Boosting capacitive charge storage of 3D-printed micro-pseudocapacitors via rational holey graphene engineering

Journal Pre-proof Boosting capacitive charge storage of 3D-Printed micro-pseudocapacitors via rational holey graphene engineering Xiaocong Tian, Kang Tang, Hongyun Jin, Teng Wang, Xiaowei Liu, Wei Yang, Zhicheng Zou, Shuen Hou, Kun Zhou PII:

S0008-6223(19)30899-1

DOI:

https://doi.org/10.1016/j.carbon.2019.08.089

Reference:

CARBON 14570

To appear in:

Carbon

Received Date: 3 July 2019 Revised Date:

6 August 2019

Accepted Date: 31 August 2019

Please cite this article as: X. Tian, K. Tang, H. Jin, T. Wang, X. Liu, W. Yang, Z. Zou, S. Hou, K. Zhou, Boosting capacitive charge storage of 3D-Printed micro-pseudocapacitors via rational holey graphene engineering, Carbon (2019), doi: https://doi.org/10.1016/j.carbon.2019.08.089. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Graphical abstract for Boosting Capacitive Charge Storage of 3D-Printed Micro-Pseudocapacitors via Rational Holey Graphene Engineering Xiaocong Tian,1,2,§ Kang Tang,1,§ Hongyun Jin,1,* Teng Wang,1,3 Xiaowei Liu,4 Wei Yang,4 Zhicheng Zou,1 Shuen Hou,1 Kun Zhou2,*

Boosting Capacitive Charge Storage of 3D-Printed Micro-Pseudocapacitors via Rational Holey Graphene Engineering Xiaocong Tian,1,2,§ Kang Tang,1,§ Hongyun Jin,1,* Teng Wang,1,3 Xiaowei Liu,4 Wei Yang,4 Zhicheng Zou,1 Shuen Hou,1 Kun Zhou2,*

1

Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan

430074, China 2

Singapore Centre for 3D Printing, School of Mechanical and Aerospace Engineering,

Nanyang Technological University, Singapore 639798, Singapore 3

Key Laboratory for Green Chemical Process of Ministry of Education, Hubei Key

Laboratory of Plasma Chemistry and Advanced Materials, School of Materials Science and Engineering, Wuhan Institute of Technology, Wuhan 430205, China 4

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing,

Wuhan University of Technology, Wuhan 430070, China

§

These authors contributed equally to this work.

* Corresponding authors. E-mails: [email protected] (H. Jin); [email protected] (K. Zhou)

1

Abstract Micro-pseudocapacitors (MPCs) are prospective power source candidates with compatible sizes as well as superior power densities for miniaturized electronic devices; however, their limited capacitive charge storage characteristics are still hindering their further development. Herein, an optimized 3D printing technique is employed to build remarkable reduced holey graphene oxide (rHGO) supported MPC microelectrodes. In such microelectrodes, the presence of macroscale pores introduced through freeze drying is beneficial to the accessibility to electrolyte ions. Moreover, with two additional pore modals including the microscale pores from pseudocapacitive nanoparticles stacking and nanoscale in-plane holes formed by rational HGO engineering, the MPC microelectrodes offer an enhancement in both electrical and ionic transports. The resultant 3D-printed planar MPCs exhibit a remarkable device specific charge storage capacity of 241.3 mC cm−2, which is approximately 1.7-fold that

of

a

non-optimized

one

and

much

higher

than

most

reported

planar

micro-supercapacitors. Superior rate capability and high capacity retention (91% capacity retention after 11000 charge and discharge cycles) are also achieved. Our porosity engineering strategy combined with 3D printing is expected to serve as a facile and general design in the roadmap for next-generation state-of-the-art customized electrochemical energy storage devices.

2

1. INTRODUCTION The growing demand for portable electronic devices is stimulating the urgent development of miniaturized energy storage systems [1-3]. Presently, planar-type electrochemical energy storage devices (EESDs) are targeted as excellent ones with compatible sizes and desired electrochemical behaviors [4-7]. In the planar configurations, the interdigitated microelectrodes are arranged in the same plane and separated in precisely controlled distance to prevent short circuiting and obtain decreased ion transport path.[8] Among various EESDs, micro-supercapacitors (MSCs) have drawn extensive attention due to their high power densities and long lifespans. However, as the footprint of planar devices is very limited, the desired device areal capacity, which depends on the microelectrode structure and manufacturing methods [8-10], is highly required but usually poses a challenge. According to the the categorization of device charge storage mechanisms, electric double-layer micro-capacitors (EDL-MCs) and micro-pseudocapacitors (MPCs) are classified as two types of MSCs [11-13]. For both types, graphene is regarded as one of the favorable electrode components [14, 15]. On the one hand, graphene is a typical EDL-MC electrode material with a large surface area, high electrical conductivity and electrochemical stability, while also plays a vital role in MPC electrode modifications to boost the device capacity. To date, a series of graphene-based planar MSCs have been produced with decent electrochemical performance [16-19]. The reported manufacturing methods include layer-by-layer deposition, laser scribing, chemical vapor deposition and so on [20-26]. These device fabrication methods are sufficiently advanced to obtain patterned graphene-derived and graphene-based composite films with controllable properties; however, they commonly rely on high-cost photolithographic techniques and are difficult to achieve the thickness-controlled microfabrication of interdigitated microelectrodes. Thus, cost-effective microelectrode fabrication methods with geometrically appealing advantages are imperative 3

for graphene-based MSCs. Three-dimensional (3D) printing, also known as additive manufacturing, has recently emerged to be applicable in the fabrication of planar EESDs with customized device geometries [27-29]. Among various 3D printing techniques, the extrusion-based technique enables the cost-effective direct writing of thickness-controlled microelectrodes in interdigitated patterns [30]. Moreover, this technique is highly capable in tuning chemical components of printable ink and building high-performance MSCs. To date, some pioneering works on graphene-based 3D printing have been conducted for planar electrochemical energy storage applications. Sun et al. reported reduced graphene oxides (rGO) planar MSCs using a micro-extrusion 3D printing method [31]. Rocha et al. built multi-material planar energy storage devices using a 3D-printable thermos-responsive ink [32]. Although significant progresses in graphene-based 3D printing have been made, the delivered capacitive charge storage performance is still low, which hinders the further development of 3D-printed graphene-based MSCs. Herein, we report an optimized extrusion-based 3D printing strategy to boost the capacitive charge storage of 3D-printed MPCs through the engineering of graphene-based composite ink. To elaborate, an additive-free high-concentration composite ink is rationally formulated by integrating exfoliated holey graphene oxide (HGO) sheets with pseudocapacitive cobalt oxide hollow nanopolyhedrons. This developed facile and unique ink preparation process enables this well-dispersed ink to exhibit desirable 3D-printable rheological properties with shear-thinning non-Newtonian and stable flowing fluid behaviors. Moreover, with a further surface modification of graphene oxide (GO) nanosheets, the resultant 3D-printed microelectrodes enable not only facilitated electrical conductivity but also continuous ionic transport channels. Compared with unmodified graphene-based 3D-printed MPCs, the holey rGO-based quasi-solid-state ones demonstrate substantially 4

improved device charge storage capacities (up to 241.3 mC cm−2), superior rate capability and robust cycling stability (91% capacity retention after 11000 cycles). To illustrate these enhanced device electrochemical behaviors, the intrinsic capacitive charge storage mechanisms are presented in terms of inner electrical and ionic transport paths.

2. EXPERIMENTAL 2.1 Materials synthesis Co3O4 hollow polyhedrons were synthesized as ZIF-67 derived products. In a typical ZIF-67 synthesis process, 5 mmol of Co(NO3)2·6H2O (Aladdin Company, Shanghai, China) and 20 mmol of 2-methylimidazole (Aladdin Company, Shanghai, China) were dissolved in 30 mL of methanol solution and were marked as solution A and B, respectively [33]. Then, solution A was mixed with solution B and set aside at room temperature for 24 h. The resulting precipitates were collected by centrifugation and washing with ethanol for several times. The ZIF-67 was finally obtained by drying in air at 60 °C. Co3O4 hollow polyhedrons were synthesized through calcination in air at 350 oC for 4 h. HGO dispersion was synthesized using a solution processable method [34]. Typically, GO aqueous dispersions (2 mg mL-1) were obtained by oxidation of natural graphite powder according to the modified Hummers’ method. 25 mL of 30% H2O2 aqueous solution was mixed with 250 mL of GO aqueous dispersions with continuous heating at 100 oC for 4 h while stirred. Then, the 10 mg mL-1 HGO dispersion was purified by washing, centrifugation, sonication and subsequent evaporation of excess water. 2.2 Device 3D Printing A 3D printable HGO-Co3O4 composite ink was obtained by a simple heating and mixing method. Before mixing, Co3O4 nanoparticles were ground using a motor and pestle. Then, 450 mg of Co3O4 powders were dispersed in 10 mL of HGO aqueous dispersion with 5

continuous heating at 120 oC for 1 h, while stirred to evaporate the excess water and form a 3D printable ink. An unmodified GO-Co3O4 ink was also prepared using the same method as the control experiment. The ink was loaded into a 5 mL syringe with a nozzle diameter of ~500 µm and fitted onto a 3-axes extrusion system with two equipped nozzles. A silver paste layer was printed as the current collector, followed by the printing of interdigitated electrodes. The printed 3D structure was calcinated at 600 °C within a H2-Ar (5 % H2, 95 % Ar) atmosphere for the thermal reduction process. For the PVA-KOH gel electrolyte preparation, 5 g of KOH was added into 10 mL of deionized water, followed by the further addition of 1 g of PVA. 2.3 Structural characterization and electrochemical measurements Phase characterization of the powder was performed using Burker D8 Discover X-ray diffractometer with Cu-Kα radiation (λ = 1.5418 Å). SEM images and TEM images were recorded with a JEOL-7100F and Titan G2 60-300, respectively. To conduct the TEM measurement, as-fabricated 3D-printed microelectrodes were first dispersed within ethanol with sonication to obtain the sample suspension. Afterwards, a TEM micro-grid was placed with a drop of the sample suspension, and the drying process in air was subsequently conducted to finally obtain the TEM samples. The rheological properties of HGO-based composite ink were measured on a HAAKE MARS 60 using 20 mm steel flat plate geometry at 25 °C where a solvent trap was used to reduce evaporation. CV, GCD and EIS were conducted on a CHI 760D electrochemical workstation. 2.4 Calculation of device electrochemical performance Based on the CV curve data, the device areal capacitance values of planar MPCs were calculated according to the following equation [16]: =

( ) ∙∆ ∙

(1)

where Cdevice-1 represents the device areal capacitance of planar MPCs calculated using the 6

CV method, ∫I(V)dV the integral of the charge and discharge processes, S the total area of the devices including the both interdigitated microelectrodes area and interspace area, △V the scan range and v the scan rate. For the GCD curve data, the device areal capacitance values of planar MPCs were calculated by =

∆ ∙ ∆

(2)

where Cdevice-2 denotes the device areal capacitance of planar MPCs calculated using GCD method, Is the galvanostatic discharge current density, △V2 the voltage range excluding IR drop and △t the discharge time. Based on the above results, the capacities of devices were calculated by

=C·∆V

(3)

where Qdevice is the device areal capacity of planar MPCs, C the corresponding capacitance, and ∆V the potential window.

3. RESULTS AND DISCUSSION Our extrusion-based 3D printing solution involved three major microfabrication processes: the printable ink preparation, microelectrode extrusion and post treatment, as illustrated in Fig. 1a. The first process was to prepare the printable ink. A modified Hummer's method was employed to obtain an aqueous suspension of GO sheets [35]. To address the critical issue of in-plane ionic transport channel blocking of graphene for energy storage, the aqueous GO suspension was treated with H2O2 to form a highly-concentration aqueous suspension of HGO sheets (10 mg mL−1) with abundant in-plane pores through a mild defect-etching reaction [34]. To further boost electrochemical performance of the MPC, pseudocapacitive Co3O4 hollow nanopolyhedrons were then introduced into the HGO suspension at a 7

temperature of 120 oC, which the mixed suspension was stirred for 1 h to evaporate excess water, resulting in an additive-free HGO-Co3O4 composite suspension for the 3D-printable ink. Differing from stereolithography 3D printing, the applied extrusion-based printing relies on merely the layered self-supporting of ink extruded from nozzle, without the photopolymerization effect. The extraction of the ink from the nozzle was driven by a pneumatic force. In this regard, the extrusion-based one possessed a high requirement on rheological properties of the loading materials (ink) without using photopolymers. Fig. 1b shows the state of a continuous extrusion from the printing syringe nozzle, implying a superior self-supporting ink.

Fig. 1. (a) Schematic of the fabrication process of 3D-printed quasi-solid-state planar rHGC MPCs. Optical image of (b) a extrusion state of the HGO-Co3O4 composite ink from a syringe, (c) a printed customized pattern and (d) a printed process of printed microelectrodes on the insulating substrate. The second process was to conduct microelectrode extrusion. Such extrusion was performed on an insulating substrate, which helped to avoid short-circuiting. Prior to the 8

microelectrodes extrusion, a thin layer of silver paste was preprinted as the current collector as well as the microelectrode anchor via a preprogrammed routine. Afterwards, following similar routines, the HGO-Co3O4 composite ink was printed as a symmetric interdigitated pattern onto the preprinted silver pattern, as shown in Fig. S1. As observed in Fig. 1c, the customized pattern could be continuously extruded with a printing nozzle moving at the speed of 4 mm s−1. It is notable that the ink self-supporting capability was further confirmed based on the fact that the macroscopic collapse of the printed structure did not occur during the printing process. For a fair comparison, a control ink mixed with unmodified GO with Co3O4 was also prepared. The GO-Co3O4 symmetric interdigitated microelectrodes and corresponding MPCs were 3D-printed using an identical experimental setup and extrusion parameters.

Fig. 2. Rheological properties of high-concentration HGO-Co3O4 composite ink. (a) Viscosity/shear rate relationship of the HGO-based composite ink. Storage modulus and loss 9

modulus aging as function of the (b) shear rate, (c) angle frequency and (d) time. After the microelectrode extrusion, a well-controlled freeze drying process was employed to remove the solvent, which acted as a hard template for the internal 3D macroporous structure formation. Meanwhile, this solvent removal also reduced the severe agglomeration of (holey) graphene oxides [36, 37]. In order to obtain an electrically conductive architecture, a thermal reduction was further utilized to form reduced HGO-CoO/Co (rHGC) planar microelectrodes (Fig. 1d). A polyvinyl alcohol (PVA)-KOH electrolyte was injected on the top of rHGC microelectrodes and finally helped to consolidate the quasi-solid-state full MPC device assembly. Since ink rheological properties were critical for extrusion-based 3D printing process, the rheological data were collected prior to the printing of the as-prepared additive-free high-concentration HGO-Co3O4 composite ink at 25 oC, which was consistent with the printing environmental temperature. As shown in Fig. 2a, the apparent viscosity of the ink was close to 105 Pa s at a shear rate of 10−2 s−1, resulting from the ultrahigh concentration of HGO and Co3O4 composites within it. Its apparent viscosity decreased with increased shear stress, indicating a shear-thinning non-Newtonian fluid behavior. These behaviors ensured a feasible extrusion process and good printability for forming desired shapes [38-40]. In addition, Fig. 2b compares the values of storage modulus G′ and the loss modulus G″ for the prepared ink, where the significant difference between G′ and G″ indicate the stable ink flowing for the extrusion process [41]. This was further confirmed by the angle frequency sweep results displayed in Fig. 2c. The G′ values were distinctly higher than those of G″, suggesting an elastic solid-like behavior for the sample. The above results show that the prepared HGO-Co3O4 ink behaved suitably for the 3D printing process. The plot of G′ and G″ aging as function of time are also presented in Fig. 2f. Almost unchanged modulus data was observed with the time changing, which demonstrates the stable rheological properties under 10

shear for HGO-based composite ink.

Fig. 3. (a) SEM image of Co3O4 hollow polyhedrons. (b) Cross-sectional SEM image and (c) SEM image of 3D-printed rHGC composite microelectrodes. (e,f) TEM image of (e) rHGC composites and (f) rHGO nanosheets. In a typical HGO-Co3O4 composite ink, Co3O4 hollow nanopolyhedrons were thermally obtained from a well-defined rhombic dodecahedral ZIF-67 precursor, whose morphology is presented in Fig. S2a [42]. Differing from the smooth surface feature of such a precursor, the surface of a thermally annealed Co3O4 nanopolyhedron was highly porous as shown in Fig. 3a. Notably, the uniform size distribution of the precursor was still retained under the formation of hollow pores. Such nanostructured hollow polyhedrons could provide void spaces and enable abundant ion diffusion channels and sites for more electrolyte penetration, thus leading to the facilitated ionic diffusion kinetics [43, 44]. The X-ray diffraction (XRD) results displayed in Fig. S3 confirmed the Co3O4 crystal structure (JCPDS card No. 42-1467, space group: Fd3m, lattice constant a = 8.084 Å). While, after the HGO compositing and printing on the substrate, the peaks in resulting XRD pattern of the resultant HGO-Co3O4 composite became much weaker (Fig. S4) which may have resulted from signal blocking due 11

to the presence of HGO. Moreover, with a further reduction post treatment, the phase was indexed as a partially oxidized cobalt (CoO/Co) hybrid structure accompanied by the reduction of HGO. This finally confirmed that a reduced HGO-CoO/Co (denoted as rHGC) composite architectures has been obtained. Since 3D printing is based on a layer-by-layer assembly principle, the printed architecture is commonly presented as a layered structure in the cross-sectional view. The stacked structures of rHGC composite architectures were observed in the cross-sectional scanning electronic microscopic (SEM) image (Fig. 3b). Notably, the stacking occurred along the direction nozzle was moving [45]. In such an rHGC structure, the cross-linked macroscopic pores with tenths of millimeter in diameter are displayed in Figs. 3c and d, which were introduced possibly due to the solvent removal during the freeze drying process. This cross-linked macroporosity was likely favorable for the contact between the electrolyte ions and microelectrode surface. As a further characterization of transmission electron microscopy (TEM), the compositing state between reduced HGO (rHGO) and CoO/Co polyhedrons is presented in Fig. 3e, where the polyhedrons were wrapped onto the rHGO sheets. Moreover, such a polyhedron was integrated with many smaller nanoparticles with abundant inherently stacked microscale pores, implying facilitated ion transport. Moreover, for the rHGO component, rationally designed in-plane pores with nanoscale sizes (5-20 nm in diameter) were observed in Fig. 3f. The introduced holes may help to address the issue of the ion blockage within the rGO in-plane structure. Overall, from the structural perspective, there existed trimodal pores, including macroscale pores introduced through the freeze drying process, microscale pores from the nanoparticles stacking and nanoscale in-plane holes formed by the rational graphene engineering. For a fair comparison, a control GO dispersion with the same quantity and concentration, but without in-plane holes, was used to construct a control unmodified 12

rGO-CoO/Co (denoted as rGC) composite planar MPCs using the same fabrication procedures.

Fig. 4. Electrochemical performance of rGC and rHGC planar MPC devices. (a) CV curves and (b) GCD curves of rGC- and rHGC- based planar MPCs at a scan rate of 10 mV s-1 and current density of 1.5 mA cm-2, respectively. (c) CV curves and (d) GCD curves of rHGC-based planar MPCs at different scan rates and different current densities. (e) Device areal capacity comparison of rGC- and rHGC-based planar MPCs at various current densities. (f) Cycling stability of rHGC-based planar MPCs after 11000 cycles at a current density of 10 13

mA cm-2. The electrochemical performance of rGC- and rHGC- based planar MPCs was first evaluated using the cyclic voltammetry (CV) method. CV curves of different as-fabricated planar devices measured at a low scan rate of 10 mV s−1 are shown in Figs. 4a and S5, where a bare silver-based interdigitated current collector delivered negligible charging-discharging current values (3 orders of magnitude lower) compared with rGC or rHGC MPCs. This observation confirmed that the capacitive charge storage behaviors were mainly attributed to the rGC or rHGC electrodes in the corresponding MPC devices. With further comparisons, the current values of rHGC-based planar MPCs obviously exceeded those of rGC planar ones, suggesting enhanced capacitive charge storage behaviors. The enhancement was directly attributed to the overall increased accessible amounts of electrolyte ions participating in the electrochemical reactions within the rHGC microelectrode structure. According to equations (1) and (3), the MPC device areal capacity was calculated to be 98.6 mC cm−2 (a corresponding device capacitance of 98.6 mF cm−2), approximately as 1.9-fold that of the rGC planar one (52.3 mC cm-2) at a scan rate of 10 mV s−1, preliminarily revealing the great advantage of graphene in-plane holes in 3D-printed microarchitectures. Such an enhancement in the capacitive charge storage was further confirmed using the galvanostatic charge-discharge (GCD) measurement result as shown in Fig. 4b. At the same current density of 1.5 mA cm−2, the charging and discharging time of rHGC planar MPCs was much longer than that of rGC ones. Based on equations (2) and (3), the MPC device areal discharge capacity values of rGC and rHGC planar MPCs were calculated to be 82.4 mC cm−2 (a corresponding device capacitance of 82.4 mF cm−2) and 199.2 mC cm−2 (199.2 mF cm−2), respectively. Notably, from these charge-discharge curves, a smaller IR drop value (0.04 V) of rHGC planar MPCs was observed compared with that (0.08 V) of rHGC ones, implying a lower internal resistance as well as a superior kinetic response in as-fabricated 14

rHGC microelectrodes [46, 47]. The CV curves of rHGC-based planar MPCs at different scan rates are further displayed in Fig. 4c, where the current increased with the growing scan rates. Based on these curves, the device areal capacity values were calculated. At a scan rate of 2 mV s−1, rHGC-based planar MPCs delivered a remarkable device capacity up to 241.3 mC cm−2 (a corresponding device capacitance of 241.3 mF cm−2), while the capacity of rGC ones could only reach 136.6 mC cm−2 (136.6 mF cm−2). As shown in Table S1, this obtained outstanding device capacity is much higher than those of reported MSCs with 3D-printing-molded multiwalled carbon nanotube (MWCNT)/polyaniline (PANI)-based MSCs [48], electro-deposited mesoporous RuOx MSCs [49], printed CoO/carbon nanotube (CNT) MSCs [50], 3D-printing-stamped Ti3C2Tx MSCs [51], 3D-printed rGO MSCs [52] and 3D-printed GO/PANI-based symmetric MSCs [53]. In addition to the CV results, the GCD curves of as-fabricated devices at different galvanostatic current densities are also shown in Figs. 4d and S6, where the elongated charging and discharging time were observed with various current densities, indicating enhanced device specific areal capacity values. This enhancement was further confirmed after the calculation shown in Fig. 4e. Moreover, with the increase of current densities, it was found that the tendency of capacity decay for rHGC MPCs was smaller than the rGC ones, implying an enhanced rate capability and superior kinetics. The cycling performance for rHGC planar MPCs was further investigated and a robust cycling stability was achieved (91% capacity retention after ultra-long 11000 charge and discharge cycles). The ion transport properties of the constructed devices were further investigated via electrochemical impedance spectroscopy (EIS) with the frequency region ranging from 1 Hz to 100 kHz. A decreased ESR of 2.7 Ω was obtained for rHGC planar MPCs (Fig. S7), which was slightly less than that of the rGC one (3.6 Ω). Such a decrease is related to both the electrical resistance and ion diffusion resistance of the microelectrodes, 15

while this is more likely resulted from the facilitated ion transport in the rHGO-based framework [54, 55]. The exhibited electrochemical performance of as-prepared planar symmetric MPCs confirmed the greatly enhanced capacitive charge storage behaviors of rHGC architecture. As illustrated in Fig. 5, these desirable behaviors may be significantly related with the printed internal microstructures. Although the 3D microelectrode geometries were the same for rGC and rHGC MPCs and the ions migration took place in a similar manner (Figs. 5a and b), the amount and transport of participating ions were highly varied among different microelectrodes.

Fig. 5. Schematic of capacitive charge storage mechanisms for rGC and rHGC MPCs. (a,b) Simple description of (a) planar MPC and (b) symmetric interdigitated microelectrodes during the charging and discharging. (c,d) Different charge transport models for (c) rGC and 16

(d) rHGC microstructures. Specifically, on one hand, the applied extrusion-based 3D printing enabled construction of the rGO- and rHGO-supported framework in rGC- and rHGC- based devices, respectively. Both of their electrical transport capability was thus enhanced. However, on the other hand, the ion transport models varied much for the two different electrodes. During the charge and discharge processes, the ions migrated in the electrolyte and the ion absorption/desorption or redox reactions took place in the electrode structure. The whole electrodes could be regarded as the gathering of plentiful composite units, which were separated by macroscale pores. The existent macroscale pores for both rGC and rHGC devices ensured an efficient contact between the electrodes and electrolyte. However, much rGO and rHGO sheets were tightly adhered to the cobalt-based small nanoparticles embedding. Compared with rGO (Fig. 5c), the rHGO ensured the electrolyte ions transfer across the graphene in-plane sheet layer to the inner cobalt-based materials. With the advantage of the hollow structured morphologies of partially oxidized cobalt nanoparticles, the ions could transit ultimately across the full single composite unit (Fig. 5d). The accessible electrolyte ions in rHGC devices greatly exceeded those of rGC ones, resulting in the enhanced charge storage behavior. Thus, the enhancement of electrochemical performance was attributed to the optimized electrical and ionic transport in the rHGC 3D-printed microstructure.

4. CONCLUSIONS In summary, a well-dispersed high-concentration 3D-printable composite ink is prepared by integrating exfoliated HGO sheets with pseudocapacitive cobalt oxide hollow nanoscale polyhedrons. Such a viscous ink not only exhibits printable rheological properties with shear-thinning non-Newtonian and stable flowing fluid behaviors, but also enables an optimized 3D construction of printed planar symmetric rHGC MPCs with rational HGO 17

engineering. In the rHGC microelectrodes, the presence of macroscale pores introduced through freeze drying is favorable to the contact between the electrolyte ions and microelectrode surface. Moreover, with two more pore modals including the microscale pores from the nanoparticles stacking and nanoscale in-plane holes formed by rational HGO engineering, the rHGC microelectrodes exhibit characteristics which facilitate the transport of both electrons and ions, implying an enhanced capacitive charge storage performance. The 3D-printed planar rHGC MPCs possess a remarkable device specific areal capacity of 241.3 mC cm−2 (a corresponding device capacitance of 241.3 mF cm−2), which is approximately 1.7-fold as high as that of its non-optimized rGC counterparts, and much higher than those of most reported planar quasi-solid-state MSCs. Superior rate capability and high capacity retention (91% capacity retention after 11000 charge and discharge cycles) are also achieved. Our porosity engineering strategy combined with 3D printing is expected to serve as a facile and general design to fabricate high-performance EESDs with desired device geometries.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51802292), the Fundamental Research Funds for the Central Universities (CUG170690), Hubei Science and Technology Innovation Project (2018AAA015, 2017AAA112) and the Research Funds for Engineering Research Center of Nano-Geo Materials of Ministry of Education (NGM2019KF015).

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