Origin of Improved Photoelectrochemical Water Splitting in Mixed Perovskite Oxides

Owing to the versatility in their chemical and physical properties, transition metal perovskite oxides have emerged as a new category of highly efficient photocatalysts for photoelectrochemical water splitting. Here, to understand the underlying mechanism for the enhanced photoelectrochemical water splitting in mixed perovskites, we explore ideal epitaxial thin films of the BiFeO3-SrTiO3 system. The electronic struture and carrier dynamics are determined from both experiment and density-functional theory calculations. The intrinsic phenomena are measured in this ideal sytem, contrasting to commonly studied polycrstalline solid solutions where extrinsic structural features obscure the intrinsic phenomena. We determined that when SrTiO3 is added to BiFeO3 the conduction band minimum position is raised and an exponential tail of trap states from hybridized Ti 3d and Fe 3d orbitals emerges near the conduction band edge. The presence of these trap states strongly suppresses the fast electron-hole recombination and improves the photocurrent density in the visible-light region, up to 16 times at 0 VRHE compared to the pure end member compositions. Our work provides a new design approach for optimising the photoelectrochemical performance in mixed perovksite oxides.Comment: 7 pages and 5 figure

Sol‐Gel‐Derived Ordered Mesoporous High Entropy Spinel Ferrites and Assessment of Their Photoelectrochemical and Electrocatalytic Water Splitting Performance

The novel material class of high entropy oxides with their unique and unexpected physicochemical properties is a candidate for energy applications. Herein, it is reported for the first time about the physico‐ and (photo‐) electrochemical properties of ordered mesoporous (CoNiCuZnMg)Fe₂O₄ thin films synthesized by a soft‐templating and dip‐coating approach. The A‐site high entropy ferrites (HEF) are composed of periodically ordered mesopores building a highly accessible inorganic nanoarchitecture with large specific surface areas. The mesoporous spinel HEF thin films are found to be phase‐pure and crack‐free on the meso‐ and macroscale. The formation of the spinel structure hosting six distinct cations is verified by X‐ray‐based characterization techniques. Photoelectron spectroscopy gives insight into the chemical state of the implemented transition metals supporting the structural characterization data. Applied as photoanode for photoelectrochemical water splitting, the HEFs are photostable over several hours but show only low photoconductivity owing to fast surface recombination, as evidenced by intensity‐modulated photocurrent spectroscopy. When applied as oxygen evolution reaction electrocatalyst, the HEF thin films possess overpotentials of 420 mV at 10 mA cm⁻² in 1 m KOH. The results imply that the increase of the compositional disorder enhances the electronic transport properties, which are beneficial for both energy applications

Single-Source-Precursor Derived Transition Metal Alloys Embedded in Nitrogen-Doped Porous Carbons as Efficient Oxygen Evolution Electrocatalysts

Carbon supported metallic nanomaterials are of great interest due to their low-cost, high durability and promising functional performance. Herein, a highly active oxygen evolution reaction (OER) electrocatalyst comprised of defective carbon shell encapsulated metal (Fe, Co, Ni) nanoparticles and their alloys supported on in-situ formed N-doped graphene/carbon nanotube hybrid is synthesized from novel single-source-precursors (SSP). The precursors are synthesized by a facile one-pot reaction of tannic acid with polyethylenimine and different metal ions and subsequent pyrolysis of the SSP. Benefiting from the heteroatom doping of carbon and formation of well-encapsulated metal/alloy nanoparticles, the obtained FeNi@NC-900 catalyst possesses lowest overpotentials of 310 mV to achieve a current density of 10 mA cm−2 for OER with a small Tafel slope value of 45 mV dec−1, indicating excellent catalytic performance due to the following features: (1) A synergistic electronic effect among metal alloy nanoparticles, nitrogen-doped carbon, and entangled carbon nanotubes; (2) penetration of electrolyte is promoted towards the active sites through the porous structure of the formed mesoporous carbon clusters; (3) the unique core-shell nanostructure of the hybrid material effectively curbs the degradation of electrocatalyst by protecting the alloy nanoparticles from harsh electrolyte. This work advances an inexpensive and facile method towards the development of transition metal-based hybrid material for potential energy storage and conversion

Polymer-derived SiOC ceramics: A potential catalyst support controlled by the sintering temperature and carbon content

A series of silicon oxycarbide ceramics with varying carbon content from ca. 10 wt% to ca. 40 wt% were prepared by thermal pyrolysis of four commercially available polysiloxanes and subsequent spark plasma sintering (SPS) at 1200 degrees C, 1400 degrees C, and 1600 degrees C. The results showed that the high carbon content led to a porous microstructure, and for SiOC with ca. 40 wt% carbon content, its porosity and specific surface area at 1600 degrees C reached 34% and 262 m(2)/g, respectively. The electrochemical behavior of materials was evaluated. It was shown that SiOC has a certain degree of electrocatalytic activity, and the sample with 10 wt% carbon content obtained at 1200 degrees C exhibited an overpotential of 450 mV vs. RHE at 10 mA.cm(-2) in acid medium. Finally, it was analyzed that the electrochemical behavior of SiOC is closely related to the phase composition and microstructure of the resulting ceramics

A facile strategy for reclaiming discarded graphite and harnessing the rate capabilities of graphite anodes

Graphite negative electrodes are unbeaten hitherto in lithium-ion batteries (LiBs) due to their unique chemical and physical properties. Thus, the increasing scarcity of graphite resources makes smart recycling or repurposing of discarded graphite particularly imperative. However, the current recycling techniques still need to be improved upon with urgency. Herein a facile and efficient hydrometallurgical process is reported to effectively regenerate aged (39.5 %, 75 % state-of-health, SOH) scrapped graphite (SG) from end-of-life lithium-ion bat-teries. Ultimately, the first cycle reversible capacity of SG1 (SOH = 39.5 %) improved from 266 mAh/g to 337 & nbsp;mAh/g while 330 mAh/g (98 %) remain after 100 cycles at 0.5 C. The reversible capacity for the first cycle of SG2 (SOH = 75 %) boosted from 335 mAh/g to 366 mAh/g with the capacity retention of 99.3 % after 100 cycles at 0.5 C, which is comparable with the benchmark commercial graphite. The regenerated graphites RG1 and RG2 exhibit excellent output characteristics even increasing the rate up to 4 C. This is the best rate level reported in the literature to date. Finally, the diffusion coefficient of Li ions during deintercalation and intercalation in the regenerated graphites have been measured by galvanostatic intermittent titration technique (GITT), determining values 2 orders-of-magnitude higher than that of the spent counterparts. Taking advantage of the synergistic effect of acid leaching and heat treatment, this strategy provides a simple and up-scalable method to recycle graphitic anodes

Sol-gel-derived Ordered Mesoporous High Entropy Spinel Ferrites and Assessment of their Photoelectrochemical and Electrocatalytic Water Splitting Performance

For driving the (photo-) electrocatalytic water splitting reaction both efficient and photostable absorber materials and electrocatalysts are needed in order to make the technology economically competitive. The novel material class of high entropy oxides with their unique and unexpected physicochemical properties is a potential candidate for energy applications. Herein, we report for the first time about the physico- and (photo-) electrochemical properties of ordered mesoporous (CoNiCuZnMg)Fe2O4 thin films synthesized by a soft-templating and dip-coating approach. The high entropy ferrites (HEF) are composed of 15 ‒ 18 nm sized and periodically ordered mesopores building a highly accessible inorganic nanoarchitecture with specific surface areas up to 170 m2/g. The mesoporous HEF thin films crystallize in the cubic spinel structure and were found to be crack-free on the meso- and macroscale. The formation of the spinel structure hosting six distinct cations was verified by means of gracing incidence X-ray diffraction, X-ray photoelectron spectroscopy, time-of-flight secondary ion mass spectrometry, and transmission electron microscopy accompanied with energy dispersive X-ray spectroscopy. Photoelectron spectroscopy gave insight into chemical state of the implemented transition metals supporting the structural characterization data. Analyzed as photoanode for photoelectrochemical water splitting, the HEFs showed only low photoconductivity owing to fast surface recombination as suggested by intensity-modulated photocurrent spectroscopy. When applied as oxygen evolution reaction electrocatalyst, the HEF thin films possess overpotentials of 420 mV vs. RHE at 10 mA/cm2 in 1 M KOH. The results imply that the increase of the configurational disorder within the spinel structure enhances the electronic transport properties. The evaluation of the energy band alignment by Mott-Schottky analysis allows for an estimation which redox reactions can be driven, showing that the materials are theoretically capable of promoting overall water splitting

Single‐source‐precursor derived bulk Si 3 N 4 /HfB x N 1‐x ceramic nanocomposites with excellent oxidation resistance

In the present work, bulk Si3N4/HfBxN1-x ceramic nanocomposites were successfully fabricated via a polymer-derived ceramic approach. The chemical reaction to form the single-source precursor was confirmed by FT-IR and XPS, in which both Si-H and N-H groups of perhydropolysilazane react with borane dimethyl sulfide complex and tetrakis(dimethylamido) hafnium(IV). The investigation of the polymer-to-ceramic transformation of the synthesized precursors indicates that Hf- and B-modified PHPS exhibits high ceramic yields of up to 100 wt % after pyrolysis at 1000 degrees C under ammonia. Moreover, XRD and TEM results show that the SiHfBN ceramics with a molar ratio of B : Hf=5 and 10 resist crystallization at temperatures up to 1500 degrees C and separate after annealing at 1700 degrees C into nanocomposites comprising of an alpha-Si3N4 matrix with embedded ternary HfBxN1-x phases, solid solutions of rock salt-type HfN and HfB. Based on the investigation, warm-pressing was applied to fabricate bulk SiHfBN specimens, and the oxidation behavior of samples annealed at 1700 degrees C was recorded at 1500 degrees C over a range of oxidation times between 1 and 50 h. The weight changes of Si3N4/HfBxN1-x ceramics with B : Hf molar ratios of 2 : 1, 5 : 1 and 10 : 1 are 4.31 %, 4.37 % and 2.57 %, respectively. The formation of HfSiO4, B2O3 and SiO2 during oxidation plays a crucial role for the improvement of the oxidation resistance of the Si3N4/HfBxN1-x ceramics

Electronic Structure, Optical Properties and Photoelectrochemical Activity of Sn Doped Fe2O3 Thin Films

Hematite (Fe2O3) is a well-known oxide semiconductor suitable for photoelectrochemical (PEC) water splitting and industry gas sensing. It is widely known that Sn doping of Fe2O3 can enhance the device performance, yet the underlying mechanism remains elusive. In this work, we determine the relationship between electronic structure, optical properties, and PEC activity of Sn-doped Fe2O3 by studying highly crystalline, well-controlled thin films prepared by pulsed laser deposition (PLD). We show that Sn doping substantially increases the n-type conductivity of Fe2O3, and the conduction mechanism is better described by a small-polaron hopping (SPH) model. Only 0.2% Sn doping significantly reduces the activation energy barrier for SPH conduction from at least 0.5 eV for undoped Fe2O3 to 0.14 eV for doped ones. A combination of X-ray photoemission, X-ray absorption spectroscopy, and DFT calculations reveals that the Fermi level gradually shifts toward the conduction band minimum with Sn doping. A localized Fe2+-like gap state is observed at the top of the valence band, accounting for the SPH conduction. Interestingly, different from the literature, only 0.2% Sn doping in Fe2O3 significantly improves the PEC activity, while more Sn decreases it. The improved PEC activity is partially attributed to an increased band bending potential which facilitates the charge separation at the space charge region. The reduced activation energy barrier for SPH will facilitate the transport of photoexcited carriers for the enhanced PEC, which is of interest for further carrier dynamics study. </p