Water oxidation kinetics of nanoporous BiVO4 photoanodes functionalised with nickel/iron oxyhydroxide electrocatalysts

Light-driven, bias-free nitrogenase-based bioelectrochemical cell for ammonia generation

Nitrogen fixation is a key process that sustains life on Earth. Nitrogenase is the sole enzyme capable of fixing nitrogen under ambient conditions. Extensive research efforts have been dedicated to elucidating the enzyme mechanism and its artificial activation through high applied voltage, photochemistry, or strong reducing agents. Harnessing light irradiation to minimize the required external bias can lower the process's high energy investment. Herein, we present the development of photo-bioelectrochemical cells (PBECs) utilizing BiVO4/CoP or CdS/NiO photoanodes for nitrogenase activation toward N2 fixation. The constructed PBEC based on BiVO4/CoP photoanode requires minimal external bias (200 mV) and suppresses O2 generation that allows efficient activation of the nitrogenase enzyme, using glucose as an electron donor. In a second developed PBEC configuration, CdS/NiO photoanode was used, enabling bias-free activation of the nitrogenase-based cathode to produce 100 μM of ammonia at a faradaic efficiency (FE) of 12%. The ammonia production was determined by a commonly used fluorescence probe and further validated using 1H-NMR spectroscopy. The presented PBECs lay the foundation for biotic-abiotic systems to directly activate enzymes toward value-added chemicals by light-driven reactions.

Organic Photovoltaic Materials for Solar Fuel Applications: A Perfect Match?

This work discusses the use of donor and acceptor materials from organic photovoltaics in solar fuel applications. These two routes to solar energy conversion have many shared materials design parameters, and in recent years there has been increasing overlap of the molecules and polymers used in each. Here, we examine whether this is a good approach, where knowledge can be translated, and where further consideration to molecular design is required.

Dual Shield: Bifurcated Coating Analysis of Multilayered WO3/BiVO4/TiO2/NiOOH Photoanodes for Sustainable Solar-to-Hydrogen Generation from Challenging Waters

The heterostructure WO3/BiVO4-based photoanodes have garnered significant interest for photoelectrochemical (PEC) solar-driven water splitting to produce hydrogen. However, challenges such as inadequate charge separation and photocorrosion significantly hinder their performance, limiting overall solar-to-hydrogen conversion efficiency. The incorporation of cocatalysts has shown promise in improving charge separation at the photoanode, yet mitigating photocorrosion remains a formidable challenge. Amorphous metal oxide-based passivation layers offer a potential solution to safeguard semiconductor catalysts. We examine the structural, surface morphological, and optical properties of two-step-integrated sputter and spray-coated TiO2 thin films and their integration onto WO3/BiVO4, both with and without NiOOH cocatalyst deposition. The J-V experiments reveal that the NiOOH cocatalyst enhances the photocurrent density of the WO3/BiVO4 photoanode in water splitting reactions from 2.81 to 3.87 mA/cm2. However, during prolonged operation, the photocurrent density degrades by 52%. In contrast, integrated sputter and spray-coated TiO2 passivation layer-coated WO3/BiVO4/NiOOH samples demonstrate a ∼88% enhancement in photocurrent density (5.3 mA/cm2) with minimal degradation, emphasizing the importance of a strategic coating protocol to sustain photocurrent generation. We further explore the feasibility of using natural mine wastewater as an electrolyte feedstock in PEC generation. Two-compartment PEC cells, utilizing both fresh water and metal mine wastewater feedstocks exhibit 66.6 and 74.2 μmol/h cm2 hydrogen generation, respectively. Intriguingly, the recovery of zinc (Zn2+) heavy metals on the cathode surface in the mine wastewater electrolyte is confirmed through surface morphology and elemental analysis. This work underscores the significance of passivation layer and cocatalyst coating methodologies in a sequential order to enhance charge separation and protect the photoanode from photocorrosion, contributing to sustainable hydrogen generation. Additionally, it suggests the potential of utilizing wastewater in electrolyzers as an alternative to freshwater resources.

Balancing charge recombination and hole transfer rates in hematite photoanodes by modulating the Co2+/Fe3+ sites in the OER cocatalyst

This work investigates the roles of Co and Fe sites in a composite cocatalyst on the performance of hematite photoanodes for photoelectrochemical (PEC) water splitting. The cobalt/iron-based composite (Co-Fe-O) cocatalyst, consisting of adjustable Co2+/Fe3+ratios, was synthesized using a one-step hydrothermal method. It reveals that Co2+ sites with a robust capacity for low-bias hole capture, which is insignificantly affected by partial substitution by Fe3+, decelerate the charge recombination process. However, it also leads to a slower charge transfer, with slower oxygen-evolution kinetics on Co sites than on Fe sites. Consequently, the modulation of the Co2+/Fe3+ ratio facilitates the redistribution of surface strap states, striking a delicate balance between charge recombination and charge transfer rates. This optimization led to the highest low-bias photocurrent density of 1.6 mA cm-2 at 1.0 V vs. RHE (a 2.4-fold increase) for the cocatalyst with a Co2+/Fe3+ ratio of 1:2 (CoFe2O4 nanoparticles). Additionally, the cocatalyst with a Co2+/Fe3+ ratio of 1:4 (mixture of CoFe2O4 and Fe2O3 nanoparticles, demonstrated an impressive high-bias photocurrent density of 3.8 mA cm-2 at 1.6 V vs. RHE (a 2.3-fold increase). This study emphasizes the promising potential of modulating active sites within a cocatalyst to achieve efficient PEC water splitting on a hematite-based photoanode.

PEDOT/FeOOH/BiVO4 Nanohybrids with Excellent Photoelectric Performance Promoted by Photothermal Effects for the Ultrasensitive Detection of MicroRNA-375-3p

Herein, a novel photoactive poly(3,4-ethyl-enedioxythiophene) (PEDOT)/FeOOH/BiVO₄ nanohybrid with excellent photoelectrochemical (PEC) efficiency was assembled for the construction of an ultrasensitive biosensor for microRNA-375-3p (miRNA-375-3p) detection. In comparison with the traditional FeOOH/BiVO₄ photoactive composite, the PEDOT/FeOOH/BiVO₄ nanohybrids exhibited markedly enhanced photocurrent due to the promoted interfacial charge separation by PEDOT, which was used not only as an electron conductor but also as a localized photothermal heater to enhance the photogenerated carrier separation. Based on this PEDOT/FeOOH/BiVO₄ photoelectrode and an enzyme-free signal amplification strategy including a target-induced catalytic hairpin assembly (CHA) and hybridization chain reaction (HCR), a PEC sensing platform for the detection of miRNA-375-3p was established, achieving a wide linear range from 1 fM to 10 pM with a low detection limit of 0.3 fM. Moreover, this work provides a general photocurrent enhancement strategy for the development of high-performing PEC biosensors for sensitive detection of biomarkers and early disease diagnosis.

Hydrothermally derived Cr-doped SnO2 nanoflakes for enhanced photocatalytic and photoelectrochemical water oxidation performance under visible light irradiation

Photocatalytic dye degradation is a method of environmental degradation that is commonly used to eliminate various pollutants produced by pharmaceutical and textile industries. Herein, pure and chromium (Cr)-doped SnO₂ nanoflakes were synthesized using a simple facile hydrothermal method and photocatalytic properties were studied under visible light illumination. In addition, photoelectrochemical (PEC) water oxidation properties were also studied using the prepared samples. Doping of transition metal ions introduces structural defects, which narrow the band gap of host sample, resulting in high catalytic activity. The synthesized doped SnO₂ displayed a rutile tetragonal crystal phase with a nanoflakes-like surface morphology having no other contaminations. The optical band gap of Cr-doped SnO₂ nanoflakes was significantly reduced (2.48 eV) over the pure sample (3.32 eV), due to successful incorporation of Cr ions into the host lattice. Furthermore, the dye removal efficiency of these nanoflakes was investigated for methyl orange (MO) and tetracycline (TC) organic contaminations. The Cr-doped SnO₂ nanoflakes exhibited superior photodegradation with 87.8% and 90.6% dye removal efficiency, within 90 min of light illumination. PEC water oxidation analysis showed that the doped photoelectrode achieved enhanced photocurrent density and showed a higher photocurrent density (1.08 mA cm^-2) over that of the undoped electrode (0.60 mA cm^-2). Electrochemical impedance spectroscopy (EIS) showed that doped electrodes exhibited lesser charge resistance than the pure electrode. The synthesized Cr-doped SnO₂ nanoflakes are suitable for water oxidation and photodegradation of organic pollutants. Thus, we strongly believe that the obtained results in this report will continue to provide new opportunities for the improvement of effective visible light photocatalysts for industrial wastewater treatment and water splitting for H₂ generation.

Single-Atom Iridium on Hematite Photoanodes for Solar Water Splitting: Catalyst or Spectator?

Single-atom catalysts (SACs) on hematite photoanodes are efficient cocatalysts to boost photoelectrochemical performance. They feature high atom utilization, remarkable activity, and distinct active sites. However, the specific role of SACs on hematite photoanodes is not fully understood yet: Do SACs behave as a catalytic site or as a spectator? By combining spectroscopic experiments and computer simulations, we demonstrate that single-atom iridium (sIr) catalysts on hematite (α-Fe₂O₃/sIr) photoanodes act as a true catalyst by trapping holes from hematite and providing active sites for the water oxidation reaction. In situ transient absorption spectroscopy showed a reduced number of holes and shortened hole lifetime in the presence of sIr. This was particularly evident on the second timescale, indicative of fast hole transfer and depletion toward water oxidation. Intensity-modulated photocurrent spectroscopy evidenced a faster hole transfer at the α-Fe₂O₃/sIr/electrolyte interface compared to that at bare α-Fe₂O₃. Density functional theory calculations revealed the mechanism for water oxidation using sIr as a catalytic center to be the preferred pathway as it displayed a lower onset potential than the Fe sites. X-ray photoelectron spectroscopy demonstrated that sIr introduced a mid-gap of 4d state, key to the fast hole transfer and hole depletion. These combined results provide new insights into the processes controlling solar water oxidation and the role of SACs in enhancing the catalytic performance of semiconductors in photo-assisted reactions.

Enhancement in the photoelectrochemical performance of BiVO4 photoanode with high (040) facet exposure

Morphology and geometrical dimensions play crucial roles in the photoelectrochemical (PEC) performance of bismuth vanadate (BiVO₄) for water splitting. Decahedral BiVO₄ was synthesized through a facile hydrothermal process, which exhibited superior charge injection efficiency to the nanoporous counterpart prepared by the traditional method. More importantly, the crystal size and facet proportion of BiVO₄ decahedrons were facilely controlled. The charge separation efficiency can be significantly improved with a reduction in the crystal size and an increase in (040) facet exposure. A new method was developed for rate law analysis: illumination intensity-modulated oxygen evolution reaction rate versus open circuit potential difference, which suggested that the surface reaction kinetics was not affected by facet regulation. Furthermore, after decorating the FeOOH and NiOOH as dual oxygen evolution cocatalysts, an enhanced photocurrent density of 3.2 mA cm^-2 at 1.23 V versus reversible hydrogen electrode and interfacial charge injection efficiency of 97.0% can be reached. Our work inspires the development of facet-regulated BiVO₄ photoanodes with high charge injection efficiency in the PEC field and provides a feasible route to enhance its charge separation efficiency.

Effect of Mn2+ incorporation on the photoelectrochemical properties of BiVO4

Mn2+–BiVO4 photoanodes (Mn2+ = 0.2–1%) to improve the charge-carrier separation and electrical conductivity of BiVO4 are reported.

Turning off the “shunt channel” by coating with CoFe layered double hydroxide nanocrystals for efficient photoelectrocatalytic water splitting

Tiny and crystalline CoFe(C) nanoparticles can close the “shunt channel” between the cocatalyst and the substrate, and the recombination of photogenerated charge caused by back-reaction is inhibited.

The effect of Fe(iii) ions on oxygen-vacancy-rich BiVO4 on the photocatalytic oxygen evolution reaction

The surface oxygen vacancies could promote the photocatalytic O2 evolution of BiVO4. Simultaneously, Fe3+ ions in solution could facilitate the holes' transfer to improve the water oxidation reaction.