Type-II Heterojunction CdIn₂S₄/BiVO₄ Coupling with CQDs to Improve PEC Water Splitting Performance Synergistically

Bismuth vanadate (BiVO4) has been considered as a promising photoelectrocatalytic (PEC) semiconductor, but suffers from severe hole recombination, attributed to the short hole-diffusion length and the low carrier mobility. Herein, a type-II heterojunction CdIn2S4/BiVO4 is designed to improve the photocurrent density from 1.22 (pristine BiVO4) to 2.68 mA cm–2 at 1.23 V vs the reversible hydrogen electrode (RHE), accelerating the bulk separation of photogenerated carriers by the built-in field from the matched energy band. With the introduction of CQDs, CQDs/CdIn2S4/BiVO4 increases the photocurrent density to 4.84 mA cm–2, enhancing the light absorption and cathodically shifting its onset potential, due to the synergetic effect of the heterojunction and CQDs. Compared with BiVO4, CQDs/CdIn2S4/BiVO4 promotes the bulk separation efficiency to 94.6% and the surface injection efficiency to 72.2%. Additionally, spin-coating of FeOOH on CQDs/CdIn2S4/BiVO4 could further improve the PEC performance and keep a long stability for water splitting. The density function theory (DFT) calculations illustrated that the type-II heterojunction CdIn2S4/BiVO4 could decrease the oxygen evolution reaction (OER) overpotential and accelerate bulk charge separation for the built-in field of the aligned band structure

Self-Driven Photoelectrochemical Splitting of H₂S for S and H₂ Recovery and Simultaneous Electricity Generation

A novel, facile self-driven photoelectrocatalytic (PEC) system was established for highly selective and efficient recovery of H₂S and simultaneous electricity production. The key ideas were the self-bias function between a WO₃ photoanode and a Si/PVC photocathode due to their mismatched Fermi levels and the special cyclic redox reaction mechanism of I^–/I₃^–. Under solar light, the system facilitated the separation of holes in the photoanode and electrons in the photocathode, which then generated electricity. Cyclic redox reactions were produced in the photoanode region as follows: I^– was transformed into I₃^– by photoholes or hydroxyl radicals, H₂S was oxidized to S by I₃^–, and I₃^– was then reduced to I^–. Meanwhile, H⁺ was efficiently converted to H₂ in the photocathode region. In the system, H₂S was uniquely oxidized to sulfur but not to polysulfide (S_<i>x</i>^n‑) because of the mild oxidation capacity of I₃^–. High recovery rates for S and H₂ were obtained up to ∼1.04 mg h^–1 cm^–1 and ∼0.75 mL h^–1 cm^–1, respectively, suggesting that H₂S was completely converted into H₂ and S. In addition, the output power density of the system reached ∼0.11 mW cm^–2. The proposed PEC-H₂S system provides a self-sustaining, energy-saving method for simultaneous H₂S treatment and energy recovery

Exhaustive Conversion of Inorganic Nitrogen to Nitrogen Gas Based on a Photoelectro-Chlorine Cycle Reaction and a Highly Selective Nitrogen Gas Generation Cathode

A novel method for the exhaustive conversion of inorganic nitrogen to nitrogen gas is proposed in this paper. The key properties of the system design included an exhaustive photoelectrochemical cycle reaction in the presence of Cl^–, in which Cl· generated from oxidation of Cl^– by photoholes selectively converted NH₄⁺ to nitrogen gas and some NO₃^– or NO₂^–. The NO₃^– or NO₂^– was finally reduced to nitrogen gas on a highly selective Pd–Cu-modified Ni foam (Pd–Cu/NF) cathode to achieve exhaustive conversion of inorganic nitrogen to nitrogen gas. The results indicated total nitrogen removal efficiencies of 30 mg L^–1 inorganic nitrogen (NO₃^–, NH₄⁺, NO₃^–/NH₄⁺ = 1:1 and NO₂^–/NO₃^–/NH₄⁺ = 1:1:1) in 90 min were 98.2%, 97.4%, 93.1%, and 98.4%, respectively, and the remaining nitrogen was completely removed by prolonging the reaction time. The rapid reduction of nitrate was ascribed to the capacitor characteristics of Pd–Cu/NF that promoted nitrate adsorption in the presence of an electric double layer, eliminating repulsion between the cathode and the anion. Nitrate was effectively removed with a rate constant of 0.050 min^–1, which was 33 times larger than that of Pt cathode. This system shows great potential for inorganic nitrogen treatment due to the high rate, low cost, and clean energy source

Selective Degradation of Organic Pollutants Using an Efficient Metal-Free Catalyst Derived from Carbonized Polypyrrole via Peroxymonosulfate Activation

Metal-free carbonaceous materials, including nitrogen-doped graphene and carbon nanotubes, are emerging as alternative catalysts for peroxymonosulfate (PMS) activation to avoid drawbacks of conventional transition metal-containing catalysts, such as the leaching of toxic metal ions. However, these novel carbocatalysts face relatively high cost and complex syntheses, and their activation mechanisms have not been well-understood. Herein, we developed a novel nitrogen-doped carbonaceous nanosphere catalyst by carbonization of polypyrrole, which was prepared through a scalable chemical oxidative polymerization. The defective degree of carbon substrate and amount of nitrogen dopants (i.e., graphitic nitrogen) were modulated by the calcination temperature. The product carbonized at 800 °C (CPPy-F-8) exhibited the best catalytic performance for PMS activation, with 97% phenol degradation efficiency in 120 min. The catalytic system was efficient over a wide pH range (2–9), and the reaction of phenol degradation had a relatively low activation energy (18.4 ± 2.7 kJ mol^–1). The nitrogen-doped carbocatalyst activated PMS through a nonradical pathway. A two-step catalytic mechanism was extrapolated: the catalyst transfers electrons to PMS through active nitrogen species and becomes a metastable state of the catalyst (State I); next, organic substrates are oxidized and degraded by serving as electron donors to reduce State I. The catalytic process was selective toward degradation of various aromatic compounds with different substituents, probably depending on the oxidation state of State I and the ionization potential (IP) of the organics; that is, only those organics with an IP value lower than ca. 9.0 eV can be oxidized in the CPPy-F-8/PMS system