Devinder Mahajan, a chemist at the US Department of Energy’s Brookhaven National Laboratory, was recently issued a US Patent for his development of a low-temperature process of producing ‘pure’ hydrogen for use in fuel cells.
The process may help address one of the most significant difficulties in developing efficient and affordable fuel cells, namely how to extend the life of the catalysts that make them work.
Fuel cells combine hydrogen and oxygen without combustion to produce direct electrical power and water. They have been pursued as a source of power for transportation applications because of their high-energy efficiency, their potential for source fuel flexibility, and their extremely low emissions.
An important problem facing today’s most promising fuel cell technologies is that the same hydrogen that feeds the reaction often contains high levels of carbon monoxide formed during the hydrogen production process. The carbon monoxide (CO) ‘poisons,’ or degrades, the platinum catalysts that convert hydrogen into electricity within the fuel cell, leading to deterioration in efficiency over time.
‘The commercial viability of fuel cells for power generation depends upon solving a number of manufacturing, cost, and durability issues,’ said Mahajan. ‘Finding a simple, inexpensive method of producing hydrogen that is essentially free of carbon monoxide would help address many of those issues.’
Fuel cell researchers have tried to solve the CO-poisoning problem in several different ways. By adding metals like ruthenium or molybdenum to the platinum, scientists have been able to formulate more tolerant catalysts, but even these are poisoned by relatively low levels of CO.
A second option is to send the hydrogen through a second process to remove most of the CO before feeding it into the fuel cell. This process typically employs a high-temperature catalytic reaction (water-gas-shift), which, due to thermodynamic constraints, leaves unacceptable levels of CO in the finished product.
In Mahajan’s new process, a ruthenium trichloride or similar metal catalyst is mixed with a nitrogen complex to form a homogenous solution in a methanol and water mixture. The hydrogen feed containing CO is then introduced, and, at relatively low temperatures (between 80 and 150 degrees C), the catalyst reacts with the CO and water to convert nearly 100 percent of the CO into carbon dioxide and, as a side benefit, additional hydrogen.
The resulting hydrogen feed contains only a few parts per million of CO and is at the correct temperature to be fed directly into a fuel cell. The process also minimises the amount of waste produced during the reaction due to low temperature operation, high product selectivity, and high catalytic activity.
‘It’s quite an economical reaction, and it happens very quickly, in just a few seconds,’ said Mahajan, ‘The process works with impure hydrogen produced by any method, including coal and biomass, and can be easily scaled up for more substantial production.’
Mahajan believes his new hydrogen production method will assist the commercialisation of proton exchange membrane fuel cells, which are the most promising fuel cells for widespread transportation use because they operate at low temperatures, produce a fast transient response, and possess relatively high energy densities compared to other fuel cell technologies.