Solar Hydrogen Generation from Ambient Humidity Using Functionalized Porous Photoanodes

Abstract

Solar hydrogen is a promising sustainable energy vector, and steady progress has been made in the development of photoelectrochemical (PEC) cells. Most research in this field has focused on using acidic or alkaline liquid electrolytes for ionic transfer. However, the performance is limited by (i) scattering of light and blocking of catalytic sites by gas bubbles and (ii) mass transport limitations. An attractive alternative to a liquid water feedstock is to use the water vapor present as humidity in ambient air, which has been demonstrated to mitigate the above problems and can expand the geographical range where these devices can be utilized. Here, we show how the functionalization of porous TiO 2 and WO 3 photoanodes with solid electrolytesproton conducting Aquivion and Nafion ionomersenables the capture of water from ambient air and allows subsequent PEC hydrogen production. The optimization strategy of photoanode functionalization was examined through testing the effect of ionomer loading and the ionomer composition. Optimized functionalized photoanodes operating at 60% relative humidity (RH) and T cell = 30−70 °C were able to recover up to 90% of the performance obtained at 1.23 V versus reverse hydrogen electrode (RHE) when water is introduced in the liquid phase (i.e., conventional PEC operation). Full performance recovery is achieved at a higher applied potential. In addition, long-term experiments have shown remarkable stability at 60% RH for 64 h of cycling (8 h continuous illumination−8 h dark), demonstrating that the concept can be applicable outdoors.

Rational Design of Photoelectrodes for the Fully Integrated Polymer Electrode Membrane–Photoelectrochemical Water-Splitting System: A Case Study of Bismuth Vanadate

“…Our group has already demonstrated the surface functionalization of photoanodes and in-depth analysis on the photoanode characteristics by the deposition of different ionomers (Nafion and Aquivion) to absorb the moisture. 11 Based on our previous studies, we employed an Aquivion based ionomer here as it possesses (i) better light-transparency in the visible region, (ii) a higher absorbance of water vapor, and (iii) better PEC stability of the photoanode. Schematic and SEM images of the cross-sectional view of all the layers involved in the PEM−PEC arrangements are shown in Figure S8.…”

“…In particular, we chose Ti-felt as the substrate to be both active in charge carrier transport across the substrate and use its natural porosity that allows light transmittance for the complementary light absorption by the counter photoelectrode. 11 The resultant W-doped BiVO 4 on a Ti-felt porous substrate developed herein demonstrates excellent PEC performance with a current density of 2.1 mA• cm −2 @ 1.23 V vs RHE. Further, we functionalized the BiVO 4 photoelectrode by the impregnation of appropriate ionomers to absorb the moisture from air and demonstrated the watervapor fed PEC water splitting.…”

“…In our previous work, we demonstrated the significant advantages of the novel Aquivion ionomer, which has better water-absorption characteristics and stability over Nafion. 11 With this demonstration, we realize the importance of combining the narrow-band gap BiVO 4 photoanodes with the Aquivion ionomer, to demonstrate the water-vapor fed hydrogen generation under atmospheric humidity levels (<100%).…”

“…10−12 Moreover, the stability of the photoelectrodes in the PEM configuration is expected to be higher since the materials are in contact with polymeric membranes and not with corrosive aqueous electrolytes, and in principle, pure water or water vapor flowing through the reactor could be sufficient to drive the reactions. 11,12 In the literature, a few research groups have attempted to employ PEM−PEC reactors for water-splitting applications. 11−19 In these reports, commercially available metal meshes such as Ti and W were partially oxidized to form a core−shell structure consisting of an outer photoelectrode (TiO 2 , WO 3 ) and an inner core of electron-conducting (Ti, W) substrate.…”

Boosting photocatalytic hydrogen production from water by photothermally induced biphase systems

“…The design exhibits biphase interfaces of self-generated steam/photocatalyst loaded on the charred wood substrates/hydrogen gas. Our strategy of the photothermally induced biphase interfacial feature differs from previous studies of the room-temperature vapor in moisture environment to reduce the catalysts corrosion (the humidity was realized through a complex microfluidic microreactor 8 – 10 , convection effect 11 , and hydrophobic effect 12 ) and plasmonic thermal effects 13 and near-infrared photothermal effects 14 , 15 in the triphase interfaces of liquid water/photocatalyst/hydrogen. This photothermal–photocatalytic biphase system kinetically lowers the hydrogen gas’s transport resistance by nearly two orders of magnitude to allow the easy escape of hydrogen gas from the system.…”