Surface and Interface Engineering of Noble-Metal-Free

Feb 16, 2017 - Recent achievements in the design and synthesis of efficient non-precious-metal and even non-metal electrocatalysts make the replacemen...
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Surface and Interface Engineering of Noble-Metal-Free Electrocatalysts for Efficient Energy Conversion Processes Yun Pei Zhu,‡ Chunxian Guo,‡ Yao Zheng, and Shi-Zhang Qiao* School of Chemical Engineering, The University of Adelaide, Adelaide, SA 5005, Australia CONSPECTUS: Developing cost-effective and high-performance electrocatalysts for renewable energy conversion and storage is motivated by increasing concerns regarding global energy security and creating sustainable technologies dependent on inexpensive and abundant resources. Recent achievements in the design and synthesis of efficient nonprecious-metal and even non-metal electrocatalysts make the replacement of noble metal counterparts for the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR) with earth-abundant elements, for example, C, N, Fe, Mn, and Co, a realistic possibility. It has been found that surface atomic engineering (e.g., heteroatom-doping) and interface atomic or molecular engineering (e.g., interfacial bonding) can induce novel physicochemical properties and strong synergistic effects for electrocatalysts, providing new and efficient strategies to greatly enhance the catalytic activities. In this Account, we discuss recent progress in the design and fabrication of efficient electrocatalysts based on carbon materials, graphitic carbon nitride, and transition metal oxides or hydroxides for efficient ORR, OER, and HER through surface and interfacial atomic and molecular engineering. Atomic and molecular engineering of carbon materials through heteroatom doping with one or more elements of noticeably different electronegativities can maximally tailor their electronic structures and induce a synergistic effect to increase electrochemical activity. Nonetheless, the electrocatalytic performance of chemically modified carbonaceous materials remains inferior to that of their metallic counterparts, which is mainly due to the relatively limited amount of electrocatalytic active sites induced by heteroatom doping. Accordingly, coupling carbon substrates with other active electrocatalysts to produce composite structures can impart novel physicochemical properties, thereby boosting the electroactivity even further. Although the majority of carbonbased materials remain uncompetitive with state-of-the-art metal-based catalysts for the aforementioned catalytic processes, nonmetal carbon hybrids have already shown performance that typically only conventional noble metals or transition metal materials can achieve. The idea of hybridized carbon-based catalysts possessing unique active surfaces and macro- or nanostructures is addressed herein. For metal−carbon couples, the incorporation of carbon can effectively compensate for the intrinsic deficiency in conductivity of the metallic components. Chemical modification of carbon frameworks, such as nitrogen doping, not only can change the electron-donor character, but also can introduce anchoring sites for immobilizing active metallic centers to form metal−nitrogen−carbon (M−N−C) species, which are thought to facilitate the electrocatalytic process. With thoughtful material design, control over the porosity of composites, the molecular architecture of active metal moieties and macromorphologies of the whole catalysts can be achieved, leading to a better understanding structure−activity relationships. We hope that we can offer new insight into material design, particularly the role of chemical composition and structural properties in electrochemical performance and reaction mechanisms.

1. INTRODUCTION A key factor in reducing society’s dependence on traditional fossil fuels relies on several electrochemical processes: the hydrogen evolution reaction (HER), the oxygen evolution reaction (OER), and the oxygen reduction reaction (ORR), which are the foundations of various prospective technologies for renewable energy conversion and storage devices like water electrolysis, fuel cells, and rechargeable metal−air batteries (Figure 1).1−3 However, the sluggish kinetics of these electrochemical processes currently utilizes noble metal catalysts (i.e., Pt for HER/ORR and IrO2/RuO2 for OER) to realize sufficient efficiency. Nonetheless, their scarcity and high price greatly inhibit their extensive commercial reach. Accordingly, a large variety of non-precious-metal alternatives consisting of earth-abundant elements have been developed to © 2017 American Chemical Society

effectively boost the reaction kinetics and offer continuous energy through direct electrochemical conversion.4−6 Although enthusiastic efforts have been made to search for novel electrocatalysts, how to make better use of the existing catalyst systems to achieve optimal performance has been neglected. Judiciously adjusting the chemical compositions of electrocatalysts is considered a critical strategy to improve their electrocatalytic activity.7−9 As an essential member of nonmetal materials, carbon with tunable molecular structures, tailorable physicochemical properties, excellent electronic conductivity, and high physical and chemical stability plays an indispensable role in exploiting advanced electrocatalysts.10 Received: December 19, 2016 Published: February 16, 2017 915

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2. CARBON-BASED NANOSTRUCTURED ELECTROCATALYSTS 2.1. Heteroatom-Doped Nanostructured Carbon Electrocatalysts

Doping nanostructured carbon (e.g., graphene, carbon nanotubes, porous carbons) with heteroatoms can cause electron modulation, which tunes their (electro-)chemical properties.22,23 Taking the ORR for example, the improved electroactivity of heteroatom-doped carbon materials can be attributed to the doping-induced charge redistribution around the heteroatom dopants, which change the O2 chemisorption mode to effectively weaken the O−O bonding and thus facilitate the oxygen reduction process.24 More specifically, the overall ORR can proceed either through the efficient 4e− reaction pathway or two less efficient 2e− reactions associated with formation of OOH− via the reduction of OOH* intermediates. For heteroatom-doped graphene nanostructures, the formation of OOH* is the largest free energy change in the ORR process, according to density functional theory (DFT) calculation and is the rate-determining step.25 As such, chemically modified graphene with strong enough binding affinity toward OOH* to induce an endothermic OOH* desorption process is needed to minimize or even eliminate the 2e− pathways. To achieve this goal, a two-step doping method was conducted to prepare N and B codoped graphene with considerably enhanced ORR activity.26 N atoms can polarize the 2p orbital of the C atoms that are situated between the B and N dopants, leading to the donation of extra electrons to the adjacent B atoms, which is beneficial to facilitate the adsorption and binding of OOH*. Therefore, this charge transfer process results in a synergistic coupling effect between B and N heteroatoms, wherein the N sites perform as electronwithdrawing groups to indirectly activate B and thereby make the latter an electroactive site to improve the oxygen reduction activity. Dual doping strategies have also been extended to prepare carbon-based HER electrocatalysts.27 Electronic structure engineering on graphene nanostructures through doping heteroatom couples with different charge densities (e.g., N and S) can efficiently adjust the electron-donor property and electrocatalytic activity of neighboring C atoms, leading to a downshift in the valence bands of active C atoms.28,29 This produces much more favorable H* adsorption−desorption ability for the dual-doped graphene models, while the free energy of H* adsorption is regarded as a major descriptor of HER activity. As a result, proof-of-concept investigations on the basis of theoretical and experimental methods exhibit that dual dopants are capable of inducing enhanced HER activity in terms of low onset potential and high exchange currents (Figure 2).29 Although the dual doping route can effectively increase the unit electroactivity, performance is still lacking when compared to metallic counterparts. One of the primary reasons for this is the generally low dopant content in the material, which limits the number of catalytic active sites. Coupling carbon nanostructures with other active components (metallic or non-metallic moieties) to form hybrid systems provides a prospective way to further improve electrocatalytic performance.

Figure 1. Schematic illustration of water splitting and fuel cell reactions.

Further, the engineering of pristine carbons by chemical replacement of carbon atoms with heteroatoms, such as N, P, B, and S, is an efficient way to alter electronic structures and (electro-)chemical properties.11 Currently, the electrocatalytic activity of single-doped carbon remains inferior to noble metallic benchmarks. However, the doping of two elements with distinct electronegativities into the carbon frameworks can create a peculiar electron-donor property, which can cause a synergistic effect between the two dopants, leading to enhancement in electrochemical performance.12,13 Carbon nanostructures have also been widely employed as crucial matrixes to support electroactive components through physical mixing to increase surface loading and electronic conductivity.14 However, the lack of long-range order or structure control and poor contact among the different components results in a substantial impediment to the realization of effective functionalized catalysts with potential practicability and scalability.15 Indeed, engineering of the interfacial chemistry between the active components and the carbon supports to realize composite structures can induce strong synergistic effects for accelerated and stable electrocatalysis.16−18 Interface engineering through various routes like chemical doping and posttreatment relates to weak (e.g., hydrogen bonding, electrostatic attraction, van der Waals interaction) or strong (e.g., covalent bonding) coupling effects.19 Such a coupling not only can impart the changes to the local coordination environment and electronic states but also allows separate reaction procedures to occur in close proximity at different active sites and collaboratively expedite catalysis.20,21 Accordingly, the aforementioned concepts may provide alternative methodologies for efficient electrocatalyst design and new possibilities to improve their catalytic efficiencies. From recent achievements in high-efficiency catalysts, it is suggested that some requirements need to be satisfied in order to obtain better functionality and performance. These include (1) abundant active or anchoring sites, which can be achieved by controlling synthetic parameters and chemical modifications, (2) the electron mobility capability that relies on the electrical conductivity of the electrocatalyst itself, and (3) superior electrode configuration, a prerequisite in ensuring accessible active sites and enhanced transport of reaction relevant species. In this Account, we will focus on material design and its impact on overall performance. Based on this, the structure− performance correlations and general protocols will be further proposed, which may provide some insight for future materials and device optimization.

2.2. Carbon−Heteroatom-Doped Carbon Composites

Indeed, non-metal materials that are stable in harsh conditions and that are less active or inactive in electrocatalysis have been coupled with carbons to impart enhanced performance. The 916

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Figure 2. (a) Molecular configurations of three kinds of dual-doped graphene model with lowest free energy of H*. (b) Three-state free energy diagram of the graphene models. (c) Polarization curves and (d) measured exchange current (i0) of the graphene materials with the MoS2 benchmark under the same conditions for comparison. Reproduced from ref 29. Copyright 2016 Nature Publishing Group.

Figure 3. (a) Photograph of N doped graphene−carbon nanotube films (NG-CNT) for water oxidation. (b) SEM image of NG-CNT. (c,d) LSV curves of NG-CNT, G-CNT, dry NG-CNT, and noble IrO2. Reproduced from ref 32. Copyright 2014 John Wiley & Sons, Inc.

elements into the carbon network can attain novel metal-free hybrid materials.14,28 It is suggested that a practically realizable metal-free material has the potential to surpass the performance of metallic counterparts through further tailoring of experimentally achievable physicochemical properties.29

availability of various non-metal materials with tunable structures and compositions can significantly broaden the selection of candidates for high-efficiency electrocatalysis. Usually, performance enhancement of non-metal combinations can be attributed to the increased number of electroactive sites on the carbon support, increased electrical conductivity that can favor fast charge transport through the support, and promoted electron-donor properties to modify surface adsorbing behaviors and reaction processes.14,30 All of these aspects can synergistically enhance catalytic activity. Chemically doped carbon nanostructures, such as N-doped graphene, usually demonstrate moderate electrocatalytic activity. This is typically due to unexpectedly low surface areas and potentially less active centers caused by the restacking effect of the nanosheets. Although chemical or physical activation of these materials is attempted by enriching their porosity, the resultant materials are generally economically and environmentally unfavorable. However, by directly introducing porous carbon frameworks like hierarchically meso- and macroporous carbons as the support for in situ growth of Ndoped graphene, the composite materials became excellent metal-free ORR electrocatalysts.31 The superior activity was correlated with the close contact between the support and Ngraphene, reducing the activation energy, and the wellstructured porosity of the material, offering more accessible active sites.31 By taking advantages of the strong π−π interaction between different carbon nanostructures (Figure 3),32 assembly of N-doped graphene and carbon nanotubes (CNTs) was shown to form a substrate-free hydrogel film, in which CNTs could effectively avoid the stacking of the graphene nanosheets and act as linkages to facilitate electron migration. This facilitated electroactivity toward water oxidation and produced considerable performance, even outperforming precious IrO2 benchmarks.32 The electrochemical activity of chemically doped carbon intensively relies on the doping site functionality, implying that doping other heteroatoms (e.g., P, B, S, I, O) or dual doping multiple

2.3. Carbon−g-C3N4 Composites

In regards to N-doped carbon, limited N doping levels (generally less than 10 atom %) tend to lead to a finite apparent activity. g-C3N4 with its high N content (theoretically about 57 atom %) is promising for introducing higher N levels into the carbon framework. However, when used for the ORR, its limited electron transport capability favors the inefficient 2e− dominated reaction pathway, resulting in the accumulation of OOH− intermediates and thus unsatisfactory reaction selectivity and efficiency.33 Therefore, further use of g-C3N4 in electrocatalysis should first address the problem of its inherent poor conductivity.33−35 After incorporating g-C3N4 onto electron-conductive matrixes like conductive mesoporous carbons, the resultant hybrid materials were found to be highly active ORR catalysts. This is because the improvement in electron transfer ability could eliminate the barrier to further reduction of adsorbed OOH− intermediate products, which promoted adsorbed O2 molecules to directly generate OH− through a 4e− pathway (Figure 4).33 In fact, we found that different synthesis routes had a significant influence on the interaction between g-C3N4 and the carbon structures and, thus, on the hybrid structures and their electrocatalytic properties. Driven by electrostatic interaction and π−π stacking, exfoliated g-C3N4 nanosheets and CNT assemblies possess porous three-dimensional nanostructure and high nitrogen content.36 Specifically, the extensive electronaccepting tertiary and pyridinic N species in g-C3N4 can cause highly positive charge density on the nearby sp2-hybridized C atoms, which could actually favor the adsorption of reactants (OH− in alkaline electrolytes), facilitate the electron transportation between the reaction intermediates and catalyst surface, and consequently improve electrochemical OER above 917

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Figure 5. (a) N K-edge NEXAFS spectrum of g-C3N4 and C3N4@NG. (b) Scheme of interfacial electron transfer in C3N4@NG. (c) Simulated free-energy diagram at equilibrium potential and (d) experimental HER polarization curves of different electrocatalysts. Reproduced from ref 37. Copyright 2014 Nature Publishing Group.

Figure 4. (a) TEM image of g-C3N4@CMK-3 nanocomposite and (b) ORR performance evaluation. (c) Free energy diagrams of electrocatalytic ORR and optimizational configurations of adsorbed species on g-C3N4 with zero, two, and four electron participation illustrated as paths I−III.

3.1. Metal−Non-metal Catalysts with M−N−C Active Sites

Emerging research regarding metal−nitrogen as a kind of organometallic material consisting of transition metal elements (e.g., Fe, Co, Ni) coordinated by macrocycles like phthalocyanine and phenanthroline could ensure the rapid electron transport process in electrochemical reactions.40 In these materials, it is believed the catalytic activity originates from the existence of delocalized π-electrons in the macrocyclic structures. However, the inferior stability of organometallic complexes in harsh environments (e.g., strong acid or alkaline in which ORR, HER, and OER proceeded), owing to metal dissolution and material decomposition, significantly limits their broad application potential. Alternatively, pyrolysis and carbonization of the transition-metal macrocycles under the protection of inert gases at temperatures usually over 700 °C can lead to the formation of M−Nx−C materials (Figure 6),41,42 which manifest largely improved activity and stability in comparison to pristine organic macrocycles. During the heat treatment, nitrogen-rich ligands can be transformed into graphitic, pyridinic, and pyrrolic nitrogen species that are chemically bonded to the metal centers and carbon atoms in the carbon framework. Simultaneously, the elementary metals or metal carbides stemming from the reduction of pyrolysis gas can reversely catalyze the generation of graphitic carbon shells to increase the resultant material’s conductivity. As a consequence, this method can not only ensure the homogeneous distribution of catalytically active sites but also elevate the electronic connectivity within the whole system. DFT calculations indicate that a decreased local work function on the carbon surface, resulting from nitrogen doping as well as the electron transfer from the embedded metallic nanoparticle to the outer carbon layer, can impart impressive catalytic activity.43 It is acknowledged that simultaneous pyrolysis of virtually any combination of nitrogen, transition metal, and carbon precursors can form the M−Nx−C materials with certain activity. Accordingly, costly organometallic macrocycles can be reasonably substituted by a wide range of cost-effective

that of the precious IrO2 benchmark. Additionally, an atomiclayered composite of g-C3N4 chemically coupled with N-doped graphene nanosheets, within which a strong interaction from both sides formed through the generation of extra interlayered bonds, manifested high HER activity, which rivalled that of conventional metallic counterparts. 37 According to the descriptor of Gibbs free-energy of intermediator H* adsorption, the synergy effect between g-C3N4 nanosheets (excessive adsorption of H*) and N-doped graphene (very weak H* adsorption) created a moderate adsorption−desorption behavior of hydrogen on the surface of the hybrid thereby facilitating the overall electrochemical hydrogen evolution kinetics (Figure 5).37 Further doping immobilized g-C3N4 with heteroatoms like P would disturb the graphitic structure of g-C3N4 and form flower-like P-g-C3N4.38 It should be noted that the P dopants can not only perform as reinforcing active sites but also tailor the electronic structure and improve the delocalized π bonds of g-C3N4 to enhance the electronic conductivity, thus inducing promising activity toward both oxygen reduction and evolution reactions.38 Material like this can potentially be used directly as cathodes in Zn−air batteries that feature low charge/discharge overpotential and long lifetime.

3. METAL−NON-METAL HYBRID ELECTROCATALYSTS Of note, the tunable composition of carbon nanomaterials through chemical methods can introduce various heteroatoms into the carbon backbones, such as N, whereas the doping sites can further serve as anchorage centers to accommodate metal moieties through strong coupling effects, forming electrocatalytically active and stable metal−nitrogen−carbon (M−N− C) species.39 918

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Figure 6. General molecular structures of the M−Nx−C material.

transition-metal sources (e.g., nitrates, sulfates, acetates, chlorides), nitrogen precursors (e.g., polyaniline, melamine, acetonitrile), and carbon supports (e.g., graphene, carbon nanotubes, activated carbons).44,45 Notably, a creative and thoughtful choice of the right precursors and proper synthetic parameters (control over heat treatment temperature and duration, precursor reaction in solution, postmodification strategies, etc.) play a pivotal role in yielding better-performing electrocatalysts. For instance, Prussian blue could be applied to synthesize Fe−N−graphitic-carbon-bulb materials at pyrolysis temperatures as low as 550 °C,46 where low treatment temperatures ensured high doping levels of N and Fe. This facilitated the formation of graphitic structures and yielded fast kinetics and high selectivity toward ORR in both acidic and alkaline electrolytes. Although the nature of the exact active centers in such M− Nx−C catalytic systems remains ambiguous, there is no doubt that well-defined porosity and micro- or nanostructures influence catalytic performance to a great extent. Toward this goal, pyrolysis products of the as-synthesized M−Nx−C are usually subject to acid washing to remove metal impurities and enlarge surface areas as much as possible. Disappointingly, the resultant catalytic activity may suffer due to the collapse of porous structures at elevated temperatures and partial loss of active sites after acidic etching. To alleviate problems of collapse and agglomeration, some rigid templates, such as silica nanoparticles and anodic alumina, are normally utilized during carbonization. However, the hard-templating route inevitably involves tedious and time-consuming procedures, such as preparation and dispersion of templates and the corresponding postsynthesis removal, which hinders production scale-up.47 As such, polymer templates, for example, polystyrene (PS) microspheres and block copolymer (e.g., F127), are capable of homogeneously penetrating into the precursor matrices.48 Subsequent high-temperature calcination results in the concurrent formation of porous Fe−Nx−C materials and the elimination of templates (wherein the PS and F127 act as templates for creating macropores and mesopores respectively). The integration of hierarchical porosity and good conductivity provides the composite electrocatalysts with outstanding ORR activity, even closely comparable to or better than commercial Pt/C.48 By taking advantage of the M−Nx interaction, we assembled two-dimensional (2D) graphitic carbon nitride (g-C3N4) and titanium carbide (with MXene phase) nanosheets to generate free-standing films that exhibited outstanding activity in efficiently catalyzing the OER, with Ti−Nx serving as the electroactive sites (Figure 7).49 Nonetheless, direct evidence of transition metal participation in the active units of the M−Nx− C system is still lacking. In either situation, nitrogen species

Figure 7. (a) Scanning (SEM) and (b) transmission electron microscopy (TEM) images of TCCN (overlapped Ti3C2 and gC3N4 nanosheets). (c) N K-edge synchrotron-based near-edge X-ray absorption fine structure (NEXAFS) of TCCN and g-C3N4 with inset showing the relative N sites. (d) OER polarization curves. Reproduced from ref 49. Copyright 2016 John Wiley & Sons, Inc.

bonded within the carbon networks are likely significant to the active-site performance, though the nature of the active sites is yet to be conclusively identified. This requires the combination of theoretical simulation and advanced material characterization techniques (e.g., X-ray absorption spectroscopy, Mössbauer spectrum) to gain deeper understanding. 3.2. Metal Oxide− and Hydroxide−Nitogen−Carbon Composites

Apart from high-temperature calcination to incorporate nitrogen dopants, relatively mild hydrothermal and solvothermal techniques are also effective in synthesizing N-doped nanocarbons. Defective carbons can react with nitrogen precursors (e.g., ammonia, urea) during hydrothermal treatment to induce the doping process and partial reduction of carbons simultaneously. Nitrogen functional sites in the carbon frameworks can effectively attach metal precursors to grow metal oxide or hydroxide nanoparticles or nanocrystals on the surface through different treatment methods.50,51 The resultant metal−carbon hybrid materials present strong interplay from the two components. Such a strong coupling effect may trigger the electronic or chemical modification between the metallic active sites and carbon matrices, producing synergistically 919

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Accounts of Chemical Research enhanced electrochemical performance. Based on a one-step hydrothermal process with the assistance of ammonia, a hybrid material composed of Mn3O4 nanoparticles uniformly decorated on nitrogen-doped graphene shows remarkable performance toward ORR, including high activity, robust stability and respectable tolerance to the methanol crossover effect.52 Moreover, simply modulating the synthesis conditions, such as solvent types and hydrothermal time, can effectively adjust the nanostructures of immobilized Mn3O4 nanocrystals from spheres to cubes and ellipsoids. This could provide a way of elucidating the shape effect of active units on electrocatalysis.53 Currently, most of the experimentally available electrocatalysts are prepared in the form of 2D planar electrodes through dip coating or solution casting with the use of polymer binders and conductive supports (e.g., glassy carbons, metal foils). This usually results in poor electrode contact, limited catalyst loadings, and uncontrollable microstructures that are detrimental to electron mobility and multiphase mass transportation.54 Alternatively, designing 3D self-supported electrodes for direct device application can enormously decrease cost and simplify the manufacturing process of the whole system. Recently, we have successfully developed a series of graphenebased 3D binder-free hydrogel films with extraordinary mechanical flexibility through a macroscopic assembly methodology.55−57 The directional self-assembly, driven by strong π−π intersheet interactions, of graphene nanosheets under vacuum suction facilitated the formation of films with extraordinary mechanical flexibility. Free-standing graphene films can interact with transition-metal clusters, which act as spacers between the graphene nanosheets to inhibit restacking, creating a large amount of out-of-plane pores and ensuring rapid transportation of reactants and products across the whole film. Different kinds of active metallic species can be inserted into the graphene scaffolds by changing treatment methods, signifying that the electrocatalytic functionalities can be easily tuned. Ni nanoparticles confined in N-graphene films, exhibiting comparable activity to noble IrO2 in electrochemical oxygen evolution, could be fabricated with the use of N2H4 as the reductant (Figure 8a,b).55 As for this hybrid system, the NiOOH/Ni complex species resulted from the partial oxidation of Ni nanoparticles during the water oxidation process and the Ni− N−C sites were responsible for the improved OER performance. If the anchored metallic precursors were immobilized on the surface of graphene sheets in alkaline environment (e.g., urea) without reduction, metal hydroxide-involved selfsupported films could be synthesized, for instance, NiCo double hydroxides deposited on N-doped graphene hydrogels for effective water electrolysis (Figure 8c,d).56 After annealing in air, NiCo2O4 nanoparticles confined in N-doped graphene nanosheets could be obtained (Figure 8e,f).57 Note that 2D layered materials (e.g., graphene) generally consist of covalently bonded and dangling-bond-free lattices, wherein the neighboring layers are weakly bound through weak interactions, such as hydrogen bonding and van der Waals interactions.58 This makes it possible to isolate and match disparate layered materials to create a wide range of hybrids without the restrictions of lattice matching and processing compatibility. This can be seen with the combination of graphene with WS2,59 wherein the coupling of intrinsically active WS2 nanosheets and heteroatom-doped graphene matrices could render abundant active units and optimize electronic connectivity contributing to facile HER kinetics, leading to competitive hydrogen evolution activity to benchmark Pt.

Figure 8. (a) SEM image of Ni-NG and (b) polarization curves of NiNG, NG, and PNi-NG (powdery Ni-NG). Reproduced from ref 55. Copyright 2013 Royal Society of Chemistry. (c) SEM image of NGNiCo (NiCo double hydroxides on N-doped graphene hydrogels) and (d) polarization curves of NG-NiCo, dry NG-NiCo, and 2D NGNiCo. Reproduced from ref 56. Copyright 2013 John Wiley & Sons, Inc. (e) SEM image of PNG-NiCo (porous N-doped graphene− NiCo2O4), and (f) polarization curves of PNG-NiCo, NG-NiCo (Ndoped graphene-NiCo2O4 film), and PNG.

Aforementioned results reveal that the presence of nitrogen in the binder-free graphene-based films can favor the affinity between metallic precursors and graphene supports, causing a uniform distribution of metallic compound nuclei. More significantly, the considerable interactions between nitrogen functional groups and metal centers, because of the high electronegativity of nitrogen atoms, may promote the electrocatalytic processes due to the formation of metal−nitrogen− carbon bonds in the composites. Remarkably enough, the rate of an electrocatalytic reaction strongly depends on the geometric features and electronic structure of the catalyst, which can act together in affecting the adsorption for reaction intermediates and activation energies for the reactions. Accordingly, engineering crystallinity and surface structure at the atomic level (e.g., introducing vacancies) of the active metallic components in the hybrid systems provides new approaches to advanced catalyst design for broad electrocatalytic applications.60−63

4. CONCLUDING REMARKS Recent trends in research have furthered the subtle use of materials design and surface science principles to achieve highly 920

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Accounts of Chemical Research efficient electrocatalytic systems with fascinating synergistic effects. Using surface and interface atomic and molecular engineering approaches, electrocatalysts with novel physicochemical properties and synergistic effects have been designed and fabricated, exhibiting efficient catalytic activities toward ORR, OER, and HER. The combination of experimental and theoretical investigations has also been used to direct the fabrication of highly active electrocatalysts and to disclose reaction pathways. The ultimate target is to develop costeffective and high performing electrocatalysts for elementary electrochemical reactions. The electroactivity of these catalysts strongly depends on the dopant type for carbon materials and the coupling patterns between active components and carbon supports for the carbon-based hybridized systems, which are generally determined by precursors ranging from metals to non-metals and synthetic strategies. Future efforts in the preparation of electrocatalysts with synergistic activity is likely to concentrate primarily on precise control of the interplay between different elements and components in order to realize efficient catalysts with optimal chemical compositions and micro- or nanomorphologies, as well as to maximize the density of electrochemically active sites. The ultimate goal of designing efficient electrocatalysts is to develop effective renewable energy devices, such as rechargeable metal−air batteries with reverse ORR/OER occurring at the cathodes. Indeed some bifunctional electrocatalysts have been preliminarily employed in metal−air batteries or overall water splitting. Further improvement of preparation technology should be conducted besides optimization of compositions and structures, aiming to realize high efficiency and long operating life. An in-depth understanding of the electrocatalytic reaction mechanism and the active sites is necessary for rational design of non-precious-metal-based and even metal-free electrocatalysts with activity, selectivity, and durability matching and eventually exceeding that of their precious metal counterparts. This will come from detailed studies on model catalysts through in situ, ex situ, and operando techniques, thus promoting our fundamental and theoretical understanding of the electrochemical reaction mechanism involved at the molecular level. Additionally, combining experimental and computational strategies will be valuable in directing the proofof-concept atomic and molecular design and understanding the origin of electrocatalytic activities, thereby providing a way to develop high-performance electrocatalysts for other electrochemical reactions besides HER, OER, and ORR, such as CO2 electroreduction and N2 electrochemical fixation.

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research interest relates to functional nanomaterials for efficient energy conversion and storage. Chunxian Guo received his Bachelor and Ph.D. degrees from Nanyang Technological University (Singapore) in year of 2007 and 2011, respectively. Now he works as a DECRA Fellow with Prof. ShiZhang Qiao, focusing on the design and synthesis of efficient electrocatalysts by surface atomic engineering of nanostructured materials, in particular two-dimensional nanomaterials. Yao Zheng received his Bachelor and Ph.D. degrees from Nanjing Tech University (China) and University of Queensland (Australia) in years 2006 and 2014, respectively. Currently he is a DECRA research fellow in University of Adelaide (Australia) working with Prof. ShiZhang Qiao. His research focuses on developing efficient catalysts for some key electrocatalysis processes like oxygen reduction, hydrogen evolution, and CO2 reduction reactions. He is trying to develop a designing procedure of electrocatalysts by combining the power from DFT computation, nanotechnology, materials characterization, and electrochemical measurement. Shi-Zhang Qiao received his Ph.D. degree in chemical engineering from the Hong Kong University of Science and Technology in 2000 and is currently a Chair Professor at the School of Chemical Engineering of The University of Adelaide, Australia. His research expertise is in nanomaterials and nanotechnologies for new energy applications. He has coauthored more than 280 papers in refereed journals with over 19000 citations (h-index 74). In recognition of his achievements in research, he was honored with a prestigious ExxonMobil Award (2016), an ARC Discovery Outstanding Researcher Award (2013), and the Emerging Researcher Award (2013, ENFL Division of the American Chemical Society). Prof. Qiao is a Thomson Reuters Highly Cited Researcher.

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ACKNOWLEDGMENTS We acknowledge the financial support from the Australian Research Council (ARC) through the Discovery Project program (DP130104459, DP140104062 and DP160104866) and the Linkage Project program (LP160100927).

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REFERENCES

(1) Mahmood, A.; Guo, W.; Tabassum, H.; Zou, R. Metal-Organic Framework-Based Nanomaterials for Electrocatalysis. Adv. Energy Mater. 2016, 6, 1600423. (2) Mistry, H.; Varela, A. S.; Kuhl, S.; Strasser, P.; Cuenya, B. R. Nanostructured Electrocatalysts with Tunable Activity and Selectivity. Nat. Rev. Mater. 2016, 1, 16009. (3) Li, S.; Lai, J.; Luque, R.; Xu, G. Designed Multimetallic Pd nanosponges with Enhanced Electrocatalytic Activity for Ethylene Glycol and Glycerol Oxidation. Energy Environ. Sci. 2016, 9, 3097− 3102. (4) Hong, W. T.; Risch, M.; Stoerzinger, K. A.; Grimaud, A.; Suntivich, J.; Shao-Horn, Y. Toward the Rational Design of NonPrecious Transition Metal Oxides for Oxygen Electrocatalysis. Energy Environ. Sci. 2015, 8, 1404−1427. (5) Higgins, D.; Zamani, P.; Yu, A. P.; Chen, Z. W. The Application of Graphene and Its Composites in Oxygen Reduction Electrocatalysis: A Perspective and Review of Recent Progress. Energy Environ. Sci. 2016, 9, 357−390. (6) Xiao, P.; Chen, W.; Wang, X. A Review of Phosphide-Based Materials for Electrocatalytic Hydrogen Evolution. Adv. Energy Mater. 2015, 5, 1500985. (7) Hunter, B. M.; Blakemore, J. D.; Deimund, M.; Gray, H. B.; Winkler, J. R.; Muller, A. M. Highly Active Mixed-Metal Nanosheet Water Oxidation Catalysts Made by Pulsed-Laser Ablation in Liquids. J. Am. Chem. Soc. 2014, 136, 13118−13121.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (S.-Z.Q.). ORCID

Shi-Zhang Qiao: 0000-0002-4568-8422 Author Contributions ‡

Y.P.Z. and C.X.G. contributed equally to this work.

Notes

The authors declare no competing financial interest. Biographies Yun Pei Zhu received his Ph.D. degree in physical chemistry from Nankai University in 2016. Currently he is a postdoctoral researcher in the University of Adelaide working with Prof. Shi-Zhang Qiao. His 921

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Accounts of Chemical Research

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DOI: 10.1021/acs.accounts.6b00635 Acc. Chem. Res. 2017, 50, 915−923

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DOI: 10.1021/acs.accounts.6b00635 Acc. Chem. Res. 2017, 50, 915−923