Featured article: Better catalysts for hydrazine fuel cells.

[Paper: From ionic liquid-modified cellulose nanowhiskers to highly active metal-free nanostructured carbon catalysts for the hydrazine oxidation reaction. Elizângela H. Fragal, Vanessa H. Fragal, Xiaoxi Huang, Alessandro C. Martins, Thelma Sley P. Cellet, Guilherme M. Pereira, Eliška Mikmeková, Adley F. Rubira, Rafael Silva* and Tewodros Asefa*. J. Mater. Chem. A, 2017,5, 1066-1077. DOI: 10.1039/C6TA09821E.]

Better catalysts for hydrazine fuel cells

Fuel cells are devices that using oxidation processes can directly convert the chemical energy of fuels, which can be renewable, into electrical energy. Fuel cells operate with high energy efficiency and low environmental impact, and can be used in a wide range of applications. The use of these cells is still limited and many research challenges remain, such as developing catalysts to obtain efficient energy conversion processes.

An international scientific team has taken a significant step in this direction by developing a material that has proven to be very efficient for catalyzing oxidation of hydrazine (N2H4) – a liquid fuel suitable to be used in fuel cells.  Different from most efficient catalysts, there are no noble metals in the composition of this new catalyst, but there are abundant, cheap and renewable materials. The study was reported in a recently published paper in the Journal of Materials Chemistry A. Materials for energy and sustainability (8,262 impact factor), by researchers from institutions in Brazil, the United States and the Czech Republic.

“In this work we highlight the synthesis of nanostructured carbon materials using nanoparticles of crystalline cellulose modified with ionic liquid as precursor,” says Rafael da Silva, professor at the Brazilian State University of Maringá (UEM) and one of the corresponding authors of the article. “The material obtained in this process was used as a catalyst for the hydrazine oxidation process,” he adds.

As cooks testing the best combination of ingredients to make a particular dish, the researchers made a series of carbon materials with small differences between them, in order to compare them and determine which would better perform as a catalyst in the oxidation of hydrazine.

To do this, the team carefully prepared the carbon materials precursors (compounds that participate in a chemical reaction to form a new compound): ionic liquid-modified cellulose nanoparticles. Commercial cotton sold in pharmacies was chosen as raw material for preparing the cellulose nanoparticles. Cellulose is an abundant natural polymer on earth with the formula (C6H10O5)n, produced by plants and bacteria. The nanoparticles were functionalized in two groups: while some received the addition of the SO3 functional group, the others received the addition of the CO2 group. In later stages, some nanoparticle groups were subjected to surface modification processes.

With the different precursors (nanoparticles) obtained, scientists were able to prepare various types of carbon materials. Both the precursors and the materials obtained therefrom were characterized using several techniques. Finally, the team investigated the catalytic activity against the hydrazine oxidation in each of the prepared carbon materials. The scientists were able to conclude that the material with the best performance in this application had been prepared with SO3 functionalized cellulose nanoparticles, subsequently modified with an ionic liquid of the formula ([C4mim][CH3SO3]) and with no metallic trace elements in its composition.

“ We used simple precursors and we were able to obtain a catalyst that is among the best reported for the hydrazine oxidation reaction,” says Professor Silva. “In fact, our material, which is based only on abundant chemical elements, is more active than noble metal catalysts,” he adds.

The authors of the article justified the good performance of the material by the synergy of cellulose and the ionic liquid, as the latter is adsorbed on the surface and also penetrates the structure of the cellulose nanoparticles, promoting the insertion of impurities and defects – phenomena that favor the catalytic activity.

The project was carried out at Rutgers University (New Jersey, USA), by the Chemistry doctoral student of UEM Elizângela Hafemann Fragal, mentored by professors Adley Rubira and Rafael da Silva. The work was developed at the beginning of Elizângela’s doctorate in 2015, during a “sandwich” stage in the group of Professor Tewodros Asefa at Rutgers. Elizângela’s “sandwich” stage took place during the same period and in the same group as her older sister Vanessa Hafemann Fragal, both authors of the article of  the Journal of Materials Chemistry A. At this time, Vanessa was also a PhD student in the same group at UEM.

Some of the authors of the paper. From the left of the reader: student Elizângela Fragal (UEM), PhD Vanessa Fragal (UEM), PhD Alessandro Martins (UEM), PhD Thelma Sley Cellet (UEM) student Guilherme Pereira (UEM), Professor Adley Rubira (UEM), Professor Rafael Silva (UEM) e Professor Tewodros Asefa (Rutgers).
Some of the authors of the paper. From the readers´left: student Elizângela Fragal (UEM), PhD Vanessa Fragal (UEM), PhD Alessandro Martins (UEM), PhD Thelma Sley Cellet (UEM) student Guilherme Pereira (UEM), Professor Adley Rubira (UEM), Professor Rafael Silva (UEM) e Professor Tewodros Asefa (Rutgers).

The cooperation between UEM and Rutgers University, as well as the genesis of the work published in this featured paper, dates back to 2010, when Silva went to Rutgers as a Ph.D. student with a Fulbright/Capes grant, after graduating with a BS and master’s degree from UEM. Then, Silva returned to UEM, where he became a professor in 2015, and six members of the group from the Brazilian institution went to work in Professor Asefa’s group (three doctoral students in “sandwich” stages and three postdocs). Moreover, Asefa is a visiting professor at UEM, with a grant from CNPq.

In the doctorate, Silva participated in the first work that demonstrated that a catalyst for electrochemical oxidation of hydrazine can be made without the use of metals. “In 2012, we published an article [SILVA, Rafael ; Al-Sharab, Jafar ; Asefa, Tewodros . Edge-Plane-Rich Nitrogen-Doped Carbon Nanoneedles and Efficient Metal-Free Electrocatalysts. Angewandte Chemie (International ed. Print), v. 51, p. 7171-7175, 2012], in which we disclosed the synthesis of a new carbon structure that we call carbon nanoneedles, which was active in relation to the oxidation of hydrazine, with activity similar to that of the best catalysts at that time,” reports Silva, who has more than 2,700 citations to his articles, according to Google Scholar, obtained in only 10 years of research.

Since the paper of 2012, new advances on the subject have been published by several groups. “The facts learned over the years have led us to build a system that is much more active than the material published in 2012. For this we used cellulose and its specific interaction with ionic liquid, which introduces doping agents to the final carbon structure,” concludes Silva. With the published paper the team showed that it is possible to efficiently recover energy stored in hydrazine molecules. “Today we dominate the synthesizing process of the best possible catalysts for the hydrazine reaction,” says Silva.

The work was carried out with resources from the Brazilian agencies CAPES, CNPq and Fundação Araucária, as well as resources from Rutgers University and the National Science Foundation (USA).