Update on Recent Designing Strategies of Transition Metal-Based Layered Double Hydroxides Bifunctional electrocatalysts

Zhengyang Cai,^a Ping Wang,^a* Junhe Yang ^a and Xianying Wang^a*

^aSchool of Materials Science and Technology, University of Shanghai for Science and Technology, Jungong Rd.516, 200093 Shanghai, China.

 

*Corresponding Author:

Email: ping.wang@usst.edu.cn; xianyingwang@usst.edu.cn

 

ABSTRACT

Fabrication of efficient, stable and cost-effective bifunctional electrocatalysts to achieve overall water splitting reaction (OWS) has become a task of high priority, in view of the urgent need for the promotion of renewable energy-based hydrogen production systems. Transition metal-based layered double hydroxides (LDHs) as one of promising bifunctional electrocatalysts have been intensely studied, due to the advantages of unique 2D layered structure and outstanding physicochemical properties. Herein, this review aims to summarize the recently advanced strategies on design of LDHs, including nanostructuring, conductive support-based hybrid, defect engineering, anion intercalation, cation doping and related derivatives. Particularly, a thorough literature overview on OWS performance improvement of LDHs is appraised detailedly. The discussions on challenges and future directions are provided, which will shed light on development of better LDHs bifunctional electrocatalysts and exploration of efficient device units for practical applications.

Table of Content

This review is mainly focused on the recent advances on designing strategies of LDHs bifunctional electrocatalysts for overall water splitting.

 

Keywords: Layered Double Hydroxides; Bifunctional Electrocatalysts; Hydro

4.1  Nanostructuring

     Fabrication of hierarchical nanostructures or ultrathin nanosheets is of great benefit to maximize the number of active sites via increase of surface areas, and thus facilitate electrolyte diffusion and gases desorption.^{78, 114} Hu and co-workers meticulously designed a hierarchical Ni-Co-P hollow nanobrick (HNBs) comprised of oriented nanosheets (Fig. 2a and b).⁸² By employing 3D Ag₂WO₄ anisotropic cuboids as sacrificial templates, HNBs were fabricated by growth of oriented 2D Ni-Co nanosheets precursor and subsequent etching/phosphorization treatments (Fig. 2c). The unique hierarchical and hollow structures were favorable for OWS performance enhancement of HNBs, as a result of significant increase in the exposed surface area and abundant mass diffusion pathways. Moreover, it was found that small amount of Ag residual also contributed to the increase in catalytic activity, based on the density functional theory (DFT) calculations (Fig. 2d and e). Upon incorporation of Ag, the ΔG_H* became relatively close to 0 eV for the catalyst-H* state. It indicated that the faster proton/electron transfer/hydrogen release rate and the much larger water adsorption energy were realized, promoting HER processes.

Fig. 2 a) FESEM and  b) TEM images of Ni-Co-O HNBs, c) Schematic illustration of construction of hierarchical Ni-Co-P hollow nanobricks, d) Calculated free-energy change and e) water adsorption energies for Ni-Co-P and Ag-incorporated Ni-Co-P. Reproduced with permission from Ref. 82. Copyright 2018, The Royal Society of Chemistry.

 

  Another interesting strategy is to fabricate self-standing 3D core-shell LDHs nanostructures. Cu nanowires@NiFe LDH nanosheets with a 3D core/shell structure were successfully synthesized via electrodeposition by Yu and the co-workers. for highly efficient OWS.⁸⁰ Cu nanowires (NWs) cores supported on Cu foams were firslty prepared by three-step pre-synthesis procedures, namely chemical oxidation, calcination for phase transformation and electroreduction processes. Subsequently, few-layer NiFe LDH nanosheets were grown on Cu NWs cores (Fig. 3a). As shown in Fig. 3b and c, the NiFe LDH uniformly and vertically grew on Cu NWs, forming a core-shell structure. The Cu@NiFe LDH electrocatalyst exhibited excellent OWS performance, as listed in Table 2. It was attributed to the synergistic combination of NiFe LDHs and Cu NWs, which increased the edge sites exposure of LDHs and facilitated desorption of gases product, but also ensured good conductivity and mechanical stability.

    Besides, an alternative approach to boost the OWS performance is to exfoliate bulk LDHs into ultrathin nanosheets. Liu and co-workers exfoliated CO₃^2- intercalated CoFe LDHs anions (CoFe LDH-C) in DMF-ethanol mixed solvents.⁸¹ The resulted CoFe LDH-F possessed much smaller thickness of ~4.5 nm, compared to that of CoFe LDH-C (~15 nm) (Fig. 3d and e). During the exfoliation process, plentiful of coordination-unsaturated transition metals associated with oxygen vacancies were created, as demonstrated by X-ray photoelectron spectroscopy (XPS) and electron spin resonance (ESR) spectroscopy (Fig. 3f and g). The generated oxygen vacancies can decrease energy barrier for adsorption of OH^- anions, because of the low-coordination sites of MO₆ and improved intrinsic electronic conductivity, thus leading to small OER and HER overpotentials.

 Fig. 3 a) Schematic illustration of the fabrication procedures of the self-standing 3D core-shell Cu@NiFe LDH electrocatalysts. (RT is denoted by room temperature.) b-c) SEM and TEM images of Cu@NiFe LDH. Reproduced with permission from Ref. 80. Copyright 2017, The Royal Society of Chemistry. d) TEM, AFM images and height profiles of CoFe LDH-C. e) TEM, AFM images and height profiles of CoFe LDH-F. f-g) O 1s XPS spectra and EPR spectra of the CoFe LDH-C and CoFe LDH-F samples, respectively. Reproduced with permission from Ref. 81. Copyright 2016, American Chemical Society.

 

4.2 Conductive support-based hybrids

     Self-supporting electrocatalysts on conductive substrates are an attractive approach for enhancing the number of catalytic active sites and the charge transfer ability, but also simplifying the electrode preparation process.^{107, 115-118} Hou et al. reported a ternary EG (exfoliated graphene foil)/Co_0.85Se/NiFe-LDH hybrid (Fig. 4a).⁹² The field emission scanning electron microscopy (FESEM) images show the thickness of NiFe-LDHs grown on EG/Co_0.85Se nanoarrays can be reduced to be about 10 nm (Fig. 4b-e). The catalyst with a high surface area of 156 m²·g^-1 can achieve a current density of 20 mA·cm^-2 at an external voltage of 1.71 V for OWS in a two-electrode cell (Table 2.). It was attributed to the strong coupling effect of the three components, ensuring low charge transfer resistance and fast vectorial electron-transport. Especially, the 3D layered structure could offer large amounts of active sites and accelerate gas bubbles release.

Fig. 4 a) Schematic illustration for the synthesis process of EG/Co_0.85Se/NiFe-LDH. b-c) FESEM images of EG/Co_0.85Se, Inset: the enlarged FESEM image of EG/Co_0.85Se. d-e) FESEM images of EG/Co_0.85Se/NiFe-LDH. Reproduced with permission from Ref. 92. Copyright 2016, The Royal Society of Chemistry.

 

   Taking the plentiful advantages of defective graphene (DG), like fast electron transfer kinetics, excellent stability and specific defect types, DG has been widely employed for OER and HER. Jia et al. reported the exfoliated NiFe LDHs nanosheets (NS)/DG by electrostatic stacking of positively charged NiFe LDH NS and negatively charged DG (Fig. 5a).⁹³ The different types of DG defects derived by the removal of heteroatoms from graphene acted as active sites, which can directly capture the transition metal atoms via strong π-π interaction and thus contribute to activities improvement for three basic electrochemical reactions (e.g., ORR, OER and HER). On the other hand, the charge distribution on the hybrid heterostructure was demonstrated by DFT calculation study (Fig. 5b and c). The electrons were re-distributed after assembling of NiFe LDH-NS and DG, and accumulated at the defect sites (blue dash frames in Fig. 5c). Therefore the outstanding OER and HER activities can be ascribed to the exfoliated NiFe LDH and the charge conductivity of DG.

    Carbon nanotubes (CNTs) are also an effective substrate to enhance the electron transfer of LDHs. The electronegative oxygen functional groups on surface of CNTs can promote efficient dispersion of catalysts. Huang and co-workers constructed 3D nicke-cobalt bimetallic phosphide anchored on CNTs (NiCo₂P_x/CNTs). The catalyst exhibited enhanced catalytic performance toward both HER and OER (Fig. 5d). ⁹⁴ The high electrocatalytic activity was attributed to the synergistic effect between NiCo₂P_x and CNTs (Fig. 5e).

Fig. 5 a) Schematic illustration of the preparation of NiFe LDH-NS@DG nanocomposite. b) The top views of optimized Ni-Fe LDH-NS@DG (DG-5, DG-585, or DG-5775) based composite interfaces. c) The side views of 3D charge density difference plot for the interfaces between a defective graphene sheet (DG-5, DG-585 or DG-5775) and a Ni-Fe LDH-NS layer. Yellow and cyan isosurfaces represent the charge accumulation and depletion layers in the 3D space with an isosurface value of 0.002 e Å^-3. Green, brown, silver, and red balls represent C, Fe, Ni, and O atoms, respectively. The different defect types and associated enhanced charge density areas are marked with a blue solid line and a blue dash line, respectively. Reproduced with permission from Ref.  93. Copyright 2017, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. d) Schematic illustration of the synthetic route for NiCo₂P_x/CNTs. e) Mechanism and the synergetic effect of NiCo₂P_x/CNTs for both the HER and OER. Reproduced with permission from Ref. 94. Copyright 2018, The Royal Society of Chemistry.

 

4.3. Defect engineering

     In recent years, much attention has been paid to the defect engineering of LDHs,^{71, 114, 119-122} i.e., creation of oxygen vacancies, cation vacancies and their mixture. Liu et al. in 2018 firstly proposed synthesis of ultrathin feroxyhyte nanosheets (δ‐FeOOH NSs) with richful Fe vacancies (V_Fe).⁹⁵ The origin of the superior HER and OER reactivity of δ‐FeOOH NSs was clearly validated. Introduction of V_Fe led to obvious upshift on the Fe K absorption edge of δ‐FeOOH NSs (Fig. 6a), showing a reduction of the electron density of Fe atoms in NSs. As seen from the R space plot (Fig. 6b), first Fe-O shell was calculated to be a coordination number (N) of 5.5, which was close to the theoretical value of 6 for an octahedral and the second Fe-Fe shell in NSs was reduced by 20% to 1.6 in comparison with 2 for the bulk. It indicated the existence of rich V_Fe in δ‐FeOOH NSs. It was also reflected by the upshift of Fe 2p peaks in δ‐FeOOH NSs (Fig. 6c). The formation of V_Fe can activate the second neighboring surface Fe atom (Fe2 site). The DFT results also demonstrated that the Fe2 site had a small ΔG_H*, giving rise to the enhanced HER performance of δ‐FeOOH NSs (Fig. 6d). The Fe2 site acted as the OER active site, which possessed the smallest rate-determining ΔG value (Fig. 6e) and the four primitive OER steps on the electrocatalyst surface were proposed, as shown in Fig. 6f.

Fig. 6 a) Fe  K-edge XANES spectra. b) the corresponding Fourier transformations of Fe K-edge EXAFS spectra for bulk δ-FeOOH and δ-FeOOH NSs. c) The high-resolution XPS spectra of Fe 2p of bulk δ-FeOOH and δ-FeOOH NSs. d) Standard free energy diagram of the HER process on the O and Fe in δ-FeOOH NSs without V_Fe (marked as δ-FeOOH (001)) and the O neighboring to V_Fe and the second neighboring Fe to V_Fe (Fe2) in δ-FeOOH NSs with V_Fe (marked as V_Fe). e) Standard free energy diagram of the OER processes on the δ-FeOOH (001), and Fe1 and Fe2 sites on δ-FeOOH (001) NSs with V_Fe. Fe1 indicates the first neighboring Fe to V_Fe, and Fe2 indicates the second neighboring Fe to V_Fe. f) Primitive steps of the OER process on the surface of δ-FeOOH NSs with V_Fe (top view). The Roman numerals represent (i) adsorption step, (ii and iii) dissociation steps, and (iv) desorption step during the OER process. Brown: Fe; red: O. Reproduced with permission from Ref. 95. Copyright 2018, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

 

    Cheng et al. developed a new type of 3D metal-vacancy-solid-solution NiAlP nanowall array (NiAl_δP) for all-pH OWS by combination of selective alkali-etching and phosphorization methods.⁹⁶ Numerous nanoholes were well distributed throughout each nanowall forming a unique porous structure and massive atomic vacancies are observed over the lattice of the NiAlP nanowalls (Fig. 7b and c). The XPS spectra revealed that the existence of surface aluminumvacancy led to a positive shift of 0.6 eV for Ni 2p_3/2 and negative shift of 0.3 eV for P 2p (Fig. 7d and e). The theoretical calculations indicated that the NiAl_δP possessed a low ΔG_H* value -0.09 eV (close to that of Pt, ~0 eV) and the highest H₂O adsorption energy, favoring the water adsorption (Fig. 7f). The oxidized Ni and negatively charged P in NiAl_δP might act as the active sites for OER and HER, respectively.

Fig. 7 a-c)   SEM, TEN and HRTEM images for the NiAl_δP nanowall array. d-e) Ni 2p and P 2p XPS spectra. f) Theoretical H* adsorption Gibbs free energy and H₂O* adsorption energy for NiAlP, NiAl_δP, and Pt or RuO₂. Reproduced with permission from Ref. 96. Copyright 2018, The Royal Society of Chemistry.

 

4.4. Ion intercalation

   In addition to spontaneously inserting different anions or molecules into LDHs layers during preparation processes, the interlayer anions can be further replaced by using ion-exchange methods. It can enlarge the interlayer spacing, but also can exfoliate the bulk LDHs into monolayer or few-layer nanosheets which can increase the number of active sites and thus enhance catalytic activity.¹²³ Li et al. demonstrated an in-situ intercalation method to increase the interlayer spacing of NiFe LDHs (Fig. 8a).¹⁰¹ As shown in Fig. 8b, XRD pattern shows that after formamide intercalation reaction at 80 °C for 3 h, the (003) diffraction peak of NiFe LDH shifted from 11.6° to t 9.3° (NiFe LDH-80 °C), corresponding to the increase of interlayer distance from 7.8 to 9.5 Å. With the assistance of ultrasound, the interlayer distance can increase to 9.3 Å at 30 ℃ for only 10 min (NiFe LDH-US-30 °C). The extended interlayer spacing offered more inner active sites and more space for diffusion of the reactants, the OER overpotential to achieve the current density of 10 mA·cm^-2 was reduced to 210 and 203 mV for NiFe LDH-80 °C and NiFe LDH-US-30 °C, compared to that of 256 mV for pristine NiFe LDH. Similarly, Na intercalated NiFeO_x (Na_1−xNi_yFe_1−yO₂) was also studied (Fig. 8c), consisting of [MO₆] (M = Ni, Fe) octahedral layers with Na atoms residual lying between the octahedral layers.¹⁰² The existence of Na led to transformation of Ni and Fe into high chemical states (Fig. 8d and e). Moreover, the extraction of Na gave rise to the exposure of more [MO₆] active sites, leading to the improved activity. However, the excessive Na intercalation will deteriorate the structural stability.

Fig. 8 a) Schematic diagram of in-situ intercalation process over electrodeposited NiFe LDH nanosheets on substrate. b) XRD patterns of the fresh NiFe LDH coated electrode (black) and intercalated NiFe LDH coated electrodes at 80 °C (red) and 30 °C with ultrasound (blue).Reproduced with permission from Ref. 101. Copyright 2017, Elsevier B.V. c) Crystal structure of O3-type NaNi_yFe_1-yO₂. d-e) Ni 2p and Fe 2p XPS spectra. Reproduced with permission from Ref. 102. Copyright 2017, The Royal Society of Chemistry.

 

4.5. Cation doping

    Cation doping is considered to be an effective pathway for increasing OWS activity, since doping of the third metal ion can effectively adjust the electronic configuration and conductivity of LDHs and generate synergistic effect between metal ions and LDHs layers.^124-126 Chen and co-workers in 2018 reported a novel strategy to accelerate HER kinetics of NiFe LDH by Ir⁴⁺-doping (NiFeIr LDHs). The water dissociation process (Volmer step) can be accelerated, thus leading to a robust catalytic activity for OWS.¹⁰⁴ The Tafel slope of NiFe LDH was estimated to be about 125 mV·dec^-1 (Fig. 9a), indicating that the Volmer reaction (H₂O + e^- → H_ad + OH^-) was the rate-determining step. While NiFeIr LDH possessed a much smaller Tafel slope of 32 m V·dec^-1, in accordance with the Volmer-Tafel mechanism (H₂O + e^- → H_ad + OH^-, 2H_ad → H₂).

Fig. 9 a) Tafel slopes of NiFeIr LDH (violet), NiFe LDH (green) and Pt/C (black). Reproduced with permission from Ref. 104. Copyright 2018, The Royal Society of Chemistry. b) Schematic fabrication process and the resulting microstructures for hollow Mo-CoP nanoarrays. c) HER free energy diagrams for the P- and Co-sites on pristine and Mo-doped CoP (111) surfaces. d) Standard free energy diagrams for the OER path on pristine (green) and Mo-doped (orange) β-CoOOH (001) surfaces. e) Bader charge analysis for surface Co ions and H in HO*adsorption on pristine and Mo-doped β-CoOOH (001) surfaces. Reproduced with permission from Ref. 105. Copyright 2018, Elsevier B.V.

 

    Rational design of hollow Mo-doped CoP (Mo-CoP) nanoarrays was recently proposed by Guan et al. (Fig. 9b).¹⁰⁵ DFT calculation results show the ΔG_H* value of the P-site on Mo-CoP (111) surface dropped dramatically to 0.07 eV, indicating much superior HER performance (Fig. 9c). For OER (Fig. 9d) the potential limiting step (PLS) for β-CoOOH was determined to be the deprotonation step (HO* → O* + H⁺ + e^-). Accordingly, the ΔG₂ value was reduced from 1.95 eV to 1.82 eV by doping of Mo. As also evidenced by the reduced Bader charge of the Co and H (Fig. 9e), the electrons will transfer from Mo dopants to the nearby Co species and the H in HO* adsorption. The Mo-induced electron transfer towards the H can reduce the strength of the H-O bond in adsorbed HO*, thus facilitating the PLS process and enhancing the OER activity.

Fig. 10 a) HRTEM images of Se-(NiCo)S/OH nanosheets. Reproduced with permission from Ref. 108. Copyright 2018, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. b) Schematic representation for synthetic procedure of FeCoNi-HNTAs. c) FESEM images of FeCoNi-HNTAs, Scale bars: 2 μm and inset: 200 nm. d-f) Normalized Fe, Co, and Ni L-edge sXAS spectra in the total electron yield (TEY) mode. g) Under-electrolyte Gas bubble adhesive force measurements of FeCoNi-HNTAs. Reproduced with permission from Ref. 107. Copyright 2018, Springer Nature Publishing AG.

 

4.6. Other LDHs derivatives

    Besides the above-mentioned design strategies, fabrication of LDHs derivatives is also of great interest by combination of two or multi design strategies for modification of LDHs (e.g. using multi-element doping and unique morphology design). Based on the theoretical expectations, transition metal chalcogenides (Se-(NiCo)S_x/(OH)_x nanosheets) for highly efficient OWS was demonstrated by Hu et al.¹⁰⁸ A large amount of Se-O and S-O bonds can be formed in the nanosheets, which accelerated the charge transfer, and introduced abundant disordering and defects (Fig. 10a). Taking consideration of the remarkable electrocatalytic HER activity of metallic monoclinic 1T phase MoS₂ and excellent OER activity of trimetallic cobalt/iron/nickel-based (Co, Fe, Ni-based) sulfides, Li and co-workers reported synthesis of hybrid nanotube arrays (FeCoNi-HNTAs) composed of crystalline structures of 1T MoS₂ and (Co, Fe, Ni-based)₉S₈ (Fig. 10b and c). ¹⁰⁷ The prominent activity was attributed to systematic optimization of chemical composition and geometric structure. Interestingly, the synergistic effects among Fe, Co and Ni ions were proved by aid of L-edge soft X-ray absorption spectroscopy (sXAS). Friebel et al.¹²⁷ further demonstrate that the Fe³⁺ indeed acted as the highly active sites for OER. The observation indicated that the Co ions can change the electronic state of Fe from Fe²⁺ to Fe³⁺ (Fig. 10d). The crystal-field coordination of Co ions can be tuned by Fe ions, that Co³⁺ ions located at octahedral sites (the small shoulder at 780.4 eV in FeCoNi-HNTAs) are the active centers for OER rather than the Co²⁺ ions at tetrahedral sites (778.2 eV) (Fig. 10e). And the XPS peak shoulder was also observed, indicating the existence of Ni³⁺ as the OER active sites (Fig. 10f). Furthermore, the FeCoNi-HNTAs electrodes showed prominent superaerophobicity and superhydrophilicity with no gases bubble adhesive force, which was greatly beneficial for gases evolution reaction and mass transfer (Fig. 10g).

 

4.7. Relevant applications

     Although the electrocatalytic water electrolysis is regarded as a promising pathway for hydrogen production, the main obstacle remains the high energy consumption for potential practice. It is suggested that the effective, reliable and sustainable solution is to combine high-efficiency bifunctional OWS electrocatalysts with renewable energy sources, such as solar energy, wind energy, hydropower, etc. The renewable energy can be converted into hydrogen energy for storage, and then burn directly as fuel or used in more efficient fuel cell devices for electricity generation.

     Many efforts have been devoted to exploiting the renewable energy-conversion systems, especially solar energy-based water splitting (Fig. 11).^{93, 128-130} Luo and co-workers successfully designed an efficient and intrinsically safe bias-free solar-driven water-splitting device composed of perovskite light harvesters, non-precious metal based catalysts, and a bipolar membrane (Fig. 11a).¹³⁰ And a current density of 10 mA·cm^-2 was obtained at the bias voltage of 1.63 V for electrochemical water splitting and the solar to hydrogen conversion efficiency is up to 12.7%. Other interesting case is to design hybrid LDHs via combined with semiconductor materials (like hematite,¹³¹ BiVO₄,¹³² g-C₃N₄,^{133, 134} etc.) for photoelectrocatalytic water splitting. For instance, Yan’s group successfully fabricated a triadic photoanode consisting of dual-sized CdTe quantum dots (QDs), Co-based layered double hydroxide (LDH) nanosheets and BiVO₄ particles (QD@LDH@BiVO₄). The catalyst possessed a remarkably enhanced PEC performance, achieving a current density of 2.23 mA·cm^-2 at 1.23 V vs. RHE.⁶⁹

Fig.11 a) Schematic diagram of the solar-driven water-splitting device composed of perovskite light harvesters, earth-abundant catalysts, and a bipolar membrane. Reproduced with permission from Ref. 130. Copyright 2016, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. b) Demonstration of a solar power assisted water-splitting device with a voltage of 1.5 V. Reproduced with permission from Ref. 93. Copyright 2017, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

 

5. Conclusions and outlook

      Although the LDHs have been widely investigated for solely electrocatalytic HER or OER, poor electrocatalytic performance of bulk LDHs restrict the practical applications, owing to the limited numbers of active sites with poor intrinsic activity and low electron conductivity. Moreover, it remains challenging to the design criteria of LDHs bifunctional electrocatalysts for OWS. Therefore, this review summarizes the recent advanced design strategies of LDHs as bifunctional electrocatalysts for OWS. In the review, The current cutting-edge technologies for rational design of LDHs and related mechanisms are provided: (1) Fabrication of nanostructures can improve the density of active sites and enhance mass transfer capacity; (2) In-situ growth of LDHs on various conductive supports can enhance charge transport capacity; (3) Manufacture of various defects can regulate the surface electronic configuration of LDHs to increase the active site types and intrinsic activity; (4) Small molecules or ion intercalation can expand the LDHs layer spacing to increase the number of active sites; (5) Cation doping can enhance the synergistic effect between different metal ions and thereby increase the intrinsic activity; (6) Development of related LDHs derivatives. These strategies not only improve the OER activity of LDHs, but also enhance their performance for HER, ultimately achieving the excellent OWS performance.

      Despite these considerable efforts, there are still some challenges: (1) DFT methods are powerful in catalytic mechanism analysis of LDHs. However, it is hard to identify the actual active sites and determine the synergistic effect in more complexed catalyst components systems. Theoretical evaluation techniques still need to be continuously improved; (2) It is also expected that in-situ or operando characterization techniques, such as ambient-Pressure XPS, in situ XANES and in-situ Raman, can be fully applied to support the real-time mechanism analysis of catalytic reactions. Therefore, it should be considered to include highly efficient and well-structured electrolyzer systems; (3) Almost all LDHs bifunctional electrocatalysts can only run in alkaline environments. It is necessary to explore new strategies to make LDHs suitable for neutral or even acidic environments.

 

Conflicts of interest

There are no conflicts to declare.

 

Acknowledgements

We greatly appreciate the financial supports from the National Natural Science Foundation of China (51572173, 51602197, 51771121, 51772297 and 51702212), Shanghai Municipal Science and Technology Commission (16060502300, 16JC402200 and 18511110600), Shanghai Academic/Technology Research Leader Program (19XD1422900) and Shanghai Eastern Scholar Program (QD2016014).

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