Nanofluidic voidless electrode for electrochemical capacitance enhancement in gel electrolyte

ARTICLE https://doi.org/10.1038/s41467-021-25817-8 OPEN Nanofluidic voidless electrode for electrochemical capacitance enhancement in gel electrolyte 1234567890():,; Kefeng Xiao 1, Taimin Yang 2, Jiaxing Liang1, Aditya Rawal3, Huabo Liu1, Ruopian Fang1, Rose Amal Hongyi Xu 2 ✉ & Da-Wei Wang 1 ✉ 1, Porous electrodes with extraordinary capacitances in liquid electrolytes are oftentimes incompetent when gel electrolyte is applied because of the escalating ion diffusion limitations brought by the difficulties of infilling the pores of electrode with gels. As a result, porous electrodes usually exhibit lower capacitance in gel electrolytes than that in liquid electrolytes. Benefiting from the swift ion transport in intrinsic hydrated nanochannels, the electrochemical capacitance of the nanofluidic voidless electrode (5.56% porosity) is nearly equal in gel and liquid electrolytes with a difference of ~1.8%. In gel electrolyte, the areal capacitance reaches 8.94 F cm−2 with a gravimetric capacitance of 178.8 F g−1 and a volumetric capacitance of 321.8 F cm−3. The findings are valuable to solid-state electrochemical energy storage technologies that require high-efficiency charge transport. 1 School of Chemical Engineering, The University of New South Wales, Sydney, NSW, Australia. 2 Department of Materials and Environmental Chemistry, Stockholm University, Stockholm, Sweden. 3 Nuclear Magnetic Resonance Facility, Mark Wainwright Analytical Center, The University of New South Wales, Sydney, NSW, Australia. ✉email: ; NATURE COMMUNICATIONS | (2021)12:5515 | https://doi.org/10.1038/s41467-021-25817-8 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-021-25817-8 T he demands for safe and fast responding solid-state energy sources continue to rise. With short seconds-to-minutes charging time and long lifespan, supercapacitors (SCs) are in principle superior to batteries for high-power energy storage applications. Solid-state SCs have emerged as high-priority energy storage solution for on-chip and flexible electronic devices1,2. Solid-sate SCs with gel polymer electrolytes impart promising advantages over the traditional SCs with liquid electrolytes, such as easy maintenance, better reliability, and improved manufacture flexibility. However, as the developing electronics go more miniature, there is a growing technological desire for electrode materials delivering both high areal and volumetric capacitance, without compromising the gravimetric capacitance. Porous electrodes are widely used in solid-state SCs. In general, embedding uniform ion penetration network in porous electrodes is key to achieving high capacitance in gel electrolytes. However, gel electrolytes are oftentimes poorly penetrative due to the entanglement and stickiness of cross-linked polymer chains. Such gel penetrability dependence of electrode capacitance causes a significant performance drop compared to electrodes in liquid electrolytes. Many reported works chose to alleviate the low gel penetrability by limiting the mass loading and thickness of electrodes (<1 mg cm−2), which results in small areal capacitance. Enhancing gel electrolyte penetration through increasing the proportion of macropores (>100 nm) in the electrode is commonly applied (some cases also use less viscous sol electrolytes)3–11, but this approach would impair the volumetric capacitance. Additionally, it has been stressed that the practical gravimetric capacitance of the electrode would drastically diminish when the mass of electrode-loaded electrolyte is counted12–14. These facts suggest that enhancing the electrode capacitance without compromising one other metric is usually constrained for solid-state SCs. We propose a different electrode design concept for solid-state SCs with the purpose of achieving universal high capacitance on all metrics (gravimetric, areal, and volumetric). Our strategy is to use nonporous two-dimensional (2D) nanofluidic structure that is intrinsically dual conductive to electrons and ions as the electrode active materials in solid-state SCs. We validate the strategy using tungstate anion-linked polyaniline (TALP), a layered 2D conductive polymer-oxyanion structure15,16. In this work, we observed the laterally confined water in the layered TALP (c = 1.18 nm), which forms intrinsically hydrated nanofluidic channels that are inherently ionic conductive. The scale of the confined hydrated ion channel in TALP is close to double Debye length (2λ)17,18, a parameter that is experimentally examined useful for high-density charge storage10,12,19–21. We also showed the robustness of the layered nanofluidic channels of TALP in keeping regular ion pathways under mechanical compaction, which is distinct from the deformable complex carbon networks22,23. This property allowed compressing powdery TALP particles to compact pellet electrode with a large apparent density of 1.8 g cm−3 and a very low porosity of 5.56%. The primary TALP particles sheared and fused in the pellets, which produced a spreading nanofluidic ion penetration network throughout the electrode even without external electrolyte flooding. The nanofluidic TALP pellet electrode showed almost equal high areal capacitances in liquid and gel electrolytes (9.10 vs. 8.94 F cm−2), as well as gravimetric and volumetric capacitance, which enables exceptional holistic capacitances that largely outstrip the state-ofthe-art porous electrodes in solid-state SCs. Results Intrinsic nanofluidic ion channels. The self-assembly of the layered structure of TALP is directed by the cooperating oxidative 2 polymerization and hydrogen bonding interactions of monomeric aniline and oxotungstate via a one-pot process in an aqueous medium15,16. The unexfoliated original TALP particles consist of stacked nanosheets with high regularity, as demonstrated through transmission electron microscopy (TEM) analyses (Fig. 1a, b). Such unidirectional arrays of self-aligned nanochannels that are in atomic proximity distinguish the corrugated texture or hierarchical connectivity of the ion channels in restacked or crosslinked porous nanosheets10,12,20,21,24,25. The 1.18 nm lamellar periodicity of nanochannels in TALP, according to X-ray diffraction (XRD) measurement15,16, is close to 2λ and is promising to store more charges and induce high ionic flux during charging26. Ultrasonic agitating the intact as-synthesized TALP particles in water or ethanol can extensively exfoliate the particles into ultrathin monolayer and few-layer nanosheets (Supplementary Fig. 1). Some strip-like cavities on the substrate nanosheets derive depth around 1.3 nm near to the monolayer thickness. The fine lattice structure of TALP nanosheet is determined through structure reconstruction of TALP particle basing on the reciprocal lattice (Fig. 1c). The lattice parameters of the averaged in-plane structure are determined to be a = 6.86 Å, b = 7.60 Å. The flake morphology of exfoliated (...truncated)