Structural basis for differing electrocatalytic water oxidation by the cubic, layered and spinel forms of lithium cobalt oxides
Structural basis for differing electrocatalytic water oxidation by the cubic, layered and spinel forms of lithium cobalt oxides
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DOI：
10.1039/c5ee02195b
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发表时间：
2016-01-01
影响因子：
32.5
通讯作者：
Dismukes, G. Charles
中科院分区：
文献类型：
--
作者：
Gardner, Graeme;Al-Sharab, Jafar;Dismukes, G. Charles
锂钴氧化物LiCoO2的两种多晶态提供了一个机会来对比可逆电荷存储（电池功能）与催化水氧化/析氧(OER; 2H(2)O -> O-2 + 4H（+) + 4e(-)）的结构要求。之前，我们报道了立方相纳米晶体的高OER电催化活性，而层状相（原型锂离子电池阴极）的低活性。通过透射电镜、电子衍射、伏安法和OER电解条件下的元素分析，我们发现Li+离子从层状LiCoO2中部分脱层，引发了立方尖晶石LiCo2O4的结构重组，同时形成了更活跃的催化相。比较立方LiCoO2 （50 nm）和铱（5 nm）纳米颗粒在碱性和中性电解质中用于OER催化（膜基系统的商业基准），发现在Tafel斜率（48 mV dec(-1)），过电位（eta =类似于420 mV@10 mA cm(-2)），法拉第产率（100%）和OER稳定性（14小时内无损失）方面具有优异的性能。立方LiCoO2和尖晶石LiCo2O4固有的OER活性归因于[Co4O4]（n+）立方体结构单元的存在，这些结构单元提供了较低的Co4+氧化电位和较低的立方体间空穴迁移率。相反，层状相缺乏立方体单元，表现出广泛的平面内空穴离域，这在熵上阻碍了四个电子/空穴协同的OER反应。真正相关的催化剂的一个基本特征是在真正的电解槽堆中高效连续运行。立方体LiCoO2在固体电解质碱性膜电解槽中的初步试验表明，在电流密度高达400毫安厘米（-2）和过电位低于已证实的PGM（铂族金属）催化剂的情况下，连续运行1000小时（无故障）。
The two polymorphs of lithium cobalt oxide, LiCoO2, present an opportunity to contrast the structural requirements for reversible charge storage (battery function) vs. catalysis of water oxidation/oxygen evolution (OER; 2H(2)O -> O-2 + 4H(+) + 4e(-)). Previously, we reported high OER electrocatalytic activity from nanocrystals of the cubic phase vs. poor activity from the layered phase - the archetypal lithium-ion battery cathode. Here we apply transmission electron microscopy, electron diffraction, voltammetry and elemental analysis under OER electrolysis conditions to show that labile Li+ ions partially deintercalate from layered LiCoO2, initiating structural reorganization to the cubic spinel LiCo2O4, in parallel with formation of a more active catalytic phase. Comparison of cubic LiCoO2 (50 nm) to iridium (5 nm) nanoparticles for OER catalysis (commercial benchmark for membrane-based systems) in basic and neutral electrolyte reveals excellent performance in terms of Tafel slope (48 mV dec(-1)), overpotential (eta = similar to 420 mV@10 mA cm(-2) at pH = 14), faradaic yield (100%) and OER stability (no loss in 14 hours). The inherent OER activity of cubic LiCoO2 and spinel LiCo2O4 is attributed to the presence of [Co4O4](n+) cubane structural units, which provide lower oxidation potential to Co4+ and lower intercubane hole mobility. By contrast, the layered phase, which lacks cubane units, exhibits extensive intra-planar hole delocalization which entropically hinders the four electron/hole concerted OER reaction. An essential distinguishing trait of a truly relevant catalyst is efficient continuous operation in a real electrolyzer stack. Initial trials of cubic LiCoO2 in a solid electrolyte alkaline membrane electrolyzer indicate continuous operation for 1000 hours (without failure) at current densities up to 400 mA cm(-2) and overpotential lower than proven PGM (platinum group metal) catalysts.