A Template Editing Strategy to Create Interlayer-Confined Active Species for Efficient and Durable Oxygen Evolution Reaction.

Substantial overpotentials and insufficient and unstable active sites of oxygen evolution reaction (OER) electrocatalysts limit their efficiency and stability in OER-related energy conversion and storage technologies. Here, a template editing strategy is proposed to graft highly active catalytic species onto highly conductive rigid frameworks to tackle this challenge. As a successful attempt, two types of NiO6 units of layered Ni BDC (BDC stands for 1,4-benzenedicarboxylic acid) metal organic frameworks are selectively edited by chemical etching-assisted electroxidation to create layered γ-NiOOH with intercalated Ni-O species. In such an interlayer-confined intercalated architecture, the large interlayer space with high ion permeability offers an ideal reaction region to sufficiently expose the OER active sites comprising high-density intercalated Ni-O species, which also suppresses the undesirable γ to ß phase transformation, thus exhibiting efficient and durable OER activity. As a result, water oxidation can occur at an extremely low overpotential of 130 mV and affords 1000 h stability at 100 mA cm-2 . The strategy conceptually shows the possibility of achieving stable homogeneous-like catalysis in heterogeneous catalysis.

Physical Basis of Multi-Energy Coupling-Driven Water Oxidation.

Hydrogen production by electrolyzing water is an important technique to store energy from renewables into chemical energy. Many efforts have been made to improve the energy conversion efficiency. In this review article, we mainly summarized the emerging ideas on water oxidation by multi-energy coupling. First, the physicochemical nature of electrolyzing water reaction is described. Then, we conceptually proposed the physical basis of energy coupling with a goal to maximize the energy conversion efficiency and showed the methods to achieve heat-electricity and magnetism-electricity coupling to drive water splitting. Finally, the material requirements for creating efficient energy coupling water splitting system were proposed. These new ideas unlock a big potential direction for developing multi-energy coupling hydrogen production devices to efficiently store the intermittent and fluctuating renewables.

N-Doped Graphene-Coated Commercial Pt/C Catalysts toward High-Stability and Antipoisoning in Oxygen Reduction Reaction.

Stability and antipoisoning effects are the main challenges for the application of commercial Pt/C catalysts. Herein, we soaked and adsorbed polydopamine to coat Pt particles on commercial Pt/C and subsequently converted the coatings to few-layer N-doped graphene by calcination to produce Pt/C@NC. The coatings effectively block the direct contact of Pt nanoparticles and electrolyte, thus enhancing the catalyst stability by avoiding Ostwald ripening and suppressing the competitive adsorption of toxicants, contributing to the enhancement of the antipoisoning ability. More importantly, the coatings do not hurt the oxygen reduction reaction (ORR) activity of commercial Pt/C, which exhibits a half wave potential of 0.84 V in an acidic electrolyte. The spectroscopic and theoretical results confirmed that the coatings originate from a strong Pt bonding to pyridinic N of N-doped graphene and that the high ORR activity results from the coordinately unsaturated carbon atoms, as the real ORR active sites, to strongly capture electrons from Pt.

Heat-Electricity Coupling Driven Cascade Oxidation Reaction of Redox Couple and Water.

High barriers of water oxidation mediated by redox couple continuously challenge to maximizing efficiency from renewables to hydrogen energy. Here, an electricity-heat complementary strategy was achieved by a heat-electricity-sensitive interconversion of the α-Ni(OH)2/γ-NiOOH redox couple. In our strategy, the thermo-activated effects significantly lower the barrier energies of initial electroxidation of Ni2+/Ni3+ and subsequent chemical water oxidation to the nearly equal value via coupling a low-grade heat field (<100 °C), thereby achieving a consecutive two-step cascade reaction without kinetic delay. As a result, the cascaded water splitting reaction can happen at an extremely low overpotential of 130 mV and affords a low cell voltage of 1.73 V at 100 mA cm-2 at 90 °C in alkaline electrolyte. Our findings open a new avenue to produce hydrogen by complementation and gain effects of different-grade energies.

Solid-state redox couple mediated water splitting.

The solid-state redox couple is a vital charge transfer medium for electrochemical water splitting. In this Frontiers article, we summarize the versatile application of redox couples in promoting OER kinetics, in decoupling the HER and OER, and in combined electrochemical-thermochemical water splitting. These new ideas unlock vast potential for applying redox-couple-mediated water splitting to the storage of the intermittent and fluctuating energy derived from renewable sources.

Silicon Photoanode Modified with Work-function-tuned Ni@Fey Ni1-y (OH)2 Core-Shell Particles for Water Oxidation.

The photoelectrochemical (PEC) water splitting determines by the light absorption and charge extraction/injection. Here, we dispersedly modified the core-shell structured Ni@Niy Fe1-y (OH)2 on Si photoanodes and in-situ electrochemically converted it to Ni@Niy Fe1-y OOH to form a Si/SiOx /Ni@Niy Fe1-y OOH assembly, exhibiting the adjustable band bending and catalytic ability in water oxidation depending closely on the composition of Niy Fe1-y OOH. Combining with the island-like dispersed distribution to maximize the light absorption and the Ni@Niy Fe1-y shell as a high work function and a catalytic layer to simultaneously enlarge charge extraction and injection, the Si/SiOx /Ni@Ni0.7 Fe0.3 OOH assembly achieved an onset potential of 1.0 VRHE , a saturated current density of 35.4 mA cm-2 and a more than 50 h stability in an electrolyte with pH 9 under AM1.5G simulated sunlight irradiation. Our findings suggested that regulating the charge energetics at Si-electrolyte interface by discontinuously modifying a composition-adjustable core-shell structure is a potential route to develop highly efficient PEC devices.

Polaron States as a Massive Electron-Transfer Pathway at Heterojunction Interface.

For heterojunction semiconductor photoelectrodes, efficient charge separation is localized in the junction-induced electric field region and charge transfer follows a band-to-band charge-transfer pathway. Here, we found that polaron states at the heterojunction interface have a function of storing and transferring electrons. As a successful demonstration, we verified that the polaron states (Ti3+OH) on TiO2 are not passivated when used to create a CdS/TiO2 heterojunction and function as an efficient pathway for massively capturing, storing, and transferring the electrons from conduction bands of both TiO2 and CdS, thus effectively enhancing the charge separation efficiency of the heterojunction photoanode. The electron throughput of polaron states remains a positive correlation with polaron state density. Interfacial electron transfer through the TiO2 surface polaron states has great potential application in the development of high-performance heterojunction devices based on TiO2.

Surface polaron states on single-crystal rutile TiO2 nanorod arrays enhancing charge separation and transfer.

Polaron states on TiO2 photoanodes provide an important electron transfer pathway at the electrode-electrolyte interface. Here, we electrochemically doped single-crystal rutile TiO2 nanorod arrays with exposed (110) facets to produce surface polaron states, Ti3+-OH, which greatly contributed to charge separation and transfer. Our results experimentally clarified the previously confused understanding of the origin of improved photoelectrochemical (PEC) water splitting performance and verified that the enhanced PEC effects mainly arise from surface polaron states instead of grain boundary passivation.

One-step synthesis of IrOx-decorated ultrathin NiFe LDH nanosheets for efficient oxygen evolution reaction.

The oxygen evolution reaction (OER) is a vital proton donor for various clean energy technologies. Here, we synthesized IrOx-decorated ultrathin NiFe-LDH nanosheets (thickness 1-2 nm) by a one-step co-precipitation method. The IrOx was uniformly dispersed on the surface of ultrathin NiFe-LDH nanosheets, greatly increasing the active sites and the electric conductivity for NiFe-LDH. As a result, the IrOx/U-NiFe-LDH exhibited excellent OER performance with a low overpotential (236 mV) and a Tafel slope (74.3 mA dec-1).

Light harvesting and charge management by Ni4S3 modified metal-organic frameworks and rGO in the process of photocatalysis.

Harvesting and charge management is obtained by means of Ni4S3 modified Metal-organic Frameworks (MOF) and rGO, namely, the Uio-66 (Zr)/rGO combined with Ni4S3 photocatalyst was successfully prepared with the solvothermal method. The Ni4S3 acted as the electron transfer agent greatly improve the electrons transmission from the excited state dye to the rGO/MOF surface for proton reduction reaction. The hydrogen production amount over EY-sensitized rGO/MOF/Ni4S3 photocatalyst has reached 280 µmol for 5 h, which is about 14 times than that of the pure Ni4S3 photocatalyst and 185 times than that of the pure rGO/MOF photocatalyst under visible light irradiation (λ ≥ 420 nm). In the composite, the rGO acts as electron-transfer mediator and Ni4S3 serves as H2-evolution active site. A series of studies shown that the Ni4S3 modified MOF and rGO provided more active sites and improved the efficiency of photo-generated charge separation by means of several characterizations such as SEM, XRD, XPS, Element Mapping, UV-vis DRS, BET, Photocurrent, Voltammetric Scanning, Fluorescence Spectra and FTIR. and the results of which were in good agreement with each other. The photoelectron migration rate and photogenerated charge separation efficiency of the composite can be obviously increased with graphene as a good electron acceptor and transfer medium and Ni4S3 as hydrogen producing active site.

Light-assisted synthesis MoSx as a noble metal free cocatalyst formed heterojunction CdS/Co3O4 photocatalyst for visible light harvesting and spatial charge separation.

In this paper, a novel photocatalyst with visible light harvesting and spatial charge separation is reported, in which cobalt oxide (acting as a hole trap) and molybdenum sulfide (acting as an electron trap) are assembled on the surface of cadmium sulfide nanorods. The MoSx/CdS/Co3O4 composite photocatalyst shows a high H2 evolution with a yield of 537.51 µmol in 5 h, which is 35.53 times greater than over pristine CdS (15.13 µmol). The detailed underlying reason was comprehensively studied and understood by means of SEM, TEM, XRD, XPS, UV-vis DRS, BET; in particular, investigation of their photoelectrochemical properties with photocurrent, voltammetric scanning, fluorescence spectra etc. The high photocurrent response, the lower overpotential (-0.37 V vs. SCE), the faster electron transfer rate constant (ket = 4.23 × 109 s-1) and the short fluorescence lifetime (0.457 ns) supported the efficient spatial charge transfer between CdS and MoSx as well as Co3O4. The simultaneous loading of bicocatalysts can significantly improve the photo-induced spatial charge separation of CdS because MoSx nanoparticles act as an electron trap to rapidly transfer electrons from CdS while Co3O4 acts as a hole trap to quickly transfer holes, enhancing its photocatalytic performance as well.

Promotion of the excited electron transfer over Ni- and Co -sulfide co-doped g-C3N4 photocatalyst (g-C3N4/NixCo1-xS2) for hydrogen Production under visible light irradiation.

A Ni- and Co- sulfide co-doped g-C3N4 photocatalyst (g-C3N4/NixCo1-xS2) was prepared by hydrothermal method and this photocatalyst, namely, g-C3N4/NixCo1-xS2 shown excellent photocatalytic properties due to the special structure of Ni-Co-S with boundary different exposure to active site of transition metal-metal (Ni-Co) active planes. With the introduction of Co atoms, the H2 production amount reached the maximum about 400.81 µmol under continuous visible light irradiation for 4 hours based on the efficiently charge separation and greatly improved electron transfer resulted from the presence of sufficient active exposure at the boundary. The serial studies shown that the existence of Ni-Co-S structure over g-C3N4 active surface is the key factor of activity affections by means of several characterizations such as SEM, XRD, XPS diffuse reflectance etc. and the results of which were in good agreement with each other. A possible reaction mechanism over eosin Y-sensitized g-C3N4/NixCo1-xS2 photocatalyst under visible light irradiation was proposed.

Under-sampling trajectory design for compressed sensing based DCE-MRI.

Dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) needs high temporal and spatial resolution to accurately estimate quantitative parameters and characterize tumor vasculature. Compressed Sensing (CS) has the potential to accomplish this mutual importance. However, the randomness in CS under-sampling trajectory designed using the traditional variable density (VD) scheme may translate to uncertainty in kinetic parameter estimation when high reduction factors are used. Therefore, accurate parameter estimation using VD scheme usually needs multiple adjustments on parameters of Probability Density Function (PDF), and multiple reconstructions even with fixed PDF, which is inapplicable for DCE-MRI. In this paper, an under-sampling trajectory design which is robust to the change on PDF parameters and randomness with fixed PDF is studied. The strategy is to adaptively segment k-space into low-and high frequency domain, and only apply VD scheme in high-frequency domain. Simulation results demonstrate high accuracy and robustness comparing to VD design.

Under-sampling trajectory design for compressed sensing MRI.

The under-sampling trajectory design plays a key role in compressed sensing MRI. The traditional design scheme using probability density function (PDF) is based up observation on energy distribution in k-space rather than systematic optimization, which results in non-deterministic trajectory even with a fixed PDF. Guidance-based method like Bayesian inference scheme is always bothered with high computational complexity on entropy. In this paper, we study how to adaptively design an under-sampling trajectory in the context of CS with systematic optimization and small complexity. Simulation results conducted on images from different slices and dynamic sequence demonstrate the effectiveness of the proposed method by comparing the designed trajectory with those by traditional method.