Electronic-Structure Tuning of Water-Splitting Nanocatalysts

Special Issue Part Two: Big Questions in Chemistry

Review

Electronic-Structure Tuning of Water-Splitting Nanocatalysts Wenxiu Yang,1 Zichen Wang,1 Weiyu Zhang,1 and Shaojun Guo1,2,* Electrochemical water splitting (EWS) represents a promising pathway for the storage of intermittent energies, such as wind and solar, in the form of hydrogen gas. The operational efficiency of EWS is governed in part by the electrocatalysts for two electrode reactions, namely, the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). In this review, we highlight recent fundamental and experimental progress on tuning the electronic structure of electrocatalysts for enhanced EWS. In particular, we discuss several strategies to adjust the electronic structure of nanoelectrocatalysts, including: alloying, doping, interfacing, incorporating oxygen vacancies, and edge-defect engineering. Finally, some invigorating perspectives for future research directions are also provided.

Background of Electrochemical Water Splitting Inspired by the irreversibly declining reserves of traditional fossil fuels and their negative effects on the environment and human life, innovative conversion and storage technologies based on new energy sources are highly anticipated in the 21st century [1,2]. The intermittent shortcomings of new energy technologies such as wind and solar energy have limited their widespread application. However, electrocatalytic generation of hydrogen through electrochemical water splitting (EWS) can effectively solve these issues [3]. Briefly, EWS can be divided into two fundamental half-reactions: the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). An electrolysis cell voltage of 1.23 V is required to drive EWS under standard conditions, corresponding to an energy input of DG = 237.1 kJ mol 1 [4]. As shown in Figure 1, the OER is a four electron–proton coupled reaction including multiple intermediates (e.g., MOH, M-O, MOOH, and O2) with O2(g) produced from the combination of 2M-O intermediates or the decomposition of MOOH intermediates in both acidic and basic media (M is a catalytically active metal center) [5,6]. The HER is comparatively simpler. It is a twoelectron process with one reaction intermediate (M-H) proceeding through two potential pathways (i.e., the Volmer–Heyrovsky or Volmer–Tafel mechanism in Figure 1) [7,8]. In acid electrolytes, the free energy of hydrogen adsorption (DG(H*)) is widely regarded as a descriptor for the HER catalyst, which should be close to zero. Except for the Volmer–Heyrovsky or Volmer–Tafel steps, the HER in basic media also requires an initial water dissociation step that may introduce an additional energy barrier and govern the HER rate [9]. Hence, it is of great importance to develop efficient, stable, and cost-effective catalysts to make EWS a viable and scalable energy-storage technology [8,10,11]. Traditionally, precious metal-based catalysts such as commercial Pt/C (for the HER) and IrO2/ RuO2 (for the OER) have been regarded as the best EWS catalysts in practical devices. Nevertheless, the high cost, low reserve, and poor stability of precious metal-based electrocatalysts limits their use in large-scale hydrogen production [12]. To date, significant effort has

Highlights The critical challenge of electrochemical water splitting (EWS) is to overcome the slow kinetics and large overpotential of the oxygen evolution reaction (OER). Although hydrogen evolution activity in acidic solutions has been achieved to a sufficient extent, acceptable activity of alkaline hydrogen evolution still remains to be achieved. Strategies such as alloying, doping, interfacing, oxygen-vacancy engineering, and edge-defect engineering can selectively adjust the electronic structure of nanocatalysts for enhanced EWS catalysis. To date, significant effort has been expended toward constructing efficient EWS electrocatalysts from two promising avenues: low-Pt precious metal (LPM) catalysts or non-precious metal (NPM) catalysts.

1 Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing, 100871, China 2 BIC-ESAT, College of Engineering, Peking University, Beijing, 100871, China

*Correspondence: [email protected] (S. Guo).

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10.1016/j.trechm.2019.03.006 © 2019 Published by Elsevier Inc.

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Figure 1. Mechanisms of the Oxygen Evolution Reaction (OER) and the Hydrogen Evolution Reaction (HER) in Electrochemical Water Splitting [5,7]. M is a catalytically active metal center.

been expended toward constructing efficient EWS electrocatalysts from two promising avenues: low-Pt precious metal (LPM) catalysts or non-precious metal (NPM) catalysts. In terms of OER catalysts, Ir/Ru-based nanocrystals, transition metal (TM; e.g., Fe, Co, Ni, and Cu) oxides, and layered double hydroxides (LDH) have demonstrated competitive activities in basic media. While for HER catalysts, Pt-free metal alloys, TM phosphides, chalcogenides, borides, carbides, and nitrides have shown promise in acidic solutions [13–20]. Despite their promise, the EWS activity and stability of the reported materials still necessitates further improvement. Beyond material type, effective regulation of both geometric and electronic structure are critical strategies for developing efficient LPM and NPM EWS catalysts. As the most fundamental geometric characteristics, nanocatalyst specific surface area (SSA) and porous microstructure have been considered critical parameters for enhancing electrocatalytic performance. For example, active-site density and mass-transfer rates have been improved greatly in high SSA materials such as: 3D super structures, 2D ultrathin nanosheets, 1D ultrafine nanowires, and 0D nanoclusters [16,21–23]. Despite these endeavors, strategies based on tuning geometric structure usually show limited achievable windows for significantly enhancing EWS performance. Tuning the electronic-structure of electrocatalysts, however, provides additional opportunities to achieve improved EWS catalytic performance. In this review, we summarize recent advances in controlling the electronic structure of LPM and NPM nanomaterials (Figure 2). In particular, we discuss several strategies for tuning electronic structure, including: alloying, doping, interfacing, incorporating oxygen vacancies, and edge-defect engineering. We hope that this mini-review sheds light on the available methods for constructing more efficient LPM or NPM catalysts for application in future EWS devices and fuel cells.

Handles for Tuning Electrocatalyst Electronic Structure Alloying The activity of EWS nanocatalysts is closely related to their electronic structure. Controlled synthesis of alloying nanocrystals represents a robust approach to more sophisticated 260

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Figure 2. Illustration of Key Handles for Tuning Nanocatalyst Electronic Structure and Promoting Electrochemical Water Splitting (EWS) Electrocatalysis [24–28].

electrocatalysts for the HER, OER, and oxygen reduction reaction (ORR) with enhanced activity [29–34]. To date, LPM hybrids such as Pt-Mn concave nanocube/Ni(OH)2 materials [35], IrCo nanoalloys [36], and rhombic dodecahedral MNi (M = Ir and Pt) nanoframes [37] have been reported as efficient EWS catalysts. Based on d-band center theory, the oxygen adsorption energy can be effectively tuned by alloying Ir with 3d TMs [38]. For example, Guo and colleagues have demonstrated IrM (M = Co, Ni) nanocrystals as bifunctional HER and OER catalysts for the development of efficient EWS devices (Figure 3A) [39]. Due to the ligand effect, the d-band center of Ir can be shifted far from its original Fermi level through alloying Ir with M, leading to the adsorption energy of IrM weaker than that of pure Ir (Figure 3B,C), thus reducing the activation energy of the rate-determining step. It was also demonstrated that except for promoting the lower binding energy of oxygen intermediates, W stabilized the formed active IrO2 during the OER process by inducing stronger bonding of Ir and O, hence hindering the dissolution of Ir [28], rendering IrW nanodendrites applicable for high-performance EWS at all pH values (Figure 3D,E). Trends in Chemistry, May 2019, Vol. 1, No. 2

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Figure 3. Morphology Characterization, Theoretical Calculation, and Proposed Catalytic Mechanism for Several Alloys. (A) Transmission electron microscopy (TEM) image of as-prepared IrCoNi nanocrystals; (B) projected density of state (PDOS) of Ir d-bands and IrCoNi models, with corresponding d-band center denoted by dash lines; (C) illustration of reaction paths for the oxygen evolution reaction (OER), indicating that alloying Ir with M leads to weaker adsorption of oxygenbased intermediates, thus decreasing the barrier of reaction path [39]; (D) TEM image of IrW nanodendrites; (E) PDOS of IrO2 (top) and W-IrO2 (bottom) with the overlapped states of Ir 5d (red) and O 2p (blue) denoted by the shaded area (yellow) [28]; (F) electronic pull mechanism of OER processes for Ni0.8Co0.1Fe0.1OxHy; and (G) electronic push mechanism of the hydrogen evolution reaction (HER) process for Ni0.9Co0.1OxHy [13].

NPM alloy catalysts are also of interest for efficient EWS devices [40–43]. For example, Wang and colleagues fabricated Ni0.75Fe0.25Se2 hollow nanochains with good OER activity. Importantly, the optimal O* and OH* adsorptions were found to be closely related to the Fe and Ni atom configuration in the crystal lattice [42]. Recently, Zhu and colleagues employed a MOF-templating approach to fabricate Co Fe P alloys. Here, the electrochemicalinduced high-valent Fe stabilizes Co in a low-valence state, which provides a synergistic effect from the high activity of CoP and high stability of FeP, making the Co Fe P alloys efficient EWS catalysts [43]. Doping Doping is an effective tactic of deliberately introducing minor amounts of foreign metal ions or heteroatoms into the host nanomaterial to improve catalytic activity [44,45]. In the area of LPM catalysts, Zhang and colleagues recently developed single-atom Au-doped NiFe LDH catalysts. These catalysts showed a sixfold OER activity improvement relative to that of the LDH system because dopant Au atoms caused charge redistribution by transferring electrons to the LDH, resulting in improved catalytic performance [46]. Mu and colleagues demonstrated that doping Ru into hollow Ru-RuPx-CoxP polyhedral [47] could induce surface reconstruction of Co2P for unstable surface termination, indicated by a combination of X-ray diffraction (XRD), Xray photoelectron spectroscopy (XPS), and line-scan electron energy-loss spectroscopy 262

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(EELS). Density functional theory (DFT) calculations further revealed that the introduction of Ru promotes the electrocatalytic reaction kinetics by increasing the density of states at the Fermi level with reduced OER intermediate adsorption energy. Various TM elements (e.g., Fe, Co, Ni, and Cu) have been successfully doped into metal-oxide, hydroxide, and LDH catalysts, which efficiently promote the formation of an active O radical intermediate and subsequent O–O coupling during the OER process [18,46]. An electronic push/ pull effect of Co and Fe doping on the HER and OER performance of Ni-based hybrids was demonstrated (Figure 3F,G) recently [13], in which Fe dopants pull partial electrons from adjacent Ni/Co sites, leading to a higher electron affinity of the Ni/Co sites that facilitate OH adsorption and faster charge transfer for the OER. In contrast, Co dopants pushed partial electrons to adjacent Ni sites and increased the number of lattice O2 groups, enhancing H+ adsorption and charge transfer for the HER. Coralloid-like W0.5Co0.5-xFex catalysts with a precisely controlled amount of Fe were directly grown on arbitrary substrates [8]. Guo and coworkers [8] found that dopant Fe reduced the oxidation state of Co in the trimetallic oxyhydroxides, which might further optimize the binding energy of oxygen intermediates and boost OER activity. Sun and coworkers produced a NiFe-LDHs by partially substituting Ni2+ with Fe2+, resulting in the formation of Fe-O-Fe couples. In situ X-ray absorption spectroscopy (XAS) and DFT results further revealed that the Fe-O-Fe motifs could make high-valent metal sites stable at atomic levels, enhancing the OER performance. Briefly, the deprotonation of OH* to O* is the rate-limiting step for the Ni site. Meanwhile, the calculated overpotentials of Ni-substituted Fe, Fe site, and Ni site are 0.32, 0.36, and 0.48 V, respectively, further highlighting the importance of regulating the local electronic structure for increasing the OER activity [48]. Oxygen-Vacancy Engineering Recently, oxygen-vacancy engineering has been explored as an effective strategy to promote EWS by activating neighboring atoms to enhance the density of states near the Fermi level, as well as the reactivity of the active site [51,53–56]. These synergistic effects further yield an accelerated electron-transfer rate and instability of the metal-oxygen bonds, leading to a faster intermediate exchange effect and superior HER/OER performance. Additionally, two electrons existing on the oxygen vacancy can be easily excited into the conduction band for an improved conductivity [24,57]. Until now, oxygen vacancies have been fabricated through different methods such as: reduction of NaBH4 [58,59], ligand-assisted polyol reduction [60], doping of ionic liquids (ILs) [51], air plasma activation [53,54,61], ion exchange [62], in situ electrochemical activation [50,55], and lower valencestate doping [63]. These findings are typically verified via XPS, Raman, and electron paramagnetic resonance (EPR) spectra [60]. Cu-doped RuO2 hollow porous polyhedrals are an example of a typical LPM catalyst. Here, rich O vacancies are formed nearby Cu, which induces the appearance of unsaturated Ru, makes part of the charge on their neighboring Ru atom less positive, and shifts the p-band center of nearby O atoms closer to the Fermi level to enhance OER activity. Recently, Peng and colleagues successfully synthesized a uniform necklace-like reduced-TM oxide catalyst by an ‘adsorption-calcination-reduction’ strategy. These NPM catalysts have rich oxygen vacancies and a controlled multishell structure (Figure 4A) [49]. DFT calculations revealed that the optimized energy barrier for the formation of HER/OER reaction intermediates should be attributed to the introduction of oxygen vacancies by the reduction of NaBH4 (Figure 4B). Guo and colleagues [50] reported porous NiO/CoN nanowire arrays with abundant oxygen vacancies for promoting the electrocatalytic performance and stability (Figure 4C). The electron spin resonance (ESR) signal at g = 2.004 was used to demonstrate electrons trapped on Trends in Chemistry, May 2019, Vol. 1, No. 2

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Figure 4. Design, Characterization, and Theoretical Calculation of Representative Oxygen Vacancy-Rich Catalysts. (A) Transmission electron microscopy (TEM) image of reduced-transition metal oxide; (B) schematic for the creation of oxygen vacancies on the surface of reduced NiCo2O4 by NaBH4 reduction [49]; (C) scanning-electron-microscopy image of NiO/CoN nanowire arrays; (D) electron-paramagnetic resonance spectra of the NiO/CoN materials before and after the oxygen evolution reaction (OER) [50]; (E) graphical comparison of the oxygen vacancies derived from X-ray photoelectron spectroscopy spectra (O3 peak area) [51]; (F) TEM image of Co-MnO2 ultrathin nanosheets; (G) projected density of state (PDOS) of MnO2 and Co-MnO2|OV; (H) reaction coordinates of OER on MnO2 and Co-MnO2|OV [52]. HER, Hydrogen evolution reaction; MNC, MoS2/NPF-CoFe2O4.

oxygen vacancies. As shown in Figure 4D, the enhanced ESR signal intensity illustrates that the NiO/CoN nanowire arrays possess more oxygen vacancies after the OER. Wang and colleagues provided an amorphous vacancy-abundant MoS2/NPF-CoFe2O4 (MNC) OER catalyst by introducing heteroatom-containing ILs [51]. Using XPS, the MNC was demonstrated to own the richest oxygen vacancies, which led to its having the lowest overpotential for the OER (Figure 4E). Co-doped MnO2 ultrathin nanosheets with abundant oxygen vacancies (CoMnO2|OV) were made by a facile spontaneous redox reaction (Figure 4F) as an efficient OER catalyst. DFT calculations revealed that the enhanced catalytic activity primarily originated from the 264

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increased conductivity and decreased adsorption energy barrier (Figure 4G,H) of OH on the O sites nearby the doped Co and oxygen vacancies [52]. Therefore, oxygen-vacancy engineering is a potential successful strategy for promoting the activity of the EWS catalysts from the following aspects: (i) increasing conductivity, (ii) activating neighboring atoms, and (iii) optimizing the energy barrier for the formation of HER/OER reaction intermediates. Interfacial-Site Engineering Interactions between different regions at interfaces has been demonstrated to elevate the chargetransfer rate and promote electrochemical reaction rates [68–71]. For example, Au/CeO2 singleatom catalysts with maximized interfacial sites show the enhanced catalytic activity for the oxidation of primary alcohols [72]. Similar concepts have been demonstrated on making PtPd-Fe3O4 interface nanoparticles for enhancing the detection of H2O2 [73], preparing welldefined cubic multilayered Pd-Ni-Pt sandwich nanoparticles [74], and PtPb/Pt core/shell nanoplates [75] for enhanced methanol electro-oxidation catalysis, which is ascribed to the abundant active interfacial sites and coordinatively unsaturated atomic sites [76]. A typical example of LPM catalysts are Pt3Ni/NiS nanowire heterostructures made by directly sulfuring highly composition-segregated Pt-Ni nanowires (Figure 5A) [64]. DFT calculations revealed that the synergistic effect between NiS and Pt3Ni enhances HER activity in potassium hydroxide (KOH) solution, where NiS promoted the original water dissociation and Pt3Ni efficiently transferred H+ to H2 [64]. Reduced graphene oxide (rGO), carbon nanotubes, and hollow carbon spheres have also been regarded as excellent substrates for designing interfacial catalysts. Xiong and colleagues reported a Pd/Pt-rGO structure as efficient HER catalysts, revealing that graphene substrates had strong electronic coupling with Pd and Pt according to their highly hybridized projected density of state (Figure 5B,C), thus providing interconnected and resistanceless network for efficient HER catalysis [65]. For NPM catalysts, optimizing the interface of TM oxides, phosphides, nitrides, and chalcogenides could potentially boost EWS activity [69,76,77]. A high-energy interfacial CoO/hiMn3O4 structure was designed as an OER catalyst. In this case, MnIII-O pairs of hi-Mn3O4 acted as electron acceptors to draw electrons from CoO, forming the Mn-O-Co interface and a high oxidation state of CoO that favors charge transfer and enhances intrinsic OER activity [78]. Interestingly, a class of CuS/NiS2 interface nanocrystal catalysts (Figure 5D,E) were also developed that are capable of binding new reaction intermediates to promote the OER [66]. Additionally, interfacial engineering was also well demonstrated in MoS2/NiO/Ni3S2 heterostructures for facilitating the synchronous chemisorption of hydrogen and oxygen-based intermediates separately, consequently boosting the overall EWS properties (Figure 5F) [67]. Interfacial effects in heterojunctions could efficiently optimize electron transfer and improve the EWS performance through the following approaches: (i) reducing interfacial resistance and the absolute value of hydrogen and oxygen-based adsorption energy, (ii) electron-withdrawing and forming metal-O-metal interfaces, (iii) synergistic effects between compositions promoting different fundamental reaction steps [79–82]. Edge-Defect Engineering Very recently, edge-defect engineering has appeared as an effective pathway to modulate the electronic structure of nanomaterials, thereby contributing to enhanced electrocatalytic activity [87–93]. Considering the atoms located at edge steps are intrinsically very active, Guo and coworkers presented ultrathin PtPdM (M = Co, Ni, Fe) nanorings as excellent LPM catalysts (Figure 6A). Importantly, excellent OER activities were ascribed to the high portion of step Trends in Chemistry, May 2019, Vol. 1, No. 2

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Figure 6. Geometric Structure and Theoretical Catalytic Sites of Defect-Rich Catalyst. (A) Transmission electron microscopy (TEM) image of PtPdCo nanorings, and (B) high-resolution TEM (HRTEM) images of an individual PtPdCo nanoring projected along the zone axes of axis [83]. (C,D) Scanning electron microscope (SEM) and HRTEM images of winged Au@MoS2 [84]; (E) TEM image of 3D graphene networks (the inset shows corresponding selected-area electrondiffraction pattern); (F) HRTEM image of vertical graphene sheets on SiOx nanowire [85]; (G) HRTEM image of the as-designed MoS2 nanosheet/carbon macro-porous electrocatalyst; (H) four optimized models for hydrogen adsorption at different sites of MoS2/carbon hybrids; (I) the calculated DGH for hydrogen adsorption on different sites [86].

atoms, efficient atom utilization, and strong ligand effect from M to Pt (Figure 6B) [83]. In another study, winged Au@MoS2 nanostructures with abundant edge-terminated active sites showed dramatically improved HER electrocatalytic activity, attributed to the optimized proton-adsorption process (Figure 6C,D) [84]. Figure 5. Structure, Theoretical Activity, and Mechanism of Interfacial Catalysts. (A) High-magnification transmission electron microscopy (TEM) image, highangle annular dark-field scanning TEM image, and corresponding energy-dispersive spectroscopy (EDS) elemental mapping of Pt3Ni/NiS heterostructures (scale bars = 20 nm, Ni in red, S in green, and Pt in blue) [64]. Projected density of state diagrams from first-principles simulations: (B) Pd(100)-graphene interface and (C) Pt (100)-graphene interface [65]. (D) TEM and (E) high-resolution TEM (HRTEM) images of CuS/NiS2 and corresponding fast Fourier transform patterns (inset), in which the interface is marked by the dashed line [66]. (F) Proposed mechanisms for the dissociation of H2O, OH, and OOH intermediates on the MoS2/Ni3S2 heterostructures: yellow (S), green (Ni), blue (Mo), white (H), and red (O) [67]. Abbreviations: HER, Hydrogen evolution reaction; OER, oxygen evolution reaction.

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For NPM catalysts, the electrochemical properties of CNTs were found to be dominated by oxygenated species at the ends of the nanotubes [94]. The graphene edge was also shown to have two times higher reactivity than that of the bulk atoms [87,95]. Loh and coworkers demonstrated that the edge sites with unpaired electrons and carboxylic groups could serve as active sites in electrocatalysis [88]. 3D graphene with a high density of sharp edge sites was produced for boosting HER (Figure 6E). DFT investigations indicated that their excellent HER performance was attributed to the abundant sharp edge defect sites of the 3D frameworks, which efficiently promote the adsorption and reduction of protons (Figure 6F) [85]. Additionally, 2D layered materials (e.g., LDH, CoSe2, MoS2, MoSe2, WSe2, and 2H-WS2) often expose basal planes with minimal roughness and dangling bonds terminating the surface, while the edge surface contains many dangling bonds that are chemically active in regulating electrocatalysis [27,41,96–100]. As shown in Figure 6G, MoS2 nanosheet/carbon macro-porous hybrid catalysts with abundant engineered unsaturated sulfur edges were reported to enhance the HER catalysis. DFT calculations verified that the high exposure of unsaturated sulfur edges could optimize DGH and significantly improve the intrinsic HER catalytic activity (Figure 6H,I) [86]. Consequently, as a key strategy for constructing highly efficient EWS catalysts, edge-defect engineering is useful from two main aspects: (i) edges act as electrocatalytically active sites that can optimize the hydrogen adsorption energy, and (ii) edges provide anchoring points for the doping of other active species.

Outstanding Questions Can we increase further the activity and stability of electrochemical water splitting (EWS) catalysts? How can we characterize EWS catalysis precisely and facilely? Despite some established reaction mechanisms based on various technologies and analysis methods, the lack of characterization tools to ‘see’ the reaction intermediates render them incomplete. Regarding this issue, in situ technologies to characterize the electrocatalysts during the EWS process are highly desired. How can we stabilize the EWS catalysts? As for electrocatalysts, longterm stability is a performance metric as important as the activity during practical electrochemical processes. Although a wide range of materials have been reported to show promising activity towards EWS, very few provide sufficient longevity.

Concluding Remarks In recent decades, interest in designing HER/OER nanocatalysts has grown significantly due to their importance in the development of renewable energy technologies, including: hydrogen-generation devices, H2-O2 fuel cells, and metal-air batteries. In this review, we highlighted recent important advances in the electronic-structure tuning of LPM and NPM catalysts for enhanced EWS catalysis. Detailed study through DFT calculations has deepened our understanding of the EWS process, where adsorption abilities toward reaction intermediates of the electrocatalysts should be regarded as important parameters to evaluate performance and guide us to design promising catalysts. Moreover, the critical issues of EWS are to expedite the slow kinetics of the OER and the initial water decomposition in alkaline HER. Several remaining challenges in EWS should be overcome in the future (see Outstanding Questions). These include: (i) design and construction of more promising nanocatalysts that exhibit optimized electronic structure and have extremely large surface area and porosities; (ii) development of more advanced characterization technologies that enable the detection of reaction intermediates and the elucidation of active sites [e.g., in situ XAS, XPS, Fourier-transform infrared spectroscopy (FT-IR) spectroscopy, XRD, Raman, and vibrational sum-frequency generation spectroscopy]; and (iii) realizing the industrial production of EWS nanocatalysts with available activity and durability. Considering the obvious advantages of large SSA and a hierarchical porous structure, reasonably regulating the electronic structure of nanocatalysts with extremely high surface area will likely provide the most promising avenue for developing more efficient EWS devices. Acknowledgments This work was financially supported by the Beijing Natural Science Foundation (JQ18005), National Key R&D Program of China (No. 2016YFB0100201), National Natural Science Foundation of China (NSFC) (No. 51671003, 21802003), the China Postdoctoral Science Foundation (No. 2018M631239), Open Project Foundation of State Key Laboratory of Chemical Resource Engineering, the start-up supports from Peking University and Young Thousand Talented Program.

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How can we produce desired electrocatalysts at practical scale? Most of the reported electrocatalysts to date are synthesized on a laboratory scale. However, in order to utilize these promising electrocatalysts for practical applications, it is also important to explore large-scale and efficient synthetic methods for mass production, without compromising catalytic performance.

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