(Invited) Insights on Scale-up of BiVO_4-Based Photoelectrochemical Water Splitting Devices

摘要

With the STH efficiencies of metal oxide-based PV-PEC water splitting devices now approaching 10%, the next step is to move beyond laboratory experiments and demonstrate large area devices. We demonstrate an unbiased 50 cm~2 PV-PEC water splitting device with an STH efficiency of 2.1%. While this is the highest reported value for a large-area BiVO_4-based water splitting device, it is still a factor of ～3 below the 6.3% efficiency we achieve for the corresponding small-area device. This illustrates that straightforward scale-up of spray-deposited BiVO_4 photoanodes from <1 to 50 cm~2 leads to significant efficiency losses. We studied the influences of variation in materials quality over larger areas, substrate conductivity, electrolyte conductivity, and cell geometry. At relatively modest photocurrent densities (2 - 3 mA/cm~2), the total voltage losses amount to ca. 600 mV for our 50 cm~2 device. The ohmic losses in the transparent conducting substrate (F-doped SnO_2) represent only ～10% of the total losses. A much larger loss mechanism is due to mass transport limitations of the ionic species in the electrolyte. Specifically, we show that lateral diffusion in the direction parallel to the electrode surface plays a more important role that one would intuitively expect. We further quantified the different loss mechanisms using a combination of finite element analysis modeling (COMSOL Multiphysics) and in-situ quantitative analysis of the fluid dynamics and pH of the electrolyte using particle image velocimetry and fluorescence techniques (Figure 1). This feedback mechanism between simulation and experiment allows a more accurate model construction and validation. Based on these insights, electrochemical engineering strategies to overcome the losses associated with scale-up are offered, which would limit the voltage loss for large-area devices to less than 50 mV.