Liu Tianying, Wang Pan, Li Wei, Wang David Z, Lekamge Damith D, Chen Boqiang, Houle Frances A, Waegele Matthias M, Wang Dunwei
Department of Chemistry, Merkert Chemistry Center, Boston College, Chestnut Hill, Massachusetts 02467, United States.
Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.
ACS Cent Sci. 2024 Dec 16;11(1):91-97. doi: 10.1021/acscentsci.4c01415. eCollection 2025 Jan 22.
As a vital process for solar fuel synthesis, water oxidation remains a challenging reaction to perform using durable and cost-effective systems. Despite decades of intense research, our understanding of the detailed processes involved is still limited, particularly under photochemical conditions. Recent research has shown that the overall kinetics of water oxidation by a molecular dyad depends on the coordination between photocharge generation and the subsequent chemical steps. This work explores similar effects of heterogeneous solar water oxidation systems. By varying a key variable, the reaction temperature, we discovered distinctly different behaviors on two model systems, TiO and FeO. TiO exhibited a monotonically increasing water oxidation performance with rising temperature across the entire applied potential range, between 0.1 and 1.5 V vs the reversible hydrogen electrode (RHE). In contrast, FeO showed increased performance with increasing temperature at high applied potentials (>1.2 V vs RHE) but decreased performance at low applied potentials (<1.2 V vs RHE). This decrease in performance with temperature on FeO was attributed to an increased level of electron-hole recombination, as confirmed by intensity-modulated photocurrent spectroscopy (IMPS). The origin of the differing temperature dependences on TiO and FeO was further ascribed to their different surface chemical kinetics. These results highlight the chemical nature of charge recombination in photoelectrochemical (PEC) systems, where surface electrons recombine with holes stored in surface chemical species. They also indicate that PEC kinetics are not constrained by a single rate-determining chemical step, highlighting the importance of an integrated approach to studying such systems. Moreover, the results suggest that for practical solar water splitting devices higher temperatures are not always beneficial for reaction rates, especially under low driving force conditions.
作为太阳能燃料合成的关键过程，水氧化反应在使用耐用且经济高效的系统时仍然具有挑战性。尽管经过了数十年的深入研究，但我们对其中详细过程的理解仍然有限，尤其是在光化学条件下。最近的研究表明，分子二元体系进行水氧化的整体动力学取决于光电荷产生与后续化学步骤之间的协同作用。这项工作探讨了异质太阳能水氧化系统的类似效应。通过改变一个关键变量——反应温度，我们在两个模型体系TiO和FeO上发现了截然不同的行为。在相对于可逆氢电极（RHE）为0.1至1.5 V的整个外加电势范围内，TiO的水氧化性能随温度升高呈单调增加。相比之下，FeO在高外加电势（>1.2 V vs RHE）下性能随温度升高而增加，但在低外加电势（<1.2 V vs RHE）下性能下降。强度调制光电流光谱（IMPS）证实，FeO上这种随温度下降的性能归因于电子 - 空穴复合水平的增加。TiO和FeO不同温度依赖性的根源进一步归因于它们不同的表面化学动力学。这些结果突出了光电化学（PEC）系统中电荷复合的化学本质，即表面电子与存储在表面化学物种中的空穴发生复合。它们还表明PEC动力学不受单一速率决定化学步骤的限制，凸显了采用综合方法研究此类系统的重要性。此外，结果表明对于实际的太阳能水分解装置，较高温度并不总是有利于反应速率，特别是在低驱动力条件下。
