Photocorrosion at Irradiated Perovskite/Electrolyte Interfaces

Metal–halide perovskites transformed optoelectronics research and development during the past decade. They have also gained a foothold in photocatalytic and photoelectrochemical processes recently, but their sensitivity to the most commonly applied solvents and electrolytes together with their susceptibility to photocorrosion hinders such applications. Understanding the elementary steps of photocorrosion of these materials can aid the endeavor of realizing stable devices. In this Perspective, we discuss both thermodynamic and kinetic aspects of photocorrosion processes occurring at the interface of perovskite photocatalysts and photoelectrodes with different electrolytes. We show how combined in situ and operando electrochemical techniques can reveal the underlying mechanisms. Finally, we also discuss emerging strategies to mitigate photocorrosion (such as surface protection, materials and electrolyte engineering, etc.).


Detailed description of the kinetic steps of photocorrosion at SC/liquid junctions
The initial step for the kinetic models describing photocorrosion processes is the stepwise breaking of the back-bonds of surface atoms (following charge carrier generation). 1-7 During anodic photocorrosion a hole generated behind the surface row of atoms migrates to the surface and is captured by a back-bond (Equation S2). In a subsequent chemical step, a component of the electrolyte stabilizes the radical-like intermediate (Equation S4). The first hole-capture can be considered reversible (or irreversible as discussed later), while the second one is irreversible as it is accompanied by the solvation of the participating species. These species involved in the corrosion possess different hole-capture cross-sections. The rate-limiting step of anodic corrosion processes is the first hole-capture of the surface bond. Energetically, the back bonds of surface atoms can be considered as surface states that have an energy level slightly below the valence band. The radical-like surface state, which is formed after the first hole-capture, has a different energy level, that lies above the valence band in the bandgap region.
It is evident from the outlined mechanism (Equation S3) that chemical species (other than the redox agent) present in the solution can have a profound effect on the corrosion process.
Through specific binding to surface atoms (that have incomplete coordination) these can alter the strength of the back bonds and therefor the energetics of corrosion. It becomes necessary to consider solution phase equilibria and adsorption/desorption behavior of electrolyte components to accurately describe these processes. 8,9 Furthermore, complex competition scenarios can occur among species in the electrolyte for the active sites of the photocatalysts and photoelectrodes, that add further reaction steps to be considered. 6,10 In some cases even electron injection from the corrosion products can be envisioned (Equation S5). These kinetic models consider the intermediate species immobile on the surface during the corrosion process.
There are models available that render them mobile, and also consider the reaction between two such intermediates on the surface (Equation S6). 11 It was found experimentally that two classes of SCs exist: one where the stabilization efficiency of redox agents is light intensity dependent 1,13,14 , and another one where it is not.
General SCs constitute the first category, while layered 2D SCs fall into the second one. This behavior was explained by reconciling the reversibility of the first hole capture step (Equation

S2
). 15 In the case of 2D transition metal-chalcogenides this step is irreversible, both the photocorrosion reaction and the redox reaction depend on hole concentration in a similar manner, therefore the stabilizing efficiency is independent from the light intensity. In the other cases, where the photocorrosion process had a higher degree of dependence on the generated holes, the stabilization efficiency decreased as the light intensity was increased at a given concentration of the stabilizing redox agent. A further explanation of this behavior is that in the case of the studied 2D transition metal-chalcogenides the light absorption induces transition between d-orbitals of the metal d-bands. These orbitals are not involved in bonding between the constituents of the 2D structure, therefore no photocorrosion occurs. From corrosion studies information can be gathered not only for the corrodible material but also for the redox species in the electrolyte. The rearrangement energy of the redox species in the solution can be determined. 2

Detailed description of the electrochemical techniques used in photocorrosion studies
In the early mechanistic studies, SC photocorrosion was studied by purely electrochemical text). The corrodible SCs that were studied by these techniques are also summarized in Table   S1. Electrochemical photocapacitance technique can map both surface and bulk states of the photoelectrode and link these states to specific electrochemical processes. 4, [24][25][26] The capacitance change of the photoelectrode is monitored, while it is subjected to dual illumination (below and above bandgap energy). By altering the wavelength of the sub-bandgap illumination, the energy position and density of states present in the bandgap region of the photoelectrode is explored.
A major difference between corrosion induced by electrochemical hole injection (from solution redox processes) and photogenerated holes was revealed through this technique. When multiple states are involved in the corrosion process (i.e., in the case of photoexcitation), the hole population is equally distributed among these states. When electrochemical hole injection occurs however, the solution redox species will inject holes preferentially on the state lying closer to the valence band edge (lower energy state).
The change in the surface composition at different stages of photocorrosion can be monitored by ex situ XPS. [27][28][29] If careful sample transfer is achieved, possible corrosion intermediates and products can be identified on the surface. As a further step, quasi-operando measurements can be carried out on the surfaces of emersed electrodes. 30,31 This technique can help to identify key intermediates on the electrode surfaces. As there is no sample transfer step, chemical contamination of the electrode surfaces can be minimized. Recently the technique operando ambient pressure "tender" electrochemical XPS was introduced, which reveals the working mechanisms of semiconductor/liquid junctions 32 and electrocatalyst surfaces. 33 Apart from studying corrosion processes this technique can be extended to evaluate the nature of band bending during the operation of photoelectrochemical cells. 34 From the side of the electrolyte, a (photo)electrochemical scanning flow cell was combined with an inductively coupled plasma mass spectrometer 35 recently, to study the photocorrosion of WO 3 and BiVO 4 photoelectrodes. 36,37 This technique is capable of identifying even small amounts of photoelectrode material loss. The end products of the corrosion in the electrolyte can be probed this way.