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

开发高效太阳能分水系统的最有前途的方法之一是使用光电化学器件，该器件由光电阴极和光阳极串联而成，具有优化的光学(带隙)、光电化学(准费米能级，电流匹配)和表面催化性能。例如，当使用两个带隙为1.1 eV和1.8 eV的吸收体时，这种串联电池可以提供大约25%的太阳能到氢气的效率。虽然已经有了基于晶体硅(1.1 eV)的高效低带隙光电阴极，但弥合这一差距到工作设备的主要挑战是开发具有良好匹配特性的高效和稳定的光阳极：带隙为1.8 eV，在低至0.4V的电势下光电流最大(即，光电流起始值约为0.2V vs.RHE)。然而，目前还没有符合这一标准的材料，必须开发新材料。为了应对这一挑战，这个合作项目旨在研究两种新型和不同类型的光阳极，它们基于两种不同的光吸收材料，基于理性考虑和理论(DFT)计算，被选为非常有前途的：i)掺杂钨酸铜，和ii)掺锑氮化镓。对组成、结构、形貌和结晶度的高度控制是高效捕光和电荷分离的关键。因此，采用溶胶凝胶法和金属有机化学气相沉积(MOCVD)技术可以制备出不同形貌的薄膜(多孔膜、致密膜、外延层、纳米管)。为了保证水氧化的快速动力学，将通过原子层沉积和固有的低温方法(电化学、光电化学和光化学金属有机沉积)沉积高效的非晶析氧电催化剂薄层，以避免热激活的层间原子扩散形成缺陷。我们将使用时间分辨太赫兹光电导探针和光诱导瞬时吸收光谱来研究电荷输运性质和复合动力学。这些研究将推动光阳极的合理设计，并有望为不同材料类别和结构中的电荷动力学提供独特的知识，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.