Our Hydrogen Future
The following is an abridged evaluation of hydrogen fuel-cell powered vehicles and the factors inhibiting progress in the field. It was written from a scientific perspective and principally deals with the technical obstacles preventing fuel-cell vehicles becoming mainstream, though I have included some of the social and political challenges this prodigal saviour must conquer. The full report was a component of my final year physics in 2009.
Do hydrogen vehicles provide the future for automotive transportation?
With oil reserves expected to dry out some time in the next 50 years, scientists, politicians and environmentalists have been searching for a viable, long-term replacement for the vast majority of vehicles running on petrochemicals. Much hubbub has surrounded hydrogen fuel cells but can they viably replace the petrol and diesel engines we depend on so much?
Automotive companies, the oil industry itself and wider society have been reluctant to accept the fact that the worldʼs supplies of oil are running out, but the necessity for a viable alternative to be implemented quickly has been compounded by the growing issue of global climate change. It would be wrong to assert that the main obstacle facing the replacement of petrochemical vehicles was industrial pressure though. Society will not willingly give up the luxury and flexibility endowed by conventional vehicles and therefore any replacement will have to exceed what has been established with the status quo. Practicality, reliability and value for money are all considerations that must be taken into account when designing hydrogen vehicles and are factors that will undoubtedly decide their fate.
The major advantage of hydrogen fuel cells is that the only waste product is water vapour. While water vapour itself constitutes 95% of the greenhouse gas effect, hydrogen fuel cells produce a smaller amount of water vapour per mile than their petrochemical combustion engine contemporaries, typically 113.4 g/mile against 176.9 g/ mile5. Crucially, in contrast with conventional vehicles, it is comparably easy for water vapour to be condensed within the vehicle itself and could either be deposited as liquid while moving or collected in a reseviour tank.
Figure 1
Hydrogen fuel cells produce electricity from reacting oxygen and hydrogen in the presence of an electrolyte. Fuel cells are similar to batteries in the sense that in that they convert energy from chemical to electrical states, the difference being that fuel cells are thermodynamically open systems as they consume the reactant (hydrogen) from an external source which must be replenished. Fuel cells catalyse the separation of hydrogen protons and electrons which force the electrons to travel through a circuit, thus converting the fuel to electrical energy). A further catalytic process reunites the electrons with the protons and combines the full hydrogen molecules with oxygen to form water (fig. 1).
Figure 2
As a typical fuel cell only produces a voltage of around 0.65 V at full load, groups or ʻstacksʼ of fuel cells must be used to provide enough power to move vehicles. Fuel cell stacks can be aligned in series for a high voltage to be yielded or alternatively in parallel for a high current. For typical automotive uses fuel stack stacks will be configured in series for high voltage required by electric traction motors. Due to the fact that stacks are made up of lots of individual fuel cells, the concept is extremely scalable meaning that as manufacturing processes improve fuel cells will be able to be implemented in consumer electronics, such as mobile phones, as alternatives to conventional chemical battery power. To match the ~80kW of power required by the vehicle, tens of thousands of fuel cells (each operating at 0.65 V, approx. 120,000 for this example) will have to be linked in series in a single stack.
One specific type of fuel cell has properties that make it most suitable for automotive power. Proton exchange membrane fuel cells (PEMFCs) allow for lower than average temperature and pressure ranges within the unit, making them suitable for vehicle power plants. Petrochemical internal combustion engines operate at around 200°C whereas PEMFCs have been designed for use between 50-100°C, reducing wasted heat energy and improving the proportion of chemical energy that eventually translates into kinetic movement.
Despite these impressive figures, there remain a lot of areas in need of improvement even at this very basic level. While a platinum catalyst is perfectly suitable for splitting the hydrogen molecule, it is not very efficient in splitting the oxygen molecule at all. Not only is the present platinum solution expensive but significant electric losses occur resulting in a typical fuel cell efficiency between 40-60% caused by trace amounts of carbon monoxide in the fuel source ʻpoisoningʼ the catalyst and low general catalytic activity.
By increasing catalytic activity of the platinum catalyst, the susceptibility to carbon monoxide poisoning may be negated or diminished significantly. Increasing the density of the platinum molecules increases the number of active sites available for catalysis. In 2008 a German study of 100 faced ‘cube’ shaped nano-particles of platinum discovered a four fold increase in oxygen reduction activity compared with naturally faceted particles of a similar size. The 100 facets bonded less strongly to rogue sulphate ions which increased the number of catalytic sites open to oxygen molecules so a greater proportion of the surface area of the platinum was available for oxygen reduction. Indeed, carbon monoxide poisoning can be further reduced by alloying platinum with another metal. Not only does this reduce susceptibility of platinum to carbon monoxide poisoning but it also increases catalytic activity. In a recent study, a platinum nickel alloy surface had a higher reduction activity than pure platinum by a factor of ten. By modifying the electronic structure of the surface through alloying the catalyst, the tendency of platinum to bond to oxygen containing ions (such as the aforementioned sulphates) is reduced which in turn increases the number of available oxygen reduction sites. Both of these methods simultaneously decrease the likelihood of membrane poisoning and improve catalytic activity to achieve impressive results. It remains to be seen whether polyfaceted nano-particles of a platinum-metal alloy are possible, but by combining these two different discoveries it seems possible that both catalytic activity and durability against poisoning can be improved manyfold. While mass produced polyfaceted platinum alloy nano-particles may be some way off, this recent research demonstrates that attaining the US Department of Energy’s goal for fuel-cell catalysts, (130 A/cm3), might be closer than we think. In 2008 the typical cost of a fuel cell stack for a family car was around $33,200 – significantly more than an equivalent internal combustion engine (around $5000), but with the research detailed here and increasing commercial viability in mass-production, it seems as though we can expect that differential to collapse.
While fuel cells themselves still have obstacles to overcome it is evident from cited research that developments are ongoing and promising. Significant inroads are being made in improving catalysts in PEM fuel cells through reducing poisoning, increasing platinum catalytic activity to the extent of maximising surface area and minimising material wastage. The argument remains however, that if prototype hydrogen vehicles are already exceeding the efficiencies of conventional petrol engines by a factor of three the issue of fuel cell inefficiency is, for the time being, a moot point provided that unit cost can be reduced to similar combustion engine levels. The fact of the matter is however, that the two will go inseparably hand in hand. As less and less platinum gets used in fuel cells (as they get smaller and alloying becomes mainstream) fuel cell efficiency will increase and stack cost will decrease proportionally. Are fuel-cells themselves viable replacements for internal combustion engines? The answer is a resounding yes. Of course this judgement does not reflect the the wider implications of hydrogen fuel-cell economy viability which I will discuss further…
Figure 3 (red electric motor, black petrol engine)
In hydrogen fuel cell vehicles, the PEMFC provides energy to an electric traction motor which has its own unique abilities. First and foremost, electric traction motors are vastly more efficient than their conventional equivalents with an average efficiency of 92% vs the 20% of a petrol combustion engine. The peak of an electric traction motorʼs torque occurs at 1 rpm and in some implementations continues until 6,000 rpm (fig. 3). This means that rate of acceleration is much more uniform with such a steady torque curve, in contrast to a petrol engine, which has very little torque at low rpm and only reaches its peak within a very narrow rpm range. Importantly this results in the redundancy of a manual or even automatic transmission with the introduction of a single speed fixed gear, further reducing maintenance requirements for hydrogen fuel cell vehicles and saving weight with no need for a transmission system. This is a major benefit as gearboxes and clutches are heavy and cumbersome.
While PEMFC stacks and electric motors seem to invite praise and approval, fuel-cell skeptics point to glaring problems in the hydrogen fuel-cell plan; namely infrastructure and acquisition. For hydrogen fuel-cells to be truly emission free the production of hydrogen itself must be green. Ideally this would occur through the separation of salt water through electrolysis, with the salt as an electrolyte, with Hydrogen gas being produced at the cathode and oxygen at the anode. While the electricity used could be derived from a renewable source, electrolysis still has efficiency obstacles to overcome. Unfortunately in water electrolysis, only about 60% of the input electrical energy is transferred into chemical hydrogen energy due to the process requiring extremely high voltages (in comparison to waterʼs reduction potential) which surplus accounts for over-potential which energy is eventually lost as heat. Once again, if an effective electrocatalyst could be developed this over-potential would be diminished, but it still rests on a development of the existing platinum catalyst.
The result is that hydrogen is presently too expensive to be derived from water through electrolysis. Until a better electrocatalyst is designed or surplus amounts of green electricity can be produced, electrolysis is not a viable method of acquisition. At present is is far cheaper to procure hydrogen through steam reformation of petrochemical hydrocarbons where either carbon monoxide or carbon dioxide is produced as waste. Some car manufacturers are touting steam reformation as an answer to some of the logistical and infrastructure challenges that hydrogen vehicles face, instead suggesting the installation of a methanol tank within the vehicle to produce hydrogen gas for the fuel cell on the go, but this approach is short sighted given the fact that emissions are only reduced and not eliminated. While steam reformation may prove to be a temporary option for a transition to hydrogen vehicles it is certainly not a lasting solution.
Figure 4
A third option for hydrogen production is biological enzymes that could catalyse the reversible oxidation of hydrogen. Hydrogenases are already found in biological anaerobic metabolism and were first discovered in the 1930s and work by proteins donating or accepting electrons resulting from the oxidation reaction of hydrogen. Researchers at Berkeley genetically engineered a strain of algae which, ʻwith further refinementsʼ, could produce vast amounts of hydrogen through photosynthesis. Presently, by restricting the microbes diet from sulphur, hydrogen production was increased by a factor of 100,000 but to make it commercially viable the efficiency needs to be increased by another factor of 100. While this unusual production of hydrogen certainly requires a lot more research and understanding it is a potential contributor to the hydrogen economy.
While developments in electrolysis and biological hydrogen production are being made, at present the only commercially accessible method is through steam reformation which is not an ideal solution. From a fuel production perspective therefore, hydrogen vehicles cannot be said to current provide a flawless future for automotive transportation – though improvements are foreseeable.
Infrastructure is one of the major challenges facing the viability of hydrogen as a replacement fuel. Hydrogen is versatile in that it can be transported through pipelines as gas, by trucks as bottled compressed gas or liquefied after cooling to -253°C and transported by cryogenic tube trailers. Each method of delivery however comes with its own set of obstacles. As hydrogen has a low volumetric energy density in comparison with
other fuels more of it needs to be transported in terms of end user mile, this means that transport as compressed cylinders on conventional trucks is extremely costly and transporting it farther than 200 miles from the point of production becomes cost-prohibitive. Cryogenically cooling and liquefying hydrogen is very expensive as well but offers the benefit of a higher energy content than gaseous hydrogen and could become a method for delivery for short distances. Pipelines are the obvious answer and the least expensive solution to transporting large quantities of hydrogen over long distances very quickly. Unfortunately however, hydrogen pipeline infrastructure in the United States is limited with only 1,200 miles of pipeline compared with over a million miles of natural gas pipelines. As the US Department of Energy points out;
“The high initial capital costs of new pipeline construction constitute a major barrier to expanding hydrogen pipeline delivery infrastructure.”
Currently it is near impossible convert existing natural gas networks to hydrogen pipelines as hydrogen has a tendency to embrittle steel and welds used to construct pipelines. As hydrogen molecules are very small, preventing leakage and permeation is a major challenge as well. In any case, several key areas of research need to be explored in order for infrastructure to become more viable. Lower cost, more reliable compression technology, more efficient, cheaper hydrogen liquefaction processes and schematics to tie production, delivery and end-use technologies together. Until further research can be undertaken and comprehensive to-the-forecourt solutions are implemented, its difficult to wholeheartedly embrace and vouch for hydrogen fuel as our all-conquering saviour.
It is of course important to critique any potential replacement fuel for problems and obstacles but equally important to examine the alternatives we have to chose from. Electric vehicles powered by battery stacks seem like an attractive proposition however they have their own draw backs. Batteries must be charged up over night which severely limits the vehicles range making them only suitable for short journeys in cities and towns. Hybrid vehicles utilising both petrol combustion engines and smaller electric motors (powered by batteries) are attractive in reducing inner city pollution but hitherto have not proved any ʻgreenerʼ than small, frugal combustion engine cars. Comparing the Toyota Prius (hybrid) to the highly efficient diesel VW Polo Bluemotion reveals some startling results, the VW achieves a combined mpg of 74 against the Priusʼs 46. Other technologies and efficiency measures introduced in recent cars have had dramatic effects on engine economy. Longer gear ratios to cut engine revs, engine management software to cut idling speed, particulate filters to cut CO2 emissions, lighter materials wherever possible, low rolling resistance tires and improved aerodynamics are all ways in which car economy has been improved by plain good design. Combining all these innovations has resulted in impressive concepts that aim to enhance to longevity of our fossil fuel supplies. VWʼs 1 litre car concept demonstrated possibilities by becoming the worldʼs most economical road car with 285 mpg. By reducing the drag coefficient (Cd) to 0.159 (compared with 0.3 on most cars) the importance of aerodynamics in automotive design was reaffirmed.
While these achievements are undoubtedly impressive they only circumvent the real problem of fossil fuel dependency. While economical vehicles will prolong our reserves of hydrocarbons they will not eliminate our need for them. As for battery powered vehicles, the problem of charging is too big to be avoided. At present electric cars do not provide a sustainable future for personal transportation. While the spirit of hybrid vehicles is admirable, in many case they serve only to limit pollution at low speed in cities. The hybrid status becomes more of a gimmick when all a car’s hybrid system consists of is a few small batteries coupled with an electric motor and big V8 petrol – all designed to earn the marque that hallowed ‘hybrid kudos’ rather than for any legitimate ends.
On hydrogen fuel-cell vehicles however, the concept of hybrid battery/fuel-cell power has numerous advantages. By using a set of rechargeable batteries or capacitors, when the vehicle is subjected to braking, kinetic energy is transferred into electrical and then chemical battery energy through the use of a small hub turbine. Instead of using the fuel-cell for power the car can then draw on the battery for propulsion until it is depleted. This is just a minor modification on the design of a hydrogen fuel-cell vehicle in a very similar way to Formula 1 style KERS systems. Regenerative braking in this way could improve efficiency a great deal – more so than the systems found in petrol hybrids.
In contrast to the slow and gradual development of petrol vehicles, we are asking for an overnight solution to the many factors that come into play when considering the viability of a fuel. Production, delivery and consumption all at maximum efficiency with zero emissions is a tall order for any contending replacement though it seems that despite numerous obstacles, hydrogen has the potential to fit the bill. The advent of the motorcar didn’t happen over night – it was a long drawn out process. As expressed throughout this report, significant progress is being made in all fields related to the hydrogen fuel viability. Hondaʼs prototype FCX Clarity is a testament to what has already been achieved from the automotive perspective. The introduction of a superior platinum catalyst outlined earlier in this report will certainly go a long way to meeting and exceeding the cost requirements the US Department of Energy outlined for hydrogen fuel-cell viability.
The obstacles faced by hydrogen fuel-cell vehicles today are no more monumental than those faced by those early car and oil companies a hundred years ago. The difference is, while we have the benefit of technology and 21st century science to implement our solutions, the oil companies of yore had the benefit of decades of natural growth. We need significant private investment from energy companies to ensure that the infrastructure challenges we are faced with are overcome quickly. As Mill would undoubtedly point out, it may be more expedient for oil companies to ignore the facts of global warming and fossil fuel shortage but in the long run, it is much more useful in achieving their ultimate end (money making) for Shell, BP and Exxon to invest in hydrogen production and infrastructure. It is in fact more expedient for oil companies not to be expedient, with the added bonus of treading the moral high ground by putting the fate of the planet ahead of short term profits.
The reluctance to adopt any new fuel is understandable as to commit to an entirely new fuel requires a lot of capital with limited short term returns. While hydrogen fuel-cell vehicles may not pose an immediate solution to the energy crisis, research should be actively encouraged and funded to ensure that infrastructure deployment can occur forthwith. Iʼm convinced that hydrogen fuel-cell vehicles provide us with the best vision for our automotive future.
