Investigating Mixed Metal Chalcogenides for Electrocatalytic Water Oxidation: An Integrated Experimental and Theoretical Approach towards Materials Innovation

非技术综述：清洁氢气的生产是大规模和长期实施氢燃料经济的主要挑战之一。利用水的裂解反应生产氧气和氢气是目前最有前途的清洁氢气生产技术之一。然而，水分解反应的效率受到动力学缓慢的水氧化过程(也称为析氧反应，OER)的限制，这就需要使用催化剂来降低活化能垒。OER催化剂需要设计成能够在较低的应用电位下促进反应，从而提高整体的能量转换效率，同时，无毒、廉价、丰富和耐用。通过这笔由材料研究部的固态和材料化学计划和化学部的化学催化计划共同资助的赠款，PI采用假设驱动的综合实验和基于DFT的理论方法来设计基于含有第一行过渡元素的混合金属硫化物(硒和碲化物)的高效OER电催化剂。它们良好的电子和结构特性表明，这些硫化物基电催化剂的性能优于传统的OER电催化剂，后者基于贵金属或过渡金属氧化物，仅显示出适度的催化活性。这种含有地球上丰富的非贵重元素的高效OER电催化剂在全球范围内具有巨大的社会影响，特别是在可预见的未来，以可持续和非成本禁止的方式进行替代能源发电是人类主要关注的问题之一。这个多学科项目涉及本科生、研究生和博士后研究人员，他们在化学、电化学、物理和表面科学的前沿方法上合作，以发现新的催化剂成分。为了传播有关替代能源发电的知识，设计了演示实验，并通过在圣路易斯科学中心设立的外联活动、通过密苏里州S和T；T开办的少数族裔工程入门(MITE)营地以及通过夏季为当地高中教师举办的讲习班与公众分享，其中包括利用该项目的成果现场演示活跃的电解水器。技术综述：本项目由材料研究部固体和材料化学计划和化学部化学催化计划共同资助，主要研究三元和四元过渡金属硫化物对OER的电催化活性，具体目的如下：(1)通过组合方法确定新的有效的OER电催化剂组成；(2)通过实验测量和电子能带结构计算了解它们的催化活性，并对结构-性质相关性进行适当的了解；(3)研究这些电催化剂在OER条件下的稳定性。从材料化学的角度，PIs研究了过渡金属硫化物比常用的贵金属氧化物对OER有更好的催化效率的假设，原因包括：(I)金属-硫族键中的共价度增加，这将改变金属原子的化学势；(Ii)由于广泛的金属-金属键引起的硫化物结构的丰富，这将引起不同的氧化态，这将影响催化剂位置的氧化还原电位；(Iii)过渡金属硫化物表现出的各种金属-硫族配位几何结构，可以影响催化活性中心的性质和产生阴离子空位；以及(Iv)复杂的电子性质以及较小的带隙，使其在可见光区域更具吸收能力。本文主要通过电沉积的方法合成了过渡金属硫化物(硒和碲化物)，包括Nixey[E=Se，Te]，三元[Ni1-xMxEn；M=Fe，Co，Mn]和四元[NixMIyMIIzEn；MI=Fe，Mii=Al，Co，Mn]，并通过详细的电化学研究考察了它们的催化活性。系统的电子能带结构计算提供了对活性催化剂位置的洞察，并创造了关于这些新催化剂的结构-性质关系的深入知识。特别强调了催化剂表面的化学成分的阐明。采用各种表面分析技术，可以对这些硫系催化剂在OER条件下的稳定性提供有价值的见解，并能够识别实际的催化活性物种。

Non-technical Summary:The production of clean hydrogen is one of the main challenges for large-scale and long-term implementation of a hydrogen fuel economy. Using water splitting reactions to produce oxygen and hydrogen is currently one of the most promising technologies for generating clean hydrogen. However, the efficiency of the water splitting reaction is limited by the kinetically slow water oxidation process (also referred to as oxygen evolution reaction, OER) which necessitates the use of catalysts that can lower the activation energy barrier. The OER catalyst needs to be designed such that it can facilitate the reaction at low applied potential to increase the overall energy conversion efficiency and, at the same time, be non-toxic, cheap, abundant, and durable. Through this grant, co-funded by the Solid State and Materials Chemistry Program in the Division of Materials Research and the Chemical Catalysis Program in the Division of Chemistry, the PIs employ a hypothesis-driven integrated experimental and DFT-based theoretical approach to design highly efficient OER electrocatalysts based on mixed metal chalcogenides (selenides and tellurides) containing first row transition elements. Their favorably tailored electronic and structural properties suggest that these chalcogenide-based electrocatalysts outperform the conventional OER electrocatalysts, which are based on precious metal or transition metal oxides and show only modest catalytic activity. Such high-efficiency OER electrocatalysts containing earth-abundant non-precious elements have a large societal impact globally, especially in the foreseeable future, when alternative energy generation in a sustainable and non-cost-prohibitive way is one of the primary concerns of mankind. This multidisciplinary project involves undergraduate, graduate and postdoctoral researchers collaborating on cutting edge approaches in chemistry, electrochemistry, physics, and surface science for discovery of new catalyst compositions. To disseminate knowledge about alternative energy generation, demonstration experiments are designed and shared with the public through outreach activities set up at the St. Louis Science Center, through Minority Introduction to Engineering (MITE) camps run by Missouri S&T, and through workshops organized during the summer for local high school teachers, which include live demonstrations of an active water electrolyser utilizing results from this project. Technical Summary:This project, which is co-funded by the Solid State and Materials Chemistry Program in the Division of Materials Research and the Chemical Catalysis Program in the Division of Chemistry, is centered on investigating electrocatalytic activities of ternary and quaternary transition metal chalcogenides towards OER with the following specific aims: (1) identifying new efficient OER electrocatalyst compositions through combinatorial approach; (2) understanding their catalytic activities through experimental measurements as well as electronic band structure calculations and developing a proper insight of the structure-property correlation; (3) studying stability of these electrocatalysts under conditions of OER. From a materials chemistry point of view, the PIs investigate the hypothesis that transition metal chalcogenides have better catalytic efficiency than the commonly used precious metal oxides for OER due to several factors including: (i) increased degree of covalency in the metal-chalcogen bonds which will alter the chemical potential of the metal atom; (ii) structural richness of the chalcogenides resulting from extensive metal-metal bonding giving rise to variable oxidation states, which will affect redox potential of the catalyst site; (iii) variety of metal-chalcogen coordination geometry exhibited by the transition metal chalcogenides that can affect the nature of active sites for catalysis as well as creating anion vacancies; and (iv) intricate electronic properties along with a smaller bandgap making it more absorptive in the visible region. Transition metal chalcogenides (selenides and tellurides), of binary, NixEy [E = Se, Te], ternary [Ni1-xMxEn; M = Fe, Co, Mn], and quaternary [NixMIyMIIzEn; MI = Fe, MII = Al, Co, Mn] compositions are synthesized (mainly by electrodeposition) and their catalytic activities are investigated through detailed electrochemical studies with support from this grant. Systematic electronic band structure calculations provide an insight into the active catalyst sites and create in-depth knowledge regarding structure-property relationships for these new catalysts. Special emphasis is placed on the elucidation of the chemical composition on the catalyst surface. Employing a variety of surface analytical techniques reveals valuable insights regarding the stability of these chalcogenide catalysts under conditions of OER and allows the identification of the actual catalytically active species.