated into their constituent parts at the anode of the fuel cell – two protons (H+) and two electrons (e-). The proton travels through the polymer electrolyte membrane, which is electrically insulating, and the electrons are forced around the external circuit, providing useful electrical power. Finally, the protons and electrons recombine with oxygen from the atmosphere at the cathode to produce water
and heat as the only by-products.  

The electrodes are made from porous carbon to allow gas diffusion and have high electrical conductivity and surface area. This carbon is coated with platinum or platinum alloy, which catalyses the reaction at the electrode surface. The catalyst significantly adds to the cost of the fuel cell, and efforts are being made to reduce the amount necessary by producing highly dispersed platinum nanoparticles and alloys, such as Pt3Co, or by replacing the platinum with cheaper materials altogether. 

The membrane material must have high proton conductivity, low electronic conductivity, high chemical and thermal stability, as well as low gas permeability to eliminate gas crossover. The best materials are typically fluorinated polymers, such as Nafion, which are functionalised with sulphonic acid moieties. However, water management in the membrane is important, since it must be kept hydrated in order to conduct protons. This limits the operating temperature of PEM fuel cells to above zero and below 80°C. 

An abundance of hydrogen?

Provided the hydrogen is produced without the use of fossil fuels, the entire process of electricity generation by hydrogen fuel cells is emission free – the only by-products are water vapour and heat. But this is an important caveat, because, although hydrogen is the most abundant element in the universe, it doesn’t occur by itself on Earth – it is tied up in compounds such as water (H2O) or hydrocarbons like methane (CH4). It must, therefore, be isolated from these compounds. Currently, around 95% of hydrogen is created from fossil fuels by steam reforming or partial oxidation of methane and coal gasification processes, which significantly taints the green credentials of hydrogen fuel technologies.

The remainder of the hydrogen is produced by sustainable processes, such as biomass gasification or the electrolysis of water. The latter method works in the exact reverse of a hydrogen fuel cell – taking water, inputting electrical energy, and producing oxygen and hydrogen gases. If this electrical energy comes from renewable energy sources such as wind power or solar power, only then is the hydrogen truly a sustainable source of power. 

A common question that crops up during UCell’s public engagement activities is, ‘Why don’t we just use the electricity from the renewable sources directly, and cut out the middle men of electrolyser and fuel cell?’ It’s all to do with demand. Hydrogen offers an excellent way to carry the energy from the time and place of electricity production, often during the day in remote offshore or sunny locations, to wherever and whenever the end user wishes to use it.

Storing up for the future

This raises the question, how can the hydrogen carry the energy? Or really, how can we, the end user, carry the hydrogen? This is an issue of hydrogen storage. The element can be stored as a gas, liquid or solid. For portable applications such as UCell’s, each presents unique challenges. Hydrogen can be compressed into a high pressure cylinder, but gaseous hydrogen has very low energy density by volume, and these tanks are large and heavy. Making them smaller doesn’t necessarily make them lighter, because higher pressures require stronger materials and better safety features. Furthermore, compressing the hydrogen in the first place costs about 2% of the energy contents of the tank. Nevertheless, compared with other methods of hydrogen storage, gas cylinders represent the best solution at present. Type IV tanks made from carbon fibre/thermoplastic composites feature in the Toyota FCHV, Mercedes-Benz F-Cell and the HydroGen4 fuel cell vehicles. 

Liquid hydrogen or slush hydrogen can be stored in cryogenic tanks, but must be cooled below -253°C, which costs about 40% of its total energy content. Again, these cryogenic tanks are large and heavy. Liquid hydrogen was used in the Space Shuttle programme, but is unsuitable for domestic applications.

Storing hydrogen as a solid takes a different approach. Compounds called hydrides, in which hydrogen is bound to other elements such as lithium, magnesium, aluminium or boron, store hydrogen in their chemical structure and it is released by heating them to around 100°C. Solid state hydrogen storage systems offer much better energy density-to-volume ratios than the other methods, but present other problems such as irreversibility and impurities.

The PEM hydrogen fuel cell in the UCell system is 40-60% efficient at converting the chemical energy stored in hydrogen into electricity. However, it takes significant time to ramp up to the required output. Therefore, the system must be hybridised by adding batteries to store the electrical energy produced, which are very quick to react to spikes in power. This means the UCell system is reliable in times when energy demand is inconsistent.  

As well as this battery bank, the UCell system contains a control unit, fuel cell fans and mass flow controllers. The fuel cell operates at 70V/DC and provides up to 3kW of power. The voltage is dropped to 24V/DC in converters, so that the electricity can be stored in the battery bank. The inverter then converts the electricity into conventional 220–240V/AC mains electricity so that appliances can be directly plugged in. The entire system (apart from the gas bottles) is contained on a small, mobile cart. The casing is made of Perspex, allowing safe operation and making the technology visible to the public.

If we are to move towards a more sustainable future, hydrogen technologies are destined to be key players. However, the economics, logistics, lack of infrastructure and, of course, public opinion, are hindrances that must be overcome before these technologies become competitive with fossil fuels. Day to day, the UCell team is working to meet the technical challenges in the lab, and with its public engagement work, is attempting to dispel the myths surrounding hydrogen and bring these future technologies into the present day. 

For the last four years, UCell has attended Green Man Festival, providing power for a performance stage with electricity from hydrogen. 2014 saw the team's debut visit to Glastonbury Festival, to provide power for a free mobile phone charging station. Together with solar panels, the hydrogen fuel cell kept hundreds of happy festival-goers connected over the weekend. Throughout these festivals, the team has delighted in engaging the public in conversation and hands-on demonstrations to communicate everything from how fuel cells work at the atomic level to the renewable energy landscape as a whole. 

Aside from music festivals, UCell has attended and powered numerous other events for school-aged children, as well as events closer to home at UCL. The team’s knowledge encompasses a diverse range of renewable energy research, with fields of study ranging from nuclear energy to batteries, and technology development from fundamental chemistry all the way to grid-scale integration.  

For more information, email ucell@ucl.ac.uk.