A record-breaking catalyst could help a new generation of fuel-cell electric vehicles to draw their hydrogen supplies from liquid fuels such as methanol. By purifying the hydrogen stream that feeds the fuel cell, the catalyst may improve the prospects for a climate-friendly technology that has struggled to gain a commercial foothold.
The catalyst is made from particles of molybdenum carbide studded with platinum (Pt/MoC) and converts carbon monoxide and water into carbon dioxide and hydrogen—an industrially important reaction known as the water-gas shift (WGS) reaction. The new catalyst is at least ten times more active than previously reported WGS catalysts and works at relatively low temperatures (Nature 2021, DOI: 10.1038/s41586-020-03130-6). “It’s a very active system which outperforms current systems,” says Bert M. Weckhuysen of Utrecht University, who designs and studies solid catalysts and was not involved in the work.
Hydrogen fuel cells generate electricity by combining hydrogen and oxygen. Most fuel-cell electric vehicles carry their hydrogen in high-pressure tanks, and are refueled at dedicated hydrogen filling stations. The need for this entirely new, costly infrastructure has been a major hurdle to wider adoption of these vehicles.
Instead, some automakers hope to produce hydrogen on board their vehicles, using chemistry to liberate the gas from liquid fuels. However, this reforming process often creates carbon monoxide as a by-product, a gas that can poison the fuel cell’s own catalyst.
That’s where the WGS reaction comes in: it scrubs the troublesome CO from the gas feed, and throws extra H2 into the bargain.
Finding a WGS catalyst that fits the bill is not easy. It needs to have high activity and stability; and it should operate at roughly the same temperature as the reforming process and the fuel cell. Yet commercial WGS catalysts are typically inactive below about 100 °C.
Back in 2017, Ding Ma of Peking University and colleagues unveiled a promising gold-molybdenum carbide WGS catalyst with high activity at 120 °C (Science 2017, DOI: 10.1126/science.aah4321). Now, they have surpassed that with their new Pt/MoC catalyst.
The team covered particles of MoC with various amounts of platinum, from 0.02% to 8% by weight, and tested their activity in a gas stream containing CO, H2O, CO2 and H2. The 2%-loaded catalyst offered the best balance between activity and stability. At 100 °C, the catalyst achieved almost complete conversion of all CO in the gas stream, but it even showed moderate activity at just 40 °C. “I think that is a remarkable finding,” Weckhuysen says.
After 100 h of continuous operation, the catalyst’s activity had fallen by about one-half. Despite this, after 263 h of reaction time, each Pt atom had helped to generate 4.3 million hydrogen molecules—the highest turnover number ever achieved for a WGS catalyst, and about 10 times higher than the team’s previous Au/MoC catalyst.
According to Ma’s team, this is the only WGS catalyst to meet the cost, weight, and activity standards set out in the US Department of Energy’s 2004 technical targets for transportation fuel cells. The researchers are already testing the catalyst to supply a working fuel cell with hydrogen sourced from a methanol reforming reaction.
In principle, Ma says, replacing some of the platinum atoms with iron or cobalt could reduce the cost of the catalyst. “The loading of the platinum is still a little bit high at 2%,” he says. “If we can lower it to 0.5% or 1% that would be better, and that is our future target.”