Photoelectrochemical (PEC) water splitting is an attractive method for the production of hydrogen from renewable energy sources. We have in recent years focused on both oxygen evolution reaction and hydrogen evolution reaction materials. In this talk, we want to summarize photoelectrode and surface engineering paths to high-performance electrodes for water splitting.

Firstly, we will report on BiFeO₃ (BFO) which has recently been identified as a promising photocathode material due to its light absorption and photo-electrochemical properties [1-3]. For practical applications, however, the PEC performance of BFO needs to be improved, which requires understanding of the aspects that limit its activity. We will present the effect of the ratio of Bi to Fe in the precursor solution of the sol-gel synthesis on the properties of BFO thin films [4]. Thin films with a stoichiometric ratio of Bi:Fe and thin films with 10% excess of Bi are prepared on fluorine-doped tin oxide. While bulk characterization techniques (XRD, RBS) show the formation of phase-pure BFO, surface characterization techniques (XPS, LEIS, TEM, ToF-SIMS) indicate Bi enrichment on the surface. The light absorption and band gap do not change upon adding 10% excess Bi in the precursor solution. However, the current density of the excess Bi samples is nearly two times larger than that of the stoichiometric BFO film at 0.6 V vs RHE. Electrochemical impedance spectroscopy explains this improved performance in terms of a lower recombination rate and a lower charge transfer resistance in Bi excess films. The lower recombination rate is attributed to less defects in the thin films, i.e. less Bi and O vacancies (XPS, TOF-SIMS). The low charge transfer resistance is attributed to a distinct Bi-oxide top layer that was detected on all samples (XPS, TOF-SIMS, LEIS, TEM). In summary, we can conclude that adjusting the Bi:Fe ratio is an effective manner to enhance the PEC performance of BFO thin films for hydrogen evolution reaction. 

Secondly, we will discuss a series of papers where we use WO₃ as oxygen evolution material for photo-electrochemical water splitting [5-8]. We start with discussing physical and chemical defects in WO₃ thin films and their impact on photo-electrochemical water splitting. We then transfer the thin film to scalable substrates, such as Si wafers, and discuss how the performance is boosted with WO₃/n-Si heterostructures. The role of Si and its impact on interface engineering is investigated. Then, we extend the study to the third dimension by investigating micro-pillars and nanowires and the impact of the three dimensional structure through surface area and light management on the performance.

We will conclude with sketching our further efforts to high-performance electrodes via photoelectrode and surface engineering using and developing further paths to scalable methods.