105 Cyclable Pseudocapacitive Na-Ion Storage of Hierarchically

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105 Cyclable Pseudocapacitive Na-Ion Storage of Hierarchically Structured Phosphorus-Incorporating Nanoporous Carbons in Organic Electrolytes Sul Ki Park, Sung Hyun Kwon, Seung Geol Lee, Min Sung Choi, Dong Hoon Suh, Purittut Nakhanivej, Hyunjoo Lee, and Ho Seok Park ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b00068 • Publication Date (Web): 13 Feb 2018 Downloaded from http://pubs.acs.org on February 15, 2018

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ACS Energy Letters

105 Cyclable Pseudocapacitive Na-Ion Storage of Hierarchically Structured Phosphorus-Incorporating Nanoporous Carbons in Organic Electrolytes Sul Ki Park,† Sung Hyun Kwon,∥ Seung Geol Lee,∥ Min Sung Choi,† Dong Hoon Suh,† Purittut Nakhanivej,† Hyunjoo Lee,*,‡ and Ho Seok Park*,†,§Δ

†School of Chemical Engineering, Sungkyunkwan University, 2066, Seoburo, Jangan-gu, Suwon 440-746, Korea

∥Department of Organic Material Science and Engineering, Pusan National University, 2, Busandaehak-ro 63beon gil, Geumjeong-gu, Busan 46241, Republic of Korea. ‡Clean Energy Center, Korea Institute of Science and Technology, 39-1 Hawolgok-dong, Sungbuk-gu, Seoul 136-791, Republic of Korea §Department of Health Sciences and Technology, Samsung Advanced Institute for Health Sciences and Technology (SAIHST), Sungkyunkwan University, 2066, Seoburo, Jangan-gu, Suwon 440-746, Korea

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Δ SKKU Advanced Institute of Nano Technology (SAINT), Sungkyunkwan University, 2066, Seoburo, Jangan-gu, Suwon 440-746, Korea

Corresponding Author * [email protected] (Ho Seok Park), [email protected] (HyunJoo Lee)

TITLE RUNNING HEAD: 105 Cyclable Pseudocapacitive Na-Ion Storage of Hierarchically Structured Phosphorus-Incorporating Nanoporous Carbons in Organic Electrolytes

BRIEFS: The hierarchically structured, P-incorporating nanoporous carbons demonstrate greatly improved rate and cyclic capabilities of Na-ion storage in organic electrolytes due to the hierarchical interconnected porosity and the pseudocapacitance of the redox active P=O bonds.


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ACS Energy Letters

ABSTRACT

Despite the significant impact of sodium (Na) storage systems in terms of the natural bundance and environmentally friendliness, high-performance pseudocapacitive mterials in organic electrolytes remain challenging. Here, we demonstrate the pseudocapacitive Na-ion storage of hierarchically structured, phosphorus-incorporating steam-activated nanoporous carbons (PaCNs) with improved rate and cyclic capabilities in organic electrolytes. The P-aCNs with a hierarchical honeycomb structure are derived from lignocellulosic biomass via a proposed synthetic process. The prominent pseudocapacitive behaviors of the P-containing groups in organic Na-ion electrolytes are confirmed by the surface area-independent and surface-confined capacitances, distinctive redox waves and strong binding with Na-ions. In particular, the P-aCN demonstrates the cyclic stability of 96.0% over 100,000 cycles in full cell, achieving a high capacitance of 265.43 F g-1 and rate capability of 75%. These Na-ion pseudocapacitive features of P-aCNs arising from the hierarchical interconnected porosity and the redox-active P=O bonds are comprehensively investigated by experimental and computational analyses.


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TOC GRAPHICS


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ACS Energy Letters

Electrical double layer capacitors (EDLCs) store charge electrostatically on the surface of a porous carbon framework in a non-faradaic manner and have advantages over conventional and lithium-ion batteries in terms of their high power, fast rate capability, and long cyclic stability.1 However, the low capacitance and high leakage current of EDLCs are regarded as critical bottlenecks for high-energy applications such as electric vehicles and energy storage systems (ESS).2 Pseudocapacitive materials that can store charge on their surfaces via faradaic processes have been suggested to improve the areal capacitances by an order of magnitude compared to those of EDLCs,3 as exemplified by transition metal oxides or nitrides and conducting polymers.4-6 However, although the capacitance can be enhanced, the rate and cyclic capabilities of pseudocapacitors are very often reduced.7 Even though the potential window of organic electrolytes is wider than that of aqueous electrolytes at high energies,8,9 it is more challenging to obtain pseudocapacitance because of the sluggish ion diffusion and different redox reactivities with the ion carriers. Porous carbons are considered promising active materials for energy storage applications because of their large surface areas, controllable porous structures, chemical and mechanical stabilities, and low cost.10-12 Along with a tailorable micro/meso porosity and surface area,13 the incorporation of heteroatoms such as oxygen, nitrogen, fluorine, sulfur, and phosphorus have been investigated to modify the electronic structures and improve the electrochemical properties of these materials.14-18 That is, the pseudocapacitance of heteroatom-incorporated carbons can be triggered by surface or near-surface redox reactions,18 the charge transfer resistance at the electrode/electrolyte interface can be reduced by the doping effect,17 and the wettability can be increased for the maximum utilization of the accessible area.16 Despite the extensive research into heteroatom-incorporating carbon materials as pseudocapacitors,19-22 the interaction of these


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materials with ion carriers has focused solely on aqueous H+ or Li+ electrolytes.23,34 Considering the promise of Na+-based organic electrolytes in terms of being more naturally abundant and environmentally friendly compared to Li,23,25,26 a rational design for the heteroatom chemistry and hierarchical structure of porous carbons and a fundamental understanding of pseudocapacitive Na+ storage are essential for the development of pseudocapacitive systems. Amongst the heteroatoms, the phosphorous (P) chemistry of carbon nanomaterials is at an early stage of investigation and has mostly focused on electrocatalysis.19,27,28 The distinct effects of P incorporation on the electrochemical properties are associated with the unexpected changes in the charge density and electrochemical reactivity. These properties are attributed to the lower electronegativity of P than that of carbon, the overlapping of the additional 3s23p3 orbital with the sp2 and sp3 orbitals of carbons, and the larger size of P atoms compared to carbon.19 Motivated by this unique chemistry, recently, P-incorporating carbon materials have been intensively investigated for pseudocapacitor applications.18,29-31 However, the pseudocapacitive mechanism of P-containing groups occurring in organic Na+ electrolytes has yet to be explored. Herein, we report the controllable synthesis of hierarchically structured, P-incorporating nanoporous carbons for pseudocapacitive Na+-ion storage devices in organic electrolytes with greatly improved rate and cyclic capabilities. Our synthetic approach is (1) to incorporate Pcontaining moieties into the porous carbon networks to obtain a controlled chemical composition and bonding configuration and (2) to construct hierarchical structures for enhanced pseudocapacitive performance. The in-situ and ex-situ spectroelectrochemical and theoretical interpretations are first highlighted to understand the interactions of P-containing groups with Na+-ions in the organic electrolytes, depending on their chemical identity, bonding configuration, and composition.


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ACS Energy Letters

Figure 1 shows the chemical procedures for the separation-free synthesis of the hierarchically structured, P-incorporating porous carbons derived from lignocellulosic biomass through a hydrothermal method followed by steam activation. Although previous studies concerning the conversion of lignocellulosic biomass into carbon nanomaterials have been intensively performed using pre-treated cellulose, lignin has been mostly handled as a useless waste due to the high cost of its chemical isolation.32 Thus, a separation process is a prerequisite for the synthesis of high-quality carbon nanomaterials because of the heterogeneity of lignocellulosic biomass. In this study, we used a hydrothermal method to synthesize a hierarchically structured, P-incorporating nanoporous carbon (P-aCN) directly without any separation or chemical reagents. The hydrothermally treated, P-containing and hydrothermally treated, and hydrothermally treated and steam-activated (without P) samples are denoted CN, P-CN, and aCN, respectively. During the hydrothermal process at a high temperature and pressure, an aqueous dispersion and solid monolith were produced at the upper and bottom parts, respectively, of the one-pot reactor. In this process, lignin was converted into a long-chain structure with abundant oxygen functional groups via condensation and polymerization and, thus, precipitated as a solid product (Figure 1b). As shown in the

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C cross-polarization magic

angle spinning (CP-MAS) NMR spectrum of the CNs, the broad peaks from 110 to 160 ppm are mainly derived from the aromatic carbon atoms of pure lignin.[33] In addition, no cellulosic carbon signals from 65 to 80 ppm were observed for the CNs and pure lignin (Figures 1c and S1).34-36 The percentage of total lignin over CNs was estimated to be approximately 94.72% after refluxing (Figure S2). The separation efficiency can be further improved by optimizing the hydrothermal conditions. The resulting solid CN product could be transformed into a P-aCN


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hierarchical structure via subsequent steam activation, where the reactive vapor etches the defect sites on either the basal planes or edge sites to generate micro- and mesopores.37-39 The morphologies of aCN and P-aCN were characterized, as shown in Figure 2, and the low magnification scanning electron microscopy (SEM) images show the honeycomb like morphology, consisting of hierarchically interconnected porous structure derived from raw palm oil empty fruit bunch (EFB), a kind of lignocellulosic biomass (Figures 2a, 2c, and S3). The EFB, which is composed of hemicellulose, cellulose, and lignin, shows a collection of individual bundles, having diameters of about 350 µm, bound together (Figure S3). The bundles of CN with a diameter of 200 µm were exfoliated from raw EFB during the hydrothermal process and converted into the rigid and crosslinked solid monoliths (Figure S3). The unique honeycomb structures of the monolithic aCN samples originated from the intrinsic structure of lignin-derived products and were constructed in the process of carbonization and steam activation. This honeycomb structure was preserved even after the incorporation of the P atoms into P-aCN (Figure 2c), providing macroporous interconnectivity for fast ion diffusion. The highmagnification SEM images reveal that both aCN and P-aCN contained many micro- and mesopores with diameters smaller than 100 nm after the steam activation process (Figures 2b and 2d). The high-magnification transmission electron microscopy (HR-TEM) images clearly show micropores with diameters smaller than 2 nm (Figure S4). Furthermore, the activated monoliths of aCN and P-aCN contain partially graphitic layers with edge termination, which are more crystalline than the non-activated carbon. These results indicate that the high-temperature steam activation process created some defect clusters that were then rearranged into a partially crystalline graphitic lattice. The P-aCN sample showed the unique honeycomb-like interconnected porous structures constructed through a distinct chemical route, which is like that


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ACS Energy Letters

of aCN except for the presence of phytic acid. In this study, phytic acid played multiple roles in creating the robust and cross-linked carbon networks, generating an enlarged porous structure and incorporating P. In particular, lignin is chemically modified by phytic acids to produce functional groups such as phosphoric acid, which interact via strong hydrogen bonding to form cross-linked networks.40 This network structure preserved the honeycomb morphology in the PaCN despite the severe activation conditions. To determine the presence and distribution of P, we conducted elemental mapping for C, O, and P from the STEM images (Figure 2e). The P signals were homogeneously distributed over the P-aCN sample, indicating a uniform distribution on the carbon surface. The X-ray photoelectron spectra (XPS) spectra of P-aCN were obtained to verify the presence of P, its bonding, and the atomic percentages (Figure 2f). The P2p peaks of the P-aCN at 132.20 and 133.05 eV were assigned to P–C and P=O bonds, respectively.41 The observation of these peaks indicates that P atoms bound at the P-aCN surface were preserved even after the generation of micro- and mesopores via steam activation. According to the elemental analysis, the atomic percentage of P atoms in the P-aCN was approximately 2.24 % (Table S1). The C/O ratio of the P-aCN sample was 5.56, which is lower than that (19.92) for the aCN, indicating that the content of oxygen atoms had increased because of the oxidation of P, forming P=O groups. The effect of the P atoms on the electronic structures was further investigated using Raman spectroscopy (Figure 2g). The two main peaks in the Raman spectra of the CN, P-CN, aCN, and P-aCN samples at approximately 1332 and 1590 cm-1 were assigned to disordered carbon (Dband) and the ordered C–C stretching vibrations (G-band), respectively.27 After the activation process, the aCN and P-aCN samples had D to G-band intensity ratios (ID/IG) of 0.94 and 0.91, respectively, much greater than the values (0.79 and 0.29) of CN and P-CN. Even though the


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rearranged thin and long crystallites formed during a steam activation process, as shown in TEM images, the overall defect sites with the porous structure are produced by etching oxygen defects. In addition, the G- and D-bands of P-aCN were slightly shifted by approximately 4 and 7 cm-1, respectively, compared to those of aCN because of the doping effect of P atoms, which corresponds to the n-type doping of carbon.43 To investigate the pore characteristics, we measured the nitrogen adsorption/desorption isotherms of the samples (Figure S5). Notably, the surface area and pore volume of the aCNs are 1299.5 m2 g-1 and 0.637 cm3 g-1, respectively, a 330-times increase after steam activation. The isotherms of both aCN and P-aCN are a mix of types I and II, indicating the presence of internal micropores and multilayer adsorption (Figure S5b). At relative pressures of less than 0.1, steep curves were observed because of the abundant micropores. At a high relative pressure, the typical features of multilayer adsorption onto the weakly interacting external surface of the honeycomb structure were observed, as supported by the SEM images. The P-aCN has a surface area of 1109.1 m2 g-1 and a pore volume of 0.633 cm3 g-1, which are comparable to those of aCN. However, the H4 hysteresis loop, corresponding to a composite nature of micro- and mesoporosity, was more prominent for the P-aCN sample. The portion of the mesoporosity of the P-aCN to the total surface area was 15.85% greater than 10.07% of the aCN because of the effect of phytic acid on the formation of the mesopores; this is also consistent with the SEM images. The creation of mesoporosity was further supported by the Barrett–Joyner–Halenda (BJH) plot (Figures S5c and S5d). In addition, the existence of microporosity for aCN and P-aCN was confirmed as shown in the MP plots derived from t-plots (Figures S5e and S5f). The portion of micropores for aCN was larger than that of P-can due to the additional generation of mesopores activated by phytic acid.


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ACS Energy Letters

In this study, the steam activation conditions were controlled varying temperatures from 600 to 900 °C at a fixed flow rate of 0.17 mL h-1. The different activation conditions are denoted as aCNXXX and P-aCNXXX, where XXX indicates the temperature of steam activation. As shown in the morphology of the aCN treated at different temperatures (Table S2), the as-synthesized carbon monoliths are composed of the honeycomb-like interconneted porous structure with a pore size of

105 Cyclable Pseudocapacitive Na-Ion Storage of Hierarchically

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