Tunable capacitance in all-inkjet-printed nanosheet heterostructures

Heterostructures constructed from two-dimensional building blocks have shown promise for field-effect transistors, memory devices, photosensors and other electronic applications1,2. 2D nanosheet crystals can be constructed into multilayer heterostructures using layer-by-layer methods3, but that method cannot be used to fabricate large-scale and thick heterostructures, due to the time-consuming nature and low efficiency of the process. An alternative approach to deposit different two-dimensional materials is by inkjet printing4-7. Here we show the fabrication of a nanosheet supercapacitor by inkjet printing Ti3C2Tx MXene nanosheets as electrodes, and graphene oxide nanosheets as solid-state electrolyte. The free water molecules trapped between graphene oxide sheets facilitate proton movement through the layered solid electrolyte8. The as-made supercapacitor shows high areal capacitance, good cycling stability and high areal energy and power densities comparable with existing printed supercapacitors. Moreover, the specific capacitance can be increased further by addition of liquid electrolytes.

2 Main 2D transition metal carbides or nitrides (MXenes) with general formula Mn+1XnTx (n = 1, 2, 3), where M is an early transition metal, X is a carbon and/or nitrogen and Tx are surface terminal groups like -F, -O, or -OH, have been attracting tremendous attention recently due to their outstanding chemical and physical properties 9,10 . MXenes with atomic thickness and high electrical conductivity have been widely studied for application in hydrogel sensors 11 , solar cells 12 and supercapacitors (SCs) 13,14 . MXenes have been combined with other 2D materials into multi-material structures with tunable properties and functionalities, showing promise for energy storage applications 15 . On the other hand, hydrated graphene oxide (GO) nanosheets are electrically insulating but exhibit high ionic conductivity, suggesting their potential as solid-state electrolyte and separator 8 .
MXene and GO are normally dispersed in a solvent like water, but traditional mechanical exfoliation and drying transfer techniques cannot be applied to fabricate MXene based heterostructures on large scale. Solution processing methods such as spray coating and vacuum filtration have been attempted, but these offer poor control over interface and surface roughness, resulting in poor device performance 15 . Inkjet printing, a simple, low-cost and versatile technique, provides an alternative route to the fabrication of large-scale vertical heterostructures with controlled thickness, interface and roughness 7 . Recently, various heterostructure devices based on printed 2D materials such as field-effect transistors 6 , capacitors 16 , photosensors and memory devices 7 have been demonstrated. However, realizing well controlled and sharp interfaces still presents a significant challenge for printed heterostructures. Full control over the heterostructure interface is key to achieving high performance, which includes avoiding redispersion of nanosheets from the interface upon deposition of a subsequent layer. The 3 preparation of non-toxic, stable and printable 2D inks is another critical issue for inkjet printing.
Owing to the wide range of physical properties present in 2D materials, 17 we demonstrate here that a combination of 2D materials can be used to realize an all-solid-state supercapacitor, without any liquid or gel electrolyte present in the system. In this work, we used a water-based additive-free MXene ink to inkjet print electrodes and current collectors on polyimide substrates, and a water-based GO ink to inkjet-print the solid-state electrolyte. Both sandwiched supercapacitors (SSCs) and micro-supercapacitor devices (MSCs) were printed on flexible polyimide substrates (Fig. 1). The SSCs achieved specific areal capacitances (CA) up to 9.8 mF cm -2 at a current density of 40 µA cm -2 . Addition of aqueous electrolytes led to enhancement of CA, due to the improved ionic conductivity of the electrolyte resulting from the presence of additional ions and a liquid phase.  Additive-free water-based MXene and GO inks were successfully prepared as shown in Fig. 2a.
Due to its high GO concentration, the GO ink has a dark brown color. The thickness of MXene and GO nanosheets were determined by atomic force microscopy (AFM) to be around 1.5 nm and 1 nm, respectively, indicating a unilamellar structure for both types ( Supplementary Fig.   S1a,b). The lateral sizes of MXene and GO nanosheets estimated from AFM images were about 0.76 µm for MXene nanosheets and 0.78 µm for GO nanosheets ( Supplementary Fig. S1c)  substrates show uniformity and continuity over large surface areas. It is worth noting that the sheets in both printed films showed a high degree of horizontal orientation and a layer-by-layer structure, which will facilitate the transport of electrolyte ions in in-plane structured devices such as MSCs. As shown in Fig. 2e, the XRD pattern of a printed MXene film shows strong ordering in the c direction with a (002) peak at 6.8°, thus confirming the horizontal orientation of nanosheets in printed films. The smaller angle than in dry MXene films, where the same peak is at 8.9° ( Supplementary Fig. S2a), indicates wider spacing between the layers in the printed film and intercalation of spatially confined H2O molecules 19 . The XRD pattern of a printed GO film shows a peak at 2θ = 9.6°, which corresponds with a d spacing of 0.92 nm, suggesting that electrolyte ion transport is predominant in horizontal rather than in vertical direction. The sheet 6 resistance Rs of printed MXene films could be tuned by the number of printed layers. As shown in Fig. 2f, the Rs of MXene films on Si/SiO2 substrates decreased rapidly from 116.7 Ω sq -1 (printed layers <N> = 1) to around 5.9 Ω sq -1 (<N> = 40) with an ink concentration of around 4.5 mg ml -1 . Electrochemical impedance spectroscopy (EIS) was conducted on both devices in the frequency range from 10 mHz to 10 kHz. The experimental data were fitted to the equivalent circuit shown in the inset of Fig. 3f To demonstrate the potential for practical applications at high voltages, the as-made 30L SSCs were connected in series and in parallel configurations. As shown in Fig. 3g, the voltage window MXene/single-walled carbon nanotube supercapacitor 25 . The 30L SSC exhibits good cycling stability with a capacitance retention of ~100% after 10000 cycles ( Supplementary Fig. S10a).
Moreover, the all-printed SSC shows high mechanical stability with a bending radius of about 1 cm, as shown in Supplementary Fig. S10b. MSCs were fabricated by printing MXene nanosheets with interdigitated structure as electrodes 11 on polyimide substrate, followed by a printed layer of GO nanosheets on top of/over the MXene electrodes to serve as solid-state electrolyte (Fig. 1, bottom).  Fig. S12). The all-inkjet-printed 30L MSCs exhibited a low RMS of ≈ 157 ± 20 nm at a device thickness of around 2.3 µm (Fig. 4c).
However, the contact at the cross-section between MXene electrode and GO electrolyte is poor ( Fig. 4a,b), which is likely the cause of low current response in the CV measurement (Fig. 4d).
Most likely, only GO sheets near the heterostructure interface contribute to the capacitance, because the protons are generated via hydrolysis of functional groups on GO.
Electrochemical measurements were performed on as-made all-solid-state MSCs and on MSCs to which excess aqueous electrolyte had been added. The electrochemical performance of 30L MSC improved considerably upon addition of water, due to enhanced proton mobility (Fig. 4d).
The CV curves of MSCs with varying electrode thicknesses demonstrate that thicker electrodes with more active surface sites show higher CA (Fig. 4e, Supplementary Fig. S13a-c). GCD further confirms that 30L MSC exhibits a higher capacitance than the other two devices (Fig. 4f,   Supplementary Fig. S13d-f). The CA of 30L MSC reached to 3.1 mF cm -2 , while 10 and 20 layer devices reached 1.2 and 1.9 mF cm -2 at a current density of 20 µA cm -2 , respectively (Fig. 4g).
EIS suggests that the charge transfer resistance (R1) of the 30 layers thick MXene electrode device is lower than the other two devices (4.1 kΩ, 11.9 kΩ and 12.0 kΩ for10L SSC, 20LSSC 12 and 30L SSC, respectively; Fig. 4h, Table S2). Similar to the SSC, the equivalent circuits in the low frequency range suggests mixed surface absorption and diffusional control, i.e. the double layer capacitance (Q1) and the charge transfer diffusion impedance (Q2). Addition of a 0.5 M H2SO4 electrolyte solution onto 30L MSC resulted in a a higher capacitance than in devices with excess water (Fig. 4i, Supplementary Fig. S14). The H2SO4 electrolyte provides additional protons that enhance the ionic conductivity, leading to lower series resistances.
In conclusion, we demonstrated all-inkjet-printed solid-state supercapacitors based on 2D  16 Titanium carbide (Ti3C2Tx) MXene was synthesized following a mild etching method as outlined (1)

82
The specific areal capacitance (CA) of film electrodes was calculated from the GCD curves by 83 using Equation (2): where I is the discharge current, dV/dt is the slope of discharge curve, and Aelectrode refers to the 86 geometrical surface area of the film electrode.

87
The specific areal capacitance (CA,device) of the MSC devices were also calculated from the GCD 88 curves using equations (3): Here Adevice refers to the total geometrical surface area of the device including the electrodes and 91 the gap between the electrodes.

92
The areal energy densities (EA, µWh cm -2 ) and power densities (PA, µW cm -2 ) were calculated 93 from equations (4) and (5) 94 Where Δt refers to discharge time.   The XRD pattern of pristine graphite shows a (002) peak with an interlayer spacing of 3.4 Å, 124 while the corresponding (002) peak in graphene oxide shows an interlayer spacing of 9.7 Å. This 125 indicates successful exfoliation of graphite to graphene oxide nanosheets (Fig. S3a). The Raman 126 spectrum of pristine graphite shows a strong and sharp G peak at 1581 cm -1 that is attributed to 127 the first-order scattering of the E2g mode 3 . This G peak becomes broader and shifts to 1599 cm -1 128 in graphene oxide. Furthermore, the appearance of a D band at 1365 cm -1 indicates the reduction 129 in size of the in-plane sp 2 domains (Fig. S3b) 4 . The XPS survey spectra reveal that the main   To achieve a high quality inkjet printing process, the preparation of printable and stable inks is 141 very important. Water-based MXene ink without any additives showed highly stable printing 142 behavior during jetting, which may be attributed to the presence of functional groups like -O, -143 OH and -F on the surface of MXene sheets (Fig. S4a) that help dispersion in water. To prepare a 144 printable GO ink, Triton X-100 was added to the water-based graphene oxide ink in order to 145 optimize the ink surface tension 5 . As shown in Fig. S4b, no satellite droplets were generated 146 during jetting, indicating a printable and stable GO ink.   (organic) binders. Annealing printed heterostructures at high temperatures can lead to removal of 164 the binder, but it limits the choice of substrates to thermally stable ones. Here, we successfully 165 inkjet printed vertical heterostructures without any sign of re-dispersion at the interface by 166 drying the printed patterns before printing the next layer with different nanosheets. Drying was 167 performed at 50 °C, which is a relatively low temperature and is applicable to most substrates 168 including paper and polymer substrates (Fig. S6a,b). The cross-sectional SEM images a sharp 169 interface with good contact between two different nanosheets ( Fig. S6c-f).           Due to the complexity in the high frequency range, we only present the low frequency fitting 209 results as shown in Table S2 and Chi-squared which represent the fitting error for high frequency 210 range.