Electrode materials with highest surface area and specific capacitance

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Electrode materials with highest surface area and specific capacitance cannot be the only deciding factor for applicability in energy storage devices – Inference of combined life cycle assessment and electrochemical studies Vikas Sharma, Sudipta Biswas, Baranidharan Sundaram, Prasenjit Haldar, Brajesh K Dubey, and Amreesh Chandra ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06413 • Publication Date (Web): 26 Jan 2019 Downloaded from http://pubs.acs.org on January 26, 2019

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ACS Sustainable Chemistry & Engineering

Electrode materials with highest surface area and specific capacitance cannot be the only deciding factor for applicability in energy storage devices – Inference of combined life cycle assessment and electrochemical studies Vikas Sharmaa, Sudipta Biswasb, Baranidharan Sundaramc,d, Prasenjit Haldarb, Brajesh Dubeyc and Amreesh Chandra*a,b a

School of Nanoscience and Technology b

c

Department of Physics

Department of Civil Engineering, Indian Institute of Technology Kharagpur, Kharagpur721302, West Bengal, India d

Presently at Department of Civil Engineering, National Institute of Technology Andhra Pradesh, Tadepalligudam-534101, Andhra Pradesh, India *Corresponding author: [email protected]

1 ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Hierachical nanosheets of Co3O4 deliver can deliver specific capacitance of ~ 402 F g-1, which is 50% higher than that obtained using simpler disc shaped (~ 230 F g-1) or conventional solid structures (~ 150 F g-1). A simple question is then asked: should the particles showing other morphologies be discarded? As the electrode material is to be used in green or renewable energy technologies, the carbon footprint of each particle morphology was determined using the life cycle assessment (LCA) studies. The results led to inferences, which were strikingly different from those generally expected. It is seen that simpler morphologies, prepared using simple synthesis protocols, had 5 times lower CO2 footprint than hierarchical morphology (nanosheets). The results become extremely critical for proposing their large scale industrial use. They clearly indicate that the choice of nanostructured metal oxides in energy storage devices will have to be relooked from the aspect of their own environmental impact. Particles with lowest environmental impact but comparable specific capacitances will win over other counterparts.

Keywords: electrochemistry, energy, nanomaterials, life cycle assessment, CO2 emission


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Introduction The performance of an energy storage device is directly attributed to the electrochemical performance of electrode materials, which are expected to have high surface area and tunable pore structures1-4. Hierarchical nanostructures, having novel hierarchical morphologies, are being proposed for the next generation energy storage devices such as supercapacitors, Li-batteries, fuel cells, etc.5-15. Many metal oxides, with fascinating morphologies ranging from hierarchical, hollow to layered type, have resulted in very high specific capacitance values16-24. Therefore, efforts to work on simpler structures have taken back seat and often ignored. It’s rare to find studies that carefully try to correlate the fashionable particle morphology with its environmental impacts or the cost viability for large-scale integration in energy storage devices. The question that is never asked: whether these morphologies are creating more damage to the environment than their corresponding simpler morphologies, which deliver slightly less performance? Life cycle assessment (LCA) is a factual study

dealing with product’s entire life cycle in terms of

sustainability. It is used to quantify the material’s environmental aspects ranging from global warming to human health25. This process is often undertaken by environmentalists to critique a product26-29. The available LCA studies of confined structures include nanomaterials, carbon nanotubes (CNTs) and metal oxide-based nanoparticles30-34. In the present study, we combine the results of electrochemical characteristics of different particle morphologies with the corresponding LCA based inferences. Co3O4 was chosen as the test material because of its high redox activity, appreciable theoretical specific capacitance, and tunable morphologies35-38. It is shown that a simple nanostructure can outshine the electrochemically better particles having hierarchical or other tailored nanostructures and larger surface area. The simpler nanostructure does return a slightly lower specific capacitance but its



impact on the environmental, in terms of CO2 footprint, is so low that use of other morphologies may actually become redundant or environmentally unacceptable. The morphology with higher surface area i.e. nanosheets show the highest electrochemical response. In terms of numbers, the obtained specific capacitance is > 2 times the value delivered by conventional solid spherical shaped particles. It is also much higher than the values observed in other hierarchical shaped particles such as nanodiscs. Following the common strategy, nanosheets should be proposed for energy storage devices. But, the LCA studies show that the environmental impact and associated carbon footprint of particles with nanosheet type morphology is more than 2 times than much simpler structures. Therefore, this morphology may actually be left redundant as its own environmental impacts will undermine the observed advantages. Therefore, the synergistic combination of LCA and electrochemical results show that the choice of particle morphology may have to be relooked.

Materials and methods Materials used Cobalt (II) nitrate hexahydrate (Co(NO3)2.6H2O) (97%), ammonium carbonate ((NH4)2CO3), glycerol (C3H8O3), sodium hydroxide (NaOH) and isopropanol were procured from Merck Specialities Pvt. Ltd. (India). All the reagents were used as purchased and without any further purification.

Synthesis of Co3O4 nanosheet (CO-NS) Co3O4 nanosheets were synthesized using a facile co-precipitation method in reflux conditions, under ambient pressure. To start, 4 g of cobalt nitrate was dissolved in de-ionized water under


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vigorous stirring to make solution ‘A’. 16 g of ammonium carbonate was added to 150 mL of deionized water under constant stirring at room temperature, to make solution ‘B’. ‘A’ was added drop-wise into ‘B’ using a measuring burette while ensuring constant stirring of 400 rpm. This resulted in the formation of a clear solution, which was subsequently kept stirring in an oil bath, at 120 °C and refluxed with tap water for 12 h. A light pink colored precipitate was obtained, which was vacuum filtered and washed alternatively with de-ionized water and ethanol, respectively. The resulting wet sample was air dried overnight in an oven at 80 °C. Finally, the dried powder was calcined at 500 °C for 4 h, under ambient conditions.

Synthesis of Co3O4 nanodisc (CO-ND) For the synthesis of Co3O4 nanodiscs, 0.05 M Co(NO3)2.6H2O was dissolved in 80 mL de-ionized water maintained at 40 °C, and 0.1 M solution of NaOH in 80 mL of deionized water was prepared separately. Temperature of cobalt nitrate solution was then raised to 70 °C. Subsequently, the prepared NaOH solution was added drop wise into the nitrate solution. A light brown precipitation was obtained, which was kept under vigorous stirring (750 rpm) for 4 h. After cooling, the precipitate was separated by centrifugation and washed with deionized water and methanol, three times each. The obtained precipitate was dried overnight at 60 °C in vacuum. The resulting dried powder was calcined at 600 °C for 2 h in air.

Synthesis of Co3O4 solid spheres (CO-SNS) Co3O4 solid spheres were synthesized using a one-step hydrothermal method. 0.011 g of cobalt nitrate hexahydrate was dissolved in 50 mL solution of isopropanol (~ 42 mL) and glycerol (~ 8 mL). The mixed solution was kept stirring until a clear pink solution was obtained. This solution



was transferred in to a teflon lined stainless steel container and heated to 180 °C for 6 h. After cooling, the obtained precipitate was collected by centrifugation and washed three times each with deionized water and ethanol. The collected precipitate was vacuum dried overnight at 80 °C and subsequently annealed at 350 °C for 2 h (at 4 oC min-1).

Materials characterization The phase formation of the synthesized nanomaterials was confirmed by analyzing the powder Xray diffraction (XRD) profiles collected using PAN Analytical diffractometer with Cu-Kα (lambda = 0.15406 Å) as the excitation wavelength and in the 2θ range 20-80°. Scanning electron microscopy (SEM CARL ZEISS SUPRA 40) and transmission electron microscopy (TEMFEITECHNAI G220S-Twin operated at 200 kV) were used for the morphological analysis. The Brunauer- Emmett-Teller (BET) surface area and pore size were estimated by analyzing N2 adsorption-desorption isotherms, collected using Quantachrome Autosorb- iQ/ MP-XR surface area analyzer. Zeta potential measurements were performed in a Horiba Scientific Nano Particle Analyzer SZ-100. Electrochemical measurements were performed by collecting cyclic voltammetry (CV), charge discharge (CD) and electrochemical impedance spectroscopy (EIS) profiles using Autolab instrument equipped with Nova 1.10 software.

Electrode preparation Slurries for forming electrode films were prepared by mixing 80 wt% of active material, 10 wt% activated carbon and 10 wt% polyvinylidenefluoride-co-hexafluoropropylene (PVDF-HMP) using acetone as the mixing media. The obtained slurries were drop casted on to the graphite sheets (1 cm × 1 cm) and dried at 80 °C, overnight in vacuum. All electrochemical measurements were


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performed using 3 M KOH as the electrolyte. Platinum wire and Ag/AgCl (in 3.0 M KCl) were used as the counter and reference electrodes, respectively.

Environmental Analysis In order to estimate the environmental impact of Co3O4, life cycle assessment methodology was utilized as per ISO 14044 and 14040 (ISO 14044:2006; ISO 14040:2006). A gate-to-gate LCA was performed using real time laboratory experimental data developed at IIT Kharagpur. The system boundaries of the LCA determine which processes, materials, energy flows and activities are included in the analysis39. The functional unit (FU) for the present study was selected based on the amount of Co3O4 NPs synthesised for each batch (in gm). In addition, the raw materials acquisition for the reaction, their transportation, use and disposal of the Co3O4 NPs were not considered within the system boundary. Life Cycle Inventory (LCI) pertains to the list of material inputs (chemicals, acids, water, and energy), outputs to air, water and soil related to the concerned product. Therefore, LCI involves data collection and calculation procedures to quantify relevant inputs and outputs. LCI data collection requires a flow diagram and a quantification of all process inputs and outputs. The list of chemicals and power requirement for the synthesis of one gram of Co3O4 NPs for three different morphologies is summarized in Table 1. The electricity consumption was primarily assumed to be from the Indian Nation electrical grid, which depends highly on fossil fuels. As on Nov 2018, the percentage share of Indian national electrical grid is as follows coal ~ 56.6 %, gas ~ 7.2 %, oil ~ 0.2%, hydro ~13.1%, nuclear ~ 2.0 % and renewable energy sources ~ 20.8% (which include small hydro project, biomass gasifier, biomass power, urban & industrial waste power, solar and wind energy)40.



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Table 1 Inventory for the synthesis of 1 g Co3O4 based three different morphologies.

Morphology CO-NS

CO-SNS

CO-ND

S. No 1 2 3 4 5 6 1 2 3 4 5 1 2 3 4

Name Co(NO3)2.6H2O (NH4)2CO3 Tap Water Distilled Heating Oil Bath heating Co(NO3)2.6H2O Glycerol Isopropanol Heating Annealing Co(NO3)2.6H2O NaOH Distilled Heating

Unit g g mL mL min min g mL L min min g g mL min

Quantity 1 4 continuous flow 12 h + cooling 300 mL 500 °C/ 240 min 120 °C/ 720 min 4.4 320 1.6 180 °C/240 min 350 °C at 4 °C/120 min 3.6257 0.997 300 240 min

The LCA was conducted using the SimaPro 8.0.3.14 software and Ecoinvent v 3.1 database. To calculate the overall life cycle impact, ReCIPe Midpoint method was used. The impact potentials, viz., climate change (CC) (kg CO2 to air), ozone depletion (OD) (kg chlorofluorocarbon (CFC-11) to air), terrestrial acidification (TA) (kg SO2 to air), freshwater eutrophication (FEU) (kg P to freshwater), marine eutrophication (MEU) (kg N to freshwater), human toxicity (HT) (kg 1,4 dichlorobenzene (14DCB) to urban air), photochemical oxidants formation (POF) (kg non-methane volatile organic carbon compounds (MVOCs) to air), particulate matter formation (PMF) (kg PM10 to air), terrestrial ecotoxicity (TE) (kg 14DCB to industrial soil), freshwater ecotoxicity (FE) (kg 14-DCB to freshwater), marine ecotoxicity (ME) (kg 14DCB to marine water), water depletion (WD) (m3 of water) and fossil fuel depletion (FD) (kg oil), were evaluated. 8 ACS Paragon Plus Environment

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Results and discussion Before the electrochemical studies of the materials, phase formation was confirmed by analysing the XRD profiles. Figure S1 shows the XRD profiles of the three different morphologies of Co3O4, which indicated single-phase polycrystalline samples. All the peaks could be indexed using the JCDPS card no. 42-1467 associated to the space group Fd3̅m (227).

Figure 1(a-c) SEM micrographs and (d-f) TEM micrographs of CO-NS, CO-ND and CO-SNS, respectively. As stated earlier, determination of particle morphology and surface area has become extremely critical for proposing the usability of a material in various applications. SEM 9 ACS Paragon Plus Environment


micrographs shown in Figure 1(a-c) clearly indicated the formation of Co3O4 particles with three different morphologies viz. nanosheet (CO-NS), nanodiscs (CO-ND) and solid nanospheres (COSNS). Nanosheets bundles, as visible in Figure 1(a), were a direct consequence of the hierarchical self-assembly of sheets with nano-sized widths. Nanodiscs, shown in Figure 1(b), had dimensions of ~200 x 150 nm. The third morphology showed uniformly sized solid sphere of diameter ~ 200 nm (see Figure 1(c)). The morphologies were also confirmed by the TEM micrographs, which are shown in Figure 1(d-f). TEM micrographs of CO-NS showed that the sheets were composed of small nanoparticles, which were aggregating to form the bigger 2-D sheet like structures. Due to imperfect assembly of nanoparticle, sheet structures with high porosity can be expected. In case of CO-ND, discs like structures, having width in the nano range were clearly visible, as shown in Figure 1(e). Figure 1(f), confirmed the formation of the solid spheres of Co3O4. Growth mechanisms involved behind the evolution of these morphologies is explained in the supporting information (see Figure S2-S4). Figure S5(a-c) shows the N2 adsorption- desorption isotherms for the three synthesized morphologies. All the nanomaterials displayed type III type isotherms with H3 hysteresis having P/P0 = 0.45 to 1. The obtained surface area for CO-NS, CO-ND and CO-SNS were 150, 34 and 37 m2 g-1, respectively. The pore size distribution indicated the presence of mesopores with average pore diameter between ~ 2.1, ~ 1.58 and ~ 1.51 nm for CO-NS, CO-ND and CO-SNS, respectively. Amongst the three morphologies, Co3O4 nanosheets showed the highest surface area. Along with surface area, the number of available active surface sites for ions adsorptiondesorption are also important in determining the electrochemical behaviour of the electrode material. For this, zeta-potential measurements were performed and the obtained values were +9, -0.5 and -19.6 mV for CO-NS, CO-ND and CO-SNS, respectively. This is shown in Figure S5(d).


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Moreover, shift of the zeta potential values towards the positive regime suggested availability of high active adsorption sites for the OH- ions (present in water) and subsequent release of H+ ions in the solution4. The trend in the shift showed that the available active surface sites were in the order: CO-NS > CO-ND > CO-SNS.

Electrochemical analysis Before the electrochemical measurements, to ensure complete wetting, the coated electrodes were pre-soaked in the electrolyte (3 M KOH) solution for 30 min. Co3O4 generally shows potential window of 0.5 V 36-37. Here, we also report an extended stable voltage window of

Figure 2(a-c) CV profiles at different scan rates and (d) specific capacitance with scan rate for CO-NS, CO-ND and CO-SNS based electrodes. 11 ACS Paragon Plus Environment


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~ 0.9 V. Figure 2(a-c) shows the CV profiles of the materials in the potential window range of -0.6 to 0.3 V. The obtained voltage window was stable for all the electrodes, with no evidence of H2/O2 evolution. CV analysis showed that the CO-NS based electrodes had the largest area enclosed within the loops, indicating the highest specific capacitance. Higher electrochemical response could be attributed to: (i) high available surface area of the sheet like structures, (ii) improved porosity allowing the ions to access interiors of the active material, and (iii) larger number of available active surface sites for redox activity. Amongst the three morphologies, CO-NS showed the highest specific capacitance of 402 F g-1 at 10 mV s-1, calculated from the CV profile using the relation4: 1

CCV = 2MS

r

+V

∫ I. dV △V −V

(1)

where M, Sr, △V and the current voltage integral represent the mass of the active material, scan rate, potential window and the absolute area under the CV curve. The other two morphologies showed specific capacitances of 233 F g-1 and 154 F g-1, respectively. Variation of specific capacitance with scan rates is shown in Figure 2(d) and values are listed in Table S1. With the increase in the scan rate, specific capacitance decreased, which is a normal trend in metal oxide based supercapacitors. This happens because the faradaic behaviour decreases and EDLC contribution starts dominating the overall specific capacitance values rate41-42. To have an idea of the faradaic and non-faradaic contribution in the specific capacitance of the electrode materials, quantification of pseudocapacitive and double layer portions was performed. When the scan rate was increased by 20 times i.e 200 mV s-1, specific capacitance retentions were 77%, 63% and 84% for CO-NS, CO-ND and CO-SNS, respectively. The total specific capacitance from an


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electrode is the sum of the specific capacitance from the redox reactions occurring in the bulk of the electrode and surface adsorbed changes, as shown below42: CS = CED + CB

(2)

where CS, CED and CB are total specific capacitance, double layer capacitance and bulk capacitance due to redox reaction, respectively.

Figure 3(a-c) Specific capacitance variation with inverse square root of scan rate to determine surface and bulk contribution for CO-NS, CO-ND and CO-SNS based electrodes. 13 ACS Paragon Plus Environment


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Cumulated capacitance graphs, plotted using the obtained specific capacitance values on the y-axis and the inverse root of scan rates on x-axis, are shown in Figure 3(a-c). It is well established that, when the scan rate tends to infinity, only the surface contributes in the specific capacitance. In contrast, at lower scan rates, the faradaic contribution dominates42. From Figure 3(a-c), it was clear that, at lower scan rates, the curve was linear but at higher scan rates its linear nature was distorted. This behavior of the graphs was observed in all three materials and could be directly related to the availability of electroactive sites and their utilization. At lower scan rates, higher electroactive sites can be utilized for performing electrochemical reactions. For determining the percentage contribution of surface and bulk portions in the observed specific capacitance, the points at lower scan rates were linearly fitted. The points at higher scan rates were excluded. The intercept obtained by extrapolating the fitted curve on the y-axis provided the estimation of pseudocapacitance part. Faradaic contribution was estimated to be 69%, 51% and 36 % for CO-NS, CO-ND and CO-SNS, respectively. Highest contribution in case of sheet like structures indicated towards higher redox activity in it as compared to other two morphologies. Performance of an electrode material during charge discharge measurements determines its commercial usage. Therefore, charge discharge profiles were collected at different current densities ranging from 2-10 A g-1. The results are shown in Figure 4(a-c). CD curves showed that the maximum specific capacitance obtained for CO-NS based electrode was ~320 F g-1 at 2 A g-1, calculated using the relation4:

I.dt

CCD = M.(V−IR)


(3)

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where ‘I/M’ denotes the current density, ‘dt’ discharge time, ‘V’ operating potential window and ‘IR’ is the voltage drop found at the interface of charging and discharging profile. For CO-ND and CO-SNS, the maximum observed specific capacitance were 178 F g-1 and 113 F g-1 at 2 A g-1, respectively. The variations of specific capacitance with current densities are shown in Figure 4(d) and the obtained values are listed in Table S2.

Figure 4(a-c) CD profiles and (d) variation of specific capacitance with current densities for CO-NS, COND and CO-SNS based electrodes.

Supercapacitors are known for their high cycling stability. Therefore, cycling performance was tested for the three electrode materials, for 2000 subsequent cycles at 3 A g-1. For CO-NS, the specific capacitance was nearly constant with minimal fluctuations during the initial 1000 cycles. 15 ACS Paragon Plus Environment


After 1000 cycles, specific capacitance increased, as shown in Figure S6(a). Once the intercalation-deintercalation channels stabilize owing to repeated cycling, the specific capacitance may show slight change from the initial stages. If the specific capacitances increase then it indicates opening of more number of channels and vice-versa. There is also the possibility of appearance new chemical structure or metallic phase of an ion, which can lead to enhanced case. In the present case, the latter two cases were not observed. Hence it can be suggested that the crystalline lattice was actually needed some time to allow the formation of highest possible number of channels for ion-motion. However, the values of coulombic efficiencies for all the electrode materials remain >90%, which proved the good repeatability of the electrode materials. Coulombic efficiency variation with cycling is shown in Figure S6(b). Capacity retentions were 99%, 98%, and 96 % for CO-NS, CO-ND, CO-SNS, respectively. To explain the charge storage kinetics of the electrodes, EIS measurements were also performed. The results showed that the ESR values before cycling were ~ 0.82 Ω, ~ 2.15 Ω and ~ 3.07 Ω for CO-NS, CO-ND and CO-SNS, respectively (Figure S7(a-c)). It was observed that the ESR values increased with cycling in all the three types of electrodes used. After 2000 cycles, the values of ESR were ~ 1.15 Ω, ~ 2.47 Ω, ~ 3.16 Ω for CO-NS, CO-ND, CO-SNS, respectively. From the low frequency region, it was found that all the materials showed good Warburg type capacitive response. Straight and linear portion was prominent for CO-SNS, which also corroborated inference of high surface contribution in the observed specific capacitance4. In the other two morphologies, low frequency portion was slightly away from 45°, indicating towards high pseudocapacitance contribution4. For CO-NS, low ESR value of 0.82 Ω was obtained. Hence, from the electrochemical results, it could be inferred that, with the enhancement in surface area and available active surface sites, specific capacitance values can be tuned. Nanosheets were found 16 ACS Paragon Plus Environment

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to have best electrochemical behaviour among the three, owing to their high surface area of ~150 m2 g-1. The question, which needs to be asked: should this particle actually be used? The answer that came out from the following analysis was that: it should probably ‘NOT’ be used. The simpler nanoparticles may be more useful for large-scale application. Life cycle impact assessment Analysis results show that, for all the synthesized morphologies of Co3O4, power consumption (contributing to over 90%) was found to be dominant on the environmental impact categories as listed in Table 2. For CO-SNS, apart from power consumed, isopropanol had a higher impact in terms of CC, TA and WD than glycerol (Figure 5(a)). Moreover, the impact on MEU and TE was mainly associated with the use of glycerol. Apart from electricity consumption, interestingly, glycerol impact on OD, FE was found to be more than isopropanol. For the other morphologies, the impact caused due to the power consumption was found to be more dominant as can be seen from Figure 5(b,c)33. Power consumed for CO-NS had four times greater impact on global warming potential (GWP) than the disc shaped particles (Table 2). This is more likely due to the energy intensive synthesis process of sheet like morphology (raising furnace temperature from 0 °C to 500 °C and maintaining it for a period of 240 min) (Table 1). Similarly, GWP impact resulting from the synthesis of sheet morphology was found to be 2.3 times greater than the disc like structures. Moreover, the comparative analysis of different morphologies revealed that, synthesis procedure of solid spheres had greater impact in all categories apart from MEU and TE (Figure 6(a)). The reason is the contribution from both isopropanol and glycerol had greater effect on both the indicators (MEU and TE). From the analysis, it has been inferred that synthesis of NPs itself has the most impact on all the environmental categories, however, irrespective of the morphologies considered, the power



consumed for the synthesis from the grid has significant effect towards GWP. In order to lower

Figure 5 Percentage contribution of all impact categories on (a) 1 g CO-SNS, (b) 1 g CO-ND and (c) 1 g CO-NS. 18 ACS Paragon Plus Environment

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the footprint on the environment, consequently, other viable possibilities should be encouraged. One such alternative scenario that was taken into consideration was to use the power generated from roof top solar panel (renewable source) and the results are reported below.

National Grid vs Roof Top Solar Panel (Renewable Energy) A more in-depth contribution analysis was conducted, particularly with reference to energy source. The analysis compared the results obtained from grid drawn power vs renewable energy (power drawn from rooftop solar panel). Surprisingly for CO-SNS, high GWP was mainly due to the impact from isopropanol (58%), followed by glycerol (23%) and electricity (19%). Previously, the main contributor was the power used, however, while using the solar panel, the impact of electricity was greatly reduced. Further, isopropanol has significant impact on CC (57%), TA (43%), POF (64%), PMF (41%) and FD (84%) than other raw materials as seen in Figure 6(b). Apart from isopropanol, impact of glycerol was found to be high on ME (93%), TE (91%) and FE (63%) while for ozone depletion, both isopropanol (38%) and glycerol (19%) had significant impact from the roof mounted solar photovoltaic cell33, 39. For CO-SNS, the impact on the categories (CC, TA, MEU, POF, TE, FE, FD) was mainly from isopropanol followed by glycerol and power consumed. This was not the case earlier, where electricity was being drawn from the national grid. Significant reduction on the impact categories when compared with power from national grid was found specifically for on TA (23 times), POF (20), PMF (29), FD (23).

The reduction on the respective impact categories is solely attributed to the electricity drawn from the roof top solar panels. Amongst all the morphologies, disc was found to have least effect on the CO2 emission followed by sheet and solid spheres. The GWP of solid spheres and



Figure 6(a) Percentage contribution of all impact categories on three different 1 g Co 3O4 based morphologies, (b) percentage contribution of all impact categories on 1 g CO-SNS (RE) and (c) percentage contribution of all impact categories on three different 1 g Co3O4 based morphologies. sheet like structures was found to be 7 and 4 times greater when compared with disc like structures of Co3O4 (Figure 6(c)). For the other two morphologies (solid and disc), the effect of power was found to have higher impact on all the impact categories. The solid spheres was found to have


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greater impact on categories such as CC, TA, MEU, POF, TE, FA and FD, while for other categories synthesis of sheet was found to have greater impacts. Table 2 Environmental burdens in synthesis of 1 g Co3O4 based three different morphologies. Impact category

Unit

CO-ND Grid

CC OD TA FE MEU HT POF PMF TE FE ME WD FD

CO-NS RE

CO-SNS

Grid

RE

Grid

RE

kg CO2 eq kg CFC-11 eq kg SO2 eq kg P eq kg N eq kg 1,4-DB eq kg NMVOC kg PM10 eq kg 1,4-DB eq kg 1,4-DB eq kg 1,4-DB eq

13.30893 0.50595 -7 1.10 × 10 7.76×10-8

53.23226 4.37×10-7

2.46417 4.31×10-7

23.08472 3.15×10-7

3.88026 2.68×10-7

0.08140 0.00066 0.00143 0.28841

0.00355 0.00012 0.00015 0.24963

0.32559 0.00262 0.00572 1.15330

0.01788 0.00067 0.00075 1.49064

0.13538 0.00115 0.00877 0.50398

0.01861 0.00034 0.00684 0.44582

0.04071 0.03955 0.00023

0.00201 0.00135 0.00074

0.16284 0.15821 0.00093

0.01049 0.00670 0.00464

0.07394 0.06349 0.01327

0.01588 0.00618 0.01403

0.00209

0.00057

0.00836

0.00273

0.00715

0.00487

0.00339

0.00333

0.01356

0.02004

0.00630

0.00621

m3 kg oil eq

11.10075 3.04162

6.36518 0.12795

45.38574 12.16558

41.51345 0.63956

18.94559 6.33854

11.84223 1.96803

From the above results, it can be inferred that the source of power plays a vital role on the environmental indicators than the raw materials used for the NP synthesis. By switching the power from non-renewable to renewable source, the impact on GWP is significantly reduced (by nearly 26 times for discs, 21 times for sheet and 6 times for solid spheres of Co3O4). This is shown in Table 2. Similar significant decrease was found on indicators such as FE (22% for discs, 18 % for sheet and 7% for solid spheres), POF (20% for discs, 15 % for sheet and 4% for solid spheres),



PMF (29% for discs, 23 % for sheet and 10% for solid spheres), FD (23% for discs, 19 % for sheet and 3% for solid spheres), FE (5% for discs, 3 % for sheet and 3% for solid spheres).

Conclusion Nanoparticles of Co3O4, with different morphologies, have been prepared. The hierarchical nanosheet like morphology, with highest BET surface area, delivers the highest specific capacitance of ~ 402 F g-1. In most cases, it would be directly inferred that electrode material (Co3O4 in this case) having nanosheet morphology would be most useful for energy storage devices such as supercapacitors or batteries. The additional results presented in the paper proves otherwise. Combining the LCA studies, it is clearly established that the applicability of simpler morphologies of Co3O4 may supersede the more hierarchical structures, which can deliver higher capacitance. Such studies must therefore also be peformed for other metal oxides based systems so as to choose an environmentally friendlier particle morphology. In the present case, simpler morphologies, prepared using simple synthesis protocols, have much lower environmental impact and CO2 footprint that is nearly 5 times lower than hierarchical morphology (nanosheets). This study also highlights the importance of exploring other available sustainable options based on renewable sources in order to reduce the carbon footprint. Acknowledgement (AC) acknowledges the funding received from DST (India) under its MES scheme for the project entitled, “Hierarchically nanostructured energy materials for next generation Na-ion based energy storage technologies and their use in renewable energy systems.


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Confict of interest The authors have no competing finacial interests.

Associated Content The Supporting Information is available free of charge on the ACS Publications website. It contains: XRD profiles, growth mechanisms, N2 adsorption-desorption isotherms, zeta potential profiles, cycling performance, coulombic efficiency variation with cycling, EIS profiles before/after cycling and table consisting of values of specific capacitance with scan rates/current densities for CO-NS, CO-ND and CO-SNS.



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For table of contents use only

Synopsis: Simpler morphologies with appreciable specific capacitance and lower CO2 footprint are suitable alternatives for green and sustainable energy resources.


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Figure 1(a-c) SEM micrographs and (d-f) TEM micrographs of CO-NS, CO-ND and CO-SNS, respectively. 35x42mm (300 x 300 DPI)

ACS Paragon Plus Environment


Figure 2(a-c) CV profiles at different scan rates and (d) specific capacitance with scan rate for CO-NS, COND and CO-SNS based electrodes. 58x45mm (300 x 300 DPI)


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Figure 3(a-c) Specific capacitance variation with inverse square root of scan rate to determine surface and bulk contribution for CO-NS, CO-ND and CO-SNS based electrodes. 19x46mm (300 x 300 DPI)



Figure 4(a-c) CD profiles and (d) variation of specific capacitance with current densities for CO-NS, CO-ND and CO-SNS based electrodes. 59x46mm (300 x 300 DPI)


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Figure 5 Percentage contribution of all impact categories on (a) 1 g CO-SNS, (b) 1 g CO-ND and (c) 1 g CONS. 40x59mm (300 x 300 DPI)



Figure 6(a) Percentage contribution of all impact categories on three different 1 g Co3O4 based morphologies, (b) percentage contribution of all impact categories on 1 g CO-SNS (RE) and (c) percentage contribution of all impact categories on three different 1 g Co3O4 based morphologies. 40x59mm (300 x 300 DPI)


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Electrode materials with highest surface area and specific capacitance

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