Kurzfassung: The influence of various synthesis conditions of a metal-nitrogen-carbon (M-N/C) catalyst material on oxygen reduction reaction (ORR) kinetics is discussed. Seven M-N/C catalysts based on cobalt are obtained by changing various synthesis conditions, such as the mixing environment, pyrolysis gas, and post-treatment. The ORR activity and stability measurements are performed using the classical three-electrode configuration in a 0.1 M HClO 4 solution. The most active and stable ORR catalyst proves to be the material obtained by mixing a cobalt salt, 2,2’-bipyridine, and a high surface area silicon carbide derived carbon together in water and pyrolyzing the mixture in argon. In a fuel cell test, however, a maximum power density value of 135 mW cm −2 is achieved with the catalyst mixed together in a planetary ball-mill at a low catalyst loading of 1.0 ± 0.1 mg cm −2 and at a test cell temperature of 60 o C despite of the fact that preparing the catalyst via dry ball-milling reduces the surface area of the material roughly 40% more than in the case of using a solution-based method. Consequently, mixing the catalyst precursors together without any additional chemicals in a planetary ball-mill instead of in a solution appears to be the most promising choice.

Kurzfassung: The performance of fuel cells is first and foremost defined by the kinetics of the oxygen reduction reaction (ORR). The electrocatalysts have to conform among the others with the following requirements set by the Department of Energy (DOE) like: reduced precious metal loading, increased activity, improved durability/stability, and increased tolerance to air, fuel and system-derived impurities [1]. One possibility to ensure the excellent performance of the electrode is using the electrocatalyst with the small size platinum nanoparticles deposited onto the hierarchically porous carbon support. In this way, the reactant gas (oxygen or air) can be uniformly distributed over the catalyst surface. Many authors reported the influence of the oxygen/air pressure on the kinetics of ORR and demonstrated that the effect of pressure on the oxygen solubility is more significant in dilute electrolyte solutions compared to the concentrated ones [2-4] . Therefore, in this work the electrocatalytic activity toward ORR for 8.3wt%Pt-C, 12.4wt% Pt-C and 20wt%Pt-Vulcan was analysed in pure oxygen and synthetic air saturated 0.1M KOH electrolyte solutions at atmospheric pressure. Platinum nanoparticles were deposited (8.3 and 12.4wt%) onto the molybdenium carbide derived carbon support [5] using the sodium borohydride method. The commercially available 20wt%Pt-Vulcan catalyst was used as a reference system for comparison [6] . The several physical and electrochemical characterization techniques like the low temperature nitrogen sorption, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, inductively coupled plasma mass spectrometry, x-ray diffraction analysis, cyclic voltammetry and rotating disc electrode methods were applied. The results clearly indicate that increasing partial pressure of the oxygen significantly influences the ORR kinetics, due to improved solubility of the oxygen in alkaline electrolyte solution. As expected, the limiting diffusion current density values for various Pt catalysts in oxygen saturated solution found to be approximately 5 times higher than that determined for the same catalysts in the syntethic air conditions. This result is in good agreement with literature data [3]. For detailed electrochemical analysis the Tafel-like plots were constructed. The results established indicate that the ORR mechanism remains unchanged and four electron reduction mechanism is valid for both catalysts, e.q. for the synthesised and commercial one. It should be emphasized that electrochemical activity toward ORR for 8.3wt% Pt-C catalyst was higher than that for 20wt%Pt-Vulcan in 0.1M KOH solution at fixed oxygen partial pressure conditions. References: [1] https://www1.eere.energy.gov/hydrogenandfuelcells/.../fuel_cells.pdf [2] M. Chatenet, M. Aurousseau, R. Durand, and F.Andolfatto, Journal of The Electrochemical Society, 150(3), D47 (2003). [3] A. Parthasarathy, S. Srinivasan, A.J. Appleby, and C.R. Martin, Journal of The Electrochemical Society 139(10), 2856 (1992). [4] W.-Y. Yan, S.-L. Zheng, W. Jin, Z. Peng, S.-N. Wanga, H. Du, and Y. Zhang, Journal of Electroanalytical Chemistry, 741, 100 (2015). [5] A. Jänes, T. Thomberg, H. Kurig, and E. Lust, Carbon, 47, 23 (2009). [6] E. Härk, R. Jäger, and E. Lust, Electrocatalysis, 6, 242 (2015). Acknowledgments This project has received funding from The Estonian Ministry of Education and Research (institutional research project IUT20-13, personal research grant PUT55) and European Regional Development Fund (The Centres of Excellence TK117 and TK141). The performance of fuel cells is first and foremost defined by the kinetics of the oxygen reduction reaction (ORR). The electrocatalysts have to conform among the others with the following requirements set by the Department of Energy (DOE) like: reduced precious metal loading, increased activity, improved durability/stability, and increased tolerance to air, fuel and system-derived impurities [1]. One possibility to ensure the excellent performance of the electrode is using the electrocatalyst with the small size platinum nanoparticles deposited onto the hierarchically porous carbon support. In this way, the reactant gas (oxygen or air) can be uniformly distributed over the catalyst surface. Many authors reported the influence of the oxygen/air pressure on the kinetics of ORR and demonstrated that the effect of pressure on the oxygen solubility is more significant in dilute electrolyte solutions compared to the concentrated ones [2-4] . Therefore, in this work the electrocatalytic activity toward ORR for 8.3wt%Pt-C, 12.4wt% Pt-C and 20wt%Pt-Vulcan was analysed in pure oxygen and synthetic air saturated 0.1M KOH electrolyte solutions at atmospheric pressure. Platinum nanoparticles were deposited (8.3 and 12.4wt%) onto the molybdenium carbide derived carbon support [5] using the sodium borohydride method. The commercially available 20wt%Pt-Vulcan catalyst was used as a reference system for comparison [6] . The several physical and electrochemical characterization techniques like the low temperature nitrogen sorption, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, inductively coupled plasma mass spectrometry, x-ray diffraction analysis, cyclic voltammetry and rotating disc electrode methods were applied. The results clearly indicate that increasing partial pressure of the oxygen significantly influences the ORR kinetics, due to improved solubility of the oxygen in alkaline electrolyte solution. As expected, the limiting diffusion current density values for various Pt catalysts in oxygen saturated solution found to be approximately 5 times higher than that determined for the same catalysts in the syntethic air conditions. This result is in good agreement with literature data [3]. For detailed electrochemical analysis the Tafel-like plots were constructed. The results established indicate that the ORR mechanism remains unchanged and four electron reduction mechanism is valid for both catalysts, e.q. for the synthesised and commercial one. It should be emphasized that electrochemical activity toward ORR for 8.3wt% Pt-C catalyst was higher than that for 20wt%Pt-Vulcan in 0.1M KOH solution at fixed oxygen partial pressure conditions. References: [1] https://www1.eere.energy.gov/hydrogenandfuelcells/.../fuel_cells.pdf [2] M. Chatenet, M. Aurousseau, R. Durand, and F.Andolfatto, Journal of The Electrochemical Society, 150(3), D47 (2003). [3] A. Parthasarathy, S. Srinivasan, A.J. Appleby, and C.R. Martin, Journal of The Electrochemical Society 139(10), 2856 (1992). [4] W.-Y. Yan, S.-L. Zheng, W. Jin, Z. Peng, S.-N. Wanga, H. Du, and Y. Zhang, Journal of Electroanalytical Chemistry, 741, 100 (2015). [5] A. Jänes, T. Thomberg, H. Kurig, and E. Lust, Carbon, 47, 23 (2009). [6] E. Härk, R. Jäger, and E. Lust, Electrocatalysis, 6, 242 (2015). Acknowledgments This project has received funding from The Estonian Ministry of Education and Research (institutional research project IUT20-13, personal research grant PUT55) and European Regional Development Fund (The Centres of Excellence TK117 and TK141).

Kurzfassung: Estonian phosphorite ore contains trace amounts of rare earth elements (REEs), many other d-metals, and some radioactive elements. Rare earth elements, Mo, V, etc. might be economically exploitable, while some radioactive and toxic elements should be removed before any other downstream processing for environmental and nutritional safety reasons. All untreated hazardous elements remain in landfilled waste in much higher concentration than they occur naturally. To resolve this problem U, Th, and Tl were removed from phosphorite ore at first using liquid extraction. In the next step, REE were isolated from raffinate. Nitrated Aliquat 336 (A336[NO3]) and Bis(2-ethylhexyl) Phosphate (D2EHPA) were used in liquid extraction for comparison. An improved method for exclusive separation of radioactive elements and REEs from phosphorite ore in 2-steps has been developed, exploiting liquid extraction at different pH values.

Kurzfassung: Hence, extensive research have been devoted to finding a non-noble oxygen reduction reaction (ORR) catalysts, such as Fe and nitrogen co-doped carbon materials (Fe-N/C) [1-3]. Fe-N/C type ORR catalysts were synthesized using FeSO 4 ∙7H 2 O as the Fe precursor, nitrogen containing organic compounds as N precursors and carbide derived carbon as the carbon support. To synthesize catalysts, precursors and carbon support were mixed into a slurry, dried and pyrolyzed at 800 °C in Ar for 1.5 h [4]. Nitrogen containing organic compounds were following: urea (Urea), ethylenediaminetetraacetic acid disodium salt (EDTA), 1,3-di(1H-imidazol-1-yl)-2-propanol (DIPO), 1,10-phenanthroline (Phen) and 2,2’-bipyridine (Bipyr). Silicon, molybdenum, titanium carbide derived carbons as the carbon supports were compared. Prepared catalysts were characterized by several physical characterization methods: the nitrogen sorption method, inductively coupled plasma mass spectrometry, X-ray photoelectron spectroscopy, transmission electrode microscopy, scanning electron microscopy, time of flight secondary ion mass-spectrometry and Raman spectroscopy. The electrocatalytic activity of the various catalysts was investigated in O 2 saturated in 0.1M KOH and 0.1M HClO 4 solutions using rotating disc electrode method. Additionally, durability of the catalysts was tested during ~150 h in both acid and alkaline solutions. For the durability test, the electrode was cycled within the potential range of 0.21 – 1.23 V vs RHE in Ar saturated solution. The ORR activity of studied catalysts in both acid and alkaline solution strongly depends on the N precursor used. The half-wave potential E 1/2 value for studied catalysts increases in the following order: C 〈 Fe-Urea/C 〈 Fe-EDTA/C 〈 Fe-DIPO/C £ Fe-Phen/C £ Fe-Bipyr/C. In acidic conditions the potential degradation rate is faster than in alkaline conditions for all studied catalysts. The fact that both catalysts showed better durability in alkaline conditions can be explained by possible dissolution of active particles or protonation of ORR active centers at low pH [2,5]. Acknowledgements: This work was supported by the projects TK141 “Advanced materials and high-technology devices for energy recuperation systems” (2014-2020.4.01.15-0011), NAMUR ”Nanomaterials - research and applications” (3.2.0304.12-0397) and by the Estonian institutional research grant No. IUT20-13. References [1] Bezerra, C. W. B.; Zhang, L.; Lee, K.; Liu, H.; Marques, A. L. B.; Marques, E. P.; Wang, H.; Zhang, J. A review of Fe–N/C and Co–N/C catalysts for the oxygen reduction reaction. Electrochimica Acta . 2008 , 53 , 4937–4951. [2] Singh, D.; Tian, J.; Mamtani, K.; King, J.; Miller, J. T.; Ozkan, U. S. A comparison of N-containing carbon nanostructures (CNx) and N-coordinated iron–carbon catalysts (FeNC) for the oxygen reduction reaction in acidic media. J. Catal. 2014 , 317 , 30–43. [3] Roncaroli, F.; Dal Molin, E. S.; Viva, F. A.; Bruno, M. M.; Halac, E. B. Cobalt and Iron Complexes with N-heterocyclic Ligands as Pyrolysis Precursors for Oxygen Reduction Catalysts. Electrochimica Acta . 2015 , 174 , 66–77. [4] Kasatkin, P. E.; Jäger, R.; Härk, E.; Teppor, P.; Tallo, I.; Joost, U.; Šmits, K.; Kanarbik, R.; Lust, E. Fe-N/C catalysts for oxygen reduction based on silicon carbide derived carbon. Electrochem. Commun. 2017 , 80 , 33–38. [5] Tylus, U.; Jia, Q.; Strickland, K.; Ramaswamy, N.; Serov, A.; Atanassov, P.; Mukerjee, S. Elucidating Oxygen Reduction Activ e Sites in Pyrolyzed Metal–Nitrogen Coordinated Non-Precious-Metal Electrocatalyst Systems. J. Phys. Chem. C Nanomater. Interfaces . 2014 , 118 , 8999–9008.