Dionigi Fabio, Reier Tobias, Pawolek Zarina, Gliech Manuel, Strasser Peter
The Electrochemical Energy, Catalysis, and Materials Science Laboratory, Department of Chemistry, Chemical Engineering Division, Technical University Berlin, 10623, Berlin, Germany.
ChemSusChem. 2016 May 10;9(9):962-72. doi: 10.1002/cssc.201501581. Epub 2016 Mar 24.
Seawater is an abundant water resource on our planet and its direct electrolysis has the advantage that it would not compete with activities demanding fresh water. Oxygen selectivity is challenging when performing seawater electrolysis owing to competing chloride oxidation reactions. In this work we propose a design criterion based on thermodynamic and kinetic considerations that identifies alkaline conditions as preferable to obtain high selectivity for the oxygen evolution reaction. The criterion states that catalysts sustaining the desired operating current with an overpotential <480 mV in alkaline pH possess the best chance to achieve 100 % oxygen/hydrogen selectivity. NiFe layered double hydroxide is shown to satisfy this criterion at pH 13 in seawater-mimicking electrolyte. The catalyst was synthesized by a solvothermal method and the activity, surface redox chemistry, and stability were tested electrochemically in alkaline and near-neutral conditions (borate buffer at pH 9.2) and under both fresh seawater conditions. The Tafel slope at low current densities is not influenced by pH or presence of chloride. On the other hand, the addition of chloride ions has an influence in the temporal evolution of the nickel reduction peak and on both the activity and stability at high current densities at pH 9.2. Faradaic efficiency close to 100 % under the operating conditions predicted by our design criteria was proven using in situ electrochemical mass spectrometry.
海水是地球上丰富的水资源，其直接电解具有不与需要淡水的活动竞争的优势。由于存在竞争性的氯离子氧化反应，在进行海水电解时，实现氧气选择性具有挑战性。在这项工作中，我们基于热力学和动力学考虑提出了一种设计标准，该标准确定碱性条件更有利于获得析氧反应的高选择性。该标准指出，在碱性pH值下，过电位<480 mV时维持所需工作电流的催化剂最有可能实现100%的氧气/氢气选择性。在模拟海水的电解质中，pH值为13时，镍铁层状双氢氧化物被证明符合这一标准。通过溶剂热法合成了该催化剂，并在碱性和近中性条件（pH值为9.2的硼酸盐缓冲液）以及新鲜海水条件下对其活性、表面氧化还原化学和稳定性进行了电化学测试。低电流密度下的塔菲尔斜率不受pH值或氯离子存在的影响。另一方面，在pH值为9.2时，氯离子的添加对镍还原峰的时间演变以及高电流密度下的活性和稳定性都有影响。使用原位电化学质谱法证明，在我们的设计标准预测的操作条件下，法拉第效率接近100%。
