Development of optimum bandgap photoanodes for tandem water-splitting cells based on doped complex metal oxides and III-V semiconductors coupled to water oxidation electrocatalysts

开发高效太阳能水分解系统的最有前途的方法之一是使用光电化学装置，该光电化学装置包括具有优化的光学（带隙）、光电化学（准费米能级、电流匹配）和表面催化性质的光电阴极和光电阳极的串联。例如，当使用带隙为1.1 eV和1.8 eV的两种吸收剂时，这种串联电池可以提供约25%的太阳能-氢气效率。虽然基于晶体硅（1.1eV）的高效低带隙光电阴极是可用的，但是将差距桥接到工作装置的主要挑战是开发具有良好匹配特性的高效且稳定的光电阳极：1.8eV的带隙，以及在低至0.4V vs. RHE（即，光电流开始于约0.2 V vs. RHE）。然而，目前还没有满足此类标准的材料，必须开发新材料。为了应对这一挑战，该合作项目旨在研究基于两种不同类型的光吸收剂的两种新型和不同类型的光阳极，基于理性考虑和理论（DFT）计算选择为非常有前途的：i）掺杂钨酸铜，ii）锑掺杂氮化镓。对组成、结构、形态和结晶度的高度控制对于有效的光捕获和电荷分离至关重要。因此，将通过溶胶-凝胶和金属有机化学气相沉积（MOCVD）技术制备具有各种形貌（多孔、致密、外延层、纳米柱）的薄膜。为了确保水氧化的快速动力学，将通过原子层沉积和通过固有低温方法沉积用于析氧的高效非晶电催化剂的薄层（电化学，光电化学，和光化学金属有机沉积）为了避免热激活层间原子扩散形成缺陷，将采用详细的机理研究来确定光电阳极的光电化学性能。时间分辨太赫兹光电导探针和光诱导瞬态吸收光谱将被用来研究电荷输运特性和复合动力学。这些研究将推动光阳极的合理设计，并有望提供有关不同材料类别和结构中电荷动力学的独特知识，其意义远远超出本项目的范围。

One of the most promising approaches for the development of highly efficient solar water-splitting systems is the use of photoelectrochemical devices comprising of a tandem of photocathodes and photoanodes with optimized optical (bandgap), photoelectrochemical (quasi-Fermi levels, current matching), and surface catalytic properties. For example, such tandem cells could provide solar-to-hydrogen efficiencies of around 25% when using two absorbers with bandgaps of 1.1 eV and 1.8 eV. While highly efficient low-bandgap photocathodes based on crystalline silicon (1.1 eV) are available, a major challenge in bridging the gap to working devices is the development of efficient and stable photoanodes with well-matched characteristics: bandgap of 1.8 eV, and photocurrent maximum at potentials as low as 0.4 V vs. RHE (i.e., photocurrent onset at ca. 0.2 V vs. RHE). However, there are currently no materials fulfilling such criteria, and new materials must be developed. In addressing this challenge, this collaborative project aims to investigate two novel and distinct types of photoanodes based on two different classes of light absorbers, selected as highly promising on the basis of rational considerations and theoretical (DFT) calculations: i) doped copper tungstates, and ii) antimony-doped gallium nitride. High degree of control over the composition, structure, morphology, and crystallinity is crucial for efficient light harvesting and charge separation. Therefore, thin films with various morphologies (porous, compact, epitaxial layers, nanopillars) will be prepared by sol-gel and metalorganic chemical vapor deposition (MOCVD) techniques. In order to ensure fast kinetics of water oxidation, thin layers of highly efficient amorphous electrocatalysts for oxygen evolution will be deposited by atomic layer deposition and by inherently low temperature methods (electrochemical, photoelectrochemical, and photochemical metalorganic deposition) in order to avoid formation of defects by thermally activated interlayer atomic diffusion.Detailed mechanistic investigations will be employed to identify the bottlenecks in photoelectrochemical performance of photoanodes. A time-resolved terahertz photoconductivity probe and photo-induced transient absorption spectroscopy will be used to study the charge transport properties and recombination dynamics. These investigations will drive the rational design of the photoanodes and are expected to provide unique knowledge on the charge dynamics in different materials classes and architectures with significance far beyond the scope of this project.