•The conducting carbon layer was rationally introduced on the metal oxide arrays.•Based on this nanostructure, a high-performed supercapacitor has been constructed.•This work can be extended to the design of other flexible energy storage devices.Flexible solid-state supercapacitors (FSSCs) have attracted increasing interest for the development of portable and wearable electronics. However, the widely application of FSSCs has been limited by relatively deficient of the electrode materials, such as low electrical conductivity and modest reaction kinetics, which limit the practical energy output and stability. Herein, we introduce the design and fabrication of a sophisticated FSSCs device which consists of conducting carbon materials modified transition metal oxides/hydroxides nanorod arrays (NRAs). The well-defined carbon conducting layer was rationally introduced on the micro NRAs substrate by using metal-organic framework (MOF) as precursor, which was epitaxial grown on the surface of ZnO NRAs. Subsequently, the as-synthesized ZnO@MOF core-shell NRAs were transformed to the well-uniformed ZnO@C@CoNi-layered double hydroxide (LDH) and Fe2O3@C core-shell NRAs through in situ electrodeposition and cation exchange strategy, respectively. Furthermore, an asymmetric ZnO@C@CoNi-LDH//Fe2O3@C FSSC device was constructed with a high energy density (1.078 mW h cm–3), power density (0.4 W cm–3) as well as outstanding cycling lifespans (retention 95.01% after 10,000 charge/discharge cycles), which is superior to majority asymmetric FSSCs.Well-aligned hierarchical carbon modified transition metal oxides/hydroxides NRAs have been successfully fabricated by using epitaxial growth of ZIF-8 as carbon precursor. Benefiting from the specific core–shell heterostructure and the conductivity of carbon layer, both ZnO@C@CoNi-LDH and Fe2O3@C NRAs electrodes exhibited excellent electrochemical performance. Based on this sophisticated nanostructure, a high-performed flexible solid-state supercapacitor (FSSC) was further successfully constructed.Download high-res image (122KB)Download full-size image

•Ultrathin 2D Fe-doped CoP arrays (FeCoP UNSAs) have been successfully synthesized.•The FeCoP UNSAs displays satisfactory activity toward overall water splitting.•The incorporation of Fe benefits adsorption of water and dissociation of OH group.Transition metal phosphides (TMPs) have shown promising performance in electrocatalytic water splitting. However, the sluggish kinetic of oxygen evolution reaction (OER) process deteriorates their activity toward overall water splitting. To overcome this issue, two-dimensional (2D) ultrathin arrays of metal-doped CoP (MCoP; M = Fe, Ni, and Mg) were successfully prepared by using layered double hydroxides (LDHs) as precursors. The as-obtained 2D ultrathin arrays exhibit an outstanding electrocatalytic activity and long-term durability toward both half-reaction in overall water splitting. As a result, the electrolyzer assembled by FeCoP UNSAs consumes a cell potential as low as 1.60 V (at 10 mA cm−2). An experimental-theoretical combination study reveals that the electronic structure of Co is modulated via the incorporation of Fe, which benefits the adsorption of water molecule and the dissociation of OH group, accounting for the largely enhanced activity toward overall water splitting.Two-dimensional (2D) ultrathin arrays of FeCoP were fabricated by using layered double hydroxides (LDHs) as precursor, which exhibited excellent electrocatalytic activity toward overall water splitting.Download high-res image (307KB)Download full-size image

The explore and development of electrocatalysts have gained significant attention due to their indispensable status in energy storage and conversion systems, such as fuel cells, metal–air batteries and solar water splitting cells. Layered double hydroxides (LDHs) and their derivatives (e.g., transition metal alloys, oxides, sulfides, nitrides and phosphides) have been adopted as catalysts for various electrochemical reactions, such as oxygen reduction, oxygen evolution, hydrogen evolution, and CO2 reduction, which show excellent activity and remarkable durability in electrocatalytic process. In this review, the synthesis strategies, structural characters and electrochemical performances for the LDHs and their derivatives are described. In addition, we also discussed the effect of electronic and geometry structures to their electrocatalytic activity. The further development of high-performance electrocatalysts based on LDHs and their derivatives is covered by both a short summary and future outlook from the viewpoint of the material design and practical application.The electrocatalysts based on layered double hydroxides and their derivatives (transition metal alloys, oxides, sulfides, nitrides and phosphides) for small molecules electrocatalytic reactions are reviewed. The synthesis strategies, structural characters and electrochemical performances for these materials are well considered.Download high-res image (212KB)Download full-size image

•Well-defined 3D nitrogen-doped carbon nanotube (NCNT) arrays have been successfully synthesized.•The 3D NCNT array electrode displays satisfactory activity and stability in both ORR and OER.•Flexible, rechargeable all-solid-state Zn–air battery is fabricated by using this 3D NCNT array as air-cathode, giving rise to excellent discharge-charge performance.The exploration of highly-efficient, low-cost bifunctional oxygen electrocatalysts for both oxygen reduction reaction (ORR) and oxygen evolution reactions (OER) is critical for renewable energy storage and conversion technologies (e.g., fuel cells and metal–air batteries). Here we report the design and fabrication of free-standing nitrogen-doped carbon nanotube (NCNT) arrays via a directed growth approach as a high-performance bifunctional oxygen electrocatalyst. By virtue of the unique hierarchical nanoarray structure, uniform N-doping and decreased charge-transfer resistance, the as-prepared NCNT array exhibits rather high activity and stability in both ORR and OER, even superior to the mono-functional commercial Pt/C (for ORR) and IrO2 (for OER). A flexible, rechargeable all-solid-state zinc–air battery is successfully fabricated by using this self-supporting NCNT electrode as air-cathode, which gives excellent discharge-charge performance and mechanical stability.A free-standing three dimensional nitrogen-doped carbon nanotube (3D NCNT) array has been successfully fabricated via a directed growth approach, which can act as high-performance bifunctional oxygen electrocatalyst for ORR and OER. By using this self-supporting electrode material as air-cathode, a flexible, rechargeable all-solid-state Zn–air battery was assembled, which display desirable operational performances for energy storage and conversion.Download high-res image (219KB)Download full-size image

Photoelectrochemical (PEC) water oxidation has received considerable attention owing to its key role in the overall water splitting. In this work, several reduced titania@layered double hydroxide (CoAl–LDH, CoCr–LDH, and CoFe–LDH) hybrid photoanodes were fabricated via electrochemical deposition of LDH on the reduced titania, and their PEC properties for water oxidation were studied systematically. The reduced titania@CoCr–LDH photoanode shows a much improved PEC performance compared with pristine reduced titania, with a photocurrent density enhancement of 43% (from 0.65 mA cm−2 to 0.93 mA cm−2) and an onset potential decrease of 21% (from 0.23 V to 0.18 V vs. the RHE). This improvement is also successfully demonstrated in the reduced titania@CoAl–LDH and reduced titania@CoFe–LDH system. The photoconversion efficiency of reduced titania is significantly enhanced after the incorporation of LDH (0.42–0.51% at ∼0.46 V vs. the RHE). Both the experimental studies and DFT calculations confirm a synergistic effect between the reduced titania and LDH. The results show that a good match of the band structure facilitates the fast electron–hole separation and the migration of holes from reduced titania to LDH, followed by the LDH catalyzed water oxidation. The CoCr–LDH has the highest driving force for oxygen evolution among these LDHs, accounting for the optimal PEC performance of the reduced titania@CoCr–LDH photoanode.

The ever-increasing demand for renewable and clean power sources has triggered the development of novel materials for photoelectrochemical (PEC) water splitting, but how to improve the solar conversion efficiency remains a big challenge. In this work, we report a conceptual strategy in a ternary material system to simultaneously enhance the charge separation and water oxidation efficiency of photoanodes by introducing reduced graphite oxide (rGO) and NiFe-layered double hydroxide (LDH) on TiO2 nanorod arrays (NAs). An experimental–computational combination study reveals that rGO with a high work function and superior electron mobility accepts photogenerated electrons from TiO2 and enables fast electron transportation; while NiFe-LDH acts as a cocatalyst which accelerates the surface water oxidation reaction. This synergistic effect in this ternary TiO2/rGO/NiFe-LDH photoanode gives rise to a largely enhanced photoconversion efficiency (0.58% at 0.13 V) and photocurrent density (1.74 mA cm−2 at 0.6 V). It is worth mentioning that the photocurrent density of TiO2/rGO/NiFe-LDH, to the best of our knowledge, is superior to previously reported TiO2-based photoanodes in benign and neutral media. In addition, the method presented here can be extended to the preparation of other efficient photoanodes (WO3/rGO/NiFe-LDH and α-Fe2O3/rGO/NiFe-LDH) toward high level PEC performance.

How to develop cost-effective electrocatalysts for the oxygen evolution reaction (OER) is one of the critical issues in renewable energy storage and conversion technology. Here, we report the preparation of well-ordered ultrathin film (UTF) electrodes based on layered double hydroxide nanosheets (LDH NSs) and iron porphyrin (Fe-PP) through an electrostatic layer-by-layer (LBL) technique, which show excellent OER performance. By virtue of the high catalytic activity of LDH NSs and good electron-transfer ability of Fe-PP, the resulting CoNi-LDH NS/Fe-PP UTF exhibits a remarkably low overpotential (264 mV) to attain an OER current density of 10 mA cm−2 and a substantially decreased Tafel slope of 37.6 mV dec−1, much superior to that of the IrO2 catalyst. Moreover, this method can be extended to the preparation of other UTFs based on LDHs and Fe-PP (e.g., CoMn-LDH NS/Fe-PP, CoFe-LDH NS/Fe-PP and ZnCo-LDH NS/Fe-PP) with significantly enhanced OER performance relative to pristine LDH NSs. To illustrate the advantage of these UTFs in practical water splitting, a prototype electrolyzer cell is also fabricated by using the (CoNi-LDH/Fe-PP)30 UTF as the anode and Pt wire as the cathode, which achieves the production of both oxygen and hydrogen by using a 1.5 V AA battery as the power source.

Recently, layered double hydroxides (LDHs) have emerged as highly active photocatalysts due to their unique structure, large specific surface area and semiconductor properties. However, the slow interfacial kinetics and fast charge recombination are the major obstacles which limit the performance of LDH-based photocatalysts. Here, we demonstrate the doping of rare earth ions into the host layer of LDHs to inhibit the charge recombination and increase the charge injection efficiency simultaneously. A series of terbium ion (Tb3+) doped ZnCr–LDHs (Tb-ZnCr–LDHs) have been successfully synthesized via a co-precipitation method, and their photocatalytic water splitting activities were evaluated under visible light irradiation. The sample with a Tb3+ doping content of 0.5% (molar ratio) shows optimal performance for oxygen evolution (1022 μmol h−1 g−1) among all these Tb-ZnCr–LDH materials. The photoluminescence and photoelectrochemistry measurements over the Tb-ZnCr–LDH samples prove effective separation of photo-induced charge carriers and high charge injection efficiency, compared with a pristine ZnCr–LDH. This strategy can be applied to modify other photocatalysts toward low-cost solar fuel generation systems.

•Well-defined carbon-based network has been successfully synthesized.•The obtained carbon materials have tunable nanostructure and active site.•The optimal carbon catalyst exhibits highly satisfactory ORR activity.Electrocatalysts for oxygen reduction reaction (ORR) play a key role in renewable energy technologies including metal-air batteries and fuel cells. Despite tremendous efforts, the development of ORR electrocatalysts with high activity and low cost remains a great challenge. Here, we report the fabrication of well-defined carbon network with honeycomb-like structure as a high-performance catalyst toward ORR, via pyrolysis of bimetallic Co, Zn-zeolitic imidazolate (Co, Zn-ZIF) crystal arrays grown on the surface of layered double hydroxide nanoplatelets (LDHs@Co, Zn-ZIF). The concentration of doped-heteroatoms (N, Co), the graphitic degree as well as the surface porosity of the resulting carbon network can be finely controlled by tuning the Co/Zn molar ratio in the LDHs@Co, Zn-ZIF precursors. The optimal carbon catalyst (CoZn-2) exhibits excellent ORR activity with an onset potential of 0.976 V vs. RHE and a limited current density of 5.8 mA cm−2, which is superior to commercial Pt/C catalyst. In addition, both an extraordinary long-term stability (∼99.5% current retention over 20,000 s) and a strong tolerance against methanol corrosion are also obtained. This work demonstrates an effective strategy to artificially regulate the nanostructure and intrinsic active site of carbon-based ORR electrocatalysts.A well-defined two dimensional carbon network with controllable heteroatom doping, graphitic degree and porosity has been successfully fabricated by pyrolysis of bimetallic Co, Zn-ZIF crystal arrays grown on the surface of LDHs nanoplatelets, which exhibits excellent ORR performances. This work demonstrates an effective strategy to artificially regulate the nanostructure and intrinsic active site of carbon-based ORR electrocatalysts.

The exploitation of highly efficiency and low-cost electrocatalysts toward oxygen evolution reaction (OER) is a meaningful route in renewable energy technologies including solar fuel and water splitting. Herein, NiFe-layered double hydroxide (NiFe-LDH) hollow microsphere (HMS) was designed and synthesized via a one-step in situ growth method by using SiO2 as a sacrificial template. Benefiting from the unique architecture, NiFe-LDH HMS shows highly efficient OER electrocatalytic activity with a preferable current density (71.69 mA cm–2 at η = 300 mV) and a small onset overpotential (239 mV at 10 mA cm–2), which outperforms the 20 wt % commercial Ir/C catalyst. Moreover, it exhibits a remarkably low Tafel slope (53 mV dec–1) as well as a satisfactory long-time stability. Electrochemical studies reveal that this hierarchical structure facilitates a full exposure of active sites and facile ion transport kinetics, accounting for the excellent performance. It is expected that the NiFe-LDH microsphere material can serve as a promising non-noble-metal-based electrocatalyst toward water oxidation reaction.Keywords: electrocatalysts; hollow microsphere; layered double hydroxide; non-precious-metal hydroxide; oxygen evolution reaction;

A new electrochemical synthesis route was developed for the fabrication of Fe-containing layered double hydroxide (MFe-LDHs, M = Ni, Co and Li) hierarchical nanoarrays, which exhibit highly-efficient electrocatalytic performances for the oxidation reactions of several small molecules (water, hydrazine, methanol and ethanol). Ultrathin MFe-LDH nanoplatelets (200–300 nm in lateral length; 8–12 nm in thickness) perpendicular to the substrate surface are directly prepared within hundreds of seconds (<300 s) under cathodic potential. The as-obtained NiFe-LDH nanoplatelet arrays display promising behavior in the oxygen evolution reaction (OER), giving rise to a rather low overpotential (0.224 V) at 10.0 mA cm−2 with largely enhanced stability, much superior to previously reported electro-oxidation catalysts as well as the state-of-the-art Ir/C catalyst. Furthermore, the MFe-LDH nanoplatelet arrays can also efficiently catalyze several other fuel molecules’ oxidation (e.g., hydrazine, methanol and ethanol), delivering a satisfactory electrocatalytic activity and a high operation stability. In particular, this preparation method of Fe-containing LDHs is amenable to fast, effective and large-scale production, and shows promising applications in water splitting, fuel cells and other clean energy devices.

•Hierarchical ZnO nanorod@nanoplatelet (ZnONR@NP) core-–shell nanoarrays were fabricated by an in situ growth method.•The surface of ZnO NR@NP was further modified by plasmonic Au nanoparticles via a photo-reduction strategy.•The resulting Au–ZnO NR@NP nanoarray exhibits promising behavior in photoelectrochemical (PEC) water splitting.•Density functional theory (DFT) calculation confirms the synergistic effect between Au and ZnO.Au nanoparticles sensitized ZnO nanorod@nanoplatelet (NR@NP) core–shell arrays have been synthesized via a facile hydrothermal method followed by a further modification using Au nanoparticles. The resulting Au–ZnO NR@NP nanoarray exhibits promising behavior in photoelectrochemical (PEC) water splitting, giving rise to a largely enhanced photocurrent density, photoconversion efficiency as well as incident-photon-to-current-conversion efficiency (IPCE), much superior to those of pristine ZnO nanorods arrays and ZnO NR@NP. This is attributed to the coordination of ZnO core–shell hierarchical nanostructure and the surface-plasmon-resonance effect of Au nanoparticles, which facilitates the exposure of active sites and utilization of visible light. Density functional theory (DFT) calculations further confirm that the photogenerated electrons of ZnO transfer to Au, which suppresses the recombination of electron–hole pairs. Therefore, this work provides a facile and cost-effective strategy for the construction of hierarchical metal/semiconductor nanoarrays, which can be potentially used in the field of energy storage and conversion.Au nanoparticles sensitized ZnO nanorod@nanoplatelet (NR@NP) core–shell arrays were synthesized via a facile hydrothermal method followed by a further modification using Au nanoparticles. The resulting Au–ZnO NR@NP nanoarray exhibits promising behavior in photoelectrochemical (PEC) water splitting, giving rise to satisfactory photocurrent density, photoconversion efficiency as well as incident-photon-to-current-conversion efficiency (IPCE).

•TiO2/ZnFe-LDH photoanode is synthesized by photo-assisted electrodeposition of ZnFe-LDH on TiO2 nanoarrays.•The obtained TiO2/ZnFe-LDH photoanode exhibits largely promoted performances in the PEC water splitting.•This work demonstrates a strategy to modify the semiconductor photoanode.A highly-matched semiconductor/cocatalyst is crucial to enhance the bulk charge separation and surface reaction kinetics of the photoelectrode in the solar water splitting system. In this work, well-aligned, hierarchical zinc-iron layered double hydroxide (LDH) is in situ synthesized on the surface of TiO2 by a facile and effective photo-assisted electrodeposition (PED) method. An experimental-computational combination study reveals that the photogenerated holes of TiO2 tend to travel to ZnFe-LDH which enhances the bulk charge separation; while ZnFe-LDH acts as a cocatalyst which accelerates the surface water oxidation reaction. The resulting TiO2/ZnFe-LDH-PE photoanode exhibits a largely enhanced PEC performance: the photocurrent density at 1.0 V vs. RHE is 2.29 and 1.31 times higher than that of the pristine TiO2 and TiO2/ZnFe-LDH-E (prepared by a conventional electrosynthesis method) photoanode, with 150 mV and 50 mV of negative shift for onset potential. This can be ascribed to the enhanced interface interaction and highly-matched band structure between ZnFe-LDH and TiO2. It is expected that this strategy can be extended to other heterostructures for advanced performance in the fields of energy conversion and storage.Well-aligned zinc-iron layered double hydroxide (ZnFe-LDH) has been in situ synthesized on the surface of TiO2 by the photo-assisted electrodeposition method. The highly-matched semiconductor (TiO2)/cocatalyst (ZnFe-LDH) system displays much enhanced efficiency in photoelectrochemical water splitting.

Photoelectrochemical (PEC) water oxidation has received considerable attention owing to its key role in the overall water splitting. In this work, several reduced titania@layered double hydroxide (CoAl–LDH, CoCr–LDH, and CoFe–LDH) hybrid photoanodes were fabricated via electrochemical deposition of LDH on the reduced titania, and their PEC properties for water oxidation were studied systematically. The reduced titania@CoCr–LDH photoanode shows a much improved PEC performance compared with pristine reduced titania, with a photocurrent density enhancement of 43% (from 0.65 mA cm−2 to 0.93 mA cm−2) and an onset potential decrease of 21% (from 0.23 V to 0.18 V vs. the RHE). This improvement is also successfully demonstrated in the reduced titania@CoAl–LDH and reduced titania@CoFe–LDH system. The photoconversion efficiency of reduced titania is significantly enhanced after the incorporation of LDH (0.42–0.51% at ∼0.46 V vs. the RHE). Both the experimental studies and DFT calculations confirm a synergistic effect between the reduced titania and LDH. The results show that a good match of the band structure facilitates the fast electron–hole separation and the migration of holes from reduced titania to LDH, followed by the LDH catalyzed water oxidation. The CoCr–LDH has the highest driving force for oxygen evolution among these LDHs, accounting for the optimal PEC performance of the reduced titania@CoCr–LDH photoanode.

A new electrochemical synthesis route was developed for the fabrication of Fe-containing layered double hydroxide (MFe-LDHs, M = Ni, Co and Li) hierarchical nanoarrays, which exhibit highly-efficient electrocatalytic performances for the oxidation reactions of several small molecules (water, hydrazine, methanol and ethanol). Ultrathin MFe-LDH nanoplatelets (200–300 nm in lateral length; 8–12 nm in thickness) perpendicular to the substrate surface are directly prepared within hundreds of seconds (<300 s) under cathodic potential. The as-obtained NiFe-LDH nanoplatelet arrays display promising behavior in the oxygen evolution reaction (OER), giving rise to a rather low overpotential (0.224 V) at 10.0 mA cm−2 with largely enhanced stability, much superior to previously reported electro-oxidation catalysts as well as the state-of-the-art Ir/C catalyst. Furthermore, the MFe-LDH nanoplatelet arrays can also efficiently catalyze several other fuel molecules’ oxidation (e.g., hydrazine, methanol and ethanol), delivering a satisfactory electrocatalytic activity and a high operation stability. In particular, this preparation method of Fe-containing LDHs is amenable to fast, effective and large-scale production, and shows promising applications in water splitting, fuel cells and other clean energy devices.

Recently, layered double hydroxides (LDHs) have emerged as highly active photocatalysts due to their unique structure, large specific surface area and semiconductor properties. However, the slow interfacial kinetics and fast charge recombination are the major obstacles which limit the performance of LDH-based photocatalysts. Here, we demonstrate the doping of rare earth ions into the host layer of LDHs to inhibit the charge recombination and increase the charge injection efficiency simultaneously. A series of terbium ion (Tb3+) doped ZnCr–LDHs (Tb-ZnCr–LDHs) have been successfully synthesized via a co-precipitation method, and their photocatalytic water splitting activities were evaluated under visible light irradiation. The sample with a Tb3+ doping content of 0.5% (molar ratio) shows optimal performance for oxygen evolution (1022 μmol h−1 g−1) among all these Tb-ZnCr–LDH materials. The photoluminescence and photoelectrochemistry measurements over the Tb-ZnCr–LDH samples prove effective separation of photo-induced charge carriers and high charge injection efficiency, compared with a pristine ZnCr–LDH. This strategy can be applied to modify other photocatalysts toward low-cost solar fuel generation systems.

How to develop cost-effective electrocatalysts for the oxygen evolution reaction (OER) is one of the critical issues in renewable energy storage and conversion technology. Here, we report the preparation of well-ordered ultrathin film (UTF) electrodes based on layered double hydroxide nanosheets (LDH NSs) and iron porphyrin (Fe-PP) through an electrostatic layer-by-layer (LBL) technique, which show excellent OER performance. By virtue of the high catalytic activity of LDH NSs and good electron-transfer ability of Fe-PP, the resulting CoNi-LDH NS/Fe-PP UTF exhibits a remarkably low overpotential (264 mV) to attain an OER current density of 10 mA cm−2 and a substantially decreased Tafel slope of 37.6 mV dec−1, much superior to that of the IrO2 catalyst. Moreover, this method can be extended to the preparation of other UTFs based on LDHs and Fe-PP (e.g., CoMn-LDH NS/Fe-PP, CoFe-LDH NS/Fe-PP and ZnCo-LDH NS/Fe-PP) with significantly enhanced OER performance relative to pristine LDH NSs. To illustrate the advantage of these UTFs in practical water splitting, a prototype electrolyzer cell is also fabricated by using the (CoNi-LDH/Fe-PP)30 UTF as the anode and Pt wire as the cathode, which achieves the production of both oxygen and hydrogen by using a 1.5 V AA battery as the power source.