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Modified Electrode to Increase Solar Fuel Production

Source: FK1954, Public Domain

Hydrogen production via electrochemical water splitting is a promising area for storing solar energy. For this technology to be economically competitive it is crucial to develop water-splitting systems with high solar-to-hydrogen (STH) efficiencies. Usually, expensive solar cells or costly semiconductors are used for this.

Now (2021), scientists at the Brookhaven National Laboratory have demonstrated that modifying the top layer of atoms on the surface of electrodes can have a decisive impact on the activity of solar water splitting. They found that bismuth vanadate electrodes containing more bismuth on the surface (relative to vanadium) produced a greater electrical current when it was absorbing energy from sunlight. This photocurrent was able to initiate the chemical reactions that split water into oxygen and hydrogen. The hydrogen could then be stored for later use as a clean fuel.

Building on the earlier research results, which showed that bismuth vanadate was a promising electrode material for solar water splitting as it could absorb sunlight across a range of wavelengths and remained relatively stable in water, the scientists designed a method for precisely growing single-crystalline thin films of it: high-energy laser pulses hit the surface of polycrystalline bismuth vanadate inside a vacuum chamber. The heat from the laser caused the atoms to evaporate and settle on the surface of substrate to form a thin film. To create a bismuth-rich surface, the scientists put one sample in a solution of sodium hydroxide. To confirm that this chemical treatment changed the composition of the top surface layer, they employed low-energy ion scattering spectroscopy (LEIS) and scanning tunneling microscopy (STM). In LEIS, electrically-charged atoms with low energy were directed at the sample. According to the LEIS analysis, the treated surface comprised almost entirely bismuth, with an 80-to-20 ratio of bismuth to vanadium. During STM, an electrically conductive tip was scanned very close to the sample surface while the tunnelling current flowing between the tip and sample was measured. By combining these measurements, the scientists could make out the electron density of surface atoms. Comparing the STM images before and after treatment, the scientists discovered a clear difference in the patterns of atomic arrangements corresponding to vanadium and bismuth-rich surfaces, respectively. After proving that the chemical treatment successfully altered the first layer of atoms, the team compared the light-induced electrochemical behaviour of the treated and nontreated samples. They also measured the photocurrent of both samples for sulphite oxidation.

A lot of research has already been conducted on the efficiency of bismuth vanadate for photocatalytic reactions. In 2014, for example, scientists demonstrated that a nanoporous morphology (with a specific surface area of 31.8 square meters per gram) effectively suppressed bulk carrier recombination without additional doping, manifesting an electron-hole separation yield of 0.90 at 1.23 volts (V) versus the reversible hydrogen electrode (RHE). They increased the propensity for surface-reaching holes to instigate water-splitting chemistry by serially applying two different oxygen evolution catalyst (OEC) layers, FeOOH and NiOOH, which reduced interface recombination at the BiVO4/OEC junction while creating a more favourable Helmholtz layer potential drop at the OEC/electrolyte junction. The resulting BiVO4/FeOOH/NiOOH photoanode reached a photocurrent density of 2.73 milliamps per square centimetre at a potential as low as 0.6 V versus RHE.

In 2019, scientists analysed BiVO4 photoanodes with predominant [010] and [121] orientations and demonstrated a crystallographic-orientation-dependent charge separation of BiVO4 for solar water oxidation. They found that a [010]-orientated BiVO4 photoanode generated a photocurrent 2.9 times better than that of the [121]-orientated one because of the improved charge separation. A thorough investigation of the surface band bending by open-circuit potential and film conductivity by contacting atomic force microscopy showed that the higher electron mobility along the [010] direction than that of [121] was responsible for the improvement in charge separation. Their work has provided a fundamental insight into charge separation in anisotropic photoanodes for rational design of efficient photoanodes for solar energy conversion.

Using vanadium and bismuth for the solar water-splitting reaction has several advantages: when bismuth vanadate absorbs light, it generates electrons and electron vacancies called holes. Both of these charge carriers need to have enough energy to do the necessary chemistry for the water-splitting reaction. Holes to oxidize water into oxygen gas, and electrons to reduce water into hydrogen gas. While the holes have more than enough energy, the electrons do not. The scientists found that the bismuth-terminated surface lifted the electrons to higher energy, making the reaction easier.

Follow-up studies are needed to understand how surface oxygen vacancies and their tendency to immobilise charges are affected when BVO is immersed in water and working together with a co-catalyst to enhance charge transfer, as well as whether transition metal oxides can effectively work as co-catalysts. The scientists also intend to explore how the activity of solar water splitting depends on what type of atoms (bismuth or vanadium) terminate the surface layer.