Researchers from The Netherlands have developed a new process to recover platinum, an expensive and increasingly rare element, from the catalysts of used fuel cells.
This process involves an electrochemical dissolution, performed in mild conditions.
This also allows the reuse of the carbon catalyst substrate.
Platinum: a Very Precious (and Scarce) Metal
Platinum (Pt) is a transition metal (see periodic table above); due to its properties, it has many important technological applications.
Pt is considered a precious metal, and it is indeed used for jewelry, due to its grey shiny color and its inertness.
Its main uses, however, are in technology, especially as a catalyst in many reactions. The catalytic converters in vehicle exhaust systems, for instance, are based on several metals, including platinum.
Platinum is not an abundant metal; it is, on the contrary, the least abundant noble metal – the 2014 worldwide reserves were estimated to be about 66,000 tonnes. For comparison, copper reserves were estimated to be about 300 million tonnes.
Proton Exchange Membrane Fuel Cells
Proton Exchange Membrane Fuel Cells (PEMFC) are a type of fuel cells; they generate electricity through chemical reactions, by converting molecular hydrogen (H2) and oxygen (O2) into water H2O. The range of applications of PEMFCs varies from portable, such as smartphones, to automotive, i.e. cars and buses, and household power generators, with some products already available commercially.
The PEMFC mechanism is based on the use of platinum as a catalyst, the Pt being in the form of nanoparticles immobilized on an appropriate porous substrate, such as carbon.
Recovering the Platinum
Platinum demand has increased steadily in recent years and it is likely that it will continue to increase. Because of this, it is essential to try to recover Pt from old/unused devices.
For the recovery, normally the substrate material is either dissolved in acid or oxidized; platinum is then recovered by separation from the solution.
This process presents several disadvantages; in fact, the support material is destroyed and the platinum separation is never complete. The level of purity of the separated platinum is also a concern.
Novel Separation Process
Researchers from the Delft University of Technology (The Netherlands) developed a new process to separate platinum from PEMFC; they published their results in ChemSusChem in May 2015.
Decoded Science spoke to Dr. Roman Latsuzbaia, recent PhD graduate in the Dutch university (TU Delft) and the main researcher in this study. Dr. Latsuzbaia tells us,
“We wanted to develop a method which allowed us to regenerate platinum catalyst, but without destroying the substrate (carbon), so we could redeposit Pt nanoparticles on the same support. In this way, the carbon and dissolved Pt can be reused, and Pt catalyst lifetime can be prolonged, making the PEMFC cheaper and more sustainable. We chose a selective electrochemical dissolution of the platinum; practically, this means that we apply a voltage to the substrate and only the platinum is dissolved, while the carbon support is not damaged.”
Choosing the Right Conditions
According to Dr. Latsuzbaia, the choice of the right conditions was crucial.
“We know from literature that we can dissolve platinum by applying a voltage; we knew, however, that we needed to optimize the conditions and the process, to be sure that the substrate was not damaged and dissolution process is selective to Pt.
To do this, we used milder conditions; this means voltages lower than those reported in literature. We considered an interval from 0.5 to 1.1 V, with a voltage scan rate of 50 mV/s.
The solution we used for the dissolution contained perchloric acid (HClO4, 0.1 M) and hydrochloric acid (HCl). For HCl we tested 3 different concentrations – 0.1, 0.25 and 0.5 M – to see the effect on both the dissolution and the remaining substrate. We also tested different temperatures of 22, 35 and 60 oC”.
Recovering Platinum: Interesting Results
These experiments led to interesting results.
“We saw that with higher HCl concentration the dissolution was faster; even with the lowest hydrochloric acid concentration, however, we managed to separate more than 90 % of platinum.
Considering the temperature, we obtained the worst results at 35 oC, as the separation efficiency was higher at 22 and 60 oC.
As for the carbon support, its surface was not significantly affected with the recovery performed with the most diluted HCl concentration (0.1 M). With higher hydrochloric acid concentration, on the contrary, the carbon surface showed some modification which could affect its functionality.”
“With this work we showed that it is possible to recover platinum from PEMFC almost completely, without damaging the carbon substrate.
We managed to perform the recover in mild conditions (low temperature and acid concentration); this is good for the substrates but also for the environment, since this process avoids use of concentrated acid mixtures, such as aqua regia, and will not have a too negative impact on the environment.” Dr. Latsuzbaia said.
Increasing Platinum Demands: Reuse of Pt
Indeed, considering the increasing demand for platinum and the necessity of reusing it as much as possible, this process represents an important development towards a more sustainable use of platinum. The fact that the PEMFC substrate can also be recovered and reused makes this method even more valuable.
Decoding Science. One article at a time.