Battery-powered electric vehicles that give off no carbon dioxide as they drive are about to become mainstream. Today they constitute less than 1 percent of all rolling stock on the road globally, but multiple innovations in features such as the battery’s cost and lifetime have made prices so competitive that Tesla has more than 400,000 advance orders for its $35,000 Model 3, which is slated to hit the road in the middle of 2018.
Unfortunately, the other great hope for vehicles that exhaust no carbon—those powered by hydrogen-fed fuel cells—remains too pricey for broad sales. (The manufacturer’s suggested retail price for the Toyota Mirai is $57,500.) A raft of laboratories and businesses, however, are determined to cut costs by replacing one of the most expensive components in the fuel cells: the catalyst. Many commercial catalysts for fuel cells contain the precious metal platinum, which aside from being expensive, is too rare to support ubiquitous use in vehicles.
Investigators are pursuing several lines of attack to shrink the platinum content: using it more efficiently, replacing some or all of it with palladium (which performs similarly and is somewhat less expensive), replacing either of those precious metals with inexpensive metals, such as nickel or copper, and foregoing metals altogether. Commercial catalysts tend to consist of thin layers of platinum nanoparticles deposited on a carbon film; researchers are also testing alternative substrates.
Stanislaus S. Wong of Stony Brook University, who works closely with Radoslav R. Adzic of Brookhaven National Laboratory, is among those leading the charge. He and his colleagues have, for instance, combined relatively small amounts of platinum or palladium with cheaper metals such as iron, nickel or copper, producing many alloyed varieties that are far more active than commercial catalysts. Wong’s group has fashioned the metals into ultrathin one-dimensional nanowires (roughly two nanometers in diameter). These nanowires have a high surface area–to-volume ratio, which enhances the number of active sites for catalytic reactions.
Naturally, platinum-free catalysts would be ideal. Work on them is newer but bustling as well. In late 2016, for instance, Sang Hoon Joo of Ulsan National Institute of Science and Technology (UNIST) in South Korea reported that an iron- and nitrogen-doped carbon nanotube catalyst has activity comparable to commercial catalysts. Also, Liming Dai of Case Western Reserve University and his colleagues have invented a catalyst using no metal at all; it is a nitrogen- and phosphorus-doped carbon foam that is as active as standard catalysts.
Inventing and preparing a material that has excellent catalytic activity is just part of the challenge, Wong notes. Researchers are also working to scale up existing laboratoryproduction methods to ensure consistency in the activity and durability of the best candidates. In all phases of their efforts, experimentalists are getting help from theorists who apply sophisticated computer models to figure out how all sorts of variables affect performance—from the chemical compositions, sizes and shapes of metal nanoparticles to the detailed architectures of the support structures. Such collaborations, Wong says, should one day make it possible to rationally design superior catalysts for affordable fuel-cell vehicles.
Of course, the goal of a sustainable transport system demands not only zero carbon emissions during driving but also during the production and distribution of the fuel, be it electricity or hydrogen. That larger challenge remains.