Pretreated mesocarp fibre biochars as carbon fuel for direct carbon fuel cells

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Pretreated mesocarp fibre biochars as carbon fuel for direct carbon fuel cells

N. Jafri

^a

, W.Y. Wong

^b,^*

, L.W. Yoon

^a,^**

, K.H. Cheah

^c

aSchool of Computer Science and Engineering, Faculty of Innovation and Technology, Taylor’s University, Subang Jaya, Selangor, 47500, Malaysia

bFuel Cell Institute, University Kebangsaan Malaysia, Bangi, Selangor, 43600, Malaysia

cSchool of Aerospace, University of Nottingham Ningbo China, Ningbo, 315100, China

h i g h l i g h t s

Pretreated mesocarp fiber biochar as fuel for DCFC.

HCl pretreated biochar reduces ash from 2.5 to 0.1 wt% and increases O/C ratio.

Higher power density of DCFC with HCl treated biochar than activated carbon.

Porous fibrous biochar increases active sites for electrooxidation.

a r t i c l e i n f o

Article history:

Received 12 June 2020 Received in revised form 4 September 2020

Accepted 5 September 2020 Available online xxx Keywords:

Direct carbon fuel cell Palm mesocarp fibre Biochar

Pretreatment

Oxygen functional groups

a b s t r a c t

This work focuses on the effect of acid and alkali pretreatment of palm mesocarp fibre (PMF) on its fuel performance in a direct carbon fuel cell (DCFC). PMF is pretreated with acid and alkali in the range of 0.1 Me4 M and followed by pyrolysis to produce biochar fuel.

Performance is evaluated in the DCFC at 750C, 800C, and 850C. This work reveals that 2.0 M HCl treated PMF biochar gives the lowest ash value (0.1 wt%) and the highest O/C ratio among all tested biochars. The acid pretreatment contributes to enhanced electrochemical reactivity of the PMF biochar, which gives a peak power density output of 11.8 mW cm²at 850C in the DCFC. This obtained peak power density is higher than the power density of untreated biochar, recorded at a value of 0.70 mW cm². The results indicate that reduced ash, the existence of oxygen functional groups, and porous fibrous structure have increased the electro-oxidation active sites of the pretreated biochar fuel in DCFC.

Introduction

Sustainable materials for energy conversion and energy de- vices have recently attracted enormous interest. Intensive efforts have been made on utilizing bio-derived and waste-

derived compounds for energy generation. For instance, biogas derived from biomass has been integrated with solar system as a stable source of power generation designed especially for a house in a remote region [1]. For biochars or activated carbon, research is underway to study their potential in various promising applications such as supercapacitors,

*Corresponding author.

**Corresponding author.

E-mail addresses:[email protected](W.Y. Wong),[email protected](L.W. Yoon).

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batteries, and hydrogen storage. Among the biomass that have been converted to biochars for applications as mentioned above are walnut shell [2], ground soybean [3], plane tree barks [4], Solanum lycopersicum plant leaves [5], apple bagasse [6], winery residues [6], peanut shells [7], wood chips [8], and Posidonia Oceanica aquatic plant [8]. More interestingly, biochars can be extended to applications in energy catalyst materials with further fine-tuning of surface properties through different pretreatment methods. He et al.

[9] took waste fish scales biomass and converted it into nanosheet catalyst through KOH activation for use in vanadium redox flow battery. The catalyst exhibited a high surface area, many active sites, and good wettability. Sengon wood has been studied as the noble-free metal catalyst support, which exhibited better catalyst stability due to the porous structure [10]. Hence, there is massive potential for the application of biomass-derived biochar in clean energy sys- tems owing to their sustainable nature and low cost.

The advancement in the field of fuel cells as the future energy generation technology has been gaining attention lately. It is of great interest to study a relatively new type of fuel cell, the direct carbon fuel cell (DCFC), which utilizes solid carbon as fuels for electricity production. Conventionally DCFC utilizes solid carbon fuels such as carbon black, coal, graphite, and activated carbon at the anode to produce electricity [11,12]. Typically, DCFC operates at a temperature range of 650 Ce1000 C. High-temperature DCFCs possess the advantage of providing higher energy efficiencies and offer the ability to recover and capture the gas products from the system for further applications [13]. DCFC based on solid ceramic electrolytes operating on solid carbon fuel has ad- vantages of simple operation, stable electrolyte, non-corrosive environment, and easy scale-up [12,14]. However, the process of delivering solid carbon to the anode-electrolyte interface is complex.

The overwhelming choice of fuel for DCFC research to date has been the conventional carbon black [15e17]. It is produced commercially and is considered a source of carbon with consistent properties. Li et al. [18e20] examined the effect of fuel properties on the performance of DCFC. According to them, fixed carbon, graphitic structure, surface area, pore volume, and functional groups were the significant characteristics that influenced the electrochemical reaction in the DCFC. While carbon black has much to contribute to this field of research, it is not a renewable energy source. Hence, there is a necessity to investigate the possibility of using carbon fuels, which are readily available and renewable.

In recent times, different biomass-based carbons have been successfully operated as fuels for DCFCs, including almond shell [21], walnut shell [21,22], palm kernel shell [23], pepper straw [24], corn straw [25], wheat straw [26], bagasse [26], corncob [26], rubberwood, and rice husk [27]. The authors achieved promising values of power densities close to 250 mW cm²within the temperature range 600e900C. It has been stated that the electrochemical performance of carbon fuels is significantly linked to their physicochemical properties and compositions. Some potential ways have been identified for the continuous improvement of solid oxide electrolyte based DCFCs operated on carbon fuels derived from waste biomass. One of the strategies is focused on the

use of novel electrode materials (cathodes and anodes) and electrolytes which could more efficiently work with carbon fuels. Li et al. [25] confirmed the potential of corn straw for increased power output through the application of novel anode perovskite material La0.8Sr0.2Fe0.9Nb0.1O_3‒d (LSFNb), obtaining power from a DCFC in the range of 200e300 mW cm². In a separate study, Ali et al. [21] used novel anode material La0.4Sr0.6M0.09Ti0.91O3-d and employed almond and walnut shells as fuels for DCFC operation. The obtained power densities were close to 70 mW cm²for the biochars with an improved anode stability.

Past studies have also revealed that various impurities in carbon fuels adversely affect the anodic reactions and the lifetime of DCFCs [18,28]. Therefore, the pretreatment of carbon fuels to eliminate potentially harmful impurities is seemingly essential. Different pretreatment methods such as alkali, acid, laser, heat, and plasma treatments have been performed on carbon fuels to improve their physicochemical characteristics, which consequently enhance their electrochemical reactivity apart from reducing the ash [29].

Biomass demineralization by using various acids has shown to be effective in removing inorganic constituents from the biomass and improving fuel quality. One such study conducted on oil palm waste showed organic constituent removal, an increase in BET surface area, and pore volumes using HCl as the pretreatment agent [30]. Other studies using biomass involved pretreatment agents such as HCl, H2SO4, HF, and HNO3[31e33]. Research involving conventional fuels such as coal has reported the appearance of surface oxygen functional groups after pretreatment with HCl and HNO3,which possibly enhanced fuel performance. Eom et al. [34] performed HNO3 pretreatment on coal. They reported that through acid pretreatment, the oxygen functional groups on fuel surface increased, and Si composition decreased.

Decreasing ash content on the fuel surface, especially Si, reduces the electrolyte resistance [34]. The current distribution was also reported to be better in the pretreated coal than raw coal due to the effect of additional functional groups as the oxygen functional groups reduce the charge transfer resistance at anode [34]. All of these improvements contributed to enhanced Boudouard reactivity of the char and subsequently improved power outputs in the DCFC. Li et al. [20] treated activated carbon and carbon black with HNO3. They stated that HNO₃-treated carbon fuels showed the highest electrochemical reactivity in the DCFC owing to a high degree of surface oxygen functional groups. Jiao et al. [35] performed KOH treatment on coal, which led to the improvement in surface area from micropores and resulted in the creation of oxygen-containing functional groups. The improvements consequently led to enhanced power outputs in the DCFC.

Nonetheless, there are limited studies reporting the difference in the contribution of acid and alkali pretreatment towards desired properties of the biochars as carbon fuel for DCFC application. Previous work by Palniandy et al. [27] found that alkali pretreated rubberwood char exhibited a higher power density than acid pretreated rubberwood char for operation in the DCFC at 850C. It was stated that alkali pretreatment led to higher fixed carbon, lower ash content, and the presence of surface oxygen functional groups, which contributed to better performance. In our recent work [23], it

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was established that acid pretreated biochar from palm kernel shell (PKS) is an effective carbon fuel in the DCFC. This outcome was confirmed in electrochemical comparative tests of DCFCs at 850C in which the PKS biochar showed superior performance over commercially obtained activated carbon due to low ash content and a higher degree of surface oxygen functional groups. However, it is worth noting that the difference in the lignocellulosic content in biomass may require different pretreatment conditions to obtain the desired properties for use as carbon fuel in DCFCs.

The palm oil industry in Malaysia generates massive amount of biomass waste each year, with an estimate of 85e110 million dry tonnes of solid biomass generation by 2020 [36]. Research involving the use of oil palm biomass for biochar production is limited, and this creates new avenues to apply biochar derived from renewable sources for energy generation in Malaysia. To the best of our knowledge, only one work has been reported on the production of PMF biochar using different pyrolysis temperatures to be used as fuel in the DCFC [37]. The reason for choosing PMF biomass for the present study lies in the fact that it contains the second-highest amount of lignin among oil palm biomass [30]. Lignin present in the biomass translates into high-quality carbon fuel, and its composition affects the final biochar weight [38e40].

Different biomass materials differ in their comparative amounts of cellulose, hemicellulose, and lignin, which affect their heating value. Hence, the pretreatment that leads to better biochar properties in different biomass will be unique.

The novelty of research work lies in the comparison of the two pretreatment methods that lead to different properties of the biochar and hence different performance in the DCFC. The prominence of this research lies in establishing the potential of waste PMF biomass as a substitute fuel source in the DCFC.

The significance of this research study also rests in building DCFC technology a viable, cleaner alternative to conventional power generation methods.

PMF has different lignocellulosic content compared to PKS used in our previous work [23]. Hence, this work aims to investigate the effect of acid and alkali pretreatment of PMF on the physicochemical properties of the biochar, as well as its performance in the DCFC. The microstructure and physicochemical properties of the acid and alkali pretreated biochar were characterized by combining proximate and elemental analysis, TGA, XRD, BET, and SEM. Based on a comprehensive characterization of the chemical properties and microstructure of the treated biochar, its electrochemical reactivity has been assessed in the DCFC. The research study also centers on the likelihood of the dependence of the electrochemical performance of the PMF biochar fuel on its intrinsic physicochemical properties, such as chemical composition, the content of oxygen-surface functional groups, thermal properties, graphitic structure, and specific surface area.

Material and methods

Biomass preparation

Raw oil palm mesocarp fibre (PMF) used in this study was obtained from an oil palm plant from Selangor, Malaysia. The

biomass samples were first sun-dried, followed by washing with distilled water. Subsequently, it was oven-dried (UN75, Memmert, USA) at 105C for 24 h as per the ASTM D2867-09 standard. The oven-dried samples were subjected to grinding using a disc milling machine and were then sieved to a size of 0.5 mm using a sieving machine (RX-812-1, W.S Tyler USA). All the samples were kept in a desiccator before use.

HCl and NaOH pretreatment on PMF

Four types of carbon fuels were used in this work, namely the commercial activated carbon (AC), PMF treated with NaOH, PMF treated with HCl, and untreated PMF that are denoted as

‘PMF-NaOH’, ‘PMF-HCl’ and ‘U-PMF’, respectively. Acid pretreatment was conducted by preparing a 1.0 M solution of hydrogen chloride (HCl). 50 mL of acid solution was added into 5 g of raw biomass. The contents were immersed for 24 h at room temperature [20]. This procedure was repeated for an acid concentration of 0.1, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, and 4.0 M.

The respective PMF biomass obtained after each HCl concentration is denoted by the molar concentration as the suffix.

The biomass was washed continually with distilled water to remove any chemical residue and oven-dried for 24 h. The alkali treatment was performed according to the method obtained from the literature [41]. 5 g of raw biomass was mixed with 20 mL of the alkali solution with different concentrations and stirred for 8 h at 95C. The concentration of NaOH used was 0.1, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, and 4.0 M. The biomass obtained after each NaOH concentration is denoted by the molar concentration as the suffix. After alkali pretreatment, the biomass was washed and dried by following the same procedure as acid treatment. All the experiments were conducted in duplicates.

PMF biochar preparation

The pretreated and untreated PMF biomass were pyrolyzed in a quartz tube (HST 12/400, Carbolite) at 500C in a nitrogen environment with a heating rate of 10C min¹. At 500C, the samples were held for 1 h before cooling down to room temperature for biochar production.

Characterization of biochar

The elemental analysis was performed in an elemental analyzer (Vario MACRO Cube CHNS, Germany) to determine the composition of the organic elements. Proximate analysis of the fuel samples was performed through thermogravimetric analysis (TGA), using a thermogravimetric analyzer equipped with a STA 6000 PerkinElmer thermo-balance. The proximate analysis through TGA evaluates the thermal decomposition of biochars, giving information on the content of fixed carbon, volatile matter, moisture, and ash. In TGA analysis, 10 mg of samples were heated from 30C to 900C at a heating rate of 10C min¹in a nitrogen environment. The nitrogen flow was switched to oxygen when the temperature reached 900 C, and the condition was held for 20 min to determine the ash content [42]. The phase composition and graphitic structure of the fuel samples were determined using X-ray diffraction analysis (XRD). Diffractograms were

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recorded in a Bruker D8 powder diffractometer, and diffraction data were collected for 2qfrom 10to 90with a step scan mode (0.05per step). Brunauer-Emmett-Teller (BET) analysis was used for the determination of the surface area and pore volumes of the samples with Autosorb 1C (Quantachrome Instruments, USA). The morphology of biochar was determined by scanning electron microscopy ((Hitachi S3400N-II, Japan). Before imaging, the samples were coated with a plat- inum coating. The SEM images were examined under mag- nifications of 500 and 1000.

The electrochemical characteristics of biochar were tested in a DCFC system, as shown inFig. 1. Electrochemical measurements were conducted using a potentiostat (Gamry, Interface 1000 E, Germany) in a frequency range of 1 KHz to 0.1 Hz. The single-cell performance was evaluated for the temperature range of 750Ce850C. The button cell, which consists of an anode (nickel-yttria-stabilized zirconia), cathode (lanthanum strontium manganite), and electrolyte (yttria- stabilized zirconia), was procured from Ningbo SOFCMAN Energy Technology, China. Silver wires were used for the measurement of current. The components in the DCFC are shown inFig. 2. Nitrogen was supplied from the anode side at 600 mL min¹(STP), and the cathode side was supplied with oxygen at 200 mL min¹. TheIeVcharacteristics of the single- cell were computed with a scan rate of 10 mV s¹_.

Results and discussion

Proximate and elemental analysis of biochars

The effect of pretreatment on biochar was analyzed by physicochemical and electrochemical characterization.Table 1presents the results of the proximate analysis of untreated and pretreated PMF biochars, which underwent pyrolysis at a temperature of 500C. All the experiments were conducted in duplicates to ensure reproducibility of the results. The mean of the experimental values, which gave less than 3% difference, is used to denote the estimation. As depicted inTable 1, the percentages of fixed carbon (FC) for the pretreated PMF biochars were higher than the untreated biochar. Meanwhile, the volatile matter (VM), moisture, and ash values were reduced for the pretreated samples. All the tested biochars

were rich in carbon, suggesting the pretreatment step made the fuels more carbonaceous. This characteristic of biochars makes them more reactive as fuel when utilized in the DCFC system. The reason being that electrochemical reactions in DCFCs require carbon as the principal substrate [43,44]. The higher amount of FC also signifies higher chemical energy, which leads to an improved DCFC performance [45]. The proximate analysis results also show that chemical pretreatment aided in the reduction of VM considerably for the pretreated PMF biochars. Typically, higher amounts of VM in the biochar would interfere with the electrochemical reaction and cause performance degradation in the fuel cell [46,47].

The FC contents for both NaOH and HCl pretreated PMF biochars increased when compared to untreated biochar.

While comparing the two pretreatment methods of HCl and NaOH on PMF biomass, as seen inTable 1, it was observed that HCl pretreatment has resulted in a slightly higher FC content for PMF biochars. This increase in FC content by the demineralization effect may be due to the reaction of the chemical agents with the concealed minerals and the dissolution of the ash holding inorganic mineral components in the biomass matrix [48], with HCl being a more effective ash removal agent that is discussed in the latter part. As the concentration of HCl and NaOH increases, a decline in the FC contents is observed for PMF biochars, which are associated with the stripping of carbonaceous matter from the biochars [49].

It is observed that HCl treated PMF biochars composed of very low ash content (0.1e0.4 wt%) with the acid concentration range between 1.5 M and 4.0 M; while NaOH treated PMF biochars recorded higher ash content (1.5e1.7 wt%) at the same concentration range used despite it being significantly lower than AC (2.7 wt%) and untreated PMF biochar (2.5 wt%).

Raw PMF biomass contains various lignocellulosic contents, typically comprising 31.8% hemicellulose, 34.5% cellulose, and Fig. 1eSchematic diagram of the DCFC system.

Fig. 2eComponents in the DCFC. 1: flange, 2: ceramic tube, 3: furnace, 4: anode current collector, 5: biochar placed on top of button cell, 6: cathode current collector.

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25.7% lignin [35]. The use of dilute acids for pretreatment is effective in the removal of hemicellulose-linked alkali and alkaline earth metals (AAEMs), while dilute alkalis help in solubilizing lignin [50]. Probably owing to this reason, the maximum removal of ash in HCl treated PMF biomass could be asserted. The difference in ash removal could also be attributed to the types of inorganic contents existing in the biomass samples. PMF biomass is shown to contain a high amount of silica, potassium, and calcium compounds [51]. The removal of these compounds is possibly associated with the solubility of the various metal oxides in acids or alkali solutions [52]. The degree of ash removal in PMF biomass is different for different concentrations of HCl and NaOH, as reported inTable 1. As the concentration of the respective treatment agents increase, there is a decline in the ash content, which is attributed to the effective leaching of inorganics by the agent. With a further increase in the concentrations of HCl and NaOH, the ash contents start to increase. This phe- nomenon occurs as a consequence of side reactions, which results in lignocellulose-derived by-products. These by- products are inhibitory and tend to accumulate as the concentration of the pretreatment agent increase [53].

The VM content of pretreated and untreated biochars de- clines markedly compared to the raw PMF. In general, both the pretreatment methods of HCl and NaOH on PMF biomass resulted in a drastic decrease in VM content for PMF biochars, as reported inTable 1. With the increase in the concentration of HCl and NaOH, there is a decline in the VM contents for both acid and alkali pretreated PMF biochars. According to earlier studies, researchers have pointed out that pretreatment of biomass reduces the ash contents, which in turn reduces or eliminates the release of volatile species during pyrolysis [54].

Also, volatile matter removal during pyrolysis further increases the fixed carbon in the biochars [55].

There have been studies done on pretreatment techniques, mostly using different types of coal. Chien et al. [45] conducted a study on the demineralization of anthracite coal with dilute

HCl. The results revealed that the superior performance of coal in the DCFC was attributed majorly to high fixed carbon content, low values of moisture and ash (particularly that of sulfur), and moderate values of volatile matter. Likewise, Jiao et al. [35] performed alkali treatment of coal with KOH. They identified the high amount of carbon in pretreated char to be one of the reasons for superior performance in the DCFC over untreated carbon fuel. These research findings are consistent with the present study and suggest that the pretreatment process is preferred to obtain biochars with the desired proximate characteristics. Palniandy et al. [27] performed alkali pretreatment on rubberwood and concluded that physicochemical characteristics post alkali pretreatment, such as higher fixed carbon content, oxygen functional groups, and lower ash content, led to the enhancement in the electrochemical reactivity of the rubberwood char. In our previous work, which used acid pretreated PKS biochar [23], the results showed higher ash content (1.0 wt%) despite acid treatment.

The reason is likely attributed to PMF biomass having a fibrous structure. The fibrous structure corresponds to higher surface sites for the chemical reaction to occur between the acid and the ash compound, which leads to such a low ash content in PMF biochar.

The elemental composition of the PMF biochars evaluates its feasibility to be used as a DCFC fuel. Typically, the high content of carbon indicates that the biochar could be used as a potential fuel for DCFC since carbon is the primary reactant in the DCFC electrochemical reaction [19,34]. The results for elemental analysis for all the carbon fuels are given inTable 2.

The ash value is another important factor in determining the effectiveness of the pretreatment method. As seen inTable 2, the lowest ash content for the HCl treated PMF biochar is recorded for PMF-2.0 sample (PMF biomass treated with 2.0 M HCl), having a value of 0.1 wt% and the lowest ash value for NaOH treated biochar is recorded for PMF-3.0 (PMF biomass treated with 3.0 M NaOH) at 1.5 wt%. These ash values for HCl and NaOH treated PMF biochars were considerably lower than Table 1eComparison of the proximate compositions of HCl-treated PMF, NaOH-treated PMF, raw PMF, untreated PMF and activated carbon (AC).

Carbon fuels FC (wt.%) VM (wt.%) Moisture (wt.%) Ash (wt.%)

Raw PMF 35.1 48.7 7.7 8.5

AC 92.0 e 5.3 2.7

Untreated PMF 70.3 21.7 5.5 2.5

PMF biochars (HCl treated) PMF-0.1 71.0 20.5 6.7 1.8±0.01

PMF-0.5 71.4 21.7 5.8 1.1±0.01

PMF-1.0 72.2 20.6 5.7 1.5±0.04

PMF-1.5 72.5 22.0 5.2 0.3±0.02

PMF-2.0 73.1 20.7 6.1 0.1±0.02

PMF-2.5 70.1 23.5 6.2 0.2±0.01

PMF-3.0 69.5 23.6 6.6 0.3±0.03

PMF-4.0 68.9 23.9 6.8 0.4±0.01

PMF biochars (NaOH treated) PMF-0.1 63.4 28.1 6.6 1.9±0.03

PMF-0.5 66.4 25.8 6.0 1.8±0.02

PMF-1.0 67.3 25.6 5.4 1.7±0.02

PMF-1.5 68.9 24.3 5.1 1.7±0.02

PMF-2.0 68.3 23.5 5.5 1.7±0.03

PMF-2.5 68.9 22.6 5.7 1.6±0.02

PMF-3.0 70.0 21.7 5.9 1.5±0.02

PMF-4.0 67.8 23.8 6.8 1.6±0.02

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the untreated PMF biochar recorded at 2.7 wt%. A high quan- tity of ash will retard the mass transfer, which occurs between the carbon fuels and the current collector at the anode side, and adversely impacts fuel cell performance [56]. HCl and NaOH pretreatments are both able to reduce the ash content, which might aid in enhancing the DCFC performance. Hence, for further physicochemical characterization, the respective PMF biochars having the lowest ash values were chosen.

Table 2also shows that PMF biochars contain low fractions of sulfur (S), which is a component present in ash. The low content of contaminations, especially sulfur, is favorable for effective and failure-free working of the DCFC [57]. It is essential to study the effect of ash components on the DCFC performance because the gathered ashes would affect the life span of DCFC [57]. In the case of investigated biomass PMF, the major components that account for ash are namely silica, potassium, calcium, aluminium and iron [58]. There are a few studies on the effects of impurities in carbon fuel on the performance of DCFC. Li et al. [18,28] performed acid pretreatment on coal and pointed out that the elimination of the mineral impurities from coal aided in protecting the electrodes and the electrolytes. Weaver et al. [59] reviewed the influence of ash addition in the DCFC system. They added 10 wt% of fly ash into the electrolytes and stated that this addition did not have a significant effect on the polarization curves in a fixed bed DCFC. Vutetakis et al. [60] demonstrated the effect of several mineral impurities in a fluidized bed DCFC. They noticed a sharp fall in current at high over- potentials, which was caused by the passivation of electrodes owing to the creation of a film on the surface of the electrode by the dissolved Al2O3, SiO2, and TiO2 from coal ashes. Cherepy et al. [57] used petroleum cokes containing 2.5e6 wt.% sulfur in their DCFC. They described that the occurrence of sulfidation corrosion at the anode (nickel) weakened the cell performance over time, thereby decreasing the carbon discharging rate and increasing of the cell resistance.

The oxygen to carbon (O/C) ratio for the carbon fuels is given inTable 2. This ratio specifies the presence of oxygen- containing functional groups in the samples [19,43]. It is un- derstood that the electro-oxidation of the biochar fuel could be promoted by the oxygen-containing surface functional groups [43]. As can be seen inTable 2, the treated biochars, PMF-2.0 (HCl) and PMF-3.0 (NaOH) have the O/C ratio at 0.26 and 0.22, respectively, which is higher than AC and untreated PMF biochar. Besides, the PMF biochars showed higher oxygen functional groups than the AC fuel, most likely due to having different microstructures and textual properties [20]. As per the data given inTable 2, treated PMF biochars are seen to contain high carbon contents and lower sulfur contents. This

indicates the possible potential of biochars as fuels for application in DCFCs [57,61].

Thermal decomposition of biochars

The thermal decomposition of the fuel samples was examined through TGA in a nitrogen atmosphere, and the thermogravimetric analyzer curves obtained are shown in Fig. 3 The samples show one noticeable mass loss below 150C, which pertains to the dehumidification phase. Overall, mass losses were seen in the lower temperature range of 25 Ce300C, which is related to dehydration and gas elimination from the samples. It can be observed that the thermal decomposition of raw PMF and untreated PMF samples took place at about 300C. This is indicative of the oxidation phase for raw PMF and untreated PMF samples to start at about 300C, a temperature value lower when compared to PMF-NaOH and PMF- HCl samples. The data analysis of the samples showed some thermal effects in the temperatures between 450 C and 900C. The weight loss at around 450C could be related to the elimination of organic compounds. At the same time, the weight loss at around 750C is associated with the decomposition of inorganic salts [62]. The PMF biochars showed weight loss between 400C and 700C due to slight thermal decomposition, suggesting the presence of carbon mostly in the solid-state. There occurred less than 10% of weight loss between the temperatures of 700Ce850C, which perhaps indicates that biochar carbon atoms will be able to participate in the electrochemical reaction of the DCFC [63]. The oxidation of the biochar takes place between 290 C and 500C. The oxidizing rate is seen to increase with the increase in the operating temperature and reaches the maximum at around 700C. Above 700 C, the carbon in the sample completely Table 2eElemental analysis of carbon fuels.

Proximate analysis C (wt.%) H (wt.%) N (wt.%) S (wt.%) O (wt.%) H/C O/C HHV (MJ kg¹)

AC 96.4 0.6 0.0 0.6 0.2 0.01 0.01 34.4

Raw PMF 47.4 6.6 4.5 0.3 41.2 0.14 0.87 N/A

Untreated PMF 73.4 3.9 1.9 4.2 15.4 0.04 0.20 27.7

PMF-2.0 (HCl) 75.6 2.2 1.6 0.3 19.8 0.34 0.26 28.1

PMF-3.0 (NaOH) 78.9 2.0 1.1 0.1 17.6 0.03 0.22 27.4

Fig. 3eThermogravimetric analyzer curves recorded for raw PMF biomass and biochars.

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oxidizes to CO₂, and the remaining component is ash. When compared to other carbon materials with high crystallinity such as carbon fibre and graphite [64], the biochar here is easier to be oxidized, and this probably will contribute to a high electrochemical reaction rate in the anode of the DCFC [19,43].

Graphitic structure of PMF biochars

Fig. 4shows the X-ray diffraction graphs of the PMF biochar samples and the graphite reference.

All the samples showed X-ray diffraction patterns having broadened peaks, which is a typical characteristic of the amorphous structure of carbon-based materials. All the PMF biochars displayed two broad peaks in the range of 10e30 and 30e50assigned to (0 0 2) and (1 0 0) plane, respectively, which is typical for the amorphous structure of carbon particles. No apparent characteristic peak of graphite at 26was observed, indicating that the biochars are non-graphitized and have a disordered structure. The effect of pretreatment is not significant on the graphitic structure of biochar. The amorphous property, however, is beneficial for facilitating the electro-oxidation of carbon at the anode during the DCFC operation [22,65]. In previous research, it was established that the electrochemical performance of carbon fuels was significantly improved when the carbon source had a more disordered structure [20,66]. In a recent study conducted on walnut shells, the authors mentioned that less graphitized carbon is a promising fuel in the electrochemical reaction for obtaining higher performance [22]. Based on the results attained through structural investigations, the obtained PMF biochar appears to be valuable solid fuel.

Surface area of carbon fuels

BrunauereEmmetteTeller (BET) analysis enables the estimation of surface areas and total pore volumes of carbon fuels.

The BET analysis results of the carbon fuels are outlined in Table 3. AC gives the highest BET surface area recorded at 805 m²g¹, while HCl and NaOH treated PMF biochar exhibits surface areas at 419.365 m² g¹ and 391.090 m² g¹,

respectively. The structural disruption of the biomass structure caused by pretreatment has possibly led to pore formation, thereby enhancing the surface area. It is seen from the results (Table 3) that although pretreated biochars have lower surface area compared to that of AC fuel, their pore volumes are recorded to be higher. This feature is likely attributed to the porous nature of the biomass. Upon chemical pretreatment, the pores on biochars were formed with the removal of the inorganic compounds and contributed to higher pore volumes (0.96e0.98 cm³g¹) compared to the untreated biochar (0.154 m³g¹). The total pore volume estimations are also essential to note as it assists the electrochemical reactivity by enhancing the interaction of carbon particles with oxide ions at the anode [19].

Surface morphology of biochars

Scanning electron microscopy (SEM) displaying surface mor- phologies and particle sizes of PMF biochars are presented in Fig. 5. The biochars displayed different structural forms with the pretreated biochars showing the apparent presence of pores, while the untreated PMF biochar shows a rugged surface with crumbled edges and retaining its fibrous structure.

The PMF biochar pretreatment with dilute HCl and NaOH indicated a porous structure. This is probably due to the leaching away of minerals and extractives while making the biochar structure to disrupt [32,67]. The hemicellulose and mineral constituents in the fibrous structure undergo partial dissolution, which led to an increase in surface areas and pore volumes [68]. This feature may help in increasing the

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000

0 10 20 30 40 50 60 70 80 90 100

Intensity

2 Theta (°

PMF-NaOH

PMF-HCl U-PMF Graphite

₁₀₀

002

Fig. 4eXRD pattern for PMF biochars and graphite.

Table 3eBET surface area and total pore volume of carbon fuels.

Carbon fuels

BET surface area (m²g¹)

Total pore volume (cm³g¹)

AC 805 0.452

Raw PMF 0.310 0.001

U-PMF 342.042 0.154

PMF-NaOH 391.090 0.960

PMF-HCl 419.365 0.980

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possibility of interaction between the fuel and electrolyte in the DCFC system. The diameter of the pores of pretreated biochars increased from 2e10mm to 2e60mm as seen through SEM analysis. This may prove useful in creating new active sites in the biochar for the electrochemical reaction in the DCFC. The biochar framework is comprised of isometric particles, which is also observed through SEM analysis.

Compared to needle or spindle-shaped particles, isometric particles facilitate better contact with the anode surface [22].

In the case of needle-shaped carbon particles, only point contact is achievable between the anode and the carbon particles [22]. The morphology of carbon particles and disorder in the carbon phase has a significant impact on the current and power density outputs in the DCFC.

Electrochemical performance of carbon fuels

Open circuit voltage

The open-circuit voltage (OCV) for an ideal working fuel cell is determined when the current I¼0 A. The OCV vs. time graphs obtained for carbon fuels are plotted inFig. 6to determine the thermodynamic potential of the redox reaction that is ready to occur in the single cell. HCl treated PMF biochar recorded the highest OCV of 0.89 V while NaOH treated PMF biochar recorded a slightly lower OCV of 0.87 V, indicating that the HCl treated PMF biochar possesses higher electrochemical reactivity for carbon oxidation to occur. Also, the stable OCV values obtained over the studied period of time indicates that

the DCFC has reached electrochemical stability prior to the performance study. Meanwhile, both the AC and untreated PMF biochar OCV were recorded at 0.79 V. The reason for the difference in the OCVs for biochars could be attributed to the differences in the molecular arrangement of carbon in the biochars. This assumption was stated by Hackett et al. [69], who stated that coal-derived fuels generated higher OCV (0.95 Ve1.05 V) than graphite fuel (0.7 Ve0.8 V). This theory has also been supported by some other researchers [22,45,70], who mentioned that a higher disorder in carbons produces higher values of OCVs together with higher current and power density outputs. The higher total pore volumes with the apparent porous structure of the biochars in the amorphous phase indicate the higher electrochemical reactivity of the biochars to undergo the electro-oxidation at the anode.

Current and power densities

The purpose of the electrochemical study is to show the applicability of PMF biochars for current and power generation in the DCFC.Fig. 7andFig. 8show theIeVandIePperfor- mance curves of the HCl treated and NaOH treated PMF biochars, respectively, studied at the DCFC operating temperatures of 750 C, 800C and 850C. The reason for selecting the temperature range of 750Ce850C in DCFC was based on the literature given by Nabae et al. [71], who sug- gested that the solid oxide electrolyte based DCFC does not work well at temperatures lower than 700C. According to Nabae et al. [71], the oxidation at anode involves the oxidation of carbon monoxide (CO), which is produced through the reverse Boudouard reaction given by C þCO242CO. This reaction gets shifted to the left if the temperature is below 700C, producing low current densities, while at higher temperatures, the current densities and electrochemical performance are enhanced.

Table 4illustrates the electrochemical data extracted from the performance curves for all the carbon fuels. The results for current and power densities are reported to be higher for HCl treated PMF biochars compared to AC fuel and the other tested biochars. Consequently, the physicochemical characteristics of HCl treated PMF biochar appear to be the basis of this performance. The acid treated PMF biochar is a better performing fuel than AC even though it has a much lower surface area.

This could be credited to the large amount of oxygen- Fig. 5eSEM micrographs of PMF biochars (a) without chemical pretreatment, (b) treated with 2.0 M HCl, (c) treated with 3.0 M NaOH (Magnification£500).

Fig. 6eChange in OCV with time for AC fuel, untreated PMF, HCl treated PMF biochars, and NaOH treated biochars.

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containing functional groups denoted by the O/C ratio, as seen through elemental analysis (Table 2). The process of electrochemical oxidation of biochar is reportedly enhanced by these oxygen functional groups, which increase the entropy of the electrochemical reaction by generating additional CO gases [43,72]. The produced CO gases are likely to aid the anodic electrochemical processes in the DCFC. In prior research, Li et al. [20] employed carbon black and AC fuel for DCFC and

treated the fuels with HNO3to enhance their surface characteristics. The authors specified that the surface oxygen functional groups acted as the active sites for the electrochemical oxidation of carbon fuels, thereby enhancing the discharge rate at the anode.

The peak power density achieved by HCl treated PMF biochar was recorded at 11.8 mW cm²at 850C, while NaOH treated PMF biochar recorded a peak power density of Fig. 7e(a)IeVcurve, (b)IePcurve of DCFC operated on 750C, 800C and 850C for HCl treated PMF biochar.

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4.8 mW cm²at 850C. AC and untreated PMF biochar recorded much lower power density values of 0.71 mW cm²and 0.70 mW cm², respectively, which correspond to the lower OCV values for these samples. The HCl treated biochars yiel- ded higher power density outputs likely due to higher contents of elemental carbon, lower amounts of ash, and high surface areas. Despite having lower surface areas than activated carbon, it is anticipated that the surface area could improve the interaction between the carbon fuel particles and the anode interface when we compare the performances between the untreated PMF biochar and pretreated PMF biochars. Untreated PMF biochar with the lower surface area of 342.042 m²g¹and pore volume of 0.154 cm³g^-1resulted in a lower power density (0.70 mW cm²) when compared to PMF- HCl (11.8 mW cm²) and PMF-NaOH (4.8 mW cm²). This is further supported by the BET, and SEM analysis, where HCl pretreatment has resulted in broader pores (pore size

>0.1 mm) of biochar, followed by NaOH treated biochar while the less porous structure was observed for the untreated PMF biochar. This represents the increased surface of carbon atoms that would interact at the anode surface. So, this may have resulted in the HCl treated biochar giving a better performance in the DCFC. DCFC performance is also significantly

dependent on cell temperature. Increasing the operating temperature from 750C to 850C increased the performance of the DCFC significantly, especially for PMF-HCl. At higher temperatures, ionic conduction is facilitated, which enhances the anodic reaction kinetics. There has been one documented study involving PMF biochar conducted by Thong et al. [37].

They investigated on different pyrolysis temperatures for the production of PMF biochar to be used as fuel in the DCFC.

Their results showed a peak power density of 2.5 mW cm², and this was attributed mainly to the biochar having an amorphous carbon structure and better thermal reactivity.

It is worth mentioning that the power density obtained in this study was still low when compared to past studies.

Nonetheless, on a similar DCFC setup, research conducted using different varieties of coal (demineralized), graphite, carbon black, and pine charcoal gave power density outputs in the range of 3e17 mW cm². Kim et al. [73] utilized coal in a solid oxide electrolyte DCFC, which was in a liquid-like phase on the anode at high temperatures and reported a low power density of 3 mW cm². Dudek et al. [74] fueled the DCFC with carbon black, which supplied a maximum power density of 3 mW cm². Large polarization impedance was observed, and this behaviour was attributed to the sluggish kinetics of carbon oxidation reaction [74]. The sluggish kinetics is mostly caused by a limited reaction region, which restricts the effi- cient contact between the carbon fuel particles and the solid electrolyte. In separate research, Kim et al. [75] used graphite as the carbon source, which resulted in the power density output of 16.8 mW cm².

Impedance study of PMF biochars

Electrochemical impedance spectroscopy measurements were conducted on the PMF biochars at the same DCFC operation temperature range to see the effect of varying DCFC temperature on the cell resistances. The resistances refer to the resistance in the way of the ions through an electrolyte.

The resistances in the cell circuit for HCl and NaOH treated PMF biochars at 750C, 800C and 850C are depicted with the help of impedance spectra (Nyquist plot) inFig. 9andFig. 10, respectively. The impedance spectra consist of small high- frequency arcs overlapped with large, low-frequency arcs.

The total cell resistance comprises two major parts, namely ohmic resistance (Ro) and total polarization resistance (Rp).

Fig. 8e(a)IeVcurve, (b)IePcurve of DCFC operated on 750C, 800C and 850C for NaOH treated PMF biochar.

Table 4eElectrochemical data for carbon fuels at 850C.

Carbon fuels

Open circuit voltage (V)

Current density (mA cm²)

Power density (mW cm²)

AC 0.79 2.5 0.71

U-PMF 0.79 1.8 0.70

PMF- NaOH

0.87 15.3 4.8

PMF-HCl 0.89 27.1 11.8

Fig. 9eImpedance spectra for HCl treated PMF biochar.

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The value of Rp at low frequency is possibly lowered by increasing the working temperature. The increase in the working temperature causes the improvement of anode and cathode activities and enhances the movement of the oxi- dants at the electrode boundary, thereby promoting electrochemical activity.

The impedance results are in line with theIeVdata. Upon increasing the DCFC operating temperature to 850 C, a noticeable decrease in the frequency arc size was observed. In all the runs, the extent of the arc distinctly reduces as the cell temperature increases. This arc reduction indicates that the cell resistances are considerably decreased as the DCFC operating temperatures increase. Both the ohmic (refers to the intercept of the high-frequency arc on the real axis) and total polarization resistances noticeably decrease. With the increase in temperatures above 700C, the production of CO is more through the Boudouard reaction at the anode, which in turn improves the electrochemical performance.

In specific, a lower resistance value for HCl treated PMF biochar is seen compared to NaOH treated PMF biochar. For the HCl treated biochar, the ohmic resistance of the cell, Ro, referring to the high-frequency intercept on the real axis, is 1.7, 0.8, and 0.4Ucm²at 750C, 800C, and 850C, respectively. For the NaOH treated biochar, the ohmic resistance of the cell, Ro, is 2.8, 2.1, and 1.8Ucm²at 750C, 800C, and 850C, respectively.

Overall, it can be deduced from the EIS spectra that HCl treated biochars resulted in much reduced resistances upon increasing temperatures from 750C to 850C. The ohmic resistance indicates the electrolyte resistance to ion flow and contact resistances, which occur between carbon fuel particles and the anode layer. It is also possible that low amounts of ash in the HCl treated biochar contributes to the lowering of ohmic resistance by enhancing contact between anode and the carbon fuel particles with lesser insulating gaps.

Conclusion

The results of this work establish the potential of waste biomass derived PMF biochar as a fuel supply for DCFCs. The performances were assessed together with a commercial

activated carbon (AC) as baseline fuel. In particular, the HCl treated biochar obtained from PMF biomass depicted superior properties and was characterized by high carbon, high O/C ratio (denoting oxygen-containing functional groups), and low ash contents, particularly that of sulfur. Based on structural studies (X-ray diffraction, BET, and SEM analysis), it was established that disordered carbon particles or, typically, particles of defected graphite are created during the processing of char. HCl treated PMF biochar is also reported to have the highest surface area and pore volume among the tested biochars. Although a higher surface area of fuels may also be helpful, it is less critical than surface chemistry. It was also seen that porous isometric carbon particles of pretreated PMF biochar are desirable for use in DCFCs to obtain suitable current and power density values. HCl treated PMF biochar recorded a peak power density of 11.8 mW cm²at an operating temperature of 850C, which was higher than the results obtained for other carbon fuels in similar conditions.

Moreover, the cell resistances were recorded to be the lowest for HCl treated biochar at 850C. This work displayed that pretreatment of the PMF biomass impacted the physicochemical properties of the biochar fuels, which led to significant improvement in the DCFC performance. The outcome suggests that the structure of the carbon fuel plays a crucial part in DCFC performance, and structural modification of biochars may strengthen the applicability of the fuels in the DCFC.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This work has been supported by Fundamental Research Grant Scheme FRGS/1/2019/TK02/TAYLOR/02/1 under the Ministry of Higher Education, Malaysia and University Kebangsaan Malaysia grant scheme DIP-2018-012 & GUP- 2018-013.

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