Researchers have developed a method to combine high-cost platinum and a low-cost rare earth element, lanthanum, as an alloy to serve as a catalyst in the next generation of fuel cells that will improve their performance and reduce their cost. The development should facilitate the decarbonization of heavy transport vehicles which are less amenable to the use of batteries to power them.
Batteries may have won the battle against hydrogen fuel cells to power cars cleanly, but a number of other forms of transportation are struggling to replace internal combustion engines with batteries due to a series of obstacles such as the weight and the volume of the batteries which be required for the type of services which they provide. This is especially true for heavy transport such as shipping, aviation and long distance trucking. In these cases, most transportation analysts suggest they will likely rely on some sort of clean fuel instead.
A fuel cell is capable of powering vehicles and other machines by transforming the chemical energy of hydrogen into electricity, the only other outputs being water and heat. Until now, the most common type of fuel cell used in a number of devices, from satellites to the space shuttle, has been the alkaline fuel cell, the invention of which dates back nearly a century. The next generation is more likely to resemble a polymer electrolyte membrane fuel cell, which also uses hydrogen to generate electricity, but is much more compact, making it particularly attractive for transport vehicles heavy.
The key to making these electrochemical reactions more efficient – and thus reducing the cost of fuel cells to make them more competitive with the use of fossil fuels – is to find better catalysts, materials that speed up these reactions.
Unfortunately, of all these “electrocatalysts” that make possible the key chemical reaction involved (the oxygen reduction reaction, or ORR), platinum is by far the best. And platinum, a rare metal, is not cheap. For PEMFCs in particular, the incredibly high cost of platinum has been a major barrier to their adoption. The rapid degradation after a relatively small number of cycles of use of this already expensive electrocatalyst in the highly corrosive environment of PEMFCs has only aggravated the situation.
“The hunt is therefore on for an inexpensive electrocatalyst that is more resistant to degradation and therefore stable over longer periods of time, while providing an impressive current density, i.e. the amount of electric current per unit of volume,” said Siyuan Zhu, one of the paper’s authors and an electrochemist at the Changchun Institute of Applied Chemistry at the Chinese Academy of Sciences, “and thus enabling us to deliver on the promise of the compactness of PEMFCs “.
The main option considered to reduce costs is to “dilute” the amount of platinum needed as an electrocatalyst by alloying it with other cheaper metals that can help or even improve the catalytic properties of platinum.
And the main candidates for alloying with platinum have so far been the so-called late transition metals. Transition metals are the elements you find in the middle, or d block, of the periodic table. Iron, manganese, and chromium are transition metals in the middle of this central block, and “late” transition metals, such as cadmium and zinc, are on the right side of it.
The late transition metals, however, were found not to be immune to dissolution in the harsh and corrosive PEMFC environment. This not only results in a constant drop in performance, but the dissolved metal reacts further with the byproducts of the oxygen reduction reaction, causing uncontrollable damage to the entire system.
However, the early transition metals, those on the left side of the central block of the periodic table, such as yttrium and scandium, are much more stable. Theoretical calculations have shown that alloys of platinum and these first two transition metals are the most stable to date.
Among the early transition metals, one group has hitherto been neglected: the rare earth elements (REE). Despite their name, REEs are actually quite common in the earth’s crust and can contribute substantially to the electrochemical activity of catalysts. Thus, the problem thus far in exploring REEs as possible alloying partners for platinum has not come from cost, but rather from their low conductivity and solubility in acidic media. In principle, both of these problems can be overcome by using synthetic methods for the production of a platinum-REE alloy, but so far there are few reports of feasible synthetic methods.
The researchers therefore designed one for the preparation of an alloy between platinum and lanthanum REE.
The technique involves only two simple steps. First, the researchers obtained readily available lanthanum salts and trimesic acid, and these two precursor materials then self-assembled into nanoscale “rods.” These nanorods were then impregnated with platinum at 900°C. This very high temperature is necessary to ensure a smooth alloying process of the two metals.
The resulting platinum-lanthanum nanoparticles were then stress tested for performance in a fuel cell. The alloy electrocatalyst exceeded researchers’ expectations, providing superior stability and activity even after 30,000 fuel cell cycles.
With lanthanum’s success as an alloying partner for platinum having been demonstrated, researchers now want to try other rare earth elements to alloy with platinum to see if they can beat the electrocatalytic performance of lanthanum.
Research report:Ultra-stable Pt5La intermetallic compound for a highly efficient oxygen reduction reaction
Related links
Tsinghua University
Powering the World in the 21st Century on Energy-Daily.com
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