Well-Defined 2D Covalent Organic Polymers for Energy Electrocatalysis

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Well-defined 2D Covalent Organic Polymers for energy electrocatalysis Peng Peng, Zhuohang Zhou, Jianing Guo, and Zhonghua Xiang ACS Energy Lett., Just Accepted Manuscript • Publication Date (Web): 05 May 2017 Downloaded from http://pubs.acs.org on May 8, 2017

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Well-defined 2D Covalent Organic Polymers for Energy Electrocatalysis Peng Peng, Zhuohang Zhou, Jianing Guo and Zhonghua Xiang* State Key Laboratory of Organic–Inorganic Composites, Beijing University of Chemical Technology, Beijing, 100029, PR, China Email: [email protected] ABSTRACT

Highly efficient electrocatalysts are vital to meet the energy and environmental challenges. Although numerous nonprecious-metal or metal-free carbon-based catalysts have been demonstrated to entirely or partially replace noble-metal-based electrocatalysis, the absence of precise design and predictable process hindered the development. Well-defined 2D Covalent Organic Polymers (COPs) as a new exciting type of electrocatalyst presented superior potentials with precisely controllable capacities, such as robust tailoring heteroatom incorporation and location of active sites. Here we demonstrate the possibilities and potential of the well-defined 2D COPs used as highly efficient energy electrocatalysts for clean and renewable energy technologies. After surveying recent developments, we further discuss the possible future directions on designed synthesis of intrinsic COPs without carbonization to modulate active sites and the density of active sites at the molecular level. COP materials as a new family of electrocatalysts offer practical possibilities to study the structure, mechanism and kinetics of energy electrocatalysis and may lead to a better solution for energy and environmental issues. 1

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As the energy consumption increases at an alarming rate, clean and renewable energy technologies, especially hydrogen energy which displays high energy density and zero pollution have attracted everlasting interest. As shown in Figure 1a, hydrogen ecosystem mainly involves water electrolysis and fuel cell. In water electrolysis system, water is split into oxygen and hydrogen via oxygen evolution reaction (OER) at anode and hydrogen evolution reaction (HER) at cathode, whereas fuel cell system converts the generated hydrogen and oxygen from water electrolysis system to water along with electricity and heat via hydrogen oxidation reaction (HOR) at anode and oxygen reduction reaction (ORR) at cathode.1 Generally, metal-based or metal oxides catalysts, especially noble metals (e.g., platinum, iridium and palladium) are used for these important reactions.2 Platinum is regarded as the best catalyst for the ORR3 and HOR,4 and shows unbeatable electrocatalytic HER properties5 with

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Figure 1. (a) The hydrogen ecosystem via famous fuel cell and water electrolysis. In water electrolysis system, OER and HER take place and water is split into oxygen and hydrogen, while Fuel cell system converts hydrogen and oxygen to water through HOR and ORR. (b) Polarization curves of typical HER, ORR and OER and the related mechanism.1 The ORR involves a multi-electron transfer process in which O2 is converted into H2O or OH-. In acidic solution, O2 can be reduced in a 4e- process and converted into H2O; or undergo a partial 2e- reduction to form hydrogen peroxide, followed by another 2e- reduction then converted into H2O. In alkaline solution, O2 can be reduced by a 4e- process to form hydroxide, or by two 2e- processes to form HO2- and then OH-. The general parameter used to evaluate their activity is half-wave potential (E1/2); the higher the potential, the better the ORR activity. The mechanism of the HER in alkaline media is typically treated as a combination of three elementary steps: the Volmer step (H++e-+*→ H*)-water dissociation and formation of a reactive intermediate, followed by either the Heyrovsky step (H*+H++e-→H2+*) or the Tafel recombination step (2H*→H2+2*). The OER in acidic medium is a complex reaction pathway. Typically, OER can be considered as the microscopic reverse of the ORR. To measure the reaction rate of OER, it is customary to use the current density at a fixed potential.

extremely high exchange current density and small Tafel slope.6 RuO2 and IrO2 have emerged as the most promising catalyst candidates for the OER among the many systems explored so far.7-8 However, noble metal-based catalysts have conspicuous disadvantages, such as low selectivity, poor durability, and susceptibility to gas poisoning.9 Moreover, large-scale commercial applications of clean and renewable

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energy technologies requires a substantial amount of catalyst, hence the high cost and scarcity of precious metals on earth are also barriers. Covalent Organic Polymers show promise as alternative highly efficient electrocatalysts with notable advantages over noble-metal-based electrodes. As minutely demonstrated in Figure 1b, the ORR involves a multi-electron transfer process in which O2 is converted into H2O or OH- and HER involves a classic two-electron transfer reaction with H* (where * denotes a site on the electrode surface) as the only intermediate. OER and HOR involve the same reaction steps as the ORR and HER respectively except in reverse. To drive the above-mentioned reactions effectively, an active catalyst is required to minimize the overpotential and to increase the catalytic efficiency. Based on this demand, researchers have demonstrated that the doping effect of hetero atoms can affect the electronic structure of neighboring carbon atoms and induce the uneven charge distribution of carbon atoms, which can enhance the electrochemical reactivity.10-13 For examples, carbon atoms next to pyridinic nitrogen are demonstrated as the active sites for the ORR14 and the embedded transition metal could considerably influence or even determine the electrocatalytic performance because of their tunable energy states.15-19 To this point, N- and other heteroatoms single/co/multi-doing approach in carbon nanomaterials has been widely studied as one of the most feasible methods to modulate and fabric candidates for electrocatalysis.20-23

Recently,

numerous

nonprecious-metal

or

metal-free

carbon-based catalysts have been demonstrated to completely or partially replace noble-metals for clean and renewable energy technologies.23-27 Particularly, the 4


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synthesis and heteroatom doping of carbon-based metal-free bi-/multi-functional catalysts for ORR/OER/HER, as well as single fuel cell performance evaluation are increasingly investigated.28-31 For example, Qiao’s group has investigated HER on various chemically synthesized non-metal heteroatom-doped graphene and shed light on the underlying activity origin of these graphene-based materials through linking the activities with DFT computed reaction energetics on representative models.32 Their work elucidated catalyst design principles of graphene-based electrocatalysts for energy conversions. However, because of the absence of precise design and predictable process, these existing methods are difficult to facilitate well-defined structures and controllable active sites for these important electrocatalytic reactions. Most recently, well-defined 2D Covalent Organic Polymers (COPs) have presented superior potentials with precisely controllable capacities, such as robust tailoring heteroatom incorporation and location of active sites, pore sizes, etc. Along with tremendous development of porous organic polymers since Yaghi and co-workers discovered the topological frameworks in 2005,33 a diversity of COP materials functionalized to contain robust catalytic sites for efficient activity and well-defined building blocks for efficient mass transport has been obtained.34-35 In this perspective, we demonstrate the possibilities and potential of these well-defined 2D COPs used as highly efficient energy electrocatalysts for performance-oriented catalysts.36-38 Electrocatalytic activity of well-defined 2D COPs can be tuned with the colorful designed-synthesis. 5


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Figure 2. Development strategies of well-defined 2D COPs as energy electrocatalysts: the intrinsic activity can be promoted by introducing active sites such as metal/non-metal dopants and hierarchy porous structures contain defects; Content of active sites can be improved by using supports with large surface area and high conductivity, or the intrinsic well-defined nano-structures.

The development strategies for electrocatalytic catalysts should aim to increase the number of active sites and increase the intrinsic activity of each active site.2 As described in Figure 2, COPs with periodic and built-in molecular ordering are capable to control the doped heteroatoms clearly,34, 39 thus promote the intrinsic activity. 2D COPs with appropriate design can stabilize the dopants and tune the binding energy value to provide rapid proton adsorption, reduction and product release process.40 Besides, COPs pave a way to form well-defined electrocatalytic moieties which adjustably increase the content of the active site.

Specifically, by choosing proper building units and synthetic routes, design and synthesis of COPs with enhanced activity for energy electrocatalysis are by no means

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Figure 3 Representative procedures for design and synthesis of 2D COPs and the probable coupling reactions.34 Generally used methods for the synthesis are listed, including solvothermal,33 ionothermal,41 microwave42 and wet chemical synthesis.34 The structures of the representative 2D COP materials include COP-4,43 COP-P-M,44 Star-COF-3,45 COF-LZU146 and 1D/2D-CAP.47

a trivial issue. As shown in Figure 3, robust tailorable building blocks and synthetic routes provide a great platform to prepare rich, promising alternatives of carbon-based and carbonaceous-transition metal hybrid catalysts which have shown considerable intrinsic activity as energy electrocatalysis.48-51 Many types of coupling reactions can be used, such as boronic acid-based coupling, amino group-based coupling, alkynyl group-based coupling, bromine group-based coupling and cyan group-based coupling.34,

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Moreover, rich synthetic methods, e.g., the solvothermal,33

ionothermal,41 microwave,42 as well as ‘bottom up’ chemical synthesis34 also can be applied for the synthesis of COP materials. With the predictable synthesis routes, COPs offer a chance to profoundly investigate the nature of catalytic sites in a simple

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Figure 4. Design of various COP catalysts with well-defined structure and adjustable activity, including: Hierarchy structures such as 2D graphene-like layered morphology53 and 3D structures with a high surface area.54 Non-metal configuration, for example, only containing covalent triazine-based frameworks (CTFs).41 Heterometal-Embedded, take the contribution of non-noble metal to optimize the catalytic activity.44 Hybrid with conductors like carbon nano tube55/graphene,56 with the COPs acting as the active sites and the conductible substrates facilitating the electron-transfer process. The image of 3D structure was reproduced with permission from ref 54, Copyright 2016 American Chemical Society. The image of hybrid with nanotube was reproduced with permission from ref 55, Copyright 2015 American Chemical Society.

but clear system with only desired elements, and guide the synthesis of highly efficient catalysts.

Meanwhile, the synthetic approach of COPs is also capable of developing catalysts with well-defined and adjustable architecture. Combined with the procedure to enhance intrinsic activity, COPs have paved a controllable way to promote the content of catalytic sites. As presented in Figure 4, hierarchy porous structures in the designed synthesis of COP materials 54 can provide larger surface area, more exposed

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active sites and better electrical conductivity. To take advantage of synergistic effect, the hybridization of COPs and conducting materials, e.g., MWCNTs/ graphene, also has been realized by in-situ polymerization.55-56 These hybrid nanostructures can utilize COPs as highly active sites and electrically conducting channel to facilitate the electron-transfer process. Moreover, the combination with controllable synthesis provides a feasible strategy to tune the location and configuration of both non-metal and hetero-metal dopants, and makes it possible to optimize the content of active sites for more efficient catalysts and mechanism study as well (vide infra). The ordered distribution and tunable pore volume of COP materials embedded by heterometal57 can increase the content of active sites and take full use of doped active centers. Meanwhile, the non-metal doped configuration is possible to fabricate catalysts with simple and clear structures, and used to desirably promote the activity.

Well-defined 2D COPs have shown comparable electrocatalytic performance with noble-metal-based electrodes.

In Figure 5 it shows some representative samples with tunable electrocatalytic performance using COPs as precursors for ORR catalysts in alkaline media43-44, 57-58, along with HER56 and OER55, 59 catalysts. As ORR catalysts, COPs have exhibited outstanding performance. The transition-metal-incorporated COPs exhibit a similar onset potential compared with the benchmarked Pt/C,57, 60 while some even displayed higher kinetic current and diffusion-limiting current than those of the benchmarked Pt/C in alkaline solution.57 Similarly, transition-metal-doped COPs with the inherent

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Figure 5. Electrocatalytic activities of COPs as highly efficient catalysts: ORR catalyst in alkaline media was summarized from C-COP-4,43 NC900,58 C-COP-P-Co44 and PCN-FeCo/C57 (Left). OER catalyst was represented by Co(II) impregnated bipyridine-containing covalent organic framework Co-TpBpy59 (Middle). HER catalyst was represented by C3N456 (Right). The figure of OER catalyst was reproduced with permission from ref 59, Copyright 2016 American Chemical Society. The figure of HER catalyst was reproduced with permission from ref 56, Copyright 2014 Nature Publishing Group.

high surface area have shown high and stable performance as promising OER catalysts.

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Additionally, the hybrid of well-defined N-doped COP and N-doped

graphene has seen great promotion in HER electrocatalysis, which originated from the synergistic effect of this hybrid nanostructure.56 Compared to noble-metal-based electrodes, the COP materials have shown better tolerance for methanol and gas-poison and better durability for long operation time and cycles. With targeted design, the tunable COPs are capable to display better performance on the value of onset potential, kinetic current, etc.

Current challenges and the outlook for COP materials as highly efficient electrocatalysts. 10


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Even though highly efficient COP-derived electrocatalysts have been developed rapidly over the past decade, many key questions concerning the catalysts remain unanswered. Compared with noble-metal, the activity of conventional energy catalysts in acid media still needs considerable improvement, especially for ORR process. Besides, the role of 2D COPs in the catalytic system is still not very clear. For real electronic applications, the stability against environmental condition of some COPs such as COPs based on Schiff-base coupling reaction and boronic acid based reactions is less practical. To this point, the synthesis of COPs with well-defined and stable structures especially which contains fused aromatic ring structures should be more useful. For example, the superstructure of 5,5’,5’’-(1,3,5-triazine-2,4,6-triyl) triisophthalonitrile (TIPN) realized using stagnate cyclotrimerization displayed unusually high thermal stability and electron-beam tolerance.61 These stable organic frameworks can be used for specific devices and structure-property studies. Some COPs such as the C-COP-443 and C-COP-P-Co44 synthesized by a nickel-catalyzed Yamamoto reaction have already acted as efficient electrocatalysts for oxygen reduction with good stability. The very recent synthesized Ru@C2N with aromatic ring structures have shown comparable and even better HER activity than that of Pt/C in both acidic and alkaline solution.40 Another issue needed to be addressed is the subsequent pyrolysis in the traditional processing methods always result in undesirable even damage structure changes, leaving the mechanism of electrocatalysis uncertain. It is also vital to develop the highly efficient electrocatalysts with intrinsic COPs without carbonization to modulate active sites and the density of active sites at 11


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Figure 6. Strategies to design promising COP materials without carbonation for electrocatalysis. The volcano plots give the ORR/OER overpotential versus adsorption free energy for all the possible active sites of the doped carbon nanostructures62. The 2D well-defined COPs can be synthesized in a predictive manner with controllable structures and adjustable dopants. Figure of volcano plots were reproduced with permission from ref 62, Copyright 2015 John Wiley and Sons.

the molecular level. As shown in Figure 6, by taking consideration of electron affinity and electronegativity of dopants on charge redistribution over the carbon surface, researchers have developed descriptors that correlated to the intermediates and intrinsic descriptors that well describe the catalytic activities of heteroatom-doped carbon nanomaterials.62 These volcano plots can guide the design of doped catalysts when choosing proper heteroatoms. Combined with the valuable information provided by computational modelling, we are able to predict the complex nature of electrocatalysts and design well-defined COPs with optimal electrocatalytic activity. Furthermore, the synthesis of 2D structures with parallel and unidirectional layered alignment53 provides promising ways to fabric highly Conjugated Covalent Polymers 12


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(CCOPs) which can be directly used as electrocatalysts without carbonization and keep the structure as precisely as possible. The highly conjugated COPs with stable and irreversible structures (for example, fused aromatic ring structures) can be helpful for predicting new prospect in 2D COPs field. These CCOPs are supposed to offer in-depth knowledge of the active site and catalytic reaction mechanism. With rational design, well-defined 2D COPs may have the capability to develop catalysts with the activity, selectivity and stability matching and ultimately exceeding those of noble-metal-based catalysts.

In summary, we should note that even though the well-defined 2D COPs are still in its infancy, these exciting materials have shown promising potentials to explore electrocatalysis and are highly possible to become a topic with extensive scientific significance. Similarly to metal-organic framework (MOF)63, COPs are well-defined and uniformly distributed at molecular level thus could serve as candidates for ORR, HOR, OER and HER catalyst preparation. We believe that continued research and development of this exciting material will lead to a better solution for energy and environmental issues.

ASSOCIATED CONTENT

AUTHOR INFORMATION Peng Peng obtained his Master degree in Macromolecular Science and Engineering at Case Western Reserve University (US) in 2015. He has been a Ph.D. student in the Center of Molecular Energy Materials R&D (co-supervised by Prof. Liming Dai and Zhonghua Xiang) since 2015 where he focused on the synthesis and catalytic 13


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application of Covalent Organic Polymers. Zhuohang Zhou is a Master student in Prof. Zhonghua Xiang's research group at Beijing University of Chemical Technology. His current scientific interests are focused on designing well-defined and highly conjugated 2D covalent organic polymers for studying fundament of mechanism and kinetics of oxygen reduction reaction. Jianing Guo obtained his B.E. (2014) in Henan Polytechnic University (China). She is a Ph.D student under the supervision of Prof. Zhonghua Xiang at Beijing University of Chemical Technology. Her research is focused on the design and synthesis of electrocatalysts for water splitting and fuel cells. Zhonghua Xiang is a professor and director of the Center of Molecular Energy Materials R&D, State Key Lab for Organic-Inorganic Composites at the Beijing University of Chemical Technology (BUCT). He received his PhD in 2013 at BUCT and was a postdoctoral researcher at Case Western Reserve University (2013-2014). His current scientific interests are focused on the design & synthesis of covalent-organic polymers (COPs) and related hybrids for clean energy storage and conversion. Notes ACKNOWLEDGMENT This work was supported by NSF of China (51502012; 21676020); Beijing Natural Science Foundation (2162032); The Start-up fund for talent introduction of Beijing University of Chemical Technology (buctrc201420; buctrc201714); Talent cultivation of State Key Laboratory of Organic-Inorganic Composites; The Fundamental Research Funds for the Central Universities (ZD1502), The ‘‘111” project of China (B14004).

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Well-Defined 2D Covalent Organic Polymers for Energy Electrocatalysis

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