Addition of dihydrogen Ruthenium complexes to MgH2: a way to improve the decomposition kinetics?

INTRODUCTION Because of the increasing worldwide energy demand, new technologies to produce power with less greenhouse gas emissions are needed. Hydrogen has been identified as a potential clean and renewable energy carrier for the future as it can be easily and efficiently converted in electricity1. However a reliable and secure way of storage is essential for the expansion of its utilization2. Materials that can reversibly absorb large amount of hydrogen by means of chemical reactions are the most suitable to reach industrial applications. Among them, metal hydrides and especially magnesium hydride showed great potential. Nevertheless, the high thermodynamic stability of the system and slow hydrogen absorption/desorption kinetics limit its practical usage3. Important advances have been observed by using nano-structured magnesium hydride with additives to accelerate the dehydrogenation process. But the progresses made up to date are not enough to meet the requirements for low temperature fuel cell applications4. EXPERIMENTAL/THEORETICAL STUDY In the present work, the use of dihydrogen ruthenium complexes for improving the absorption/desorption properties of Mg/MgH2 system is for the first time reported. Dihydrogen ruthenium complexes in which one, two or more dihydrogen molecules are coordinated to a transition metal center without H-H bond breaking represent an ideal class in terms of binding strength midway between metal hydrides and physisorption5, 6. Two different ways of preparing the MgH2 and Ru complex mixtures have been tested: ball-milling and solvent impregnation. Microstructure, phase composition and morphology of commercial, nanostructured and doped MgH2 powders were fully characterized by XRD and SEM-TEM. Temperature programmed techniques, TPD, TGA and DSC, were used to investigate the hydrogen desorption kinetics and a PCT device allowed to build the absorption/desorption isotherms and study the reversibility of the process. RESULTS AND DISCUSSION The impact of doping MgH2 with dihydrogen ruthenium complexes is presented in figure 1. MgH2 was doped by ball milling with 5wt% of a ruthenium complex able to coordinate two dihydrogen molecules. The hydrogen desorption was followed by TPD analysis. The figure shows that the complex addition has a significant impact on the dehydrogenation properties of MgH2. It reduces the desorption temperature and allows a better homogeneity in comparison with nano-structured MgH2. Fig. 1: TPD analysis at 2°C/min of (1) MgH2 doped with 5wt% of complex (2) milled MgH2 (3) commercial MgH2 TGA, DSC and PCT analysis confirmed that using a dihydrogen ruthenium complex as doping agent has a positive impact on the hydrogen storage properties of MgH2. TEM analysis showed that a protective layer of complex is formed at the surface of MgH2. Its role may therefore be to homogenously disperse the ruthenium in the mixtures and to prevent the sintering of the MgH2 particles after milling, allowing faster absorption/desorption kinetics. CONCLUSION The efficiency of using dihydrogen ruthenium complexes as doping agent to MgH2 will be discussed. The concept that increasing the number of dihydrogen ligands for better hydrogen release kinetics will be demonstrated in this work. Important kinetic improvements are expected by adding metallic nanoparticles (Ni, Ti…) to the “MgH2 + complex” samples. REFERENCES 1. K. L. Lim, Chem. Eng. Technol. 33, 213-226 (2010) 2. S. Niaz, Renew. Sust. Energ. Rev. 50, 457-469 (2015) 3. Z.X. Guo, J. Eur. Ceram. Soc. 28, 1467-1473 (2008) 4. H. Yu, Int. J. Hydrogen Energ. 39, 11633-11641 (2014) 5. M. Grellier, Chem. Commun. 48, 34-42 (2012) 6. M. Grellier, Inorg. Chem. 52, 7329-7337 (2013) ACKNOWLEDGMENTS The authors acknowledge the ANR (French National Research Agency) for the financial support (grant 3H2/2016).