Understanding the role of nanostructuring in photoelectrode performance for light driven water splitting

The analysis of capacitance data for regular nanostructured photoelectrodes is revisited using a hematite nanorod array as an example. The effects of the cylindrical nanorod geometry on the capacitance voltage behaviour are outlined, and the limiting case of complete depletion is discussed in terms of the residual geometric capacity at the base of the nanorods. Since nanorod arrays generally leave areas of the substrate exposed, it is necessary to consider the parallel capacitance associated with the fraction of uncovered surface. The sensitivity of the capacitance fitting to parameter variation is explored. The enhancement of external quantum efficiency EQE by nanostructuring is also discussed using hematite nanorod arrays as experimental examples. It is shown that, although very substantial EQE enhancement should be achieved by simple geometric effects, the performance of nanostructured hematite electrodes in the visible region of the spectrum is considerably lower than predicted if all charge carriers generated in the space charge region SCR were collected. Further analysis reveals that the internal quantum efficiency increases with photon energy, suggesting that the probability of generating free, rather than bound, electron hole pairs in hematite depends on the excess energy hv Ega

Bubble induced convection stabilizes the local pH during solar water splitting in neutral pH electrolytes

Using neutral pH buffer solutions as electrolytes offers a safe and sustainable operational condition for photoelectrochemical water splitting. However, a major challenge lies in minimizing the voltage loss due to the presence of local pH gradient during the proton coupled electron transfer reactions. In this study, Euler Euler multiphase fluid dynamics simulations are introduced to investigate the interplay between convection driven by the photo electrochemical reactions and the resulting pH gradient. Bubble induced convection is found to dominate fluid dynamics at regions close to the electrodes and significantly suppress the local pH gradient. The influence of bubble parameters and orientation of solar water splitting devices to the local velocity and the concentration overpotential is further discussed. Finally, the positive aspects from product gas bubbles are quantitatively compared with the competing negative effects, such as surface coverage by gas bubbles and Ohmic loss. These negative effects have to be minimized to fully capitalize on the beneficial contribution from bubble induced convectio

Heterogeneous photocatalysts an overview of classic and modern approaches for optical, electronic, and charge dynamics evaluation

The functionality of a photoactive semiconductor i.e., photocatalysts, photoelectrodes, etc. is largely dictated by three key aspects i band gap; ii absolute potentials of the conduction band minimum and the valence band maximum; and iii bulk and surface charge carrier dynamics. Their relevance to governing the energetics and the photo electro chemical mechanisms of the semiconductor has prompted development of a multitude of characterization tools to probe the specific characteristic of the material. This review aims to summarize the current experimental techniques, including the conventional and the state of the art tools, directed at examining the key aspects i , ii , and iii of semiconductors. Although not being exhaustive, this didactic review can be useful to apprise the research community of the sophisticated research tools currently available for characterization of photo electro catalyst semiconductors as well as to bridge the multidisciplinary knowledg

Multiphase fluid dynamics simulations of product crossover in solar driven, membrane less water splitting

Efficient separation of product gases is essential for the safe operation of large scale, solar driven water splitting devices. To date, most demonstration devices use membranes, but a membrane less configuration that separates products via hydrodynamic control offers an attractive alternative without the complexity associated with using membranes. In the present study, multiphase fluid dynamics simulations are introduced to investigate product crossover in a membrane less, solar driven water splitting device. Specific emphasis is placed on implementing a realistic tilt condition of the device and the buoyancy effects on product gas bubbles. Our simulations reveal that gas bubbles, often disregarded, can be a major source of crossover rather than dissolved gases. Controlling the bubble formation and characteristics e.g., diameter therefore plays an important role in achieving efficient product separation. Finally, universal design criteria to control the product crossover are further discussed based on dimensionless analysi

Mitigating voltage losses in photoelectrochemical cell scale up

In solar water splitting, efforts in scaling up the photoelectrochemical cell beyond laboratory scale have started to attract significant attention. Several large area devices have been demonstrated, but typically the efficiencies are much lower than their small area equivalent. Here, two dimensional finite element modeling is used to evaluate the different sources of voltage loss specifically related to scale up in solar water splitting devices operated in neutral pH solutions. We quantitatively investigate the influence of the electrode area to these scale up associated losses substrate ohmic loss, electrolyte ohmic loss, and local pH gradient related losses . About 600 mV additional overpotential is needed due to these losses for a cell with electrodes of height of 8 cm at a current density of 10 mA cm amp; 8722;2. We show, however, that by applying engineering and cell design strategies, the voltage losses can be mitigated, resulting in an acceptable amp; 8764;50 mV overpotential. Overall, this study highlights the additional challenges to be considered in photoelectrochemical cell scale up and provides strategies to manage and mitigate scaling related losse

Nanoscale metal oxides 2D materials heterostructures for photoelectrochemical water splitting a review

Photoelectrochemical PEC water splitting is an interesting approach to harness clean and renewable solar energy to generate green hydrogen. To this end, metal oxides MOs have been investigated as photoelectrodes photoanode and photocathode due to their tunable optoelectronic properties and abundance, but achieving an efficient overall performance based on single MOs is very challenging due to their narrow visible light absorption, unfavorable band position, low charge mobilities, and limited stability. Heterostructuring MOs with other materials has therefore been proposed in the literature. In this review, we specifically highlight photoelectrodes based on the heterostructure of MOs and an emerging group of 2D materials consisting of mono elemental Xenes , carbides nitrides carbonitrides MXenes , boron carbon nitride BCNs , transition metal dichalcogenides TMDs , metal organic frameworks MOFs , and bismuth oxyhalides BiOX . The benefits of the formation of MOs 2D materials heterostructures are outlined, and the state of the art of MOs 2D materials heterostructures as photoelectrodes with various architectures are extensively discussed. Finally, a critical outlook on fundamental challenges and potential future directions in the development of MOs 2D materials heterostructures is presente

Chemical Treatment of Sn Containing Transparent Conducting Oxides for the Enhanced Adhesion and Thermal Stability of Electroplated Metals

A surface treatment process, named ReTreat, is presented, and is shown to enhance the adhesion of electroplated metals on Sn containing transparent conducting oxides TCOs . The ReTreat process uses Zn powders, FeSO4 and glycine buffered aqueous solutions pH 3 to 5 in order to regulate a controlled and uniform conversion of SnO2 surfaces to SnO, Sn metal, and FexSny alloys. These surface metallic and intermetallic layers selectively enrich the electroplating of metallic films including Ni, Au, and Ag . Subsequently, the process has been used to fabricate thermally stable metal films on rigid FTO coated glass and flexible ITO coated PET substrates. Standardized testing confirms that the metallic coatings exhibit sufficient adhesion to the underlying TCO with high thermal stability and tolerance to flexural strain. A reaction mechanism for the heterogenous surface treatment is deduced from X ray diffraction, X ray photoelectron spectroscopy, and in situ transmittance measurements. These investigations show how the process parameters e.g., Zn powders, FeSO4 concentration, pH, and TCO type impact the reaction rate, morphology, and composition of the treated TCO surface. This report provides detailed insights necessary for the future implementation of this innovative surface treatment, which has the prospect to be a customary process for electroplating onto Sn containing TCO

Light Induced Surface Reactions at the Bismuth Vanadate Potassium Phosphate Interface

Bismuth vanadate has recently drawn significant research attention as a light absorbing photoanode due to its performance for photoelectrochemical water splitting. In this study, we use in situ ambient pressure X ray photoelectron spectroscopy with Tender X rays 4.0 keV to investigate a polycrystalline bismuth vanadate BiVO4 electrode in contact with an aqueous potassium phosphate KPi solution at open circuit potential under both dark and light conditions. This is facilitated by the creation of a 25 to 30 nanometers thick electrolyte layer using the dip and pull method. We observe that under illumination bismuth phosphate forms on the BiVO4 surface leading to an increase of the surface negative charge. The bismuth phosphate layer may act to passivate surface states observed in photoelectrochemical measurements. The repulsive interaction between the negatively charged surface under illumination and the phosphate ions in solution causes a shift in the distribution of ions in the thin aqueous electrolyte film, which is observed as an increase in their photoelectron signals. Interestingly, we find that such changes at the BiVO4 KPi electrolyte interface are reversible upon returning to dark conditions. By measuring the oxygen 1s photoelectron peak intensities from the phosphate ions and liquid water as a function of time under dark and light conditions, we determine the timescales for the forward and reverse reactions. Our results provide direct evidence for light induced chemical modification of the BiVO4 KPi electrolyte interfac

The synergistic effect of cation mixing in mesoporous BixFe1 xVO4 heterojunction photoanodes for solar water splitting

Mixed metal vanadates are an interesting class of materials due to their favorable bandgap for visible light absorption and their catalytic activity. Here, we report a novel BixFe1 amp; 8722;xVO4 mixture system fabricated by electrospinning, which upon annealing is composed of triclinic FeVO4 and monoclinic BiVO4 phases. The mixture demonstrates extended optical absorption and a clear bandgap shift as compared with a pure BiVO4. This is also accompanied by an extended wavelength range for its photoactivity as evident from the incident photon to current efficiency. In addition, the mixture with a Bi Bi Fe ratio of 0.5 i.e., x 0.5 shows superior charge transfer and charge separation efficiency. The improved charge transfer efficiency is attributed to the higher catalytic activity of the mixed cation, while the presence of a BiVO4 FeVO4 heterojunction enhances the charge separation. The formation of the heterojunction is verified through detailed microscopic investigations revealing BiVO4 particles intimately surrounded by FeVO4. Our results demonstrate the advantage of establishing a mixture of complex metal oxides in extending optical absorption and boosting the photoelectrochemical performanc