materials Article Nanostructured Thick Electrode Strategies toward Enhanced Electrode–Electrolyte Interfaces Anukriti Pokhriyal, Rosa M. González-Gil , Leandro N. Bengoa and Pedro Gómez-Romero * Novel Energy-Oriented Materials Group at Catalan Institute of Nanoscience and Nanotechnology (ICN2) CSIC and BIST, Campus UAB, Bellaterra, 08193 Barcelona, Spain; anukriti.pokhriyal@icn2.cat (A.P.); rosamaria.gonzalez@icn2.cat (R.M.G.-G.) * Correspondence: pedro.gomez@icn2.cat Abstract: This article addresses the issue of bulk electrode design and the factors limiting the performance of thick electrodes. Indeed, one of the challenges for achieving improved performance in electrochemical energy storage devices (batteries or supercapacitors) is the maximization of the ratio between active and non-active components while maintaining ionic and electronic conductivity of the assembly. In this study, we developed and compared supercapacitor thick electrodes using commercially available carbons and utilising conventional, easily scalable methods such as spray coating and freeze-casting. We also compared different binders and conductive carbons to develop thick electrodes and analysed factors that determine the performance of such thick electrodes, such as porosity and tortuosity. The spray-coated electrodes showed high areal capacitances of 1428 mF cm−2 at 0.3 mm thickness and 2459 F cm−2 at 0.6 mm thickness. Keywords: nanostructure; thick electrode; electrolyte–electrode interface; energy storage; energy density; supercapacitor 1. Introduction Citation: Pokhriyal, A.; González-Gil, We are witnessing an accelerated transition from fossil fuels to renewable energies, R.M.; Bengoa, L.N.; Gómez-Romero, and energy storage in all its forms and facets is being called on to play a central role in this P. Nanostructured Thick Electrode transition. Among all possible technologies, electrochemical energy storage (EES) systems Strategies toward Enhanced provide great versatility, from large stationary storage to electric vehicle (EV) batteries to Electrode–Electrolyte Interfaces. minuscule flat and flexible supercapacitors. However, there are many challenges still to Materials 2023, 16, 3439. https:// be met for these solutions to be truly optimal and allow for the massive deployment of doi.org/10.3390/ma16093439 EES systems. Academic Editors: Alessandro When it comes to the development of better EES, emphasis is generally on electrode DellEra and Chunhua Chen composition, especially on the nature of the active phase, its microstructure and reactivity. Received: 30 March 2023 When it comes to electrode engineering, electrodes are invariably fabricated as thin solid Revised: 13 April 2023 coatings on thicker current collectors, both for rechargeable batteries and for supercapac- Accepted: 25 April 2023 itors. It is evident that this is not an optimal design. However, the implementation of Published: 28 April 2023 thicker electrodes has traditionally run into dead ends. On one hand, a mere increase in the thickness of electrodes using conventional formulations systematically leads to poorer performance. On the other hand, better performance is only achieved by designing ultra- sophisticated nanostructures that are difficult to scale up and implement. Therefore, it Copyright: © 2023 by the authors. is necessary and urgent to address the need for thicker electrodes through fundamental Licensee MDPI, Basel, Switzerland. studies that could help understand the factors hampering their performance. This article is an open access article Carbon-based supercapacitor electrodes generally have a thickness of a few microns, distributed under the terms and and the ratio of active components (energy storing) to non-active components (binder, cur- conditions of the Creative Commons rent collector, separator, casing, etc.) is very low. In order to reduce costs and make compact Attribution (CC BY) license (https:// energy storage devices with high power densities [1–3], it is necessary to increase the active creativecommons.org/licenses/by/ material loading through the development of thick or high mass loading electrodes and 4.0/). Materials 2023, 16, 3439. https://doi.org/10.3390/ma16093439 https://www.mdpi.com/journal/materials Materials 2023, 16, 3439 2 of 17 subsequently lower the quantity of non-active components. The industry standard for the mass loading of a thick electrode is generally >10 mg cm−2 ; however, such thick electrodes suffer from low capacitance mainly due to poor kinetics, reduced structural stability and high tortuosity [4]. Furthermore, delamination of the electrode from the current collector constitutes a possible complication leading to higher internal cell resistance [1]. Many approaches have been followed to develop thick electrodes that perform as well as conventional thin electrodes. Common methods to improve charge transfer include the development of hierarchical porous composites though pyrolysis [5–8], design of current collectors [9–12], addition of conductive materials [13–16], and modification of binders [17–20]. The introduction of carbon additives to activated carbon (AC) has long been studied to improve conductivity in supercapacitors and develop electrodes with higher areal and volumetric capacitance. AC has high porosity and thus good capacitance; however, it has low conductivity [21]. Commercial conductive materials such as carbon blacks (carbon super-P (CSP) or C65), nanofibers (NFs) and carbon nanotubes (CNTs) are added to improve electronic conductivity [22–24]. For example, a 500 µm thick CNT-based supercapacitor with bottom-up gel-filled electrolyte showed an areal capacitance of 2662 mF cm−2 at a scan rate of 2 mV s−1 and 2038 mF cm−2 at 10 mV s−1 [25]. In another study, a millimetre-thick supercapacitor electrode using poly(3,4-ethylenedioxythiophene) as the active material showed a specific capacitance of 6596 mF cm−3 at 25 mV s−1 and demonstrated high mechanical strength. In another example, a supercapacitor developed using single PANI- incorporated ultra-thick carbon nanotube fibre exhibited a high specific capacitance of 335 F g−1 and volumetric capacitance of 523.3 F cm−3 at 1 A g−1 [26]. A free-standing flexible electrode made of Co-UTBiOBr and CNTs with mass loading of 15 mg cm−2 showed rate capability of 61 mAh g−1 at 1 A g−1 after 3000 cycles [27]. Improving ion transfer throughout the bulk of the thick electrode is another approach to improve thick electrode performance. Reducing the tortuosity of the electrode by pore engineering has provided some promising results [28,29], and various techniques can be used to optimize the nano/micro pore structure of the electrode to improve ion trans- fer [2,30,31]. For example, electrolyte saturation can be improved through lyophilization or freeze-casting to create nanochannels and reduce tortuosity [4,32,33], and a ∼1 mm thick cellulose-based asymmetric supercapacitor with a RuO2 /cellulose carbon aerogel anode achieved a high areal capacitance of 4284 mF cm−2 at 2 mA cm−2 [34]. In another study, a two-step tape-casting/freeze-casting method resulted in the formation of low tortuosity graphene electrodes that demonstrated enhanced ionic transfer [30]. Apart from introducing carbon additives to improve conductivity and creating poros- ity to improve ionic conductivity, different coating and electrode formulation methods may also improve the performance of thick electrodes. The thick electrodes made using conventional knife casting methods have shown poor adhesion between carbon particles and higher series resistance with cycling, leading to reduced capacitance [35]. Moreover, thicker slurries may not allow good dispersion of carbon particles and binder. To overcome this, fine inks can be developed and sprayed on collectors in a controlled manner to allow better dispersion and good contact between carbon particles [24,36] as well as to control the thickness of the electrode in an efficient way [37,38]. Despite these various methods intended to solve the issues related to thick electrodes, there are few or no reports on thick electrodes with good rate capability that could be prepared by a scalable method. In this article, a multi-faceted approach was followed to develop high mass loading multilayer electrodes (up to ∼0.6 mm). This approach included testing two different coating methods, as well as different conductive additives and binders. Simple scalable methods such as spray coating and freeze casting were tested, and the effects of carbon additives such as CSP and CNTs as well as binders such as cellulose and PVDF-HFP were analysed and compared. This study also sought to shed some light on the barriers that hinder the performance of thick electrodes, thus helping to define design strategies. Materials 2023, 16, 3439 3 of 17 2. Materials and Methods 2.1. Materials All raw materials were commercially available and were used without additional purification or modification. Activated carbon for supercapacitor applications (YP50F) was purchased from Kuraray (bulk density of 0.3 g mL−1 and surface area of 1692 m2 g−1 ) and Carbon black Super P (≥99%) was purchased from Alfa Aesar (density of 160 ± 20 kg m−3 ). The remaining materials, Multiwalled Carbon Nanotubes (CNTs, 6–9 nm × 5 µm), 1-Methyl- 2-pyrrolidone (NMP), Poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) and Carboxymethylcellulose (CMC) 90,000 Mw, were purchased from Sigma Aldrich, Madrid (Spain). Aluminium foil of 0.05 mm thickness was used as current collector (Goodfellow). The fabricated electrodes were assembled in CR2032 coin cells from MTI by using What- man grade 4 cellulose paper separator and 1M tetraethylammonium tetrafluoroborate (TEABF4 ) in acetonitrile (all purchased from Sigma Aldrich) as electrolyte. 2.2. Active Slurry for Electrode Preparation Different compositions of the active ink were prepared for comparisons. It has been observed that a conductive carbon proportion of 10–15% shows the best performance [39] and hence, a 10% conductive carbon percentage was chosen for our electrodes of all thicknesses and compositions. CMC was selected as binder due to its environmental friendliness and flexibility [40], while PVDF-HFP was also tested because it has previously been used in the development of gel-electrolyte flexible supercapacitors [41]. Finally, it is well known that binders can block AC pores, causing a reduction in the capacitance of electrodes. Thus, binder mass was kept as low as possible while maintaining structural stability of the coatings for both CMC and PVDF-HFP binders. AC was combined with either CSP or CNTs as conductive additives, and CMC as the binder material. An AC: conductive additive: CMC ratio of 85:10:5 in de-ionized water and stirred for 12 h until a spreadable slurry was obtained. The compositions of AC, CSP and PVDF-HFP (80:10:10 and 75:10:15) was prepared by stirring with NMP for 12 h (Table S1). 2.3. Coating Methods Spray coating and freeze casting (or lyophilisation) were chosen as the two scalable methods for making thick electrodes with a controllable structure. The knife casting method was not considered due to the formation of cracks in the thicker coatings upon drying. Materials 2023, 16, x FOR PEER REVIEW Figure 1 depicts a schematic of the electrode preparation processes by spray coating 4 ofand 20 freeze casting. Figure Figure 1. 1. Electrode-making Electrode-making processes processes through through spray spray coating coating and and freeze freeze casting. casting. 2.3.1. Spray Coating For the AC (CSP/CNT) CMC electrodes, the aluminium current collector was placed on a heating plate with a constant temperature maintained at 60 °C. The slurry was loaded into a spray gun, and coatings were produced by controlling the number of sprays (4, 15, 35 and 50, resulting in electrode mass between 1–2, 6–8, 10–14, and 17–24 mg, respec- Materials 2023, 16, 3439 4 of 17 2.3.1. Spray Coating For the AC (CSP/CNT) CMC electrodes, the aluminium current collector was placed on a heating plate with a constant temperature maintained at 60 ◦ C. The slurry was loaded into a spray gun, and coatings were produced by controlling the number of sprays (4, 15, 35 and 50, resulting in electrode mass between 1–2, 6–8, 10–14, and 17–24 mg, respectively). The deposit was allowed to dry for 30 s between each coating to prevent solvent concentration and subsequent cracking. The coated film was then pressed at 3 Mt in a mechanical press and dried overnight at 100 ◦ C. This helped maintain consistency in mass and volume between different coatings and samples (Table S3). In the case of PVDF-HFP as the binder, only CSP was used as the conductive material. AC, CSP and PVDF-HFP were dissolved in NMP in ratios of 80:10:10 and 75:10:15. The process was the same as above except for a few differences. For example, the coatings were produced at a constant heating temperature of 75 ◦ C, and the slurry was sprayed 3 and 27 times to obtain coatings of different thicknesses and mass loadings (Tables S1 and S3). To test the limits of the spray coating process, even thicker electrodes (0.6 mm) with AC:CNT:CMC (85:10:5) as well as AC:CSP:PVDF-HFP (80:10:10) were developed with mass of 48 mg and 46 mg, respectively. The electrode names have a number as a prefix to differentiate different coatings on the basis of mass. For example, electrodes between 1 and 2 mg have a prefix of 1, while those between 6 and 8 mg have a prefix of 2, and so on. 2.3.2. Freeze Casting For freeze casting, AC, CSP and CMC were mixed in a ratio of 85:10:5 in water. The slurry was coated onto the current collector using a casting knife and then frozen in a container with liquid nitrogen. To obtain a thicker coating, the slurry was subsequently coated and frozen three times. The coatings were then placed in a lyophiliser for 24 h to completely remove the solvent. The electrodes were not pressed except for the cell “3B_LYO_5%CMC”. 2.4. Cell Assembly Symmetrical CR2032 coin cells were assembled for each type of thick electrode pre- pared in this work. After drying in a vacuum oven overnight, electrodes were cut to Φ = 14 mm size and then pressed. The thickness was measured with a micrometre, and the electrodes were further dried at 100 degrees for 6 h. They were transferred to an argon-filled glovebox (Jacomex GP with O2 < 5 ppm and H2 O < 5 ppm), and electrodes with similar mass were placed with a cellulose separator (Φ = 16 mm) in the coin cell with 1M TEABF4 in acetonitrile (ACN) as the electrolyte for electrochemical characterization. 2.5. SEM Analysis Electrode surface morphology was analysed by SEM (FEI Quanta 650 FEG) without metallization. Cross-sections were prepared by blade cutting and placed onto aluminium stubs at an angle of 70◦ to analyse the electrode structure of the different casting methods and characterize their thickness. Images were taken at 5 kV at different magnifications for each sample. 3. Results and Discussion 3.1. Morphology Characterization Figures 2 and 3 show the cross-sections of thin electrodes (left column) and thick electrodes (right column). For spray-coated electrodes, comparisons were made between electrodes having thicknesses in the range of 20–30 µm (suffix 1) and 317–333 µm (suffix 4). For the freeze-casted electrodes, the thinnest 271 µm (2_LYO_5%CMC) and thickest 241 µm (3A_LYO_5%CMC) electrodes where selected. Even surfaces and good contacts with the current collectors could be observed for all electrodes. As the thickness increased, some thick electrodes showed minor unevenness on the surface due to the hand-coating process. Nevertheless, the structural integrity of the Materials 2023, 16, 3439 5 of 17 Materials 2023, 16, x FOR PEER REVIEWelectrodes was maintained with the increase in thickness. A compact layering 6ofofcarbon 20 could also be seen for all spray-coated electrodes. Figure Figure2. 2.SEM SEM imagesimages of thin of electrodes (a) 1_CSP_5%CMC, thin electrodes (b) 1_CNT_5%CMC, (a) 1_CSP_5%CMC, (c) (b) 1_CNT_5%CMC, 1_CSP_10%PVDF-HFP, (d) 1_CSP_15%PVDF-HFP, (e) 2_LYO_5%CMC. (c) 1_CSP_10%PVDF-HFP, (d) 1_CSP_15%PVDF-HFP, (e) 2_LYO_5%CMC. In the case of the thin freeze-casted electrodes (Figure 2e), a distinct macro pore structure was not observed. The electrode structure resembled the thin electrodes made with the spray-casting method without showing any pattern. However, in the case of Materials 2023, 16, 3439 6 of 17 thicker freeze-casting electrodes, a characteristic macro pore structure could be observed, which was inherent to the freeze-drying process. This macrostructure was more open and distributed like an accordion, in which nano-channels may allow for better electrolyte percolation. Nevertheless, the macro pores were unevenly distributed and randomly Materials 2023, 16, x FOR PEER REVIEWoriented, creating dead-end pores that could be ascribed to the casting of multiple 7 layers of 20 at inconsistent temperatures. Figure Figure3. 3.SEM SEM imagesimages of thick electrodes of thick (a) 4_CSP_5%CMC, electrodes (b) 4_CNT_5%CMC, (a) 4_CSP_5%CMC, (c) (b) 4_CNT_5%CMC, 4_CSP_10%PVDF-HFP, (d) 4_CSP_15%PVDF-HFP, (e) 3A_LYO_5%CMC. (c) 4_CSP_10%PVDF-HFP, (d) 4_CSP_15%PVDF-HFP, (e) 3A_LYO_5%CMC. 3.2. Porosity Analysis A porous electrode allows better electrode percolation throughout the bulk and, therefore, better utilisation of active material [42]. Therefore, having an ideal pore struc- ture becomes an important consideration for thick electrodes. The porosity (𝑷) of an elec- Materials 2023, 16, 3439 7 of 17 3.2. Porosity Analysis A porous electrode allows better electrode percolation throughout the bulk and, there- fore, Materials 2023, 16, x FOR PEER REVIEW better utilisation of active material [42]. Therefore, having an ideal pore 8 of structure 20 becomes an important consideration for thick electrodes. The porosity (P) of an electrode is given by Equation (1):   ρg P = 1− ·100 (1) (1) 𝝆𝒈 ρt 𝑷 = [𝟏 − ] ∙ 𝟏𝟎𝟎 𝝆𝒕 where ρg is the geometrical density of the electrode (g−3cm−3 ) based on the thickness and where 𝝆 is the geometrical density of the electrode (g cm ) based on the thickness and loading 𝒈of the electrode, and ρt is the true electrode density (g−3cm−3 ) based on the density loading of the electrode, and 𝝆𝒕 is the true electrode density (g cm ) based on the density of the slurry. of the slurry. mass loading o f electrode −2g cm−2  ρ𝑚𝑎𝑠𝑠 g = 𝑙𝑜𝑎𝑑𝑖𝑛𝑔 𝑜𝑓 𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑑𝑒 (𝑔 𝑐𝑚 ) (2) (2) 𝜌𝑔 = d 𝑑 and and 1 ωi 1ρ = ∑ 𝜔 (3) t= ∑ 𝑖 ρi (3) 𝜌𝑡 𝜌𝑖 d is thickness of electrode without the aluminium foil in cm. ρt is the true density of 𝑑 is thickness the materials, ωi isofthe electrode withoutof mass fraction the aluminium each materialfoil in cm. of the 𝝆𝒕 is theand electrode, trueρ density i is the density of the materials, 𝜔𝑖 is the mass fraction of each material of the electrode, and 𝜌𝑖 is the of each component. density of each component. All electrodes had porosity values of >80% (Figure 4). For spray coatings, the thin All electrodes had porosity values of >80% (Figure 4). For spray coatings, the thin electrodes had a greater porosity (≥85%), which decreased with the increase in thickness electrodes had a greater porosity (≥85%), which decreased with the increase in thickness (Table 1). On the other hand, electrodes produced with the freeze-casting method showed (Table 1). On the other hand, electrodes produced with the freeze-casting method showed very high porosityeven very high porosity evenatathigher higher thicknesses, thicknesses, duedue to their to their moremore open open structure. structure. The thickest The thick- electrode produced by freeze casting ( ∼ 600 µm) had the highest est electrode produced by freeze casting (∼600 µ m) had the highest porosity of all (96%). porosity of all (96%). When the thick electrode was compressed and the macro-pore When the thick electrode was compressed and the macro-pore structure was destroyed, structure was destroyed, the porosity the porositywas reduced was reducedto to85%. However,asas 85%. However, will will be be shown shown belowbelow (Figure (Figure 4), the4), the increase increase in areal arealcapacitance capacitanceshowed showed that that thethe porosity porosity waswas sufficient sufficient to allow to allow adequateadequate electrolyte electrolyte infiltration and,therefore, infiltration and, therefore, ionion transfer. transfer. Gravimetric Gravimetric and and volumetric volumetric capacitance capacitance variance variance with porosity with porosityandandthickness thickness were alsoalso were calculated calculatedand can andbe seen can be in Figures seen S4 and S4 in Figures S5.and S5. Taking into into account accountonlyonlythe thegravimetric gravimetriccapacitance, capacitance, thinner thinnerelectrodes electrodes( 300 µ m)µm) ( 300 hadhad the the better better performance, performance, andand porosity porosity did did not not playa asignificant play significantrole. role.However, However, when when vol-volumetric umetric capacitance capacitance was observed, was observed, an independence an independence from fromthethe thicknessappeared, thickness appeared, and andthethe most most “compact” electrodes performed better. It was thus observed that “compact” electrodes performed better. It was thus observed that the porosity values the porosity values between 80–90% between 80–90%were were desirable desirable to have to havegood contact good between contact carbon between grainsgrains carbon and increase and increase the capacitance of the thick electrodes. the capacitance of the thick electrodes. Figure Dependence Figure 4.4.Dependence of electrode of electrode areal capacitance areal capacitance (at 10 mV(at s−110 ) onmV s−1 ) on thickness andthickness and porosity (10 mVporosity (10 − s ). mV s ). −1 1 Materials 2023, 16, x FOR PEER REVIEW 9 of 20 Table 1. Specifications of electrodes. Materials 2023, 16, 3439 8 of 17 Capacitance Capacitance Specific Specific Electrode Thickness (10 mV s−1) (10 mV s−1) Porosity Tortuosity Energy (Wh Power (W Mass (mg) (µm) (mF of Table 1. Specifications cm −2) (F g−1) electrodes. kg−1) kg−1) 1_CSP_5%CMC 1.37 31 87 98 89% 2.5 22 679 1_CNT_5%CMC 1.70 31 Capacitance 103 Capacitance 94 89% 4.6 Specific 23 Specific 686 Electrode Thickness (10 mV s−1 ) (10 mV s−1 ) Porosity Tortuosity Energy Power (W 1_CSP_10%PVDF-HFP 1.43(mg) Mass 23 (µm) 83cm−2 ) (mF 89 (F g−1 ) 85% 4.3 22 (Wh kg−1 ) 681 kg−1 ) 1_CSP1_CSP_5%CMC 15%PVDF-HFP 0.84 1.37 21 31 58 87 107 98 91%89% 8.1 2.5 27 22 784679 4_CSP_5%CMC 1_CNT_5%CMC 17.09 1.70 31731 1098103 99 94 87%89% 1.0 4.6 26 23 770686 1_CSP_10%PVDF-HFP 4_CNT_5%CMC 1.43 23.88 36923 1428 83 92 89 86%85% 0.5 4.3 26 22 760681 1_CSP 15%PVDF-HFP 0.84 21 58 107 91% 8.1 27 784 4_CSP_10%PVDF-HFP 4_CSP_5%CMC 25.74 17.09 322317 1351 1098 81 99 81%87% 0.4 1.0 20 26 744770 4_CNT_5%CMC 4_CSP_15%PVDF-HFP 23.88 21.18 333369 1428 1285 93 92 86%86% 0.3 0.5 24 26 716760 4_CSP_10%PVDF-HFP 25.74 322 1351 81 81% 0.4 20 744 5_CNT_5%CMC 4_CSP_15%PVDF-HFP 47.81 21.18 629333 2459 1285 79 93 84%86% 0.2 0.3 19 24 942716 5_CSP_10%PVDF-HFP 5_CNT_5%CMC 46.32 47.81 543629 2261 2459 75 79 80%84% 0.1 0.2 21 19 927942 5_CSP_10%PVDF-HFP 46.32 543 2261 75 80% 0.1 21 927 2_LYO_5%CMC 2_LYO_5%CMC 7.54 7.54 271271 407407 83 83 93%93% 0.9 0.9 21 21 704704 3A_LYO_5%CMC 3A_LYO_5%CMC 10.78 10.78 647647 567567 81 81 96%96% 0.1 0.1 20 20 732732 3.3. EISEIS 3.3. andand Tortuosity TortuosityCalculation Calculation Electrochemical Electrochemical impedancespectroscopy impedance spectroscopy(EIS) (EIS)can can be be used used for detailed information for detailed informationon onthe thekinetics kineticsofofthethe charge–discharge process as well as the pore structure of charge–discharge process as well as the pore structure of electrodes electrodes [43,44]. [43,44]. The EIS experiment was conducted over a frequency range The EIS experiment was conducted over a frequency range from 500 kHz to 50 from 500 kHz MHz to with 50 MHz with a 10 mV perturbation amplitude using a BioLogic VMP3 potentiostat a 10 mV perturbation amplitude using a BioLogic VMP3 potentiostat controlled by the controlled byEC-Lab the EC-Lab software. software. The equivalent The equivalent circuit circuit was obtained was obtained by fitting by fitting the impedance the impedance spectra spectra usingusing a simplified a simplified transmission-line transmission-line modelmodel with constant-phase with constant-phase elements elements (TLM-Q, (TLM- Ma3 ) Q,that Ma3neglected ) that neglected the charge the charge transfertransfer across across the the electrode/electrolyte electrode/electrolyte interface interface (i.e., (i.e., blocking blocking conditions) conditions) [45]. 5a–c [45]. Figure Figure 5a–cashows shows a semicircle semicircle followedfollowed by a transmission-line by a transmission-line behaviour behaviour for all of for all of the samples.the samples. Figure 5. EIS Figure fitting 5. EIS forfor fitting (a)(a) thin electrodes, thin (b)(b) electrodes, thick electrodes thick and electrodes (c)(c) and freeze-casted electrodes freeze-casted and electrodes and (d)(d) equivalent circuit. equivalent circuit. The high-frequency semicircle has been previously observed in the analysis of both thick and thin electrodes and has been ascribed to different phenomena. For example, it has been assigned to poor contact between the electrode and the current collector [45,46], to the electrolyte resistance within the pores of the electrode [44], or even to redox processes of heteroatoms present in the carbon materials [46]. The smaller semicircle for thick electrodes Materials 2023, 16, 3439 9 of 17 could represent lower interface charge loss as compared to the thin electrodes [47,48]. Further, Figure 5a,b show that the resistances of 1_CSP_5%CMC were smaller than those of 1_CNT_5%CMC whereas the resistances of 4_CSP_5%CMC were larger than those of 4_CNT_5%CMC. This anomaly could be due to the fact that CSP was observed to provide slightly better charge transfer between AC particles than CNTs, leading to lower resistance for thin electrodes. However, CNTs provided better mechanical strength (and, therefore, slightly better adhesion to the current collector), which was a more important factor in the case of thick electrodes (4_CSP_5%CMC and 4_CNT_5%CMC). Thus, CNT-based thicker electrodes had a lower resistance. Regardless of the cause leading to this electrochemical response, it has been proved that, as long as the transmission-line (TL) behaviour can be observed, the ionic resistance and so the tortuosity of electrodes can be determined [45]. Therefore, the circuit pro- posed is still valid to estimate the electrode tortuosity to correlate this parameter with the electrochemical performance. Together R1, R2 and Q2 represent the internal resis- tance of the electrode. Ma3 is the modified restricted diffusion element that represents ion diffusion within the electrode together with the capacitive charging of the surfaces, modelled with constant-phase elements to account for non-ideal behaviour that arises from geometric effects. The slope of the line at intermediate frequencies differed for different thicknesses, indicating a longer ion diffusion process in the thicker electrodes, as could be expected [44]. The model also encompasses the diffusion resistance Rd3, which gives the total ion transport resistance or Rion . The inductor L1 represents a negligible inductance in the electrode [49]. The flow of electrolyte through the pores of the electrodes can be determined by the tortuosity (τ), which can be calculated from Equation (4) [30]: Rion · A·κ · P τ= (4) 2d where A, d, and P are surface area, thickness, and porosity of the electrode, respectively, while κ is the conductivity of the electrolyte (in our case, 0.05614 S cm−1 ). The tortuosity values for the electrodes are reported in Tables 1 and S3. The thin electrodes with lower binder percentages reported lower tortuosity values (in the case of using CMC as binder); however, tortuosity nearly doubled each time as binder percentage increased from 5% to 10% to 15%, being indistinct to the binder nature. On the other hand, as thickness increased, the tortuosity of the electrodes decreased and, in some cases, the values observed were below 1 (tortuosity for through-pores). This could be possible due to the presence of dead-end pores, which would contribute to an increase in the surface area causing this decrease in tortuosity [50]. The study by Nguyen et al. points out that there is no need for ions to percolate all the way to the current collector as long as they reach the active surface of the electrode. Therefore, dead-end pores might not only lead to an increase in the active area but also provide more easily accessible paths for ions. According to this study, the latter translates into a reduction in the tortuosity value, which could even take values lower than one (more accessible than straight pores). It is worth mentioning that the layer-by-layer deposition used in this work may contribute to the formation of these pores. Likewise, dead-end pores may be caused due to the addition of conductive carbon as well as the binder. The results show that the electrodes with CSP had lower tortuosity values compared to the other spray-coated electrodes. Further, freeze-casted electrodes also had a very low tortuosity value due to the presence of micropores and more open structure. However, as can be seen from Figures 2e and 3e, these pores were not aligned or well-ordered, which could have resulted in dead-end pores mentioned earlier and hence tortuosity values < 1. 3.4. Electrochemical Characterization Electrochemical tests were performed with a VMP3 multichannel potentiostat (Bio- Logic) using the EC-Lab software. Cyclic polarization (CP) was performed for symmetrical two-electrode cells between 0 < V < 2.7, at different scan rates (2 < mV s−1 < 200) to evaluate Materials 2023, 16, 3439 10 of 17 the capacitive behaviour and calculate the potential window, stability, and cell capacitance (F) of the supercapacitors and the specific (F g−1 ), areal (F cm−2 ), and volumetric (F cm−3 ) capacitances of each electrode. Galvanostatic charge–discharge (GCD) was also performed to characterize the charging behaviour and obtain another estimate of the gravimetric capacitance. The cell capacitance (Ccell ) of a symmetrical capacitor can be calculated from cyclic polarisation (CP) as: 1 ∆Q   R I (V )·dV Ccell = = (5) 2 ∆V 2·υ·∆V R In the case of symmetrical supercapacitors, I (V )·dV is the total integral of the area under the CP (ACP ) (Qcharge + Qdischarge ), υ is the scan rate of the CP, and ∆V is the working potential window. The capacitance of an electrode (Ce ) is twice that of the cell (Ccell ); therefore, Ce = 2Ccell (6) The electrode volumetric capacitance (Cv ), areal capacitance (Ca ) and gravimetric capacitance (Cg ) can be calculated as: Ce Cv = (7) v Ce Ca = (8) A Ce Cg = (9) m where v is the volume of the electrode without the current collector (cm3 ), A is the area (cm2 ), and m is the mass of the electrode (g) considering only the active material. Finally, gravimetric energy density (Wh kg−1 ) and gravimetric power density (W kg−1 ) of the device were calculated at 1 A g−1 from the equations below, where t is the discharge time. " # 1 Cg ∆V 2 Eg_cell = (10) 8 3.6 Eg_cell Pg_cell = (11) t 3.4.1. Capacitance Figure 6 shows the CP curves of different electrodes with increasing thicknesses at a scan rate of 10 mV s−1 . The graphs show that as the thickness of electrodes increases, the area under the graph also increases, signifying and increase in areal capacitance. Comparing methods, it is noted that the freeze-casted electrodes had significantly lower capacitance compared to the spray-coated electrodes; however, the freeze-casted and pressed electrode (3B_LYO_5%CMC) showed a better areal capacitance and rate capability than its unpressed counterpart (3A_LYO_5%CMC), revealing that while the macro pore structure allows better electrolyte percolation, it reduces the contact between carbon particles, leading to poor electron transfer. On the other hand, the effect of scan rate on the areal capacitance of different samples is shown by Figure 7. An expected trend of reduction in capacitance with increase in scan rate was observed for all materials. The thick electrodes demonstrated high areal capacitances at high scan rates. However, the spray-coated thick electrodes’ capacitances rapidly declined after 20 mV s−1 . This was not seen in the case of thin electrodes, which retained their areal capacitances even at high scan rates. Materials 2023, 16, x FOR PEER REVIEW 12 of 20 Materials 2023, 16, 3439 11 of 17 Materials 2023, 16, x FOR PEER REVIEW 13 of 20 CPsofofsymmetric Figure6.6.CPs Figure symmetriccoin coincells cellsatat10 mVss−1−with 10mV 1 with 1M TEABF in ACN. 1M TEABF4 in 4 ACN. On the other hand, the effect of scan rate on the areal capacitance of different samples is shown by Figure 7. An expected trend of reduction in capacitance with increase in scan rate was observed for all materials. The thick electrodes demonstrated high areal capaci- tances at high scan rates. However, the spray-coated thick electrodes’ capacitances rapidly declined after 20 mV s−1. This was not seen in the case of thin electrodes, which retained their areal capacitances even at high scan rates. Figure 7. Areal Figure 7. Areal capacitance capacitance at at different different scan scan rates rates for for each eachtype typeof ofelectrode. electrode. The thickest electrode produced using CNTs as carbon material and 5%wt CMC The thickest electrode produced using CNTs as carbon material and 5%wt CMC as −2 ) showed as binder (5_CNT_5%CMC, ∼600 µm and 31 mg −2)cm the best aerial capac- binder (5_CNT_5%CMC, − 2 ∼600 µ m and 31 mg cm showed the best aerial capacitance of itance of 2459 F cm . The only electrode with a similar mass loading and thickness 2459 F cm−2. The only electrode with a similar mass loading −and thickness (5_CSP_10%PVDF-HFP) had a slightly lower capacitance of 2261 F cm 2 at 10 mVs−1 , (5_CSP_10%PVDF-HFP) had a slightly lower capacitance of 2261 F cm−2 at 10 mVs−1, using 10% wt of PVDF-HFP as binder. However, these electrodes did not perform well at high scan rates. The variation in aerial capacitance (mF cm−2) with changes in mass loading (mg) and thickness (µ m) of the electrodes with different materials can also be visualized through Figure 8. Both gravimetric and volumetric capacitance dependences are also de- Materials 2023, 16, 3439 12 of 17 using 10% wt of PVDF-HFP as binder. However, these electrodes did not perform well at high scan rates. The variation in aerial capacitance (mF cm−2 ) with changes in mass loading (mg) and thickness (µm) of the electrodes with different materials can also be visualized through Figure 8. Both gravimetric and volumetric capacitance dependences are Materials 2023, 16, x FOR PEER REVIEW also depicted in Figures S6 and S7 from Supporting Information. As 14 in ofthe 20 comparison of the porosity, volumetric capacitance is independent of the thickness, being comparable to Materials 2023, 16, x FOR PEER REVIEW the performance of thick electrodes and the ones between 100–200 µm,14and of 20 gravimetric capacitance is better for thin electrodes. Figure 8. Capacitance (at 10 mV s−1) of various electrodes as a function of thickness and mass. Figure 8. Capacitance (at 10 mV s−1 ) of various electrodes as a function of thickness and mass. 3.4.2.Figure Specific Energy and(atPower 8. Capacitance 10 mV s−1) of various electrodes as a function of thickness and mass. 3.4.2. FigureSpecific 9 shows Energy andenergy the specific Power versus power for selected electrodes at a current 3.4.2. density Specific of Figure Energy 1 A g−1. Overall, and as Power before, 9 shows the specific spray-coated energyelectrodes performed versus power better than for selected freeze- electrodes at a current casted electrodes. Figure density Depending of 91shows − A g the 1 upon the thickness, specific energy . Overall, there was a versusspray-coated as before, large variation power for selected in electrodes electrodes the at aspe- currentbetter than performed cific density power of ofthe 1 Aelectrodes. g−1 . Overall,The thickestspray-coated asDepending before, electrodes outperformed electrodes the thin better performed electrodes, as freeze-casted electrodes. upon the thickness, there was a than largefreeze- variation in the they casted had theelectrodes. highest specific power, Depending at 942 upon theWthickness, kg−1. there was a large variation in the spe- specific power of the electrodes. The thickest electrodes outperformed the thin electrodes, cific power of the electrodes. The thickest electrodes outperformed the thin electrodes, as as they had the highest specific power, at 942 W kg−1 . they had the highest specific power, at 942 W kg−1. Figure 9. Ragone plot of electrodes with specific energy and specific power measured at 1 Ag−1 . Here, Figure 9. Ragone plot of electrodes with specific energy and specific power measured at 1−Ag −1. circles represent spray-coated thin electrodes with mass loading ≤ 1 mg cm 2 , squares represent Here, circles represent spray-coated thin electrodes with mass loading ≤ 1 mg cm−2, squares repre- spray-coated sent spray-coated thick thick electrodes electrodes withwith average average mass loading mass loading of 14 mgof cm14 −2, mg cm−2shapes diamond , diamond indi- shapes indicate Figure 9. Ragone plot of electrodes with specific energy and specific power −2measured at 1 Ag−1. cate spray-coated spray-coated thick electrodes thick withwith electrodes average massmass average loading of 31 mg loading of cmmg 31 −2, and cm inverted , and triangles inverted triangles depict Here, circles represent spray-coated thin electrodes with mass loading ≤ 1 mg cm , squares repre- −2 depictfreeze-casting freeze-casting electrodes. electrodes. sent spray-coated thick electrodes with average mass loading of 14 mg cm−2, diamond shapes indi- cate spray-coated thick electrodes with average mass loading of 31 mg cm−2, and inverted triangles depict freeze-casting electrodes. Materials 2023, 16, 3439 13 of 17 It was observed that the specific energy values of devices made with different thick- nesses, carbon additives and binders were largely comparable. However, the thin electrode with CSP as the additive and 15% PVDF-HFP as a binder had slightly better, but com- parable, specific energy (27 Wh kg−1 ) than the thick electrodes with CMC as the binder (26 Wh kg−1 ). 3.4.3. Impact of Carbon Additives, Binders and Thickness The role of binder in the capacitance of electrodes can also be assessed from Figure 7. CNT-based electrodes had slightly better areal and volumetric capacitance than CSP-based electrodes, as they are known to disperse better with CMC. Interestingly, while the overall areal capacitance of CNT electrodes was higher than that of CSP, the slope of capacitance vs. scan rate was also higher. That is, CSP-based electrodes demonstrated a lower reduction in performance as the scan rate increased. Moreover, despite the better bonding and better mechanical strength of CNT electrodes, CSP-based electrodes of similar thickness had better gravimetric capacitance (Figure S1). Therefore, carbon-to-carbon contact in thick electrodes is better with CSP, although the thickness cannot be increased over a certain limit due to the development of cracks, unlike CNT-based electrodes. Overall, thick electrodes with 5% CMC as a binder had superior performance, con- sidering gravimetric capacitance (Figure S1), than those with 10% or 15% PVDF-HFP. Nevertheless, the PVDF-HFP-based thick electrodes were slightly more flexible than CMC electrodes. Further, it can be noticed that the increase in the amount of PVDF-HFP binder (from 10% to 15%) had a minor impact on the capacitance of the electrode. Thick electrodes produced using 15% PVDF-HFP had slightly better performance at high scan rates. This could be attributed to their better adherence to the current collector. It has been well established that using large amounts of binder can block carbon pores and reduce capaci- tance, which is also supported by the tortuosity analysis [50]. However, in this case, the improvement in gravimetric capacitance of the thick electrodes could be explained by the possibly better adherence with the current collector. 3.4.4. Discussion The use of carboxymethylcellulose (CMC) as binder with CNTs as conductive additives led to electrodes with good mechanical strength but, not especially flexible, whereas thick electrodes developed using PVDF-HFP copolymer as the binder not only had high mechanical strength but were also flexible. Nevertheless, these latter electrodes required a higher percentage of the binder, which increased the percentage of non-active components in the supercapacitor. Moreover, since the areal capacitance of the CMC and PVDF-HFP thick electrodes was similar, CMC with CNTs was judged to be a better option to develop thick electrodes, since CMC is environmentally friendly, soluble in water, and is required in lower quantities. Thick electrodes developed using these methods featured increased areal capacitances with increases in mass loading and thickness. However, this was not observed in the case of gravimetric capacitances at high scan rates. While the thick electrodes had high gravimetric capacitance at low scan rates, at higher scan rates, the thin electrodes not only performed much better, but also did not suffer loss in performance. Nevertheless, comparing the areal capacitance alone (as is common practice for thick electrodes) can be misleading. Volumetric capacitance could give a more realistic picture of the material performance, as it would consider the bulk of the electrode and, in a more practical sense, it would be more useful for device design. Nevertheless, it ignores the utilisation of active material, which can be determined with the analysis of gravimetric capacitance. Thus, a comprehensive analysis of gravimetric, areal and volumetric capacitance should be carried out for each specific device to be developed to assess the performance of thick electrodes and ensure compact cells, as well as the optimal utilisation of the active material. In line with this conclusion, carbon-based supercapacitor electrodes with mass loading between 5 Materials2023, Materials 2023,16, 16,3439 x FOR PEER REVIEW 16 14 of 20 of 17 between and 8 mg 5cmand−2 8 andmgwithcm−2thickness and withbetween thickness between 100 and 200100µmand had200theµbest m had the best per- performance at formance high at high scan rates. scan rates. Porosity estimates Porosity estimates and and tortuosity tortuosity analyses analyses from from impedance impedance measurements measurements were were also also performed performedtotodetermine determinethe thepore porestructure of of structure thethe electrodes. It was electrodes. observed It was thatthat observed dead-end dead- pores in thick end pores electrodes in thick couldcould electrodes be causing anomalously be causing low tortuosity anomalously values, low tortuosity and thus, values, and other methods such as electron tomography and X-ray absorption thus, other methods such as electron tomography and X-ray absorption spectroscopy spectroscopy could be used couldinbethe future used to further in the future toreveal the reveal further pore structure. the pore Furthermore, freeze-casted structure. Furthermore, electrodes freeze-casted had high porosity, electrodes had highbut they did porosity, butnot demonstrate they high capacitances. did not demonstrate Therefore,Therefore, high capacitances. a higher degree a higher ofdegree porosity of in the material porosity does not does in the material necessarily ensure better not necessarily ensureperformance, as there better performance, needs to be as there a balance needs to be abetween balanceporosity betweenand the contact porosity and thebetween contactcarbon betweenparticles carbonwithin the particles bulk of the electrode material. Thus, an ordered pore structure with good within the bulk of the electrode material. Thus, an ordered pore structure with good con- contact between carbon particles, tact between as depicted carbon particles,inasFigure 10d,inwould depicted Figureideally have better 10d, would ideallyperformance across have better perfor- all metrics studied in this paper. mance across all metrics studied in this paper. Figure 10. Figure 10. Schematic Schematicrepresentation representationofofmultilayer multilayerelectrode structures electrode exemplifying structures different exemplifying micro- different mi- structures, each leading to different porosities and electrode–electrolyte active areas. crostructures, each leading to different porosities and electrode–electrolyte active areas. This is This is the the key key issue issue inin the the development development of of thick thick electrodes. electrodes. Traditionally, Traditionally, the the good good balance between balance between electrolyte electrolyte impregnation impregnation of of an an electronically electronically conductive conductive carbon carbon network network has been has been “solved” “solved” by by fabricating fabricating thin, thin, single-layer single-layer electrodes electrodes (Figure (Figure 10a). 10a). However, However, thisthis obviously represents obviously representsrenouncing renouncingthe thegreat greatpotential potential of of large, large, “bulk” “bulk” electrodes electrodes to to provide provide much higher much higher capacity capacityand and energy energydensity densityperperdevice. device. This This ideal ideal goal goal is is represented represented by by the the idealized idealizeddrawing drawingininFigure Figure 10d, 10d,with porosity with fully porosity extending fully extendingthroughout throughoutmultilayer-thick multilayer- electrodes. In thisIn thick electrodes. work, this we were work, weable to highlight were that the that able to highlight laterthe goallater is possible goal is (though possible by no means (though by no too easy),too means and at the easy), andsame time, at the same wetime, witnessed the many we witnessed thebarriers that canthat many barriers be found to reach it. For instance, the fabrication of freeze-casted electrodes, can be found to reach it. For instance, the fabrication of freeze-casted electrodes, as carried as carried out by outusbyand reported us and here,here, reported led toledmultilayer to multilayerelectrodes electrodeswithwith largelarge but but dead-end porosity dead-end (as porosity shown (as shown in Figure 10c) 10c) in Figure and poor performance. and poor performance.On the Onother hand,hand, the other multilayer-sprayed thick multilayer-sprayed electrodes led to led thick electrodes much-improved to much-improved areal capacitance areal capacitancewith respect to thin with respect toelectrodes. The thin electrodes. fact Thethat fact this that improvement this improvement is notis realized not realizedwhen normalizing when normalizing by mass by massmeans meansthat there that thereis still room for the improvement of our thick electrodes. Finally, when is still room for the improvement of our thick electrodes. Finally, when the normalization the normalization of capacitance of capacitance is conducted is conducted per unit of volume, per unit of volume,an interesting an interestingsituation arises:arises: situation thick electrodes thick elec- (multi-spray method) feature higher volumetric capacitances than trodes (multi-spray method) feature higher volumetric capacitances than thin electrodes, thin electrodes, but only at low scan rates. This tells us that presently our thick electrodes conform but only at low scan rates. This tells us that presently our thick electrodes conform to an to an intermediate situation intermediate suchsituation as that depicted in Figure such as that depicted10b,inwhere Figureelectrolyte 10b, whereimpregnation of the bulk electrolyte impregnation solid electrode can be good but where other factors impede ionic of the bulk solid electrode can be good but where other factors impede ionic diffusion diffusion at high rates. at Determining and solving these impeding hurdles must be our high rates. Determining and solving these impeding hurdles must be our next collective next collective goal. goal. Materials 2023, 16, 3439 15 of 17 4. Conclusions Thick electrodes (up to 0.6 mm) were developed using commercial carbons and additives and low-cost, easily scalable methods, namely, multi-spraying and freeze-casting methods. The multiple spray coating method was developed and successfully tested to obtain electrodes with high mass loadings (31 mg cm−2 ). Different binders and their compositions were also tested to ensure coatings with high mechanical strength. Freeze- casting was applied to pre-formed electrodes to test the effect of water sublimation in the formation of macropores and the corresponding effect on performance. This latter method led to large macropores but much lower mass loading. The electrodes developed using these methods showed very high areal capacitances. For instance, a 0.6 mm multilayer spray-coated electrode with CNTs featured an areal capacitance of 2459 F cm−2 at 10 mV s−1 , and the corresponding symmetrical devices showed good specific energies (19 Wh g−1 at 1 A g−1 ) that were comparable with less-scalable thick electrodes previously reported. Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/ma16093439/s1, Table S1: Description of various coatings de- veloped for spray casting; Table S2: Theoretical density of materials for slurry preparation; Table S3: Specifications of electrodes; Figure S1: Gravimetric capacitance at different scan rates; Figure S2: Volumetric capacitance at different scan rates; Figure S3: Cyclic polarisations of different electrode materials at 10 mV s−1 ; Figure S4: Dependence of electrode gravimetric capacitance (at 10 mV s−1 ) on thickness and porosity; Figure S5: Dependence of electrode volumetric capacitance (at 10 mV s−1 ) on thickness and porosity; Figure S6: Gravimetric capacitance (at 10 mV s−1 ) of various electrodes as a function of thickness and mass; Figure S7: Volumetric capacitance (at 10 mV s−1 ) of various electrodes as a function of thickness and mass. References [51–55] are cited in the supplementary materials. Author Contributions: Conceptualization, A.P., R.M.G.-G. and P.G.-R.; methodology, A.P.; validation, A.P., R.M.G.-G., L.N.B. and P.G.-R.; formal analysis, A.P., R.M.G.-G. and L.N.B.; investigation, A.P.; resources, R.M.G.-G. and P.G.-R.; data curation, A.P.; writing—original draft preparation, A.P., R.M.G.-G. and L.N.B.; writing—review and editing, A.P., R.M.G.-G., L.N.B. and P.G.-R.; visualization, A.P.; supervision, P.G.-R.; project administration, R.M.G.-G.; funding acquisition P.G.-R. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Spanish MICINN, the State Research Agency (AEI) and the European Regional Development Fund (FEDER) (Project Nanopeistorage PID2021-128390OB- I00), and by H2020-MSCA-COFUND-2016, DOC-FAM, Grant Agreement No. 754397. The ICN2 is supported by the Severo Ochoa Centres of Excellence programme (Grant SEV-2017-0706 and CEX2021-001214-S, funded by MCIN/AEI/10.13039.501100011033). Acknowledgments: A.P. acknowledges her fellowship funded by H2020-MSCA-COFUND-2016, DOC-FAM, Grant Agreement No. 754397. This work has been carried out within the framework of the doctoral program (PhD) of Material Science of Universitat Autònoma de Barcelona (UAB). L.N.B. acknowledges funding from Horizon-TMA-MSCA-PF-EF (ref 101062498). Conflicts of Interest: The authors declare no conflict of interest. References 1. Stojanovska, E.; Kilic, A. Carbon nanofibers as thick electrodes for aqueous supercapacitors. J. Energy Storage 2019, 26, 100981. [CrossRef] 2. Cronau, M.; Paulus, A.; Pescara, L.P.; Kroll, M.; Renz, D.; Mekontso, J.A.; Marx, A.; Roling, B. What Limits the Rate Capability of Ultrathick Composite Electrodes in Lithium-Ion Batteries? A Case Study on the Thickness-Dependent Impedance of LiCoO2 Cathodes. Batter. Supercaps 2022, 5, e202200194. [CrossRef] 3. Kuang, Y.; Chen, C.; Kirsch, D.; Hu, L. Thick Electrode Batteries: Principles, Opportunities, and Challenges. Adv. 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