New Patented Process Produces Pure Hydrogen for Fuel Cells
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 because of their high energy efficiency, potential for source fuel flexibility, and extremely low emissions.
An important problem facing today’s most promising fuel-cell technologies is that the same hydrogen feeding the reaction often contains high levels of carbon monoxide (CO), which is formed during hydrogen production. Within a fuel cell, CO “poisons,” or degrades, the expensive platinum catalysts which convert hydrogen into electricity, deteriorating their efficiency over time and requiring their replacement.
“The commercial viability of fuel cells for power generation depends upon solving a number of manufacturing, cost, and durability issues,” says Brookhaven chemist Devinder Mahajan, “including finding a simple, inexpensive method of producing hydrogen that is essentially free of carbon monoxide.” Mahajan has recently done just that, by patenting a process that can produce hydrogen with a very low CO content, which results in extended catalyst life.
Fuel cell researchers have tried to solve the CO-poisoning problem in several different ways. One way is by adding metals such as ruthenium or molybdenum to the platinum to formulate more tolerant catalysts. But even these are poisoned by 100 parts per million or more of CO, which are relatively low levels.
A second option is to send the hydrogen through an additional process to remove most of the CO before feeding it into the fuel cell. This process typically employs a high-temperature catalytic reaction known as water-gas-shift, which, because of 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 the relatively low temperatures of between 80 and 150 degrees Celsius, the catalyst reacts with the CO and water to convert nearly 100 percent of the CO into carbon dioxide — plus 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 minimizes the amount of waste produced during the reaction because of the 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,” says Mahajan, “The process works with impure hydrogen produced by any method, including coal and biomass, and it can be easily scaled up for more substantial production.”
Mahajan believes his new hydrogen production method will assist in the commercialization of proton exchange-membrane fuel cells, which are the most promising 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.