DOI: 10.3303/CET2292057 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Paper Received: 4 December 2021; Revised: 27 March 2022; Accepted: 3 May 2022 
Please cite this article as: Leal Silva J.F., Lopes E.S., Anchieta C.G., Wolf Maciel M.R., Doubek G., Maciel Filho R., 2022, Biomass in Li-o2 
Batteries? Deep Discharge Performance of an O2 Electrode Based on Humins, Chemical Engineering Transactions, 92, 337-342  
DOI:10.3303/CET2292057 
  

 CHEMICAL ENGINEERING TRANSACTIONS  
 

VOL. 92, 2022 

A publication of 

 

The Italian Association 
of Chemical Engineering 
Online at www.cetjournal.it 

Guest Editors: Rubens Maciel Filho, Eliseo Ranzi, Leonardo Tognotti 

Copyright © 2022, AIDIC Servizi S.r.l. 

ISBN 978-88-95608-90-7; ISSN 2283-9216 

Biomass in Li-O2 Batteries? Deep Discharge Performance of 

an O2 Electrode Based on Humins 

Jean Felipe Leal Silva, Emília Savioli Lopes, Chayene Gonçalves Anchieta, Maria 

Regina Wolf Maciel, Gustavo Doubek, Rubens Maciel Filho 

Advanced Energy Storage Division – Center for Innovation on New Energies (AES/CINE), School of Chemical Engineering, 

University of Campinas (UNICAMP). Rua Michel Debrun, s/n, Campinas, SP, Brazil, ZIP code 13083-841. 

jefelipe@outlook.com; jefelipe@feq.unicamp.br 

Humins are a by-product of the hydrolysis of carbohydrates in the production of levulinic acid. With the 

development of new large-scale processes to produce levulinic acid, the production of large amounts of this by-

product will demand new applications beyond burning in biomass furnaces. Owing to its carbonaceous nature, 

many value-added applications have been considered. One of these applications is in the production of carbon-

based electrodes for batteries. Such application has great potential because of the increasing need for the 

development of new cost-competitive energy storage technologies with enhanced specific energy. In this 

context, Li-O2 batteries are a technology with potential specific energy higher than current state-of-the-art 

lithium-ion batteries. This work reports the synthesis of a humins-based O2 electrode for a Li-O2 battery. Humins 

synthesized as a by-product of hydrolysis of sugars were purified via leaching, dried, and thermally treated 

under an inert atmosphere to enhance their electronic conductivity. The obtained product was then mixed with 

a binder and templated over a stainless-steel mesh to obtain the O2 electrode. Batteries were assembled with 

electrodes based on humins from both sugarcane bagasse and molasses. Humins were analyzed via X-ray 

photoelectron spectroscopy and scanning electron microscopy before and after treatments, and the electrodes 

were analyzed before and after the discharge of the battery. Results of deep discharge demonstrate the potential 

for this material for the manufacturing of carbon-based electrodes for batteries. 

1. Introduction 

The development of biorefineries for the use of biomass and renewable resources for the production and 

replacement of fossil-based products has been shown as an effective strategy for the reduction of emissions of 

greenhouse gases (Hossain et al., 2017). An important component of the biorefinery approach is to seek value-

added applications for all biomass fractions and the by-products of their processing. One of the possible 

biorefinery layouts is the thermochemical conversion of biomass which can be used to produce levulinic acid 

and furfural from cellulose and hemicelluloses (Tacchini et al., 2020). This process consists on the hydrolysis of 

polysaccharides and dehydration of sugars, and these conditions also lead to the production of humins, a by-

product of condensation reactions of sugars and furans. 

Currently, the main potential application for humins is as a supplementary boiler fuel (Leal Silva et al., 2021a). 

Nevertheless, other applications as catalyst support have been already proposed because of the low cost of 

this material and the possible properties of carbon-based materials. In this context, a potential application for 

this by-product is the production of electrodes for Li-O2 batteries (Thakkar et al., 2021). Li-O2 batteries represent 

one of the next frontiers in the development of electrochemical energy storage devices because of their 

potentially high specific energy (Leal Silva et al., 2021b). In the development of this battery, several materials 

have been being tested for the manufacture of the O2 electrode. This part of the battery must present high 

surface area, porosity, catalytic activity, and conductivity to act as a suitable support for the deposition of 

discharge product, which is Li2O2 (Tan et al., 2017). Among the possible candidates under consideration by the 

scientific community, carbon-based materials deserve special attention because of the possibility of fine-tuning 

many of their properties to make them suitable for this application. 

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Based on this possible application of a biorefinery residue in the production of an electrochemical energy storage 

device, this work presents the synthesis and testing of an experimental O2 electrode for a Li-O2 battery based 

on humins from sugarcane bagasse and sugarcane molasses. Humins were synthesized, purified, and thermally 

annealed. The chemical composition and morphology of samples from each step were analyzed via several 

methods. Discharge tests of these electrodes demonstrate the potential of this material in the production of 

electrodes, thus showing a new application for humins produced during the synthesis of levulinic acid from 

biomass. 

2. Methodology 

2.1 Synthesis and purification of humins 

Humins were synthesized from two sources. Sugarcane bagasse was obtained from Usina São João (Araras, 

São Paulo, Brazil), and sugarcane molasses was obtained from Usina Ester (Cosmópolis, São Paulo, Brazil). 

Samples were stored in the freezer without any treatment. Humins from sugarcane bagasse were synthesized 

by mixing sugarcane bagasse with a 1 w/v% H2SO4 aqueous solution with 10% solids loading. The mixture was 

heated and kept at 170 °C for 75 min (Lopes et al., 2020). The resulting solid fraction (H-SCB) was recovered 

by filtration for further treatment. Humins from molasses were synthesized by mixing 100 parts of 9 w/v% H2SO4 

aqueous solution with 123 parts of sugarcane molasses, and the mixture was heated and kept at 160 °C for 80 

min (Lopes et al., 2021). The resulting solid fraction (H-SCM) was recovered by filtration for further treatment. 

Both solid fractions were dried at 80 °C under vacuum for 12 h. Then, Soxhlet extraction with water was 

performed in 3 cycles of 8 h each, and the solids were dried at 80 °C under vacuum for 12 h. 

2.2 Preparation of electrodes 

Purified humins were ground using an agate mortar and pestle and then thermally annealed on a horizontal tube 

furnace (LT3.3010, Jung, Brazil) under argon flow in a two-step process. In the first step, the samples were 

heated to 375 °C (heating rate of 5 °C min-1) for 2 h and left to cool. Then, the samples were heated to 850 °C 

(heating rate of 5 °C min-1) for 3 h and left to cool (van Zandvoort et al., 2013). The resulting carbon samples 

(C-SCB or C-SCM) were ground once again. A slurry consisting of 18 w% treated carbon and 2 w% Nafion 

(solution, 20.7 w% Nafion™ perfluorinated resin in 34:45 water:1-propanol, Sigma-Aldrich) in isopropanol 

(>98%, Sigma-Aldrich) was sonicated for 2 minutes in a tip sonicator (Eco-sonics QR 550W, Ultronique, Brazil). 

The slurry was dropped into previously weighted 16 mm discs of stainless-steel wire mesh (wire diameter: 40 

μm, opening size: 50–55 μm) over a Petri dish, which was left to dry for 24 h in an oven at 50 °C. The electrodes 

were peeled off, weighed, and stored in a desiccator. 

The batteries were assembled with a Li foil (Tob Machine, 99.99%), a glass microfiber membrane (GF/A, 

Whatman), and the O2 electrode (based on H-SCB or H-SCM) using a solution of 0.1 mol L-1 lithium perchlorate 

(LiClO4, ≥99.99% trace metal basis, Sigma-Aldrich) in dimethylsulfoxide (DMSO, ≥99.9%, Sigma-Aldrich) as the 

electrolyte whose water content was measured via coulometric Karl Fischer titration (852 Titrando Metrohm AG, 

Switzerland). Batteries were assembled inside a glovebox workstation (H2O and O2<10 ppm, Ar atmosphere, 

MBraun MB10, M. BRAUN Inertgas-Systeme GmbH) and left to rest for 1 h before testing. After the assembling 

process, the batteries were filled with pure O2 at a pressure of 3 bar to initiate the deep discharge test. 

2.3 Deep discharge test and analysis of electrodes 

After a resting period of 30 min, the battery was discharged at a current of 50 μA using an Arbin 

Potentiostat/Galvanostat (LBT21084, Arbin Instruments LLC, USA) until the cut-off potential of 2.2 V. Then, the 

cell was purged with N2 and disassembled inside the glovebox workstation. The discharge capacity was 

normalized by the weight of treated carbon deposited onto the stainless-steel template. The morphology of the 

treated carbon samples of H-SCB and H-SCM and the electrodes after discharge were analyzed using scanning 

electrode microscopy (SEM, Quattro S, Thermo Fischer Scientific Inc., USA) at a beam energy of 10 kV, and a 

working distance of 10 mm. The chemical composition of the surface of samples before and after treatment was 

analyzed using X-ray photoelectron emission spectroscopy (XPS, K-Alpha, Thermo Fischer Scientific Inc., USA) 

and energy-dispersive X-ray spectroscopy (UltraDry, Thermo Fischer Scientific Inc., USA). 

3. Results and Discussion 

3.1 Synthesized humins and carbon electrodes 

Figure 1 shows pictures of the humins samples after purification and after thermal annealing, and Figure 2 

shows SEM micrographs of the humins after purification, after thermal annealing, and after battery discharge. 

According to these results, it is possible to observe that the mild conditions used in the hydrolysis of sugarcane 

bagasse produced a sample consisting of unconverted biomass covered with humins. Results from SEM show 

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that grinding the samples after purification produced powders with an average particle size of 3.9 ± 2.8 μm for 

H-SCB and 4.0 ± 1.3 μm for H-SCM. After thermal annealing and grinding, particle size was 3.7 ± 2.7 μm for C-

SCB and 3.9 ± 1.2 μm for C-SCM. Therefore, thermal annealing did not have any impact on particle size. In the 

case of C-SCB, the particle size distribution followed a Weibull distribution, with 66% of them smaller than 

average and some particles as large as ~50 μm. In the case of C-SCM, particle size distribution followed a 

normal distribution and generally presented a spherical shape of similar size. Many spheres were grouped into 

clusters of 23 ± 11 μm by sintering, which is a problem because it reduces the surface area of the material. 

 

 

Figure 1: Humins samples (a) after purification (left: H-SCB, right: H-SCM) and (b) after thermal annealing (left: 

C-SCB, right: C-SCM). 

 

Figure 2: SEM micrographs of samples a, b) H-SCB, c, d) C-SCB, e, f) H-SCM, and g, h) C-SCM in different 

magnifications using the Everhart–Thornley detector 

SEM micrographs in Figure 2 were obtained using integration mode to avoid anomalies because of charging 

effects in samples with poor electronic conductivity. Micrographs at live and integration modes to compare the 

electronic conductivity of both materials before and after thermal annealing. According to these analyses, 

thermal annealing increased the electronic conductivity of both samples, which is a fundamental property in the 

development of carbon-based electrodes. This process step was more important for the electrode based on 

sugarcane bagasse because of the very low conductivity observed in this sample before thermal annealing. 

According to EDS results, thermal annealing increased the C/O atomic ratios from 2.06 to 20.4 in the samples 

based on sugarcane bagasse and from 1.69 to 24.9 in the sample based on sugarcane molasses. This reduction 

in the oxygen content greatly increases the conductivity of carbon-based materials (Morimoto et al., 2016). 

Moreover, the reduction in oxygen content of the samples without the reduction of particle size as discussed 

before might indicate a porosity increase after thermal annealing. Sulfur was observed to be a contaminant of 

the samples in trace amounts because of the catalyst employed in hydrolysis (H2SO4). XPS analysis of S (not 

shown here) indicates a higher detection of this element bound to C only in the thermally treated samples in 

trace amounts. 

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XPS spectra of carbon for all samples are shown in Figure 3 and indicate the predominance of C-C bonds in all 

samples. Peak integration also shows the reduction of O after thermal treatment of samples. Based on these 

results, it is possible to see that thermal annealing reduced the functionalization of carbon. Moreover, a π–π* 

satellite peak is observed with a binding energy that is 6 eV higher than the main C-C peak, thus indicating the 

presence of extended delocalized electrons (Morgan, 2021). The presence of these delocalized electrons 

contributes to the theory that thermal treatment increases the conductivity of humins-derived carbon. 

 

 

Figure 3: XPS C1s spectra of a) H-SCB, b) H-SCM, c) C-SCB, and d) C-SCM 

3.2 Deep discharge tests 

Figure 4 shows the deep discharge curves for electrodes based on C-SCB and C-SCM. According to these 

results, the specific capacity of electrodes based on C-SCM is 51% higher than electrodes based on H-SCB. In 

both cases, the discharge occurred with a potential plateau mostly above 2.75 V, indicating a low discharge 

overpotential. However, as the discharge product covered the entire carbon surface (Figure 2), the potential of 

the cell dropped fast to the cut-off potential of 2.2 V because of the high resistivity of the discharge product 

(Li2O2). This potential drop to the cut-off potential at the end of discharge occurs faster for the electrode based 

on C-SCM because of the higher uniformity in particle size, as shown in Figure 2. 

 

340



 

Figure 4: Deep discharge curves for O2 electrodes based on (a) C-SCB and (b) C-SCM 

Figure 5 presents the morphology and distribution of the discharge product in both electrodes. It is possible to 

observe that the discharge product covered uniformly both electrodes. According to the results of Karl-Fischer 

titration, the electrolyte contained 97 ppm of water. The morphology of the discharge product observed in Figure 

5b and Figure 5f is similar to results previously reported in the literature (Aetukuri et al., 2015) for an electrolyte 

with low water concentration over a carbon-based electrode. Based on this comparison, discharge tests with 

electrolytes with more water might lead to increased capacity.  

 

Figure 5: SEM micrographs of electrodes a-d) C-SCB and e-h) C-SCM in different magnifications; images in the 

bottom show the same area using the Everhart–Thornley (c and g, left) and backscatter (d and h, right) detectors 

In the case of the electrode based on C-SCB, the discharge product also covered large particles of unhydrolyzed 

biomass (Figure 5b). In the case of the electrode based on C-SCM, the spherical particles were evenly covered 

by the discharge product, with the formation of crystals that resemble the shape of a red blood cell. In the case 

of smaller agglomerates, more discharge product accumulated over their surface. Another interesting finding is 

that the particle size of the Li2O2 particles forming at the surface is small compared to the size of the humins-

based carbon spheres. The use of smaller spheres can increase the available surface area and improve the 

specific capacity of the material. Therefore, humins obtained in different hydrolysis conditions, with different 

particle sizes (van Zandvoort et al., 2013), should be tested as well to determine the impact of particle size on 

the specific capacity of this electrode material. Another point that deserves attention is the presence of sulfur, 

shown in Figure 5d and Figure 5h, because the impact of trace amounts of this element should be considered 

as well in the study of the stability of this electrode material. 

 

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4. Conclusion 

This work presented the first results of discharge tests of O2 electrodes for Li-O2 batteries based on sugarcane-

derived humins, which is a by-product of the thermochemical conversion of bagasse or molasses to levulinic 

acid. Results of specific capacity demonstrate the potential of using this material for the manufacturing of O2 

electrodes with low discharge overpotential. Images of the discharge product indicate that adjusting the 

electrolyte composition and particle size can lead to increased discharge capacity. Results also indicate high 

functionalization of this material. Additional analyses should be performed on these materials to assess their 

stability in the potential window that is suitable for most electrolytes of Li-O2 batteries, and tests should also 

consider humins obtained in different reaction conditions with possible different particle sizes. 

Acknowledgments 

The authors gratefully acknowledge the support from FAPESP (the Sao Paulo Research Foundation, Grant 

Numbers 2015/20630-4, 2016/10450-1, 2017/11958-1, and 2018/16663-2), Shell, the strategic importance of 

the support given by ANP (Brazil’s National Oil, Natural Gas, and Biofuels Agency) through the R&D levy 

regulation, the Brazilian National Council for Scientific and Technological Development and (CNPq, Grant 

Number 150313/2020-9), and the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil 

(CAPES) - Finance Code 001. 

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