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Ni- and Sn-based Colloidal Nanoparticles for Electrochemical Energy Technologies

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Universitat de Barcelona. Facultat de Física

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[eng]  In this project, nanoparticles were produced by solution-based one-pot synthesis, particularly colloidal methods. A series of powerful tools were used to characterize the structure and surface compositions before they were tested as anode materials in the field of energy conversion and storage. 
Firstly, we produced colloidal Ni polyhedral (16 ± 2 nm) and spherical (13 nm) NCs. The electrocatalytic properties of electrodes based on these NCs were first investigated in variable concentrations of KOH. Electrodes based on Ni polyhedral NCs displayed impressive current densities (59.4 mA cm 2) and mass activities (2016 mA mg 1) at 0.6 V vs. Hg/HgO in the presence of 1.0 M methanol and 1.0 M KOH, which corresponds to a twofold increase over electrodes based on spherical Ni NCs and over most previous Ni-based electrocatalyts previously reported. Electrodes based on faceted polyhedral NCs displayed a 30% loss of activity during the first few operation hours, but activity stabilized to around a 65% of the initial value after ca. 20000 s operations. 
And also, we developed a new synthetic route to produce NiSn intermetallic NPs with composition control. Detailed electrochemical measurements showed that these NPs exhibited excellent performance for MOR in alkaline solution. Ni-rich NiSn-based electrocatalysts displayed slightly improved performances over Ni-based electrocatalysts. Most notorious was the significantly improved stability of NiSn catalysts compared with that of Ni. This work represented a significant advance in developing cost-effective electrocatalysts with high activity and stability for MOR in DMFCs. 
In another sub-section of the energy conversion, a series of Ni3 xCoxSn2 (0 ≤ x ≤ 3) NPs were produced based one the protocol. A preliminary optimized catalyst composition, Ni2.5Co0.5Sn2, showed a current density of 65.5 mA cm 2 and a mass current density of 1050 mA mg 1 at 0.6 V vs. Hg/HgO for the MOR in 1.0 M KOH containing 1.0 M methanol. While the introduction of Co slightly decreased the durability with respect to Ni3Sn2, Ni2.5Co0.5Sn2 NP-based electrodes demonstrated significant stability during continuous cycling and increased activity at high methanol concentrations. The presence of Sn was found to be essential to improve stability with respect to elemental Ni, although Sn was observed to slowly dissolve in the presence of KOH. 
In the energy storage field, we focused on the performance as anode material in LIBs and SIBs. Among the different NixSn compositions tested, best performances toward Li+ ion and Na+ ion insertion were obtained for Ni0.9Sn NP-based electrodes. This optimized cycling charge-discharge performance for LIBs provided 980 mAh g 1 at 0.2 A g 1 after 340 cycles. Additionally, Ni0.9Sn NP-based electrodes were tested in Na+-ion half cells, exhibiting 160 mAh g 1 over 120 cycles at 0.1 A g 1. The pseudocapacitive charge-storage accounted for a high portion of the whole energy storage capacity, which was associated to the small size and the composition of the NixSn NPs used.
In the last chapter of the project, the CoxSn NP composition was adjusted over the range 1.3 ≤ x ≤ 0.3. These Co-Sn solid solutions were tested as anode materials in LIBs on a half-cell battery system. Among the different compositions tested, Co0.9Sn and Co0.7Sn NPs provided the best performance, with a charge-discharge capacity above 1500 mAh g 1 at a current density of 0.2 A g 1 after 220 cycles and up to 800 mAh g 1 at 1.0 A g 1 after 400 cycles. Through the kinetic analysis of Co0.9Sn NPs by the CV measurement, we found these charge-discharge capacities to include a very large pseudocapacitive contribution, up to 81% at a sweep rate of 1 mV s 1, which we related to the small size of the particles.

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Ni- and Sn-based Colloidal Nanoparticles for Electrochemical Energy Technologies