Understanding and Mitigating Degradation Phenomena in Photoelectrochemical Devices
Authors: Nandjou Fredy , Haussener Sophia 
Published: February 2020

Photoelectrochemical (PEC) water splitting is an attractive and clean method for the production of solar hydrogen. The main challenges for industrialization and large-scale implementation of PEC devices are [1]: the efficiency, the cost, the sustainability, and the durability. During the last decades, researches have been mainly focused on the development of high-performing, low cost, and earth abundant materials for solar hydrogen generation [2]. However, limited efforts are conducted to understand the degradation mechanisms occurring in the device or to predict approaches to increase the device’s lifetime, robustness and reliability.
Here, we review the degradation of the PEC device’s components (semiconductor, catalyst, and solid and liquid electrolyte). For the semiconductor, the different degradations mechanisms considered include chemical corrosion, electrochemical corrosion, photoelectrochemical corrosion, chemical destabilization, protonation, and hydroxylation. For the electrocatalyst, the presented degradation mechanisms include dissolution, agglomeration, deactivation, and catalyst support corrosion. For solid electrolytes, chemical attack, poisoning by foreign ions, mechanical degradation, phototendering, photolysis, and embrittlement are considered. For the liquid electrolyte, the different degradations mechanisms include pollution, salt deposits, and saturation. Figure 1 summarizes the different degradation mechanisms occurring in the different components and interfaces, and the interplay between the degradation effects of the different components. In Figure 1.a, degradation mechanisms are organized according to the components and interfaces suffering from it. The intrinsic degradations of the components are indicated with red circles, while the degradations arising from the degradation of the other components are circled in green. In Figure 1.b, the interplay between the degradation effects of the different components is presented. At the center of the semiconductor-electrocatalyst-electrolyte triangle is the solar radiation. The direct interaction between the photons and the different components is responsible for all degradation mechanisms specific to photo-driven applications.
We then discuss the impact of the PEC device design and operating conditions on degradation and performance loss. A bottom-up modeling approach, using degradation rates of the device components reported in literature, was used to predict the degradation-related performance loss in three different, illustrative PEC device design examples under different operating conditions [3]. The reference designs are: i) a monolithic design (wireless), ii) a wired design with two separated electrodes in a liquid electrolyte, and iii) a wired design with a separated membrane-electrode assembly. The operating conditions considered include: i) no irradiation concentration and no current dilution (reference condition), ii) irradiation concentration and no current dilution, iii) current dilution and no irradiation concentration, and iv) irradiation concentration and current dilution. The impact of the PEC device design and operating conditions on the salience of different degradations mechanisms and on the overall lifetime of the device is discussed. It is highlighted how heat and water management, device maintenance, and component replacement can help in mitigating degradation and the corresponding effects on device performance.
[1] M. Dumortier, S. Y. Tembhurne and S. Haussener. Holistic design guidelines for solar hydrogen production by photo-electrochemical routes, in Energy Environmental Science, vol. 8, p. 3614 - 3628, 2015.
[2] J. W. Ager, M. R. Shaner, K. A. Walczak, I. D. Sharp, and S. Ardo, Experimental demonstrations of spontaneous, solar-driven photoelectrochemical water splitting. Energy Environ. Sci.8:2811–2824, 2015
[3] F. Nandjou and S. Haussener. Degradation in photoelectrochemical devices: Review with an illustrative case study. Journal of Physics D, Special issue on Solar Fuels(Accepted with minor revision)

Figure 1