Department for Neutron Characterization, Institute for Energy Technology, P.O. Box 40, NO-2027 Kjeller, Norway.
Molecules. 2020 Feb 12;25(4):780. doi: 10.3390/molecules25040780.
In order to improve the suitability of NaBH as a clean fuel, its decomposition temperature needs to be decreased to below 535 °C, while its hydrogen release must be as high as possible. In this work, the influence of a collection of first and second period transition metal fluorides on the destabilization of NaBH is studied on samples produced by ball milling NaBH with 2 mol% of a metal fluoride additive. The effects obtained by increasing additive amount and changing oxidation state are also evaluated for NbF, CeF, and CeF. The as-milled products are characterized by in-house power X-ray diffraction, while the hydrogen release and decomposition are monitored by temperature programmed desorption with residual gas analysis, differential scanning calorimetry, and thermogravimetry. The screening of samples containing 2 mol% of additive shows that distinctive groups of transition metal fluorides affect the ball milling process differently depending on their enthalpy of formation, melting point, or their ability to react at the temperatures achieved during ball milling. This leads to the formation of NaBF in the case of TiF, MnF, VF, CdF, NbF, AgF, and CeF and the presence of the metal in CrF, CuF, and AgF. There is no linear correlation between the position of the transition metal in the periodic table and the observed behavior. The thermal behavior of the products after milling is given by the remaining NaBH, fluoride, and the formation of intermediate metastable compounds. A noticeable decrease of the decomposition temperature is seen for the majority of the products, with the exceptions of the samples containing YF, AgF, and CeF. The largest decrease of the decomposition temperature is observed for NbF. When comparing increasing amounts of the same additive, the largest decrease of the decomposition temperature is observed for 10 mol% of NbF. Higher amounts of additive result in the loss of the NaBH thermal signal and ultimately the loss of the crystalline borohydride. When comparing additives with the same transition metal and different oxidation states, the most efficient additive is found to be the one with a higher oxidation state. Furthermore, among all the samples studied, higher oxidation state metal fluorides are found to be the most destabilizing agents for NaBH. Overall, the present study shows that there is no single parameter affecting the destabilization of NaBH by transition metal fluorides. Instead, parameters such as the transition metal electronegativity and oxidation state or the enthalpy of formation of the fluoride and its melting point are competing to influence the destabilization. In particular, it is found that the combination of a high metal oxidation state and a low fluoride melting point will enhance destabilization. This is observed for MnF, NbF, NiF, and CuF, which lead to high gas releases from the decomposition of NaBH at the lowest decomposition temperatures.
为了提高硼氢化钠作为清洁能源的适用性，需要降低其分解温度至 535°C 以下，同时保持尽可能高的放氢量。本工作通过机械球磨法在硼氢化钠中添加 2mol%的金属氟化物添加剂，研究了一系列一、二周期过渡金属氟化物对硼氢化钠的非稳定化作用。研究了通过增加添加剂用量和改变氧化态对 NbF、CeF 和 CeF 的影响。采用原位粉末 X 射线衍射对球磨产物进行了表征，利用程序升温脱附-残气分析、差示扫描量热法和热重分析法监测了放氢和分解过程。筛选了含有 2mol%添加剂的样品，结果表明，不同的过渡金属氟化物根据其生成焓、熔点或在球磨过程中达到的温度下的反应能力，对球磨过程的影响不同。这导致 TiF、MnF、VF、CdF、NbF、AgF 和 CeF 形成了 NaBF，而 CrF、CuF 和 AgF 则保留了金属。过渡金属在元素周期表中的位置与观察到的行为之间没有线性关系。球磨后产物的热行为由剩余的硼氢化钠、氟化物和中间亚稳化合物的形成决定。大多数产物的分解温度都有明显下降，而 YF、AgF 和 CeF 除外。NbF 的分解温度下降最大。当比较相同添加剂的增加量时，发现 10mol%的 NbF 使分解温度下降最大。更高的添加剂用量导致硼氢化钠的热信号丢失，最终导致晶体硼氢化钠的损失。当比较具有相同过渡金属和不同氧化态的添加剂时，发现氧化态更高的添加剂最有效。此外，在所研究的所有样品中，发现氧化态更高的金属氟化物是对硼氢化钠最具非稳定性的试剂。总的来说，本研究表明，没有单一参数能影响过渡金属氟化物对硼氢化钠的非稳定性。相反，过渡金属电负性和氧化态、氟化物生成焓及其熔点等参数相互竞争，影响非稳定性。特别是，高金属氧化态和低氟化物熔点的组合将增强非稳定性。MnF、NbF、NiF 和 CuF 就是这种情况，它们导致在最低分解温度下从硼氢化钠的分解中释放出大量气体。
