Institut für Anorganische Chemie und Strukturchemie, Heinrich-Heine-Universität Düsseldorf, 40204 Düsseldorf, Germany.
Molecules. 2023 May 31;28(11):4464. doi: 10.3390/molecules28114464.
Metal-organic frameworks (MOFs) have been investigated with regard to the oxygen evolution reaction (OER) due to their structure diversity, high specific surface area, adjustable pore size, and abundant active sites. However, the poor conductivity of most MOFs restricts this application. Herein, through a facile one-step solvothermal method, the Ni-based pillared metal-organic framework [Ni(BDC)DABCO] (BDC = 1,4-benzenedicarboxylate, DABCO = 1,4-diazabicyclo[2.2.2]octane), its bimetallic nickel-iron form [Ni(Fe)(BDC)DABCO], and their modified Ketjenblack (mKB) composites were synthesized and tested toward OER in an alkaline medium (KOH 1 mol L). A synergistic effect of the bimetallic nickel-iron MOF and the conductive mKB additive enhanced the catalytic activity of the MOF/mKB composites. All MOF/mKB composite samples (7, 14, 22, and 34 wt.% mKB) indicated much higher OER performances than the MOFs and mKB alone. The Ni-MOF/mKB14 composite (14 wt.% of mKB) demonstrated an overpotential of 294 mV at a current density of 10 mA cm and a Tafel slope of 32 mV dec, which is comparable with commercial RuO, commonly used as a benchmark material for OER. The catalytic performance of Ni(Fe)MOF/mKB14 (0.57 wt.% Fe) was further improved to an overpotential of 279 mV at a current density of 10 mA cm. The low Tafel slope of 25 mV dec as well as a low reaction resistance due to the electrochemical impedance spectroscopy (EIS) measurement confirmed the excellent OER performance of the Ni(Fe)MOF/mKB14 composite. For practical applications, the Ni(Fe)MOF/mKB14 electrocatalyst was impregnated into commercial nickel foam (NF), where overpotentials of 247 and 291 mV at current densities of 10 and 50 mA cm, respectively, were realized. The activity was maintained for 30 h at the applied current density of 50 mA cm. More importantly, this work adds to the fundamental understanding of the in situ transformation of Ni(Fe)DMOF into OER-active α/β-Ni(OH), β/γ-NiOOH, and FeOOH with residual porosity inherited from the MOF structure, as seen by powder X-ray diffractometry and N sorption analysis. Benefitting from the porosity structure of the MOF precursor, the nickel-iron catalysts outperformed the solely Ni-based catalysts due to their synergistic effects and exhibited superior catalytic activity and long-term stability in OER. In addition, by introducing mKB as a conductive carbon additive in the MOF structure, a homogeneous conductive network was constructed to improve the electronic conductivity of the MOF/mKB composites. The electrocatalytic system consisting of earth-abundant Ni and Fe metals only is attractive for the development of efficient, practical, and economical energy conversion materials for efficient OER activity.
金属有机骨架（MOFs）因其结构多样性、高比表面积、可调节的孔径和丰富的活性位点而被研究用于析氧反应（OER）。然而，大多数 MOFs 的导电性差限制了其应用。在此，通过简便的一步溶剂热法，合成了镍基金属有机骨架[Ni（BDC）DABCO]（BDC=1,4-苯二甲酸，DABCO=1,4-二氮杂二环[2.2.2]辛烷）、其双金属镍铁形式[Ni（Fe）（BDC）DABCO]及其改性的 Ketjenblack（mKB）复合材料，并在碱性介质（KOH 1 mol L）中对其进行了 OER 测试。双金属镍铁 MOF 和导电 mKB 添加剂的协同作用增强了 MOF/mKB 复合材料的催化活性。所有 MOF/mKB 复合样品（7、14、22 和 34 wt.% mKB）的 OER 性能均明显高于 MOFs 和 mKB 单独使用的性能。Ni-MOF/mKB14 复合（mKB 为 14wt.%）在电流密度为 10 mA cm 时的过电位为 294 mV，塔菲尔斜率为 32 mV dec，与商业 RuO2 相当，RuO2 通常用作 OER 的基准材料。Ni(Fe)MOF/mKB14（0.57wt.%Fe）的催化性能进一步提高，在电流密度为 10 mA cm 时的过电位为 279 mV。低的塔菲尔斜率为 25 mV dec 以及由于电化学阻抗谱（EIS）测量而导致的低反应阻力，证实了 Ni(Fe)MOF/mKB14 复合材料具有优异的 OER 性能。对于实际应用，将 Ni(Fe)MOF/mKB14 电催化剂浸渍到商业镍泡沫（NF）中，在电流密度为 10 和 50 mA cm 时，分别实现了 247 和 291 mV 的过电位。在 50 mA cm 的应用电流密度下，活性可保持 30 小时。更重要的是，这项工作增加了对原位转化的基本认识Ni(Fe)DMOF 转化为 OER 活性的α/β-Ni(OH)、β/γ-NiOOH 和 FeOOH，具有 MOF 结构中继承的残余孔隙率，如粉末 X 射线衍射和 N 吸附分析所示。受益于 MOF 前体的多孔结构，镍铁催化剂由于协同效应而优于仅基于 Ni 的催化剂，并在 OER 中表现出优异的催化活性和长期稳定性。此外，通过将 mKB 作为导电碳添加剂引入 MOF 结构中，构建了均匀的导电网络，提高了 MOF/mKB 复合材料的电子导电性。由地球丰富的 Ni 和 Fe 金属组成的电催化体系对于开发高效、实用和经济的能源转换材料以实现高效 OER 活性具有吸引力。
