Super-Reduced Polyoxometalates: Excellent Molecular Cluster Battery Components and Semipermeable Molecular Capacitors
Super-Reduced Polyoxometalates: Excellent Molecular Cluster Battery Components and Semipermeable Molecular Capacitors
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DOI：
10.1021/ja5032369
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发表时间：
2014-06-25
影响因子：
15
通讯作者：
Irle, Stephan
中科院分区：
文献类型：
--
作者：
Nishimoto, Yoshio;Yokogawa, Daisuke;Irle, Stephan
理论研究了锂基分子团簇电池放电过程中，α-Keggin型聚氧乙烯酸盐（POM 3-）团簇[PM 12 O 40]（3-）（M = Mo，W）向超还原态POM 27-转变时的分子和电子结构变化.采用密度泛函理论进行几何优化，并采用第一性原理分子动力学模拟方法研究了在中性POM团簇表面随机配置Li原子作为电子供体时，团簇势能面上的局部极小值.在结构，电子密度和分子轨道研究的基础上，我们提出的证据表明，超还原是伴随着金属-金属键的形成，从第12至第14个多余的电子转移到集群。之后，金属-金属键的数量随着额外转移的过量电子的数量几乎线性地增加。在α-Keggin型POM中，金属三角形是一个突出的新兴结构特征。超还原过程中金属三角形形成的起源源于三角形金属原子位点中特征性三中心双电子键的形成，该特征性三中心双电子键是在POM骨架的保护下通过当底层金属原子形成共价键时桥接两个金属原子的氧原子的“挤出”而产生的。这种不寻常的几何和电子结构变化的驱动力是在单个过渡金属八面体位置的局部Jahn-Teller畸变，其中三重简并t（2）d轨道在还原过程中部分填充，并通过八面体的畸变以形成金属-金属键的方式获得能量。成键轨道显示出与金属-氧反键轨道混合的强烈贡献，从而将多余的电子从团簇中心“拖走”到笼外。在超还原的P0 M27-聚阴离子的表面上的高密度的带负电荷但大部分分离的氧原子允许由于过量电子的存在而导致的巨大库仑排斥被Li抗衡阳离子的存在所抵消，Li抗衡阳离子部分地渗透到外部氧壳层中。这种“半多孔分子电容器”结构可能是POM中有效电子吸收的原因。
Theoretical investigations are presented on the molecular and electronic structure changes that occur as alpha-Keggin-type polyoxometalate (POM3-) clusters [PM12O40](3-)(M = Mo, W) are converted toward their super-reduced POM27- state during the discharging process in lithium-based molecular cluster batteries. Density functional theory was employed in geometry optimization, and first-principles molecular dynamics simulations were used to explore local minima on the potential energy surface of neutral POM clusters adorned with randomly placed Li atoms as electron donors around the cluster surface. On the basis of structural, electron density, and molecular orbital studies, we present evidence that the super-reduction is accompanied by metal-metal bond formation, beginning from the 12th to 14th excess electron transferred to the cluster. Afterward, the number of metal-metal bonds increases nearly linearly with the number of additionally transferred excess electrons. In alpha-Keggin-type POMs, metal triangles are a prominently emerging structural feature. The origin of the metal triangle formation during super-reduction stems from the formation of characteristic three-center two-electron bonds in triangular metal atom sites, created under preservation of the POM skeleton via "squeezing out" of oxygen atoms bridging two metal atoms when the underlying metal atoms form covalent bonds. The driving force for this unusual geometrical and electronic structure change is a local Jahn-Teller distortion at individual transition-metal octahedral sites, where the triply degenerate t(2) d orbitals become partially filled during reduction and gain energy by distortion of the octahedron in such a way that metal-metal bonds are formed. The bonding orbitals show strong contributions from mixing with metal-oxygen antibonding orbitals, thereby "shuffling away" excess electrons from the cluster center to the outside of the cage. The high density of negatively charged yet largely separated oxygen atoms on the surface of the super-reduced POM27- polyanion allows the huge Coulombic repulsion due to the presence of the excess electrons to be counterbalanced by the presence of Li countercations, which partially penetrate into the outer oxygen shell. This "semiporous molecular capacitor" structure is likely the reason for the effective electron uptake in POMs.