A semiconductor-electrocatalyst nano interface constructed for successive photoelectrochemical water oxidation

High-efficiency oxygen evolution on γ-Fe2O3 catalysts with BiVO4 photoabsorbers and TpAQ hole transport layers for photoelectrochemical water splitting

The interfacial energy levels between oxygen-excavating co-catalysts (OECs) and BiVO4 often lead to carrier recombination. Modulating the interface using a hole transport layer (HTL) can effectively inhibit interfacial recombination, realizing efficient photoelectrochemical (PEC) water splitting. Herein, we design BiVO4@γ-Fe2O3/TpAQ photoanodes by one-step solvothermal insertion of TpAQ COF between BiVO4 and γ-Fe2O3 co-catalysts as HTL layer. The positive transient surface photovoltage signals indicate that the introduction of TpAQ COF provides an additional driving force for photogenerated hole transfer, which effectively improves the carrier transfer efficiency of BiVO4. Meanwhile, the fastest transfer rate of BiVO4@γ-Fe2O3/TpAQ in the intensity-modulated photocurrent spectroscopy (IMPS) test confirms the excellent charge transfer kinetics of TpAQ COF HTL. In addition, a combination of photoluminescence and energy band calculations showed that a type II heterojunction was constructed between the TpAQ COF and BiVO4, thus avoiding photogenerated electron-hole pair recombination. BiVO4@γ-Fe2O3/TpAQ exhibited the highest PEC water oxidation capability, achieving a photocurrent density of 6.3 mA cm-2 at 1.23 VRHE under the optimized photoanode. Attributed to the TpAQ COF HTL, the BiVO4@γ-Fe2O3/TpAQ photoanode exhibits excellent incident monochromatic photon-electron conversion efficiencies (up to 95.23% at 420 nm) and charge injection efficiencies (up to 94.6% at 1.23 VRHE).

One-Step Preparation of Trinary Co(III)/Co(II)/Co(0) Bifunctional Catalysts with Controllable Co Valence Distribution via Solution Combustion Synthesis and Its Application to Oxygen Reduction and Evolution in Zinc-Air Batteries

Regulating the electronic structure and chemical valence of the active site for electrocatalysis is highly crucial and challenging. In this work, a cost-effective and facile strategy for regulating the cobalt valence distribution through one-step solution combustion synthesis via cobalt nitrate (oxidizer) and glycine (fuel) was developed to prepare a bifunctional Co(III)/Co(II)/Co(0) catalyst for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in a zinc-air battery. Experimental findings show that when increasing the fuel/oxidizer ratio φ, the composition of the synthesized catalyst gradually changes from binary Co(III)/Co(II) to trinary Co(III)/Co(II)/Co(0), and the average Co valence keeps decreasing. As the content of Co(II)/Co(0) increases, the ORR performance of the sample gradually improves. The sample synthesized at φ = 1.2 shows the best bifunctional catalytic activity and was employed to assemble a rechargeable zinc-air battery. Overall, compared with the published data, the proposed catalyst has a comparable ORR/OER activity and catalyst stability, with an excellent battery life, efficiency, and stability (up to 160 h). This work provides a promising pathway for designing the valence state distribution of non-noble cobalt-based catalysts, which can be easily prepared on a large scale with low cost and used in various technologies involved in ORR and OER reactions.

Highly Coupled Dynamically Modulated Electrocatalysts on Wafer-Scale InGaN/GaN Nanowires on Silicon for Successive Acidic Photoelectrochemical Water Oxidation

Photoelectrochemical water splitting is considered one of the most promising paths for sustainable hydrogen production. However, the sluggish kinetics of the water oxidation reaction and poor stability of the photoanode significantly limit the overall performance of the photoelectrochemical device, particularly under acidic conditions, which poses great challenges for practical applications. Herein, the coupling of unique CoRuOx nanoclusters with dynamic electronic modulation effects to wafer-scale InGaN nanowires is proposed, demonstrating superior photoelectrochemical activity and stability for acidic water oxidation. Compared with InGaN nanowires loaded with typical RuO₂ cocatalysts, CoRuOx/InGaN photoanodes achieve a remarkable improvement in applied bias photon-to-current efficiency from 0.77% to 2.25%, with stable operation for over 500 min under strongly acidic conditions. Such boosted performance is attributed mainly to Co induced dynamic electronic modulation, which enhances oxygen evolution while maintaining the stable operation of CoRuOx/InGaN photoanodes. Initially, the Co sites increased the oxidation state of Ru, enhancing the activity of oxygen evolution. Moreover, during PEC operation, the Co sites stabilized the Ru sites, preventing dissolution of cocatalyst. This unique self-adaptive process significantly enhances the stability and activity of the photoanode, opening an effective avenue to achieve efficient and durable photoanodes for PEC applications.

Regulating the Electronegativity Difference and Piezoelectric Strain of the S-Mo-S Structure via Introducing Mo Vacancies for Boosting Piezo-Photoelectric Activity

Recently, piezoelectric and photocatalytic processes have shown excellent synergistic effect addressing environmental remediation challenges. Herein, a nanoflower-like Mo vacancy-modulated MoS2 (VMo-MoS2) piezo-photocatalyst with different VMo densities has been successfully synthesized using a one-step hydrothermal method. The high VMo density (12%) facilitates the enhancement of the photocatalytic activity but compromises its structural stability, resulting in unsatisfactory piezoelectric activity. Among all VMo-MoS2 piezo-photocatalysts, VMo-MoS2 with 6% VMo density exhibits the highest piezo-photocurrent density (15.50 μA cm-2), the largest potential difference (0.188 V), and the best carbamazepine (CBZ) degradation efficiency (95.81%) in only 10 min under light-ultrasound action, exhibiting a remarkable synergistic effect of the piezoelectric and photocatalytic processes. The synergistic performance originates from the simultaneous modulation of the charge distribution and the self-polarization capability of the S-Mo-S structure by VMo, as confirmed by the molecular theory calculations and finite-element simulation results. This work provides a defect engineering strategy for achieving the synergistic effect of the piezoelectric and photocatalytic processes, which opens a new research avenue for the design and application of the piezo-photocatalyst.

Size Effects on Bubble Dynamics during Photoelectrochemical Water Splitting

The rapid bubble removal from electrodes diminishes the reaction resistance within photoelectrochemical water splitting, and the coalescence between bubbles accelerates their detachment. To delve into the size effects on bubble coalescence dynamics, the Marangoni effect is utilized as a noninvasively controlling means of bubble sliding and coalescence. The study reveals that the encounter of capillary waves induces bubble detachment. A quantitative correlation has been established to elucidate the relationship between the oscillation time of coalescence and the capillary wave propagation. Then, the bubbles undergo damping oscillations due to fluid resistance after detaching, characterized by the same dimensionless oscillation periods. Additionally, the detachment velocity of the bubbles follows a power law relationship of 1/2 with the ratio of dimensionless surface energy to the equivalent radius. Considering the viscous dissipation and adhesive effect of the electrode on the bubbles, the critical radius of large bubbles enabling jumping is deduced from the perspective of energy conservation. Our research provides a strategy for the management of bubble dynamic behavior and the practical application of electrolysis technology.

Designing In2S3/FeVO4/CNT Photoelectrode for Enhanced Visible Light Driven Oxygen Evolution

The development of efficient and stable photoelectrodes is essential for the advancement of photoelectrochemical (PEC) water-splitting technologies, which hold promise for efficient oxygen evolution reaction (OER), necessary for sustainable hydrogen production. In this study, the synthesis of a ternary composite, In2 ${_2 }$ S3 ${_3 }$ /FeVO4 ${_4 }$ /CNT has been reported, designed for highly efficient PEC oxygen evolution. The formation of In2 ${_2 }$ S3 ${_3 }$ /FeVO4 ${_4 }$ heterostructure enhances PEC performance significantly due to the type-II band alignment, which minimizes electron-hole recombination and improves charge separation. The addition of CNTs further enhances performance by providing conductive pathways that improve electron transport and reduce charge transfer resistance. The resulting In2 ${_2 }$ S3 ${_3 }$ /FeVO4 ${_4 }$ /CNT ternary composite achieves a current density of 14.70 mAcm- 2 ${^{ - 2} }$ at 1.8 V vs. RHE, representing a notable increase in performance. Electrochemical impedance spectroscopy (EIS) shows that the ternary composite has the lowest charge transfer resistance, while Bode phase analysis indicates a longer carrier lifetime, emphasizing the synergistic effect of heterostructure formation and CNT inclusion. The ternary composite also demonstrates excellent stability and responsiveness during transient photocurrent cycling, maintaining performance under repeated chronoamperometric ON/OFF cycles, making it a strong candidate for water-splitting applications driven by visible light.

Direct photoelectrochemical detection of ethanol in complex biological sample

The development of advanced photoelectrochemical (PEC) technology for the direct detection of ethanol in complex biological sample, has become a hot topic. However, the photo-active nanomaterials, which could generate the photo-induced carriers under illumination, are susceptible to biofouling and interference in complex bio-matrices. In this work, the silica nanochannel-protected TiO2 nanomaterials was reported for the first time that enables the direct sensing of ethanol in real fruits and untreated whole blood. The modification of SNC enhanced the sensitivity of ethanol detection by promoting light absorption, electron-hole separation, and surface reaction rate of photo-active materials. Meanwhile, the biofouling macromolecules and interference signals can be effectively excluded due to the hydrophilic properties, size, and electrostatic exclusion of nanochannels. As a result, without any complex sample pretreatments, the proposed PEC sensor can be directly immersed in complex biological samples for ethanol detection, exhibiting a broad linear range (1.775 μM-20 mM) and a low detection limit (1.2 μM), as well as excellent reproducibility and stability. This work paves a new path for PEC sensors in real biomedical applications.

p-n-Type LaCoO3/NiFe LDH Heterostructures for Enhanced Photogenerated Carrier-Assisted Electrocatalytic Oxygen Evolution Reaction

The oxygen evolution reaction (OER) poses a significant kinetic challenge for various critical energy conversion and storage technologies including electrocatalytic water splitting and metal-air batteries. In this study, a LaCoO3/NiFe layered double hydroxide (LDH) catalyst was synthesized through the in situ growth of n-type NiFe LDH on the surface of the p-type LaCoO3 semiconductor, resulting in a p-n heterostructure for a photogenerated carrier-assisted electrocatalytic OER (PCA-eOER). The alignment of their band structures facilitates the formation of an internal electric field at the heterojunction interface, which promotes the creation of oxygen vacancies and enhances electron transport. Under illumination, the expanded visible-light absorption range and built-in electric field work synergistically to improve the generation and separation of photogenerated carriers. Meanwhile, the accumulation of photogenerated holes on the surface of NiFe LDH results in an enhancement in the concentration of high-valent active metal sites, resulting in a boost in the PCA-eOER efficiency. The LaCoO3/NiFe LDH has achieved an overpotential of 260 mV at the current density of 10 mA cm-2, 50 mV lower than in the absence of illumination. In addition, LaCoO3/NiFe LDH was assembled into an alkaline water electrolyzer and zinc-air batteries (ZABs), showing excellent practical application capability. We explored the application of LaCoO3 in a PCA-eOER, which provides a concept for designing PCA-eOER catalysts and advancing the development of perovskite-based catalysts for clean energy conversion technology.

Metal Hydroxide-Organic Framework Mediated Structural Reengineering Enables Efficient NiFe Interaction for Robust Water Oxidation

NiFe layered double hydroxide (NiFe LDH) derived oxyhydroxides are promising electrocatalysts for the alkaline oxygen evolution reaction (OER). However, NiFe LDH with a stable metal-oxygen-metal (M-O-M) structure suffers from inadequate NiFe interaction, leading to undesirable activity and stability. Herein, we develop a NiFe hydroxide-organic framework (NiFe HOF) via modification of NiFe LDH with an organic linker to break the structural constraint of M-O-M and thus boost the OER. NiFe HOF with reconfigurable metal sites facilitates structural reengineering under the OER condition to form abundant NiFe interaction and prolonged M-O bonds, stimulating lattice oxygen mechanism. Therefore, NiFe HOF shows a distinctly decreased overpotential at 50 mA cm-2, which is 68 mV lower than that of NiFe LDH. The anion exchange membrane electrolyzer using NiFe HOF as anode electrode displays ultralong stability exceeding 1050 h at 1 A cm-2 with a low attenuation of 0.16 mV h-1.

CdS-based Schottky junctions for efficient visible light photocatalytic hydrogen evolution

Heterojunctions photocatalysts play a crucial role in achieving high solar-hydrogen conversion efficiency. In this work, we mainly focus on the charge transfer dynamics and pathways for sulfides-based Schottky junctions in the photocatalytic water splitting process to clarify the mechanism of heterostructures photocatalysis. Sulfides-based Schottky junctions (CdS/CoP and CdS/1T-MoS2) were successfully constructed for photocatalytic water splitting. Because of the higher work function of CdS than that of CoP and 1T-MoS2, the direction of the built-in electric field is from CoP or 1T-MoS2 to semiconductor. Therefore, CoP and 1T-MoS2 can act as electrons acceptors to accelerate the transfer of photo-generated electron on the surface of CdS, thus improving the charge utilization efficiency. Meanwhile, CoP and 1T-MoS2 as active sites can also promote the water dissociation and lower the H+ reduction overpotential, thus contributing to the excellent photocatalytic hydrogen production activity (23.59 mmol·h-1·g-1 and 1195.8 mol·h-1·g-1 for CdS/CoP and CdS/1T-MoS2).

Identifying Root Origin of Insulating Polymer Mediated Solar Water Oxidation

Rational construction of high-efficiency photoelectrodes with optimized carrier migration to the ideal active sites, is crucial for enhancing solar water oxidation. However, complexity in precisely modulating interface configuration and directional charge transfer pathways retards the design of robust and stable artificial photosystems. Herein, a straightforward yet effective strategy is developed for compact encapsulation of metal oxides (MOs) with an ultrathin non-conjugated polymer layer to modulate interfacial charge migration and separation. By periodically coating highly ordered TiO2 nanoarrays with oppositely charged polyelectrolyte of poly(dimethyl diallyl ammonium chloride) (PDDA), MOs/polymer composite photoanodes are readily fabricated under ambient conditions. It is verified that electrons photogenerated from the MOs substrate can be efficiently extracted by the ultrathin solid insulating PDDA layer, significantly boosting the carrier transport kinetics and enhancing charge separation of MOs, and thus triggering a remarkable enhancement in the solar water oxidation performance. The origins of the unexpected electron-withdrawing capability of such non-conjugated insulating polymer are unambiguously uncovered, and the scenario occurring at the interface of hybrid photoelectrodes is elucidated. The work would reinforce the fundamental understanding on the origins of generic charge transport capability of insulating polymer and benefit potential wide-spread utilization of insulating polymers as co-catalysts for solar energy conversion.

Hybrid Overlayer of Defective Ni-MOF and NiO Nanoparticles toward Efficient TiO2/CdS Type-II Heterojunction

The severe photocorrosion of cadmium sulfide (CdS) restricts its practical application for solar hydrogen production. Although remarkable progress has been achieved with an overlayer strategy for isolating the CdS surface, the lifetime of CdS-based photoanodes is still far from the actual requirements. Herein, a hybrid overlayer of defective Ni-MOF and NiO nanoparticles has been developed through the chemical bath deposition method with postannealing. This hybrid overlayer of Ni-MOF-d is coated on the surface of the TiO2/CdS type-II heterojunction. The composite photoanode exhibits a photocurrent density of 4.41 mA cm-2 at 1.23 VRHE, which is 3.47- and 1.32-fold that of CdS and TiO2/CdS, respectively. The Ni-MOF-d overlayer gives rise to a negative shift of the onset potential by 59.51 mV. After a long-term stability test of 11 h, a photocurrent retention of 70% is observed, which is among the most robust CdS-based photoanodes. The kinetics studies reveal that the performance improvements can be attributed to the multiple functions of the Ni-MOF-d hybrid overlayer, including isolating the CdS surface from the electrolyte, cocatalyzing the electrode oxidation processes, passivating the surface defect states of CdS, and facilitating the charge injection from the photoanode to the electrolyte.

Selective oxygen reduction reaction: mechanism understanding, catalyst design and practical application

The oxygen reduction reaction (ORR) is a key component for many clean energy technologies and other industrial processes. However, the low selectivity and the sluggish reaction kinetics of ORR catalysts have hampered the energy conversion efficiency and real application of these new technologies mentioned before. Recently, tremendous efforts have been made in mechanism understanding, electrocatalyst development and system design. Here, a comprehensive and critical review is provided to present the recent advances in the field of the electrocatalytic ORR. The two-electron and four-electron transfer catalytic mechanisms and key evaluation parameters of the ORR are discussed first. Then, the up-to-date synthetic strategies and in situ characterization techniques for ORR electrocatalysts are systematically summarized. Lastly, a brief overview of various renewable energy conversion devices and systems involving the ORR, including fuel cells, metal-air batteries, production of hydrogen peroxide and other chemical synthesis processes, along with some challenges and opportunities, is presented.

Platinum Nanoparticles Bonded with Carbon Nanotubes for High-Performance Ampere-Level All-Water Splitting

Platinum nanoparticles loaded on a nitrogen-doped carbon nanotubes exhibit a brilliant hydrogen evolution reaction (HER) in an alkaline solution, but their bifunctional hydrogen and oxygen evolution reaction (OER) has not been reported due to the lack of a strong Pt-C bond. In this work, platinum nanoparticles bonded in carbon nanotubes (Pt-NPs-bonded@CNT) with strong Pt-C bonds are designed toward ultralow overpotential water splitting ability in alkaline solution. Benefit from the strong interaction between platinum and high conductivity carbon nanotube substrates through the Pt-C bond also the platinum nanoparticles bonded in carbon nanotube can provide more stable active sites, as a result, the Pt-NPs-bonded@CNT exhibits excellent hydrogen evolution in acid and alkaline solution with ultralow overpotential of 0.19 and 0.23 V to reach 1000 mA cm-2, respectively. Besides, it shows superior oxygen evolution electrocatalysis in alkaline solution with a low overpotential of 1.69 V at 1000 mA cm-2. Furthermore, it also exhibits high stability over 110 h against the evolution of oxygen and hydrogen at 1000 mA cm-2. This strategy paves the way to the high performance of bifunctional electrocatalytic reaction with extraordinary stability originating from optimized electron density of metal active sites due to strong metal-substrate interaction.

Photo-Assisted Li-N2 Batteries with Enhanced Nitrogen Fixation and Energy Conversion

Li-N2 batteries have received widespread attention for their potential to integrate N2 fixation, energy storage, and conversion. However, because of the low activity and poor stability of cathode catalysts, the electrochemical performance of Li-N2 batteries is suboptimal, and their electrochemical reversibility has rarely been proven. In this study, a novel bifunctional photo-assisted Li-N2 battery system was established by employing a plasmonic Au nanoparticles (NPs)-modified defective carbon nitride (Au-Nv -C3 N4 ) photocathode. The Au-Nv -C3 N4 exhibits strong light-harvesting, N2 adsorption, and N2 activation abilities, and the photogenerated electrons and hot electrons are remarkably beneficial for accelerating the discharge and charge reaction kinetics. These advantages enable the photo-assisted Li-N2 battery to achieve a low overpotential of 1.32 V, which is the lowest overpotential reported to date, as well as superior rate capability and prolonged cycle stability (≈500 h). Remarkably, a combination of theoretical and experimental results demonstrates the high reversibility of the photo-assisted Li-N2 battery. The proposed novel strategy for developing efficient cathode catalysts and fabricating photo-assisted battery systems breaks through the overpotential bottleneck of Li-N2 batteries, providing important insights into the mechanism underlying N2 fixation and storage.

Harnessing the electronic structure of active metals to lower the overpotential of the electrocatalytic oxygen evolution reaction

Despite substantial advancements in the field of the electrocatalytic oxygen evolution reaction (OER), the efficiency of earth-abundant electrocatalysts remains far from ideal. The difficulty stems from the complex nature of the catalytic system, which limits our fundamental understanding of the process and thus the possibility of a rational improvement of performance. Herein, we shed light on the role played by the tunable 3d configuration of the metal centers in determining the OER catalytic activity by combining electrochemical and spectroscopic measurements with an experimentally validated computational protocol. One-dimensional coordination polymers based on Fe, Co and Ni held together by an oxonato linker were selected as a case study because of their well-defined electronic and geometric structure in the active site, which can be straightforwardly correlated with their catalytic activity. Novel heterobimetallic coordination polymers were also considered, in order to shed light on the cooperativity effects of different metals. Our results demonstrate the fundamental importance of electronic structure effects such as metal spin and oxidation state evolutions along the reaction profile to modulate ligand binding energies and increase catalyst efficiency. We demonstrated that these effects could in principle be exploited to reduce the overpotential of the electrocatalytic OER below its theoretical limit, and we provide basic principles for the development of coordination polymers with a tailored electronic structure and activity.

Fast Interfacial Carrier Dynamics Modulated by Bidirectional Charge Transport Channels in ZnIn2 S4 -based Composite Photoanodes Probed by Scanning Photoelectrochemical Microscopy

Limited charge separation/transport efficiency remains the primary obstacle of achieving satisfying photoelectrochemical (PEC) water splitting performance. Therefore, it is essential to develop diverse interfacial engineering strategies to mitigate charge recombination. Despite obvious progress having been made, most works only considered a single-side modulation in either the electrons of conduction band or the holes of valence band in a semiconductor photoanode, leading to a limited PEC performance enhancement. Beyond this conventional thinking, we developed a novel coupling modification strategy to achieve a composite electrode with bidirectional carrier transport for a better charge separation, in which Ti2 C3 Tx MXene quantum dots (MQDs) and α-Fe2 O3 nanodots (FO) are anchored on the surface of ZnIn2 S4 (ZIS) nanoplates, resulting in markedly improved PEC water splitting of pure ZIS photoanode. Systematic studies indicated that the bidirectional charge transfer pathways were stimulated due to MQDs as "electron extractor" and S-O bonds as carriers transport channels, which synergistically favors significantly enhanced charge separation. The enhanced kinetic behavior at the FO/MQDs/ZIS interfaces was systematically and quantitatively evaluated by a series of methods, especially scanning photoelectrochemical microscopy. This work may deepen our understanding of interfacial charge separation, and provide valuable guidance for the rational design and fabrication of high-performance composite electrodes.

Ni, Co, Zn, and Cu metal-organic framework-based nanomaterials for electrochemical reduction of CO2: A review

The combustion of fossil fuels has resulted in the amplification of the greenhouse effect, primarily through the release of a substantial quantity of carbon dioxide into the atmosphere. The imperative pursuit of converting CO2 into valuable chemicals through electrochemical techniques has garnered significant attention. Metal-organic frameworks (MOFs) have occured as highly prospective materials for the reduction of CO2, owing to their exceptional attributes including extensive surface area, customizable architectures, pronounced porosity, abundant active sites, and well-distributed metallic nodes. This article commences by elucidating the mechanistic aspects of CO2 reduction, followed by a comprehensive exploration of diverse materials encompassing MOFs based on nickel, cobalt, zinc, and copper for efficient CO2 conversion. Finally, a meticulous discourse encompasses the challenges encountered and the prospects envisioned for the advancement of MOF-based nanomaterials in the realm of electrochemical reduction of CO2.

Nanoflake NiMn Layered Double Hydroxide Coated on Porous Membrane-like Ni-Foam for Sustainable and Efficient Electrocatalytic Oxygen Evolution

Layered double hydroxides (LDHs) have gained vast importance as an electrocatalyst for water electrolysis to produce carbon-neutral and clean hydrogen energy. In this work, we demonstrated the fabrication of nano-flake-like NiMn LDH thin film electrodes onto porous membrane-like Ni-foam by using a simple and cost-effective electrodeposition method for oxygen evolution reaction (OER). Various Ni1-xMnx LDH (where x = 0.15, 0.25, 0.35, 0.50 and 0.75) thin film electrodes are utilized to achieve the optimal catalyst for an efficient and sustainable OER process. The various composition-dependent surface morphologies and porous-membrane-like structure provided the high electrochemical surface area along with abundant active sites facilitating the OER. The optimized catalyst referred to as Ni0.65Mn0.35 showed excellent OER properties with an ultralow overpotential of 253 mV at a current density of 50 mAcm-2, which outperforms other state-of-the art catalysts reported in the literature. The relatively low Tafel slope of 130 mV dec-1 indicates faster and more favorable reaction kinetics for OER. Moreover, Ni0.65Mn0.35 exhibits excellent durability over continuous operation of 20 h, indicating the great sustainability of the catalyst in an alkaline medium. This study provides knowledge for the fabrication and optimization of the OER catalyst electrode for water electrolysis.