One of the main limitations of renewable energy sources is their reliance on technologies that require platinum-group metals, but replacing these materials could provide a solution. Rachel Lawler reports.
Platinum, palladium, rhodium, iridium, ruthenium and osmium are the six least-abundant elements on Earth. Unfortunately, these materials also provide the best catalysts for applications including fuel cells, electrolysers and lithium-air batteries – all necessary for making the most effective use of renewable energy. And supply is not meeting demand, even with increased recycling.
Researchers at MIT are looking for a solution to this problem by using more plentiful metals in catalysts. Professor Yuriy Román, Assistant Professor of Chemical Engineering, explains, ‘To modify early transition metals, such as tungsten to mimic platinum, we carburised them at elevated temperatures using methane as the carbon precursor’. This allowed the team to synthesise tungsten carbide without excess surface carbon, which is critical to maintaining the catalytic activity.
Using this method, the team was able to replicate the performance of conventional catalysts. Román says, ‘Our transition metal oxide nanoparticles were more than 100 times more active for hydrogen evolution and for methanol electro-oxidation than either commercial tungsten carbide powders or nanoparticles. For methanol electro-oxidation, our transition metal carbide nanoparticles achieved an activity that was 33% that of platinum nanoparticles.’
These processes could be used in a variety of renewable energy applications. Román says, ‘As electrocatalysts, they could be used directly or as highly active co-catalysts in fuel cells, electrolysers and lithium-air batteries. In thermal catalysts, transition metal oxides are unique in that they resist poisoning by carbon monoxide and sulphur, making them suitable for biomass reforming reactions. Outside of catalysts, these materials have garnered attention in supercapacitors, optoelectrics, medical implants and as high temperature materials for nuclear and aerospace industries.’
Early transition metal carbides, such as the tungsten carbide used by the team, are around 1,000 times cheaper than platinum. But reducing cost was not the researchers’ key aim. ‘There is physically not enough platinum in Earth’s crust to support a global renewable energy economy, even if platinum were less expensive,’ Román says.
As thermal catalysts, transition metal oxides will oxidise and lose activity if exposed to moisture or oxygen at high temperatures. Despite this, the team hopes the use of transition metal oxides could bring more renewable energy technologies closer to market. Román says, ‘Currently, the market for these technologies looks promising but is not very large. This can partially be attributed to the fact that most developments require platinum group metals.’ The team is working on optimising its lab-scale process for commercial use.
Author: Rachel Lawler
Materials World Magazine, 01 Jul 2014