Interface engineering for photoelectrochemical water oxidation
Creators: name: Cui, Junyi
Publisher: name: Imperial College London
Year: 2023

description: Photoelectrocatalytic (PEC) water splitting is a promising hydrogen production process that utilizes sunlight to produce hydrogen from water without any pollution. However, this process is limited by its poor efficiency due to serious charge recombination, especially surface recombination at the semiconductor/electrolyte interface. Loading electrocatalysts as co-catalysts on the semiconductor is a popular choice as it simultaneously improves photocurrent and onset potential. However, the decoupling of the enhancement of photocurrents from the catalytic activities of co-catalysts is widely reported. Electrocatalysts loaded onto a semiconductor photoelectrode serve multiple functions beyond catalysis, which is due to the difference between a metal electrode usually used to research electrocatalysts and the semiconductor substrate used in photoelectrocatalysis. As a result, the decoupling highlights the importance of understanding the roles of co-catalysts and developing novel architectures compatible with semiconductors, emphasizing that the approach should go beyond a mere combination of highly active electrocatalysts and semiconductors. Hence, this PhD project aims to develop novel architectures to boost the photoelectrochemical performance of promising candidates and to elucidate the intricate interfacial processes resulting from the chemical and physical properties of both semiconductors and co-catalysts for rationalising the design of semiconductors and co-catalysts. To achieve this aim, firstly, extensive research on BiVO4 photoanodes was conducted on various types of BiVO4 synthesized using different methods and revealed that the origin of efficiency loss is mainly from surface recombination. Next, a composite photoanode consisting of a light absorber of BiVO4 along with two functional layers of dispersive two-dimensional single-layer bismuthene islands enveloped with an amorphous layer of NiFeOOH, that achieves an impressively high photocurrent of 5.0 mA cm-2 at 1.23 VRHE was successfully designed. Results from comprehensive characterizations revealed the synergetic positive effect of bismuthene and NiFeOOH on improving the interfacial charge injection by impacting on two different surface states individually. Various properties of co-catalysts are all possibly linked to the PEC performance enhancement. To further relate the properties of co-catalysts to the performance, a series of core-shell structured bimetallic metal phosphides was employed as co-catalysts on BiVO4 photoanodes as a case study to reveal the relationships between the properties of co-catalysts and their functions on improving PEC performance. The properties of the co-catalysts were tuned by changing the Co and Ni ratios, and the contribution of each property was related to the PEC performance enhancement using both experimental and computational characterization methods. The improvement in the PEC performance is found to stem from a combination of factors including work function, particle size, catalytic activity, and capacitive ability. The interaction between the core-shell structured metal phosphides and BiVO4, as well as their contact with the electrolyte, creates various interfaces that facilitate three independent routes for hole injection into the electrolyte. The kinetics of these hole injection routes are significantly influenced by the properties of the co-catalysts, leading to varying degrees of enhancement in photocurrent density. Discovering the potential of new emerging promising electrocatalysts in PEC water splitting is key to this PhD project. Taking single-atom electrocatalysts, iron- and nitrogen-doped carbon (Fe-NC), as an example, this rising star in fuel cells was used as a co-catalyst on BiVO4 to boost the PEC performance and the enhancement is found mainly from reducing surface recombination, rather than accelerating OER kinetics. Overall, the findings in this thesis offer deep insight into the loss mechanism of BiVO4 photoanodes, open new avenues to tune the surface properties of photoanodes for water oxidation, and inspire the design and development of novel functional interlayers and advanced devices, appealing to a broad audience across advanced functional materials for energy harvesting.