Search for articles:
Mengmeng Zhang
, Cheng Zhong
Corresponding authors:
Received: 2019-06-5
Revised: 2019-07-19
Accepted: 2019-07-23
Online: 2020-02-01
Copyright: 2020 Editorial board of Journal of Materials Science & Technology Copyright reserved, Editorial board of Journal of Materials Science & Technology
More
Abstract
Surface/interface engineering plays an important role in improving the performance and economizing the cost and usage of electrocatalysts. In recent years, substantial progress has been achieved in designing and developing highly active electrocatalysts with the deepening understanding of surface and interface enhanced mechanism. In this review, recent development about optimizing the surface and interfacial structure in promoting the electrocatalytic activity of noble-metals and transition metal compounds is presented and the chemical enhancements are also described in detail. The relationship between the surface/interface structures (both atomic and electronic configuration) and the electrochemical behaviors has been discussed. Finally, personal perspectives have been proposed, highlighting the challenges and opportunities for future development in tuning the surface/interface active sites of electrocatalysts. We believe that this timely review will be beneficial to the construction of highly active and durable electrode materials through optimizing surface atomic arrangement and interfacial interaction, which can largely promote the development of next-generation clean energy conversion technologies.
Keywords：
Environmental problems and energy issues are becoming an increasingly serious problem with the rapid development of the modern industry. The exploitation of high-efficient and cost-effective catalysts has received extensive attention for multifarious photo/electrochemical catalytic reactions [[1], [2], [3]]. In general, the catalytic properties of materials are strongly associated with the active sites exposed at the surface and interface. Thus, surface/interface regulation of noble and non-noble catalysts is crucial to improve the catalytic activity [4,5]. However, related summaries and explicit characterizations for the regulation of surface structures and interface features are deficient.
Recently, noble-metal catalysts (e.g., Ag, Au, Pt, Pd, and their alloys) have drawn considerable attention [6,7]. Numerous reports focus on tuning the size, shape, construction, composition, hybrid, and microstructure to adjust the surface texture of noble metal materials, thus enhancing their catalytic performances [[8], [9], [10]]. For instance, Pt nanomaterials with high-index facets can exhibit higher electrocatalytic activity [11]. Moreover, the revealed surfaces of catalysts are related to morphologies, and their catalytic activity for methanol oxidation increased in the order of nanosheet > nanoflower > prickly surface > smooth spherical [12]. The introduction of other metals (such as Au [13], Ir [14], Pd [15], and Ni [16]) into Pt is another effective strategy to perfect the exposed active sites, which can increase the availability of Pt and lower the price. However, the insufficient reserves and expensive cost limit the large-scale applications of precious metals. Therefore, pursuing abundant and cheap alternatives with high activity and stability catalysts are urgent, which can afford enough active sites for catalytic reactions [17,18].
Recent reports proposed that coupling noble-metals with metal compounds is a prospective approach to reduce price and boost the availability of noble-metals [19,20]. Furthermore, transition metal-based compounds are considered as one of the most promising candidates to replace noble metal catalysts with the advantage of abundance, including metal oxides, sulfides, selenides, phosphides, carbides and nitrides [21,22]. Taking spinel cobalt oxide (Co3O4) as an example, it has been widely studied as electrocatalysts owing to its relatively high activity and stability in the alkaline electrolyte as well as low cost [2]. Controlling its micro-nanostructure and surface-exposed crystal planes is favorable to maximize the exposure of active sites of Co3O4 [[23], [24], [25]]. Unfortunately, the catalytic activity of Co3O4 is hindered by the intrinsically poor conductivity. Several strategies such as introducing vacancy [26], doping heteroatoms [27] and coupling with conductive materials [28] are proposed to address this issue. What’s more, other metal compounds such as sulfides, selenides are also reported as the catalysts with high activity and better conductivity [29,30].
Recently, substantial progress has been achieved in the development of surface and interface engineering for diverse catalytic reactions. However, a comprehensive summary is scarce. Herein, an overview of surface engineering in noble-metals and surface/interface engineering in transitional metal compounds for representative electrocatalytic applications is provided. The introduction of surface/interface modulation has been divided into three parts: noble-metals, noble metals coupled with transitional metal compounds and transition metal materials. Moreover, the further specific elaboration of surface design and interfacial construction in each part is extended. Finally, challenges and prospects in the evolution of designing surface and interface to expose more active sites for electrolysis are discussed. This review aims at summarizing the recent achievements in surface/interface engineering of noble metal and transition metal-based electrocatalysts as well as the in-depth mechanism about the relationship between active structures and the electrochemical properties, providing guidance on the rational design and development of cheap, efficient and durable electrode materials for next-generation high-performance energy conversion technologies.
Noble metals, with unique electrochemical activity, have been utilized to catalyze HER [31], ethanol/formic acid oxidation (FAO) [9,32], ammonia oxidation [14,16,33], and methanol oxidation [12,34]. In general, the noble-metal atoms as catalytic sites in single noble-metal catalysts, which absorb intermediate products and reduction/oxidation reactions occur [35]. Meanwhile, the electronic effect is produced as the other introduced metals, resulting in the d-band center shift of noble-metals, and the metals with the d-band center more close to zero as catalytic sites [14,36]. However, the catalytic sites on surface maximally contact electrolyte and realize its catalytic ability. Therefore, the precise surface engineering of noble-metal nanoparticles (NPs) has practical significance for the improvement of catalytic activity. To enhance the utilization efficiency of noble metals with increased reactivity, four crucial factors including size, morphology, surface structure, and composition are applied to regulate the surface structure of noble metals.
The size effect was first proposed to be effective in tuning the catalytic activity of noble metal Au in 1997 [37], and the Au nanocatalyst smaller than 5 nm displays the best activity. Other works about size effect have been reported subsequently. For instance, the relationship between electrodeposition parameters (e.g., El means lower potential, t1 and tu represent the time of the lower and upper potentials) and Pt NPs size was clarified by Li et al. towards ammonia oxidation [38], as shown in Fig. 1(a) and (b). The high activity can be achieved when the size of Pt NPs locate at 5 nm (Fig. 1(c) and (d)), which resulted from the nanosize effect and more exposed active sites.
Fig. 1. SEM images of Pt nanoparticle/ITO electrodes electrodeposited at various parameters. (a1‒a4) Various tl (e.g. 0.01, 0.1, 1, and 2 s) at the constant of El (-0.6 V) and tu (0.1 s). (b1‒b4) Various tu (e.g. 0.01, 0.1, 1, and 2 s) with the constant of El (-0.8 V vs SCE) and tl (0.1 s). The Pt loading-normalized CV curves measured at the constant of El (0.8 V) and tu (0.1 s). (c) t1 equal to 0.1 s and 1 s in 0.5 M H2SO4 solution at 0.05 V s-1. (d) t1 equal to 0.1 s and 1 s and contrast samples, which are measured in 0.1 M NH3 + 1 M KOH solution at 0.01 V s-1. (Ref. [
Moreover, Zhou et al. observed a volcano curve between the specific surface activity (SSA) for ORR and the size of Pd NPs [39]. The maximum value is obtained at about 5.0 and 6.0 nm. The particle size effect is originated from the combination of the distribution changes of surface low index planes, the relative abundance of low coordination sites, and the electronic states of Pd. According to the size effect, Wang et al. synthesized polycrystalline Pd NPs with the size ranging from 10 to 60 nm via controlling the concentration of Pd precursor [9]. The results reveal that the polycrystalline Pd NPs with an average size of 24 nm exhibit the highest FAO activity and stability. Moreover, the electron transfer and CO tolerance are markedly improved during the FAO, which can be mainly attributed to the larger electrochemical surface area (ECSA), more appropriate inter-particle distance and boundary terrace.
Considerable reports reveal that surface structures [34], crystal planes [40], and atomic arrangements [16] are the critical parameters to improve the electrochemical activity of catalysts. According to the thermodynamical principle, the produced noble metals with special exposed crystal planes and atomic arrangements can be developed by minimizing surface area and total surface free energy. Therefore, additive agents and preparation methods can be used to modify the noble metals’ surface structures.
For instance, Zhang et al. verified that various facets exposed on the surface can be obtained by HCl oxidative etching with different concentrations [8], as shown in Fig. 2(a). The facet-enclosed Pd nanocubes possess much higher activity for FAO than that of octahedrons [2,41] (Fig. 2(b) and (c)). In addition, noble metal NPs exposed preferential crystal planes can be prepared via electrodeposition, which is a simple and clean method without any organic additives [33]. It is reported that the fraction of Pt (100) sites increases with increasing the current density of electrodeposition [33]. Hydrogen desorption characterization shows that electrochemical treatment is also a quick and sustainable technique to produce noble metals with preferential orientation [35,42]. Periodic square-wave voltage was employed to treat the Pt disk electrode, and the proportion of (100) sites in cube Pt NPs is significantly improved, which can be verified by the much more intense H desorption peaks corresponding to (100) step sites and terrace borders [35].
Fig. 2. (a) Schematic illustration of shape evolution of the Pd nanocrystal and the corresponding TEM images for various morphologies, respectively (scale bars, 10 nm), where slight truncation at the corner of cubic Pd was induced by HCl oxidative etching in the early stage and then continuous atomic addition to {100} facets promotes the enlargement of {111} facets and finally results in the formation of octahedral Pd bounded by {111} facets. (b) Cyclic voltammograms of Pd nanocrystals in 0.5 M H2SO4 + 0.5 M HCOOH solution for formic acid oxidation at 50 mV s-1. (c) UV-vis absorbance for TMB oxidation after mixing with TMB for 5 min. (Reference [
The specific surface area is strongly related to the morphology of materials since the active sites are exposed on the surface [12,34,35,43]. Pt NPs with different morphologies may exhibit diverse electrocatalytic activity. Liu et al. demonstrated that various shape of Pt NPs have a significant effect on methanol oxidation activity [12], and the mass-specific activity (MA) decrease following the order of nanosheet > nanoflower > prickly surface > smooth spherical, as revealed in Fig. 3. Furthermore, the proportion of ample Pt(100) sites decrease in the order of cubic Pt particles (47.8%) > prickly Pt particles (26.9%) > nearly spherical Pt particles (13.8%) [44].
Fig. 3. SEM images of Pt/ITO fabricated at the different lower potential pulse duration (tl). (a‒e) 1, 0.5, 0.1, 0.05 0.01 s. (f) Schematic diagram of the potentiostatic pulsed electrodeposition. CVs of the Pt/ITO electrodes with the different tl at 0.05 V s-1: (g) In 0.5 mol L-1 H2SO4 solution containing 1 mol L-1 CH3OH. (h) In N2-saturated 0.5 mol L-1 H2SO4 solution. (Ref. [
Doping another metal into noble metals (i.e., changing the composition of metals [15,45]) is put forward to enhance the utilization efficiency of noble metals, and improve their electrocatalytic activity as a result of the synergistic effect and the modified electronic structure. Different shape of Pt can modify the supporting core of Au [34], and the active sites (e.g., step edge and kink sites) are provided by the sharp edges and tips on the surface, as can be observed in Fig. 4(a). The Pt monolayer (PtML) flower-like Au particles show superior ethanol electrooxidation performance than PtML-decorated spherical Au particles (Fig. 4(b)), which can be ascribed to more active sites in PtML flower-like Au particles. Moreover, Pt-decorated flower-like Ni particles were synthesized [16], which own abundant nanopores and discontinuous structures. Pt grains with a small size of 2-3 nm distributed uniformly on the petals of flower-like NPs (Fig. 4(c)). Besides, compared to the relatively smooth surface of Pt NPs, a large number of protruding tips and edges lie on the surface of flower-like Pt NPs, thus affording higher specific activity for various electrooxidation reactions (Fig. 4(d)).
Fig. 4. (a) TEM image of PtML-decorated flower-like Au particles (insert images: top right corner is the corresponding elemental mapping of Pt and Au obtained by EDS, bottom right corner is the higher magnification HAADF-STEM image of the area near the tip of the branch, revealing the surface atomic steps. (b) Cyclic voltammograms (scan rate 0.05 V s-1) of the ethanol electrooxidation on PtML-decorated spherical and flower-like Au particles in 0.5 M H2SO4 + 1 M C2H5OH. (Ref. [
Recently, dendritic core-shell, layered core-shell and mesoporous core-shell structures of PdPt bimetallic NPs were synthesized through controlling the concentration of the introduced halide ions [40], as revealed in Fig. 5. Due to the synergistic effect from atomic and electronic structure, the activity and durability of the dendritic core-shell PdPt NPs are both improved. For mesoporous core-shell PdPt NPs, despite more active sites from large surface area and more pore channels, the best activity but poor durability is displayed because of easier contact of electrolyte with the interior Pd. Additionally, the controllable composition of PdPt bimetallic nanodendrites was obtained via a one-step method and used for methanol oxidation [15]. Dendritic Pd52Pt48 has the most outstanding activity, which is resulted from the following reasons: the accessible active sites on the surface, highly open structure, well-developed synergistic effect, good electrical connection across the entire surface, and the modified electronic structure of Pt with the incorporation of Pd.
Fig. 5. TEM images (insert is the corresponding mapping) of (a) dendritic core-shell PdPt bimetallic nanoparticles; (b) layered core-shell PdPt bimetallic nanoparticles; and (c) mesoporous core-shell PdPt bimetallic nanoparticles. (d) The schematic of PdPt bimetallic nanoparticles with dendritic core-shell, layered core-shell, and mesoporous core-shell structures synthesized by employing different halide ions with the same concentration of 80 mM and the corresponding TEM images (the scale bars are 20 nm). (e) CV curves of commercial Pt black (A), commercial Pt/C (B), the obtained dendritic core-shell (C, D), layered core-shell (E) and mesoporous core-shell (F) PdPt bimetallic nanoparticles synthesized by using 80 mM NaCl, HCl, NaBr and NaI, respectively. (Ref. [
In another research, Pt-Ir nanocubes with preferential (100) orientation were controllably developed as shown in Fig. 6(a) and (b) [14]. The analysis of (100)-orientated Pt-Ir nanocubes shows that the introduction of Ir changes the electronic structure of Pt, and largely maintains the highly active of Pt(100) sites, thus generating a positive synergistic effect. Otherwise, hollow nanocrystals (HNCs) with cost-saving feature and large specific surface have sparked tremendous research attention. Based on these advantages, PdNi HNCs were synthesized by NiO-induced strategy and applied to ethanol oxidation and FAO [32], as exhibited in Fig. 6(c) and (d). The obtained PdNi HNCs with rough shell comprised of dendrite-like PdNi NPs have a larger surface area and more potential active sites (Fig. 6(e) and (f)).
Fig. 6. (a) TEM image for Pt-Ir nanocubes (insert images are the size distribution). (b) The high-resolution TEM image of Pt-Ir nanocubes. CVs measured on Pt-Ir nanocubes, polycrystalline Pt-Ir NPs, and Pt nanocubes, respectively. (Reference [
As aforementioned, the application of noble metal Pt is limited by the scarcity and high-cost, which calls for enhancement of the usage efficiency. Until now, supporting Pt on nanostructured transitional metal compounds is regarded as a prospective method to provide abundant surface active sites and excellent atomic efficiency [19,46].
For example, a novel ultrafine Pt NPs anchored on the surface of CoS2/CC (carbon cloth) had been controllably synthesized through step-wise approaches [19]. The highly distributed Pt NPs with proper amount and size are illustrated in Fig. 7(a). X-ray absorption fine spectroscopy (XAFS) was conducted to reveal the structural and electronic characteristics of the Pt/CoS2 heterointerfaces. In Fig. 7(b), the difference in Co K-edge extended XAFS (EXAFS) k3χ(k) oscillation curve between Pt-CoS2/CC and Pt-free CoS2/CC demonstrates that the loaded Pt affects the local atomic arrangement of Co sites, which is further clearly verified by the converted Fourier transform (FT) in R space (Fig. 7(c)). Moreover, the first dominant peak of Co-S bond shifts to a higher value in Pt-CoS2/CC when compared with that of Pt-free CoS2/CC, implying the structural lattice distortion from the partial oxidation of the Co species. Therefore, the interfacial effect contributing to the electronic modulation of CoS2-supported Pt hybrid can be proved, thus both Pt and Co species with variable electronic configurations are obtained.
Fig. 7. (a) HRTEM of Pt-CoS2/CC. (b) EXAFS k3χ(k) oscillation functions. (c) The corresponding FT curves of CoS2/CC and Pt-CoS2/CC from
Similarly, the ultrafine Pt nanoparticles decorated MoS2 nanocomposite was reported by researches [47]. The corresponding TEM image in Fig. 7(d) confirms the good distribution of Pt. The results manifest that the Pt/MoS2-80 exhibits an excellent HER activity even exceeding 20% Pt/C, indicating the introduction of ultrafine Pt nanoparticles can boost the activity of inert surfaces and retain the active edge sites in MoS2. Another single-atom Pt decorated nanoporous Co0.85Se (Pt/np-Co0.85Se) with excellent HER activity had been designed by Jiang et al. [48], and the well-dispersed blight spots in Fig. 7(e) suggest the existence of Pt atoms. The uniform distribution of Co, Se, and Pt can be observed by the EDS mapping (the right TEM images in Fig. 7(e)), and the loading amount of Pt was ∼1.03 wt%. The XANES spectra are also an evidence to reveal the presence of Pt (Fig. 7(f)), in which Pt atoms with positive valence state (0.8, insert in Fig. 7(f)) in Pt/np-Co0.85Se can be known, which may be attributed to the electron transfer from Pt to Se in Pt-Se bonds of Pt/np-Co0.85Se. Therefore, local electron structure tuned by Pt can synergistically promote thermodynamics and kinetics for HER performance.
For the oxide compounds, the introduction of noble metals such as Pt can also generate the synergistic effect and then improve catalytic activity. For instance, single atoms (SA) Pt stabilized on WO3 surface by amorphous tungstenic acid (H2WO4) layer was proposed by Zhang et al. [49], and the individual Pt atoms are highly dispersed on WO3 surface as shown in Fig. 8(a)‒(c). The H2-TPR (temperature-programmed reduction) was conducted to confirm the strongest interaction between H2WO4 layer and Pt atoms in the SA-Pt/WO3 hybrid (Fig. 8(d)). Furthermore, the result of butanone response in Fig. 8(e) manifests the advantage of high atomic efficiency of Pt in SA-Pt/WO3. As a result, the highly dispersed Pt species with positive charges would afford more active sites and improve electronic coupling effect.
Fig. 8. (a) STEM-EDS elemental mapping of SA-Pt/WO3 hybrid. (b) Top and (c) side view of HAADF-STEM images of SA-Pt/WO3. Atomically dispersed Pt species are marked by the red circled in (b) and (c). The inset of (c) is the representative TEM image of SA-Pt/WO3; (d) H2-TPR curves of bare WO3 and Pt/WO3 hybrids. (e) Butanone response in the range of 125 ppb - 5 ppm, represents dynamic butanone oxidation sensing of SA-Pt/WO3 and measured at 150 °C. (Ref. [
Moreover, a high and uniformly distributed Pt nanoparticles on nitrogen-doped carbon nanotube (NCNT) surface (Pt/NCNT) can be obtained by the atomic layer deposition (ALD) method [50]. From the TEM images in Fig. 9(a)‒(c), the ALD ZrO2 with 50 cycles on Pt/NCNT exhibits holey nanocage structure, which is marked with white circles in Fig. 9(b) and (c). The synthetic strategy by area-selective ALD for Pt encapsulated in ZrO2 nanocage is demonstrated in Fig. 9(d), which can stabilize Pt catalysts encapsulated in stable ZrO2 nanocages and enhance the stability and activity of Pt. However, the ADT has an extreme effect on ORR activity in Pt/C and ALDPt/NCNT rather than ALD50ZrO2-Pt/NCNT600 °C (Fig. 9(e)), indicating the special structure of ALDPt/NCNT can enhance the stability and activity of Pt NPs.
Fig. 9. (a‒c) TEM and HRTEM images of ALD ZrO2-Pt/NCNT catalyst with 50 cycles of ALD ZrO2. (d) Schematic diagram of platinum encapsulated in zirconia nanocages structure fabricated by area-selective ALD. (e) Mass activity at 0.9 V (vs RHE) for ALD Pt/NCNT, ALD50ZrO2-Pt/NCNT600 °C, and E-TEK Pt/C catalysts. ALD Pt/NCNT and E-TEK Pt/C catalysts after 50-cycle electrochemical activation act as fresh catalysts. ALD50ZrO2-Pt/NCNT600 °C after 500-cycle electrochemical activation acts as a fresh catalyst. (Ref. [
In order to find cost-effective alternatives of noble metal catalysts, non-noble metal catalysts which own high electrocatalytic activity and remarkable stability have been investigated extensively. Nowadays, transition metal compounds including metal oxides, sulfides, selenides, phosphides, carbides, and nitrides are considered as the most promising electrode materials for various energy-storage devices, such as Li-ion batteries, electrochemical supercapacitors, and metal-air batteries. The catalytic mechanism remains controversial in transition metal compounds. In detail, the traditional point views that the metal site is the main catalytic center in transition metal compounds [51,52]. Furthermore, the electronic state and catalytic site can be regulated as other doped elements [53]. Meanwhile, as the development of advanced characterization technologies, another point of the metal complex resulted from nonmetallic anions and metal cations bonded as the catalytic center is proposed [54,55]. The specific regulation of catalytic sites in various transition metal compounds is elaborated next.
Transition metal oxides, especially cobalt hydroxides and oxides are favorable candidates owing to their natural abundance, remarkable reversibility, and durability in alkaline electrolyte. Up to now, a series of nanostructures of Co3O4 and Co(OH)2 had been synthesized, such as nanocubes [56], nanosheets [[57], [58], [59]], nanofilms [60,61] or nano/microspheres [62,63], and their properties were tremendously investigated. Interestingly, Feng et al. synthesized sub-3 nm atomic Co3O4 nanofilms using a hydrothermal method [60]. The ultrathin nanofilms were obtained through reconstructing cobalt-ammonia complexes (Fig. 10(a)). The specific capacitance of sub-3 nm atomic Co3O4 nanofilms can reach 1400 F g-1 and exhibit superior stability (Fig. 10(a)). Similar Co(OH)2 nanofilms were also prepared as pseudocapacitive material, which possesses maximum specific capacitance of 1076 F g-1 [60].
Fig. 10. (a) The formation of atomic layer Co3O4 nanofilm and cycle test of atomic layer Co3O4 nanofilm with a galvanostatic charge/discharge current density of 2 A g-1. (Ref. [
Moreover, hollow Co3O4 microspheres were also fabricated using a one-step hydrothermal method at 120 °C [63]. Experimental results show that the hollow Co3O4 microspheres own high specific capacitances of 1227, 1169, 1116, 1035 F g-1 at current densities of 1, 2, 4, 8 A g-1, respectively. Chen et al. obtained atomic thin Co3O4 nanosheets by a facile chemical method and its thickness is only 1.6 nm [58] (Fig. 10(b)). Besides, the different local atomic arrangements between atomic thin Co3O4 nanosheets and commercial Co3O4 are shown in Fig. 10(c), which also reveals the surface structural disorder in atomic thin Co3O4 nanosheets. A flexible and portable 1D Zn-air battery was subsequently assembled using the atomic thin Co3O4 nanosheets as the ORR/OER catalyst. The battery shows not only high rate capability and exceptional cycling stability but also excellent stability during the large deformation and knotting conditions (Fig. 10(d)).
However, poor conductivity is the intrinsic problem of transition metal oxides, restricting the electrochemical activity of Co3O4. In order to improve the conductivity of Co3O4, coupling with substrate materials with better conductivity is a common and efficient strategy, such as carbon fiber [57,59,65] and graphene-based materials [64,66]. Meanwhile, carbon-based materials also benefit from the flexibility and favorable durability. Chen et al. developed a binder-free method for the in situ growth of ultrathin mesoporous Co3O4 layers on carbon cloth (Co3O4/CC) [65]. Electrochemical measurements reveal that the mass activity for ORR/OER of ultrathin mesoporous Co3O4/CC is more than 10 times than that of the carbon cloth loaded with commercial Co3O4 NPs due to the high utilization degree of active materials and rapid charge transport. Besides, the flexible Zn-air battery using ultrathin Co3O4/CC electrode exhibits excellent rechargeable performance and high mechanical stability. Recently, a high-yield method was developed to prepare atomically thin layer-by-layer mesoporous Co3O4 [64], which is coupled with nitrogen-doped reduced graphene oxide (N-rGO) (Fig. 10(e)‒(g)). The catalyst shows excellent ORR/OER performance owing to the high surface area, mesoporous structure, and synergetic effect between Co3O4 and N-rGO nanosheets. Moreover, various Co3O4 nanocrystals were produced anchored on nitrogen-doped reduced graphene oxide (N-rGO) [66], including nanocube (NC), the nanotruncated octahedron (NTO), and nanopolyhedron (NP). Different crystal faces can be exposed via controllably synthesizing the shape of nanocrystals, and the results indicate that unusual {112} plane of Co3O4 NP on rGO exhibits higher ORR/OER activity than that of {001} and {001} + {111}, which optimizes the adsorption, activation, and desorption features of oxygen species. The result is further confirmed by theoretical simulation studies.
Although transition metal oxides have emerged as front-runners serving as cheap and efficient alternatives for OER and ORR in alkaline solution, the intrinsic poor conductivity is still an obstacle and there is still substantial space for improving the electrochemical performance of catalysts. Compared with transition metal oxides/hydroxide, other potential candidates for OER and ORR were developed, such as their phosphides [[67], [68], [69], [70]], sulfides [[71], [72], [73]], selenides [4,29,74,75], carbides [[76], [77], [78]], nitrides [[79], [80], [81]], possessing higher electronic conductivity and exhibiting remarkable bifunctional catalytic activity. Transition metal phosphides have emerged as a new class of low-cost and earth-abundant catalysts and their electrocatalytic properties towards for HER have been evaluated [[67], [68], [69], [70]]. Generally, the ultrathin precursor nanosheets usually tend to aggregate owing to the presence of lattice strain during the formation. However, Li et al. obtained ultrathin nickel phosphide nanosheets with exposed (001) facets for HER via an additive-assisted solvothermal strategy which can decrease the lattice strain [82]. Various experimental results show that the covered functionalized single-layer graphene in the Ni(OH)2 can efficiently decrease the lattice strain, thus leading to the stabilizing ultrathin Ni2P 2D nanostructure (Fig. 11(a)‒(c)). The resulting nanostructure shows not only remarkable hydrogen evolution performance but also good stability under visible-light irradiation (Fig. 11(d)). Moreover, three Ni-P phases with various Ni:P ratios were prepared at a low-temperature (250 ℃) [70]. Electrochemical results show that the HER property of Ni5P4 + Ni2P nanoparticles with P-rich phase is higher than those of pure Ni2P and Ni-rich Ni12P5 + Ni2P samples. This can be attributed that Ni5P4 + Ni2P with P-rich phase can provide more active sites.
Fig. 11. (a) Synthesis of ultrathin Ni2P using FGR to reduce the lattice strain during phosphorization with different precursors. (b) AFM image of NP-FG and section analysis along the line. (c) TEM image of NP-FG and the inset image is corresponding SAED pattern. (d) Photocatalytic H2 evolution with different catalysts under visible-light irradiation for 2 h. (Ref. [
Unlike metal phosphides, transition metal sulfides are potential catalysts for both HER, OER, and ORR. In particular, cobalt sulfides have attracted increasing attention because of their low cost, high abundance, easy availability, and remarkable chemical stability. Liu et al. synthesized CoS nanosheets on carbon cloth using a facile and clean electrochemical technique [73], in which the use of a binder, surfactant, and other organic additives are avoided. With quick thermal treatment, the obtained sample shows superior electrocatalytic performance for ORR (-1.51 mA cm-2 at 0.2 V) and OER (148 mA cm-2 at 1.9 V). Recently, three CoSx hollow nanospheres compounds (i.e., Co9S8, Co3S4, and CoS2) were obtained via controlling the molar ratio of carbon disulfide during the hydrothermal process [72] (Fig. 11(e)‒(g)). Interestingly, the electrochemical activity both for HER and OER of the three CoSx hollow nanospheres follows the order of Co9S8 < Co3S4 < CoS2 HNSs (Fig. 11(h)). As seen from the corresponding crystallographic structures (Fig. 11(f)), two-third cobalt sites of Co3S4 are at the center of the CoS6 octahedra while the ratio reaches one-ninth in the Co9S8 phase. However, in the CoS2, cobalt ions are all arranged in the form of CoS6 octahedra (CoS6 Oh). The higher HER performance can be attributed to that the pyrite CoS2 phase can provide more proton-acceptor on account of its S-rich nature, while the reason for higher OER activity is that CoS6 octahedra more active than CoS4 tetrahedron (CoS4 Td). More recently, researches have demonstrated the elemental doping in Co-S compounds is a prospective means to optimize the electronic structure of materials, thus further improving the electrocatalytic performance [30,83,84].
Unfortunately, the unsatisfied conductivity for transition metal sulfides is detrimental to its electrochemical performances, thus coupling with other materials like carbon nanostructures with excellent conductivity is needed. For example, a nanocomposite of homogeneous NiCo2S4 nanocrystals supported on nitrogen-doped carbon nanotubes (NiCo2S4/N-CNT) was designed with a simple solvothermal method [30], and the integrated and high-discrete NiCo2S4 nanocrystals are evenly dispersed on N-doped CNT (Fig. 12(a)). The constituent of C, N, Ni, Co, and S equably dispersed in the NiCo2S4/N-CNT hybrid is revealed in Fig. 12(b), implying that the NiCo2S4 is uniformly deposited on nitrogen-doped carbon matrix. When applied as oxygen electrocatalyst for ORR and OER, the NiCo2S4/N-CNT exhibits an △E value of 0.80 V, which is comparable to the most active electrocatalysts previously reported (Fig. 12(c)). Meanwhile, a slight decay of discharge/charge property in the optimized NiCo2S4/N-CNT nanocomposite is observed (Fig. 12(d)). Specifically, the existing of N-doped CNT can not only provide efficient electron transfer pathway but also alter the electronic structure of the catalyst. Moreover, the sample assembled rechargeable Zn-air battery shows longer stability and lower discharge/charge overpotential in comparison with those cells catalyzed by Pt/C and RuO2.
Fig. 12. (a) TEM image of NiCo2S4/N-CNT. (b) Elemental mapping of NiCo2S4/N-CNT. (c) Comparison of OER and ORR bifunctional activity of NiCo2S4/N-CNT with representative electrocatalysts in references. The dotted lines show the △E at constant values. (d) 1 st and 150th discharge/charge curves of NiCo2S4/N-CNT electrode. (Ref. [
Furthermore, an integrated flexible electrode based on NiCo2S4/graphitic carbon nitride/carbon nanotube (NiCo2S4@g-C3N4-CNT) was also prepared by the same group [86]. The TEM image in Fig. 13(a) shows the coexistence of conductive CNTs and closely interconnected NiCo2S4@g-C3N4. Notably, the digital image inserted in Fig. 13(a) displays that NiCo2S4@g-C3N4-CNT can be integrated as a free-standing film and directly applied as flexible electrodes for flexible energy conversion devices. The rechargeable ZABs catalyzed by NiCo2S4@g-C3N4-CNT electrode exhibits low overpotential, high efficiency and excellent rechargeability (Fig. 13(b)). To identify the mechanism of activation in NiCo2S4@g-C3N4-CNT, DFT computations were performed. As can be seen from Fig. 13(c) and (d), the designed NiCo2S4@g-C3N4-CNT can reduce the reaction barriers and increase the reaction kinetics, which are resulted from the synergistic effect between bimetallic Ni/Co sites in NiCo2S4 and pyridinic N dopants in underlying g-C3N4.
Fig. 13. (a) The TEM image of NiCo2S4@g-C3N4-CNT. (b) Cycling performance of rechargeable ZABs with a duration of 20 min per cycle at 10 mA cm-2 for NiCo2S4@g-C3N4-CNT and Pt/C + RuO2. (c) Calculated d-band positions of metallic Ni and Co sites and (d) Free energy diagram of ORR and OER processes on bare NiCo2S4, NiCo2S4@N-GN, and NiCo2S4@g-C3N4. (Ref. [
Compared with metal oxides and sulfides, transition metal selenides (MxSe, M = Co, Ni, and Fe) can potentially further enhance the catalytic performance due to their intrinsically conductive metallic property. Transition metal selenides are well documented in several recent works [29,74,75]. For example, Yang et al. prepared carbon-supported nickel selenide (Ni0.85Se/C) hollow nanowires as anode materials for sodium-ion batteries [75]. Experimental results reveal that Ni0.85Se/C hollow nanowires exhibit significantly enhanced cycle and rate performance compared to Ni0.85Se NPs. The excellent cycle stability and the superiority in rate capability can be attributed to the carbon framework, which not only improves the electric conductivity but also provides void space to accommodate the volume expansion during the electrochemical process. Moreover, NixSe (0.5≤ x ≤1) nanocrystals with different crystal structures and compositions had been controllably synthesized using a novel hot-injection method [74]. The ORR, OER, and HER of samples in alkaline conditions were investigated. The electrochemical analysis shows that Ni0.5Se nanocrystals exhibit superior OER activity compared to other counterparts, which might be ascribed to pyrite-type crystal structure and Se enrichment. However, Ni0.75Se NPs exhibit the best performance for OER and HER as a result of higher electrochemical active area and electronic conductivity. Furthermore, a heterostructured core-shell Ni3(S1-xSex)2@NiOOH (0≤ x ≤1) nanoarray was in-situ grown on nickel foam [29]. Thanks to the vertical standing architecture, Ni3(S0.25Se0.75)2@NiOOH heterojunctions exhibit superior OER and HER activity and small overpotential for water splitting at a suitable electrochemical polarization for 8 h.
Except for abovementioned metal compounds, transition metal carbides and nitrides are other attractive electrochemical catalysts, since their catalytic activity can be comparable with Pt-based catalysts. Transition metal carbide nanocrystalline M3C (M: Fe, Co, Ni) with vertically aligned graphene nanoribbons (GNRs) and graphitic shells were synthesized via a hot filament chemical vapor deposition (HF-CVD) method [76]. The iron group metals carbide-GNRs show excellent electrocatalytic activity towards for HER and OER, as well as superior stability in acidic media. These are the consequence of the small size of M3C nanocrystalline with more active catalytic sites, the synergistic effect between M3C and graphitic shells as well as GNRs, together with the favorable charge and mass transport properties. A secondary metal (Fe, Ni, Mn, Zn) doped Co5.47N nanosheets were reported for OER [81]. Electrochemical test results manifest that Fe-doped Co5.47N nanosheets possess outstanding OER performance. The doped Fe could promote higher binding energy of Co 2p compared with doped Ni, Mn, Zn, facilitating the formation of more Co4+ species that are considered as real OER active sites.
The conception of single atoms is proposed as a model to investigate the size effect at the atomic level, which aroused growing attention in exploring mono-atoms stabilized on support to optimize their catalytic properties [[87], [88], [89]]. The advantageous influence of size effect on catalytic reactions can be concluded as (1) metal centers with low-coordination [90]; (2) quantum size effects [91]; (3) metal-support interactions [92]. Therefore, the investigation of supported transition-metal single atoms as a new research frontier and has caused considerable concern recently.
To clarify the size effect of transitional metal NPs on electrocatalytic performance, Co species with various size (Co NPs, atomic Co clusters, and Co single atoms) on N-doped porous carbon can be realized by regulating zinc dopant content in ZnCo-ZIF (Fig. 14(a)) [93]. In detail, the atomic level cobalt species with spatial isolation can be prepared as the Co2+ in ZnCo-ZIFs is partially replaced by dopants Zn2+, and the subsequent heat-treatment at 1173 K facilitates Zn species evaporated, thus the Co species stabilized on N-doped porous carbon was obtained. The surfaces of Co-SAs@NC (Zn/Co = 8:1) became rougher with the presence of obvious porous structures (Fig. 14(b)), and the element mapping in Fig. 14(c) verify the homogeneous dispersion of C, N, and Co species. Moreover, the Co element with the highest degree of isolation leads to Co single atoms (Fig. 14(d)), which exhibits the superior bifunctional ORR/OER activity, durability, and reversibility in Zn-air batteries. The reasons for high catalytic reactions in Co-SAs@NC can be ascribed to the single Co atoms with high reactivity and stability.
Fig. 14. (a) Preparation of the Co-NPs@NC, Co-ACs@NC, and Co-SAs@NC catalysts. (b) TEM image and (c) elemental mapping of Co-SAs@NC. (d) HAADF-STEM image of Co-SAs@NC. (Ref. [
Moreover, Xue et al. used a simple two-step strategy prepared isolated nickel/iron atoms anchored on graphdiyne (GD) [94], as shown in Fig. 15(a). The even distribution of Ni and Fe can be revealed in Fig. 15(a) and (b), respectively. The bar graphs indicate that both of Ni (1.23 ± 0.40 Å, inset of Fig. 15(b)) and Fe (1.02 ± 0.33 Å, inset of Fig. 15(c)) with very narrow size distributions, implying the atomic sizes of Ni and Fe. The theoretical calculation results for bond length in Fig. 15(d) demonstrate that a powerful chemisorption character formed and a forceful charge transfer from Ni/Fe to GD emerged (Fig. 15(d)), suggesting that the transition metals are successfully intercalated in GD layers. In addition, the variation of Gibbs free energy is presented in Fig. 15(e) to illustrate the capacity of hydrogen chemisorption/desorption energy that is related to HER performance, and the better HER performance for Fe can be obtained. The reason can be described as the over-binding effect of Ni-on-GD increases the desorption efficiency of H0. Therefore, higher Fe or lower Ni amount can contribute to the optimized HER performance. The excellent HER properties may be ascribed to the powerful electronic coupling and chemical coactions between the GD support and Ni/Fe mono-atom catalytic active sites.
Fig. 15. (a) Protocols for the synthesis of Ni/GD and Fe/GD. A two-step strategy for anchoring-isolated Ni/Fe atoms on GD, including the in situ growth of GD layers on 3D carbon cloth (CC) surfaces via Glaser-Hay cross-coupling reaction, followed by the electrochemical reduction of metal ions (Ni2+ and Fe3+) into zerovalent metallic species [Ni(0) and Fe(0), respectively]. (b) Additional HAADF-STEM image of Ni/GD, and inset is the size distribution of Ni atoms counted from HAADF-STEM images (>560 Ni atoms considered, the most probable value is 1.23 ± 0.40 Å). Scale bar, 2 nm. (c) Additional HAADF-STEM image of Fe/GD, and insert is the size distribution of Fe atoms counted from HAADF-STEM images (>1070 Fe atoms considered, the most probable value is 1.02 ± 0.33 Å). Scale bar, 2 nm. (d) Electrostatic potential maps of pristine GD, Ni/GD, and Fe/GD, respectively. (e) The chemisorption energy of H for HER performance related to the free energy profile (ΔG). The Ni/FeC1 and Ni/FeC2 mean the H adsorption on the C1 and C2 sites within Ni/Fe-on-GD system, respectively. The GDC1 and GDC2 denote the H adsorption on pristine GD system. (Ref. [
The single-atom nickel sites confined in 3D nanoporous graphene was synthesized by Deng et al. [95], which displays excellent HER performance in acid solution as the improved sp-d orbital charge transfer among the Ni atoms and the ambient carbon atoms. Cui and co-authors reported that the introduction of Fe, Co, Ni, or Cu into MoS2 can enhance the exchange current densities by at least twice [96]. Moreover, atomic level dual active sites were developed by Li et al. [97], the explored CoXNi-N/C (X means P or D) with prominent catalytic activities for ORR and OER due to the presence of Co/Ni dual sites, abundant micropores, and nitrogen. The DFT calculation verifies the existence of atomic Ni/Fe dual sites and the ability of electronic activation in N atoms enhanced by Ni-N bonds in the CoXNi-N/C hybrid material. Meanwhile, the utilization of single atoms towards CO2 reduction was also developed. Such as Co-N5/HNPCSs with the mono-atom Co active sites were reported by Pan et al. [98], which show remarkable selectivity (nearly 100% for CO) and stability for CO2 reduction. Cheng and co-authors found that the Ni-N-MEGO with the edge-anchored unsaturated nitrogen coordinated Ni single atoms shows perfect CO2 reduction ability [99]. The other applications of single atoms also proposed, such as nitrogen fixation [100], CO oxidation [101], benzene oxidation [102], NO reduction without NH3 [103], and others.
To help promote the investigation and development of surface/interface engineering of noble metal and transitional metal compounds, a synthetical overview is provided by this work. As discussed above, surface structure and interfacial regulation exert extremely effect on the electrochemical activity. Although the design of surface and interface has made major strides in the past years, precisely engineering still need unremitting pursuit. In this review, various strategies to perfect surface texture and interfacial construction for noble metals and transitional metal compounds have been summarized in detail based on the contributed research. The corresponding characterizations of the surface/interface are also mentioned. Furthermore, an enhanced electrochemical mechanism via the surface/interface modulation is revealed. Therefore, rational design surface and interface to expose more catalytic sites and optimize binding interaction are fundamental research for improving the activity of catalysts but still remains a challenge to meet the requirements for next-generation energy devices.
We expect that this summary could provide new thoughts on the rational design and construction of the surface/interface structures in noble metals and transitional metal compounds with exceptional properties. For realizing further achievements, future research trends for tuning the surface structure of noble metals and interfacial character of transitional metal compounds are proposed as follows in detail:
(1) Active sites at the surface/interface are difficult to be characterized. Direct observations about the catalytic sites are limited at present and the controversy points still exist: metallic atoms are recognized as the electrochemical sites, but the non-metallic species also has a critical influence on the catalytic activity. Further research for deeper characterizations is urgently required and the related understanding of the identification and functions of active sites should be further elucidated. Advanced in-situ/ex-situ spectroscopic and microscopic techniques (e.g., scanning probe microscopy, Raman, and X-ray absorption spectroscopy, etc) have promising potential for the corresponding studies since they may provide more information regarding the redox states of active sites and interaction with the intermediates.
(2) For the integrated composite materials containing two or more phases, the so-called “synergetic effect” is proposed to explain the reasons for improved activity. However, there is a doubt that where the active sites located in the compounds and what is the detailed mechanism for a synergistic effect. Therefore, continuous efforts are urgently required to identify the synergetic enhancement between various components from both theoretical and experimental investigations.
(3) Recent observations reveal that catalysts are actually unstable during the electrochemical polarization (e.g., the OER process), resulting in a difference between the real and original surface/interface structure of the as-prepared nanocatalysts. Special attention should be paid to exploring the real catalytically active structure of the as-prepared NCs, thus establishing a clear relationship between the active centers at the surface/interface and the apparent electrochemical properties.
(4) Great progress has been achieved in the academic area at the electrode material level. However, it still has a long way for the consideration in industrial production and application of these materials, which is caused by the traditional expensive precursors, complicated preparation strategies and small quantities of products. Further scalable and suitable production route is extremely needed by transferring the promising materials in the laboratory to the large-scale practical implementation in an industrial company, which should attract more attention in the next research stage.
In summary, it is so attractive to investigate thoroughly the engineering of surface/interface active structures and the corresponding enhanced electrocatalytic mechanism, which will absolutely further push the development of high-performance functional nanomaterials as well as the applications of related energy storage and conversion technologies.
This work was supported financially by the Joint Funds of National Natural Science Foundation of China and Guangdong Province (No. U1601216), the National Natural Science Foundation of China (Nos. 51602216 and 51472178), Young Elite Scientists Sponsorship Program by CAST (No. 2018QNRC001), and Tianjin Natural Science Foundation (No. 17JCQNJC02100).
WeChat
/
| 〈 |
|
〉 |
