Recent progress on production technologies of food waste–based biochar and its fabrication method as electrode materials in energy storage application

Abstract

The abundance of food waste across the globe has called for the mitigation and reduction of these discarded wastes. Herein, the potential of biochar derived from food waste is unquestionable as it provides a sustainable way of utilizing the abundance of available biomass, as well as an effective way of preserving the ecosystem through the reduction of concerning environmental issues. This review focuses on the food waste–based biochar as advanced electrode materials in the energy storage devices. Efforts have been made to present and discuss the current exploration of the food waste utilization, along with the biochar production technologies through thermochemical conversion, including combustion, gasification, and pyrolysis method. Finding its limitation in literatures, discussion on the food waste–based biochar fabrication method as the electrode materials is elaborated, alongside the current food waste–based biochar that has been explored in the energy application thus far. Towards the end, the outlook and perspective on the further development of food waste–based biochar have been outlined.

Introduction

The rapid depletion of fossil resources such as coal, petroleum, and natural gas and its association with concerning environmental changes has anticipated the search for sustainable and clean energy [1]. With that, there has been a huge increase in the demand for renewable energy sources such as solar and wind. Besides, the rising demand for renewable sources has called for an intermittency of electricity to be implemented as energy storage devices. Rechargeable batteries and supercapacitors are the leading energy storage devices as they came with high energy and power densities, respectively [2]. Indulging in the current development of technology, these electronic devices demand for a more flexible characteristic. Hence, the research interest is more focused on the enhancement of electrochemical energy storage (EES) devices, especially in exploring new materials that are affordable and efficient. Referring to Ehsani and Parsimehr [3], the EES devices are categorized into three groups according to their energy density and power density, which are batteries, supercapacitor, and hybrid device. Comparing these three, batteries have the highest energy density, the supercapacitor has the highest power density, whereas hybrid is a combination of both. The reliability of the EES device is dependent on both the energy density and power density of the electrode. Figure 1 illustrates the Ragone plot that shows the difference in the energy and power density for capacitor, supercapacitor, batteries, and fuel cell. Based on the Ragone plot, it is difficult to produce a single material that possesses high power and energy density at the same time.

Fig. 1.

Ragone plot representing the distribution of energy density and power density for EES [4]  

These energy storage devices power up a wide range of technologies from portable devices to electric vehicles [5]. Hence, the supply requires a huge increment to meet the demand, while the development of materials ought to exhibit high electrochemical performance, inexpensive, and easy to be scaled up at an industrial level. Furthermore, the electrode materials are the vital components in the electrochemical performance testing, as it determines the energy storage systems’ performance [6, 7]. The parameters that are often studied and reported in the EES application are gravimetric specific capacitance (F/g), energy density (Wh/kg), power density (W/kg), and cycle stability, though some literatures have also been reported in a volumetric scale, i.e., Wh/L and W/L. Comparing these dual approaches, the gravimetric scale is more useful in reporting the EES device performance [3].

Herein, the selection and fabrication of electrode materials play an important role in improving the capacitive performance of supercapacitors and have drawn the search for abundant precursor availability, excellent electrical conductivity, and tunable control of intrinsic properties [8]. Carbonaceous materials match the said criteria and serve as a predominant choice for the electrode materials, which can be churned out from different sources of biomass. Herein, the utilization of biomass as an alternative for adaption in the energy application is a plus point to promote sustainability and cost-effectivity, which also has risen magnificently in this aspect [8, 9]. With an abundance of these resources, biomass has been utilized for the preparation of carbon-based materials, and upon processing, it serves as a valuable precursor for the energy generation. Often referred to as an energy source, biomass is made up of carbohydrate and chemically composed of carbon, hydrogen, oxygen, nitrogen, and traces of sulfur and chlorine [10]. In addition, the biomass typically refers to organic matters either plant or animal-derived that has been considered waste, wherein it can come from a variety of sources such as the forestry residues, agricultural waste, and food waste [11]. Furthermore, considering the food waste is a readily available resource, it assists in lowering the search for another potential precursors. At the same time, reducing the abundance of available natural resources and making them more valuable upon conversion to the functional material. Indeed, the accumulation of biomass waste particularly the food waste has been one of the principal challenges in the environment.

Food waste refers to the remaining of unconsumed biodegradable food, often discarded at either retail or consumption phases due to its inedibility, overproduction, processing problems, and market distribution issues [12]. The US Department of Agricultural Economic Research Service defines food waste as “food discarded by retailers due to color or appearance and plate waste by consumers”. Typically, food waste is discharged from various resources including households, food processing industries, and the agricultural sector. Furthermore, the abundance of the food waste that has been generated globally is equivalent to a total of 1.3 billion tons of food waste per year [13]. In Malaysia, almost 17,000 tons of food waste have been generated daily [14], and such amount is projected to accelerate in the next 20 years due to the growing population as well as rapid economic growth. Being classified under the municipal solid waste and biomass due to their biodegradable characteristics, the food waste has imposed a huge problem as nearly 80% of the generated food waste has not been disposed of in an appropriate manner [14]. Conventionally, these food wastes have been discarded via incineration or dumped in an open area that results in severe health hazards and environmental problems Paritosh et al. [15] indeed reported that the excessive food waste will result in severe environmental pollution, health risk, and scarcity of dumping land. Due to the drawbacks, it prompts the proper mitigation and reduction of food waste. Basically, recycling the food waste into the functional materials is integral to circumvent the associated environmental concerns. Moreover, high volume and diversity of food waste make them as ideal candidate to be applied in various environmental applications, including the energy storage field.

The food waste conversion into biochar is one of the promising methods to reutilize such wastes. The role of biochar in several environmental applications such as in soil amendment, carbon sequestration, removal of contaminants, and electrodes for energy storage devices has been extensively recognized. Basically, biochar is composed of carbon matrix with a fine structure produced from the thermochemical conversion of biomass in absence of air or oxygen-limiting environment [16]. Irrespective of the types of food wastes, common thermochemical technologies that direct the formation of biochar include combustion, gasification, and pyrolysis process [17, 18], which will be further discussed in Sect. 3. Thereby, this review focuses on the valorization of food waste–based biochar as advanced electrode material in the energy storage devices. Biochar exhibits a unique porous structure for the electrolyte ion transfer that promotes electrochemical performance in the energy storage. With abundant functional groups present, it accelerates the Faradic reaction and electrons transfer between the interaction of electrode and electrolyte ions. Overall, Chu et al. [19] reported that the biochar-based electrode materials are promising and have an utmost attraction thus far due to their distinct properties, such as being environmental-friendly, low cost, and renewability factor. Moreover, the attractiveness of a biochar-based electrode is attributed to its inexpensiveness compared to granular activated carbon and graphite electrode (US$ 51–381 vs. US$ 500–2500), respectively, aside from having comparable surface area and porous properties [20]. Besides, Cheng et al. [21] reported that the other types of carbon-based electrode such as fullerene, graphene, and carbon nanotube require a complicated synthesis method and are impractical for a massive production; accordingly, their utilization is rather limited. With respect to cost and power, it is evident that the power output of the biochar that is ranging from 457 to 532 mW/m² is comparable to the porous carbon (674 mW/m²) and graphite (566 mW/m²). Meanwhile, in terms of power costing, biochar-based power output is approximately 90% lower than the porous carbon and graphite ($17–35/W vs. $392–402/W) [22].

Hence, throughout this review, the current exploration of food waste conversion to biochar, along with the advantages and limitations of each thermochemical conversion (i.e., combustion, pyrolysis, gasification) technology will be discussed. In addition, emphasis has been given to the typical physiochemical characteristics to qualify them as the electrode materials. Furthermore, a discussion on the fabrication method of the electrode materials is elaborated alongside the current food waste–based biochar that has been explored in the energy application. Referring to Senthil and Lee [23], the progress in the biochar-derived electrode in electrochemical energy storage still received a limited attention presently, thus justifies the rationale of this manuscript. Towards the end, an outlook, as well as a perspective on the future development of food waste–based biochar as the electrode material, has been provided.

Food waste

The kick-off question when discussing the idea of utilization of biochar from food waste is often related to the availability of the feedstock or precursors to produce the biochar. The availability and accessibility to biochar feedstock will determine the economic feasibility of the technologies and measure the future potential of biochar utilization from food waste. According to the World Bank’s report in 2018, solid waste is predicted to rise from 2.1 × 10⁹ to 3.4 × 10⁹ tonnes per year by 2050, and food waste took a major composition in the municipal solid waste, which is about 44% worldwide or 70% in low-income countries [24]. According to Woon et al. [25], about 17, 000 tons per day of food waste were generated in Malaysia in the year 2016 and it also comprised of 45% of the total municipal solid wastes, which acts as a signpost that signifies the food waste in Malaysia lacked in utilization and have high resource availability. To relate with this current issue, Lahath et al. [26] mentioned that during the COVID-19 pandemic, the amount of food waste has escalated in Malaysia due to an impulse-buying and the restricted movement. Furthermore, the number of food waste could probably be double or triple in number during the festive seasons as the Malaysian celebrates numbers of festivals such as the Eid celebration and Chinese New Year celebration, where there will be a lot of food being served.

The classifications of food waste types and their valorization potentials along with examples are presented in Table 1. Based on Table 1, mixed food waste and packaging are difficult to be processed into biochar as mixed food can only be possibly segregated while most of the food packaging is made from plastics. The idea of waste-to-energy should at least contribute to the mitigation of pollutants as the conversion of food waste to biochar could help to reduce the usage of landfills and increase the utilization of inexpensive and renewable materials. Till now, the development and valorization of biochar from food waste have managed to emerge into energy storage application (i.e., supercapacitor, batteries, and fuel cell) and as environmental control material in carbon capture, removal of heavy metal, and removal of atmospheric pollutant [1]. The research and development of biochar from various feedstock are quite popular among the researchers due to their high adsorption abilities to remove pollutants and tunable physiochemical properties [20]. Besides, Senthil and Lee [23] stated that the food waste from household such as fruit peel or seeds is rich with some elements such as calcium, magnesium, phosphorus, and potassium, whereas animal skins, bone, and fish scales are rich in collagen protein and nitrogen sources that could enhance the biochar physiochemical properties.

Table 1.

Classification of food waste types and their valorisation potentials [29]

Classification	Examples	Valorization routes
Organic crop residue	Fruits, vegetables	1. Animal feed 2. Extraction of valuable compounds 3. Biofuel production
Catering waste	Cooked/processed meat/fish, dairy	1. Biofuel production 2. Biological treatment 3. Land spreading and composting
Animal by-products	Waste materials from slaughterhouses	1. Disposal and treatment 2. Recovery of valuable component
Packaging	Plastic	1. Recycling and composting 2. Landfills
Mixed food waste	–	3. Energy recovery 4. Anaerobic digestion 5. Segregation and recycling 6. Landfills

Perceptibly, the different feedstock will have different performances depending on their element composition of the feedstock. An experiment has been done by Huang et al. [27], focusing on shrimp shell waste as biochar material for supercapacitor electrodes, in which it successfully achieved a high specific capacitance of 201 F/g. The prepared electrode undergone a self-template approach as the shrimp shell is already rich with chitin (β-(1, 4)-2-acetylamino-2-deoxy-D-glucopyranose) and the activation process led to surface area of 401 m²/g with high nitrogen (8.2 wt.%) and sulfur content (1.16 wt.%). Also, a study written by Zhang et al. [28] found that the biochar from banana peel can produce a higher specific capacitance up to 1670.3 mAh/g with specific surface area of 729.03 m²/g. Thus, the rising issue here is on how to select the type of food waste and does the type really plays an important role in the materials’ performance in energy storage applications?

Food waste categories

According to Cecilia et al. [29], food waste can be collected along the food supply chain until the end-consumer, as every stage of the food preparation step might produce wastes. However, one of the challenges to recycle and utilize food waste as the biochar material is to separate the waste into its categories before being utilized. The usage of mixed food waste is also doable; however, there is an inconvenience in a large-scale production as it will be difficult to maintain a similar proportion. Accordingly, food wastes can be divided into five categories comprising of organic crop residue, catering waste, animal by-products, packaging, and mixed food waste.

Organic crop residue

The first category is organic crop residue which is composed of fruit and vegetable by-products waste such as stones, husks, peels, straw, and pomace. The important step in utilizing organic crop is the drying process as some organic residues are prone to microbial spoilage due to the moisture. Based on literatures, organic crop residue is the most used material as biochar due to its abundant availability and rich with the lignocellulose components [30]. Moreover, collection of fruits and vegetable by-products such as peels or straw can be done easily from local farmers or plantation areas. The interaction of lignocellulose components differs with the temperature that result in either enhancing or reducing the biochar formation. For example, hemicellulose and lignin promote the production of lignin-derived phenols but it hinders hydrocarbon generation [31]. Almost similar with the interaction between hemicellulose and cellulose, these two components have a weak effect on the food waste–based biochar production.

Catering waste

Catering waste is generally composed of multiple components from the food preparation process such as organic waste, packaging (plastic or cardboard), and used cooking oil. These wastes are mainly generated from restaurants, cafes, and canteens which involve in food preparation and serving. A study made by Serio et al. [32] shows that pyrolysis of mixed waste of food, paper, and plastic could produce high gas yields, combustible hydrogen, methane, and also char which indicates that the product composition is relatively independent of the waste composition. However, a recent study mentioned that interactions between these components in the biomass pyrolysis process are observed as a complex procedure as many other factors should be taken into account concurrently [33, 34]. The varieties of chemical compositions in lignocellulosic biomass which are determined by nature, origin, and environmental conditions have a significant effect on the properties of biochar when being selected as the feedstock [35].

Animal by-products

The next most well-known food waste apart from vegetables and fruits is animal by-products such as meat, poultry, and fish industries. The wastes generally consist of animal skins/shells, bones, and dairy. Similar to fruits, animal by-products are required to be pre-treated to prevent pathogens and pollutants. The usage of animal skins and bones is very common among researchers due to the mineral contents (calcium and nitrogen) and also rich in protein [27]. Besides, the heavy fats contained in the animal meat can also be converted to biochar and biofuel through pyrolysis. Hassan-Trabelsi et al. [36] has managed to produce about 32.2% biochar yield from waste animal fats with an appreciable amount of carbon content at the degradation temperature of 400–500 °C.

Mixed food waste and packaging

The food packaging waste that has been thrown away is usually recycled to be reused. Nevertheless, the ones that are mixed with food waste will eventually end up in the landfill sites. Currently, there are limited studies on the degradation of food packaging independently to produce biochar as the food packaging is consisting of synthetic polymer. However, the pyrolysis of food waste that is mixed with the plastic packaging could probably tune the surface properties of the biochar due to the polymeric attributes of the packaging. Yet, the optimum mixture ratio of packaging waste and food waste still needs to be discovered as an excessive amount of packaging waste will reduce a small amount of biochar due to the carbon content. Furthermore, with various components in mixed food waste, there are different effects on the production of biochar as it is based on the nature of the food waste itself. For instance, a certain type of agricultural waste-based biochar often has higher carbon content than biochar produced from kitchen food wastes [37]. The different elemental compositions in mixed food wastes greatly influence the potential of biochar implementation as carbon sources for the electrode material development.

Production technologies of food waste–based biochar

Biochar is a well-known carbon material due to its tunable properties and abundant porosity apart from low price and environmentally-friendly properties. Thereby, the synthesis of food-waste biochar involves various approaches, ranging from a facile to an intricate technique. Basically, the appropriate selection of processing techniques for the food waste biomass will be significant in ensuring the desired physicochemical properties of the biochar can be attained. It is undeniable that different techniques and feedstock variations will produce different attributes and performances of biochar, specifically in this energy storage application. Indeed, Yaashikaa et al. [38] reported that in order to attain the maximum yield of biochar, the technique selected must be appropriate and depend on the type of biomass, as well as the optimum operating conditions, i.e., heating rate, temperature, operating time. Moreover, careful feedstock selection and optimization in the processing conditions are significant in ensuring the biochars’ stability and meeting the specific requirements [39]. On top of that, Elkhalifa et al. [40] reported that the selection of conversion method should maximize the desired products (outputs), while at the same time reduce the associated operational expenses and environmental burden.

Besides, surface modification on the carbon chain can plausibly enhance the electrochemical performance towards the desired reaction. For instance, carbon feedstock that is rich with certain minerals or heteroatoms (such as nitrogen, boron, phosphorus, and sulfur) is feasible, given that such characteristics can contribute to the biochar properties’ enhancement, whereupon it will positively affect the performance. Since production technology is known to hugely influence the biochar’s properties and corresponding performance, several thermochemical conversion technologies have been outlined for the food waste biochar production, with a strong emphasis towards the textural characteristics [41, 42]. The common thermochemical technologies for the biochar production that have been carried out thus far include combustion, gasification, and pyrolysis [42]. The working mechanism, benefits, and limitations (challenges) of each thermochemical conversion technology will be further discussed in the following subsection.

Combustion

Being the oldest method of thermochemical conversion, direct combustion takes up about 97% of the world’s bioenergy production [43]. Combustion is indeed considered the most basic and frequently employed technique, whereby the feedstock will be burnt in presence of oxygen to produce heat or electricity as the main product. The combustion process is basically defined as a chemical reaction between a fuel (hydrocarbon) and oxidizing agent, which then produces oxidized products such as carbon dioxide (CO₂), water (H₂O), and heat [44]. Simplified, the combustion process can be described by Eq. (1) [45].

$$C_x{H}_{y(s)}+O_{2(g)}\to H_2{O}_{(g)}+{\mathit{CO}}_{2(g)}$$ 1

In the context of the biochar production, the combustion process takes place at an operating temperature of 800–1000 °C [46]. During the combustion process, biomass (food waste) feedstock with a preferred moisture content of < 50 wt.% will be directly burnt in presence of air, and the stored chemical energy then will be converted to thermal (heat) energy [47]. In relation to food waste as the feedstock, biochar will be produced upon hydrocarbons’ emission during the reaction. Though, Amalina et al. [7] reported that the combustion process is unfavorable for the biochar synthesis as it converts most of the carbon in biomass to CO₂. Typically, combustion in a boiler produces about 1.5–2.0% of biochar [17].

Despite being the oldest thermochemical method, process upscaling to an industrial scale is challenging due to the operational and environmental issues. The problems are often attributed to the type of combustors that cause corrosion and fouling [42]. In addition, the combustion process also affiliates with the release of pollutant such as nitrogen oxide (NO_x), sulfur oxide (SO_x)_, and heavy metal into the environment [48]. Therefore, proper controlling measures must be implemented to prevent such releases into the atmosphere. Accordingly, several research studies had suggested that a pretreatment stage of feedstock (i.e., drying, shredding) prior to combustion could influence the process efficiency as compared to the direct combustion process [40, 49].

Gasification

Opposite to the combustion process, gasification occurs when biomass is burned in a limited oxygen environment, whereupon gas fuel or product gases are to be produced from an incomplete combustion of carbon feedstock. Gasification is a process where the carbonaceous materials are heated at an elevated temperature in between 700 and 900 °C in the presence of gasifying agents, i.e., air, steam, oxygen, nitrogen, CO₂, or a mixture of these gases, and will produce gas products as described in Eq. (2) [50, 51]. These gas products or also commonly referred to as synthesis gas or syngas, which consists of the mixtures of steam, CO₂, carbon monoxide (CO), hydrogen (H₂), and methane (CH₄) [52]. Nevertheless, syngas production and composition are subjected to variation in the reaction temperature, together with variation in particle size, elemental composition, ash content, moisture content, and volatile matter.

$${\mathit{CH}}_x{O}_{y(s)}+O_{2(g)}\to {\mathit{CH}}_{4(g)}+{\mathit{CO}}_{(g)}+{\mathit{CO}}_{2(g)}+H_{2(g)}+{\mathit{char}}_{(s)}+{\mathit{tar}}_{(s)}$$ 2

In gasification, biochar is one of the process by-products along with particulate matters, tars, ash, and oil. In the context of the food waste biochar production, the gasification process is unfavorable due to minimal biochar yield, merely about 10 wt.%, while the remaining are liquid (5 wt.%) and gases products (85 wt.%) [20, 53]. Therefore, there is a limited work on the potential of the gasification process to transform the food waste into biochar materials [54].

Pyrolysis (or carbonization)

The pyrolysis process that is well-established for biochar production has been performed at an elevated temperature of 250–900 °C under the absence of oxygen, where heat has been externally supplied to convert the organic material into solid, liquid, and gas products [38]. Throughout the process, the thermal decomposition of polymeric compounds in the food waste occurs in a series of temperature zone. With flexible operational conditions, the operating parameters such as heating temperature, heating rate, residence time, and gas flow rate are considered prominent conditions in the pyrolysis mechanism as well as the product (biochar) yield [47]. The pyrolysis process can be divided into two parts which are fast pyrolysis and slow pyrolysis. This term is varied due to different operating conditions involved that influenced the desired product. According to Sakhiya et al. [55], typical operational parameters for the fast pyrolysis are at the temperature of 300–1000 °C with a residence time of 5–30 s. Whereas for slow pyrolysis, 300–550 °C of heating temperature with residence time between min to days will be involved. Furthermore, total biochar yield for fast and slow pyrolysis is ranging from 10 to 30 wt.% and 25–35 wt.%, respectively [55]. Table 2 shows recent works on the difference in pyrolysis operating conditions of various food wastes and their corresponding biochar yield.

Table 2.

Recent work on pyrolysis characteristics of biochar production from food waste (2019–2022)

Food waste type	Pyrolysis conditions	Reactor type	Biochar yield (%)	References
Walnut shell	Temperature: 500 °C Heating rate: 15 °C/min Residence time: 1 h	Fixed-bed reactor	~ 30	[88]
Waste pomegranate peel	Temperature: 300 °C Residence time: 20 min	Tubular furnace	54.9	[89]
Food waste	Temperature: 400 °C Residence time: 30 min	Muffle furnace	60.03	[90]
Coconut flesh waste	Temperature: 350 °C Heating rate: 5 °C/min Residence time: 1 h	Tubular furnace	23.54	[91]
Pine nutshell	Temperature: 550 °C Residence time: 20 min	Fixed-bed reactor	34.11	[92]
Grape pomace	Temperature: 300 °C Heating rate: 10 °C/min Residence time: 2 h	Tubular furnace	55.1	[93]
Digestate food waste	Temperature: 500 °C Heating rate: 10 °C/min Residence time: 2 h	Tubular furnace	42.97	[94]
Rice husk	Temperature: 300 °C Heating rate: 20 °C/min Residence time: 1.5 h	Fixed-bed reactor	37.71	[95]
Coffee husk	Temperature: 350 °C Heating rate: 0.5 °C/min Residence time: 30 min	Muffle furnace	39.82	[96]
Raw food waste	Temperature: 600 °C Heating rate: 5 °C/min Residence time: 1 h	Fixed-bed reactor	28.4	[97]
Food waste	Temperature: 200 °C Residence time: 30 min	Mini batch reactor	57	[98]
Olive tree pruning residues	Temperature: 650 °C Residence time: 15 min	Continuous screw-based reactor	23.38	[99]

The food waste pyrolysis process embraced the food waste components that primarily consist of the lignocellulosic materials: cellulose, hemicellulose, and lignin. However, the degradation pathways of each component are distinct from each other, where there will be different degradation temperatures during the pyrolysis process [31]. During this process, the lignocellulosic components will undergo several processes such as depolymerization, decomposition, and cross-linking at specific temperatures that result in the biochar production. The reaction stages in the of food waste–based biochar production consist of moisture removal at 100 °C, hemicellulose degradation at 200–260 °C, cellulose decomposition at around 240–350 °C, and eventually, at a temperature of around 280–500 °C, breakdown of lignin occurs [23]. Hence, the whole pyrolysis process removes all volatile components of syngas (CO, CO₂, CH₄, etc.), thus, leaving behind solid residual of carbon materials (biochar). However, due to the complex physicochemical structure of food waste, there will be resistance for the degradation process at 400–700 °C, thus resulting in biochar with lower surface area, minimal pore volume, and low crystallinity properties [23]. Though such materials can still be applied in various environmental applications, food waste–based biochar is ought to be improved with an activation process or modification process (i.e., template, heteroatom doping). As such, this will enhance the physical properties such as surface morphology, specific surface area, and porosity that will positively influence the biochar’s performance as the electrode materials. Furthermore, it is noteworthy to mention that all the common thermochemical technologies are effective for certain materials with certain calorific value and moisture content, where these technologies require feedstock with less than 15% moisture content.

Since the food waste contains a high proportion of moisture content (> 80 wt.%), pyrolysis is reported to be non-promising due to the high energy penalty for the preliminary drying stage [56]. Specifically, Kaur et al. [57] reported that drying of the food waste consumes energy of about 7.75 MWh per ton. Thus, hydrothermal pyrolysis (or carbonization) is deemed more feasible, given this process is taken place in presence of water, and no drying stage is required. Fundamentally, the hydrothermal carbonization process that takes place in an autoclave reactor at a temperature of 120–260 °C and residence time of 5–240 min [19, 38] is a relatively new process to convert the moisture-rich food waste feedstock to char production.

Process conditions

The process conditions of the pyrolysis process have significant impacts on the production of food waste–based biochar and its characteristics. With the aim of achieving biochar products with specified yield and quality, process conditions can be altered to tailor the needs. Thereby, process parameters such as heating temperature, residence time, heating rate, and particle size of feedstock greatly influence the quality of biochar’s materials. These process parameters are the key role in enhancing the biochar’s performance which subsequently produces good electrode materials. Throughout the process, operating temperature plays a major control in the biochar yield and chemical properties as compared to the residence time, heating rate, and particle size [33]. As the temperature increases in pyrolysis process, the biochar production tends to be negatively affected due to the high thermal cracking of hydrocarbons, which leads to the increase of liquid and gas products instead of the solid [31]. Opatokun et al. [58] reported that as the temperature varied from 300 to 500 °C, there is almost a 22.5% reduction in the biochar yield of food waste during the pyrolysis process. At high temperature, biochar yield may be decreased but its physicochemical properties, i.e., pH value, specific surface area, and ash content, are increased [31]. For instance, the process temperature from 250 to 700 °C has a positive effect on the biochar surface area from 10 to 500 m²/g, but with a further increase of temperature to 800 °C, the BET surface area notably reduced to 150 m²/g [59]. There are many recent studies reported on the effect of temperature on the biochar yield; however, it is a challenging task to conclude the optimum temperature for production of food waste–based biochar, as it is highly dependent on the selection of food waste as feedstock [55, 60].

The heating rate is another delicate parameter in the process conditions of pyrolysis for biochar production. Typically, it is either a fast or slow pyrolysis process. The heating rate influences the yield of biochar and its physicochemical properties as well. Higher process temperature alongside faster heating rate results in the increase of syngas and bio-oil production, whereas lower process temperature and slower heating rate result in the increase of biochar yield. The formation of porous structure in the biochar matrix during the organic matter devolatilization from the biomass feedstock is reliant on the heating rate and process temperature [61]. Mohamed et al. [62] reported the pyrolysis of cassava wastes from 400 to 600 °C at different heating rate of 5 to 25 °C/min, in which a significant reduction in the biochar’s yield and surface area. Basically, such phenomenon is attributed to high heating rate and high process temperature that causes the melting of cell structures. Meanwhile, in contrast to a research finding by Mohanty et al. [63], increasing the heating rate on wheat straw for the biochar production resulted in a slight increase in surface area from 178 to 184 m²/g. Nevertheless, Chen et al. [64] observed no significant trend in the effects of pyrolysis heating rate for the biochar production at 10, 30, and 50 °C/min. Thus, the effects of heating rate on the biochar production are inconclusive and further thorough study is required.

In order to achieve the optimum biochar yield, longer residence time in the pyrolysis process (ranging from a few minutes to hours) is favorable as it is affecting the rate of removal of organic material. Horsfall et al. [65] reported that in maximizing the tar decomposition, high residence time and low temperature are appropriate. During the pyrolysis process of biomass conversion at high residence time, heat transfer on the materials increases, which then leads to an improvement in biochar yield. The optimization of residence time, in line with other parameters, may assist the production quality of biochar. Other process parameters that influence the production of biochar are the particle size of biomass (food waste) during pyrolysis. The effect of particle size on biochar product determines the residence time of the particles inside the reactor. If the smaller particle size of organic matter has been used, the heating process released more volatile matters, thus lead to lower amount of biochar yield but contributed to higher amount of bio-oil production as well as syngas [60].

Physiochemical properties of food waste–based biochar electrode materials

The pyrolysis process has an influence on the characteristics of food waste–based biochar as it mainly determines the composition and physicochemical properties. It is a prerequisite requirement for electrode materials’ fabrication, while structural exploration is crucial for energy application. Physicochemical properties of food waste–based biochar include the BET surface area, surface functionalization, and other properties related to the adsorptive abilities of the materials. The production of biochar typically produced the specific surface area within the range of 1.5 to 500 m²/g [66]. The change in the surface area and porosity of biochar is mainly controlled by the pyrolysis temperature, apart from the selection of biomass feedstock. The specific surface area value increases as the pyrolysis temperature increases within a certain range. At high temperatures, during the thermal decomposition of biomass, the release of volatile matter results in the reduction of pore size but increases the number of pores in the synthesized biochar. This phenomenon results in the microporous structure and high specific surface area. The increase in surface area and pore volumes on the amorphous carbon structures during the pyrolysis process occurs throughout the progressive decomposition of celluloses and hemicelluloses in food waste feedstock. However, as the temperature increases to a certain point, the specific surface area of biochar will reach a plateau. Furthermore, when temperature exceeds a critical value, excessive carbon burn-off results in the destruction of a microporous structure [67]. In investigating the performance of electrode material surface area, it is significant to promote electrolyte ions transfer into the carbon structure. Wang et al. [68] reported that by increasing the BET surface area of carbon materials from 621 to 2685 m²/g, the specific capacitance of electrodes improved significantly from 17.68 to 171.2 F/g, respectively. Figure 2 illustrates the relationship between the specific surface area of electrodes with their capacitive value. Other factors like pore size distribution may influence the electrolyte ion transfer with the carbon electrode materials. According to Arenillas et al. [69], carbon material with 0.8 nm pore size is suitable for organic electrolytes while 0.4 or 0.7 nm is useful for aqueous electrolytes due to the difference in the efficiency of pore filling based on ion sizes of the electrolytes. In a study by Raza et al. [70], carbon materials with a pore size of < 0.7 nm are beneficial in achieving high specific capacitance in an aqueous electrolyte as it exceptionally promotes the ion transport, therefore, achieving high capacitive value associated with integration between the pore size of carbon electrode materials and the electrolyte ion size [71]. Surface functionalization of biochar results in an increased Faradaic redox reaction with an enhancement of a 5 to 10% increment in the specific capacitance value [72]. Typically, oxygen-containing functional groups are reported to influence the capacitance value of activated biochar by being electrochemically active [73]. The mechanism of oxygen-functionalized electrode materials for the supercapacitor relied upon the reversible oxidation–reduction of hydroquinone or quinone groups [74]. Particularly, surface oxygen-containing functional groups can enhance the wettability, considering their hydrophilic nature between the electrode materials and electrolytes.

Fig. 2.

The relationship between the BET surface area (m²/g) and pore volume (cm.³/g) with specific capacitance (F/g) [127]

Food waste–based biochar needs to be properly activated to achieve a high capacitive value of electrode materials. Typically, surface modification of the biochar is done with either physical or chemical activation in producing the activated biochar. Activated biochar often generates a higher surface area due to the presence of activating agent that is acting upon the initial chemical compositions of biochar. The enhancement in activated biochar is usually seen in the improvement of physicochemical properties and electrochemical performance. The porous network on microporous or mesoporous structures of activated biochar remarkably improves the surface area and the specific capacitance. Acid treatment on surface modification of biochar increases the surface oxygen-containing functional groups, which then promotes the pseudocapacitance through redox reactions of carbonyl-surface in the oxygen-containing functional groups [75]. The biochar-based electrode materials with microporous structures can generate high specific capacitance of more than 200 F/g [21].

It is evidenced that the food waste biochar for electrode materials through surface modification displayed a high capacitive value. Yuan et al. [76] reported that electrode material from corn stalks with nitrogen-doped modification produced a high specific capacitance of 203.5 F/g at 1 A/g with a specific surface area of 1136.31 m²/g. Potato waste–based biochar was activated with ZnCl₂ and melamine as nitrogen dopants and the specific capacitance is as high as 255 F/g in 2 M KOH electrolyte [77]. The carbon material circulated 5000 times under a current density of 5 A/g with a cycle efficiency of 93.7%. An interesting finding has been made recently, whereupon waste coffee powder–derived electrode has capability to produce high power and energy density of 6640 W/kg and 1280 Wh/kg, respectively [78]. Such improvement is primarily attributed to melamine usage as a nitrogen-rich precursor, in which the synthesized electrodes can work up to 10,000 cycles, while maintaining columbic efficiency of 100%. Besides, improvement has also been found in expired milk that consists of casein and serum, which act as a natural source for doping agents of nitrogen and sulfur atom. Specifically, Dat et al. [79] reported that the highest N and S elements of 2.09% and 0.81%, respectively, are capable to increase the specific capacitance up to 186.3 F/g. Table 3 summarizes the food waste–based activated biochar as electrode materials in a supercapacitor for the energy storage application. Overall, the waste-to-electrode materials help to reduce the solid waste problems and at the same time, to minimize the production cost.

Table 3.

Summary of food waste–derived activated biochar electrodes for supercapacitor applications from 2016–2022

Year	Biochar precursor	Activation	Specific surface area (m2/g)	Specific capacitance (F/g)	Energy density (Wh/kg)	Power density (W/kg)	CS (cycle)	References
2016	Banana-peel	-	194	0.047	40.7	8400	1000	[28]
2017	Orang-peel	KOH	2521	407	100.4 × 10−6	6.87 × 10−2	5000	[100]
2017	Used tea leaves	KOH	2532	292	20	2560	5000	[101]
2019	Waste coffee powder	Melamine	1824	148	1280	6640	10,000	[78]
2020	Expired fresh milk	KOH-H3PO4	465	186.3	5.4	119	1000	[79]
2020	Spent coffee grounds	-	492	200	-	-	-	[102]
2020	Sapindus Mukorsossi peel	KOH	1254.5	314.5	6.68	3250	5000	[103]
2021	Garlic peel	KOH	3325.2	424.4	-	-	5000	[104]
2021	Pomegranate	KOH	2189	190	68	11,316	10,000	[105]
2021	Waste boiled rice	-	1505	158.6	-	-	2000	[106]
2021	Peanut shell	KCl		355	883	141 × 10−2	5000	[107]
2021	Tamarind shell	KOH	-	245.03	-	-	7000	[108]
2021	Shrimp shell	KOH	401	201	-	-	1000	[27]
2022	Melon seed shells and peanut shells	-	1367.2	533.7	69.7	780	-	[109]
2022	Jicama peel	ZnCl2 and H2SO4	-	179	12.60	105.86	-	[110]
2022	Salam leaves	ZnCl2	-	166	15.77	86.02	-	[111]
2022	Shrimp shell waste	HCl	1508	81.7	11.23	248.8	10,000	[112]
2022	Date seeds	NiCl2	234	508	-	-	10,000	[113]
2022	Watermelon rinds	NiFe2O4	-	374	28.33	750		[114]

^*CS, cycle stability; KCl, potassium chloride; KOH, potassium hydroxide; ZnCl₂, zinc chloride; H₂SO₄, sulfuric acid; HCl, hydrochloric acid; NiCl₂, nickel chloride; NiFe₂O₄, nickel ferrite

Current method of food waste based-biochar electrode fabrication

The device flexibility can be increased by reducing the thickness of the electrode or replacing the rough metal oxide with soft polymer active material [80]. However, to produce electrodes in a large-scale production, a thin film is not highly suitable due to high production cost even though it is quite established in a laboratory scale. Due to technological advancement, the demand for the flexible energy storage devices such as battery, supercapacitor, and fuel cell has sparked the interest among researchers to produce a working electrode that is not only cost-effective and stable but also soft (bendable) and lightweight. Thus, in this section, several electrode fabrication techniques for the energy storage application and their advantages will be further discussed. The fabrication process of the electrode can be divided into several methods, including coating, paving, soaking, spraying and sputtering, 3D-printing, and electrochemical deposition and plating. Herein, the selection of fabrication technique mostly depends on the application of electrode and reactor design. For example, the production of a commercial battery uses a coating method to fabricate the electrode and deposit the active material onto the substrates (current collector). In contrast, the production of a thin-film lithium-ion battery through the chemical deposition does not require any substrates, as it uses solid electrolyte material such as zirconia-based material. Hence, it is crucial to determine the properties of the active material and reactor design first, prior to deciding the fabrication technique. The comparison of a different current collector (substrates) and its flexibility is tabulated in Table 4.

Table 4.

Comparison of different current collector materials for energy storage device, with satisfactory electrochemical stability

Materials	Cost [$/m2]	Density	Flexibility	References
Carbon cloth	~ 650	Low	Soft	[115]
Carbon paper	~ 875	Low	Soft	[116]
Stainless steel cloth	~ 160	High	Soft	[117]
Ni mesh	~ 150	High	Hard but bendable	[82]
Ni foam	~ 73	High	Hard but bendable	[118]
Stainless steel mesh	~ 9	High	Hard but bendable	[119]

Based on the comparison, carbon cloth has higher flexibility characteristics with good electrochemical stability. Despite higher purchasing cost as compared to stainless-steel mesh, its high performance and efficiency have the potential to compensate for the high costing. Moreover, the selection of the fabrication method is also dependent on the types of the active material as well as the substrate used. For instance, for porous substrates, i.e., Ni foam or sponge-like materials, soaking is the most promising technique since a uniform deposition can be obtained and does not destroy the porous or hollow characteristics. Summary of the electrode fabrication methods is summarized in Table 5 and Fig. 3.

Table 5.

Current research findings on the fabrication method of food waste–based biochar as electrode materials

Active material	Substrate	Fabrication method	Solvent/other additives	Mixing ratio	References
Mantis shrimp shell	Ni foam	Paving	PTFE/acetylene black	8:1:1	[27]
Toreyya grandis inner shell	Ni foam	Coating	PVDF/carbon black	8:1:1	[120]
Peanut shell	Ni foam	Coating	-	-	[107]
Pomelos	Carbon paper	Coating	PVDF/acetylene black	8:1:1	[81]
Acacia leucophloea wood sawdust	Ni foam	Paving	PVDF/carbon black	8:1:1	[121]
Spent coffee ground	Carbon cloth	Coating	Carbon black/nafion	7:2:1	[102]
Sugarcane bagasse	Graphite plate	Microwave-assisted	PVDF/DMAc	9:1:40	[122]
FW (cooked rice/fruit/fish/bone/meat)	FTO films	Sputtering	Nafion/ethanol	5:0.01:1	[123]
Sewage sludge/soft wood pallets/rice husk	Ti foil	Coating	Nafion/water-isopropanol	4:0.4:1	[124]
Potato peels	Graphite sheet	Coating	PVDF/carbon black	8:1:1	[125]
Kudzu vine	PI film	3D-print	Nafion/isopropanol	20:1:4	[87]
Waste boiled rice	GCE	Paving	Ethanol/nafion	3:0.2:0.02	[106]
Hibiscus sabdariffa sticks	Ni mesh	CVD/paving	PTFE/carbon black	8:1:1	[126]
Tamarind shells	Ni foam	CVD/coating	PTFE/carbon black	8:1:1	[108]
Expired fresh milk	Stainless steel mesh	Hydrothermal/paving	PTFE/carbon black	4:1:5	[79]
Shrimp shell waste	Ni foam	Coating	PVDF/carbon black	75:15:10	[112]
Date seeds	Ni foam	Drop-casting	Nafion/carbon black	8:1:1	[113]
Watermelon rinds	Ni foam	Drop-casting/paving	PVDF/carbon black	75:15:10	[114]

Fig. 3.

The schematic diagram of physical and chemical fabrication methods. A Coating, B paving, C soaking, D spraying and sputtering, E 3D printing, and F electroplating and deposition

Coating

Coating is one of the well-known methods among researchers due to its process simplicity and effectiveness. Often, this coating technique is implemented as a post-synthesis step as most of the active materials that have been synthesized via hydrothermal or microwave-assisted method are in powdered form. Herein, the powdered sample is mixed with conducting additives and binder to produce a slurry, followed by a drying step. For example, Li et al. [81] previously prepared a working electrode by mixing the pomelo based-biochar (active material) with acetylene black (conducting additives) and PVDF (binder) in a ratio of 8:1:1. The mixing process is usually carried out by using an agate mortar with the assistance of solvents, i.e., N-methyl pyrrolidinone. However, Othman et al. [82] recently mentioned that the coating method via brushing has an unfavorable effect since the catalyst loading can be plausibly reduced. Moreover, specially designed material such as hollow structure will not be suitable for this coating method as the grinding process to convert the active material into powder form may destroy the original structures.

Electroplating (electrodeposition) coating

Electroplating or deposition is a fabrication technique in which a solid or thin film will be deposited on the surface of the substrates through the electrical movement of ions inside the electrolyte. The chemical reaction of electroplating (or deposition) follows the electrochemistry principle, in which a higher potential difference (voltage) between two materials will result in the spontaneity of the plating reaction. Compared to other methods, electrodeposition and plating offer more control on the morphology of the catalyst layer, as the reaction is highly influenced by current density, temperature, and concentration. Moreover, such process does not require any complex equipment and can be easily acquired in a single-step synthesis including the mixed ion solutions.

Paving

Paving that is used for thin film materials paved on the substrate or current collector is usually implemented to an active material that possesses low mechanical stability. Briefly, this paving technique can be done through hot-pressed or compressed under high pressure on the substrates. Huang et al. [27] fabricated shrimp-shell biochar electrode by pressing the dried film of (1 cm × 1 cm) squares on (1 cm × 2 cm) foam-nickel under 10 MPa for 30 sec. Other facile and inexpensive paving method is through a “doctor blade method,” which was invented by Glenn Howwatt back in the 1940s [83]. Using this method, the thin film is produced by preparing a slurry and placing it in front of the blade that will be moved linearly along the Mylar sheet, and to form a film behind it. Such processing technique is promising due to the feasibility of laminating a variety of green tapes to produce a double-layer dense/porous composite. Though, this paving method requires a strong adhesive strength, wherein substrate must be mechanically strong to ensure the stability of the electrodes’ framework.

Soaking

The soaking or infiltrating method is best applied to a liquid or solution of active materials. In this process, the substrates are dipped inside the liquid to ensure a uniform deposition of the catalyst on the substrates’ surface. For porous substrates, this technique can be easily done as the solution will be absorbed into the pores of the substrates during the dipping process and will produce a thin layer upon drying. Moreover, the formation of a thin layer on the porous substrates is significant as the high viscosity of the coating will cause pore blockage and reduce the surface area. On the other hand, for non-porous substrates, the solution will not infiltrate the substrates; rather, it forms a uniform thin layer while maintaining the original shape of the substrates. While Wang et al. [80] reported that both the thickness and uniformity of thin layer are difficult to be controlled, Ge and He [84] found that the dipping layer-by-layer method is effective to control the catalyst loading rate and produce a more homogenous surface. However, the main disadvantage of this methodology is that the process is rather time-consuming, as one needs to repeat the coating several times in order to obtain an appropriate thickness.

Spraying and sputtering

Spraying and sputtering are another fabrication techniques that are much simpler and faster and are considered one-step synthesis in producing the electrode material. According to Wang et al. [80], sputtering is a process where the active particles are ejected onto the solid substrates due to the bombardment of the substrates by charged particles. Correspondingly, spraying techniques use a solution of active materials that are sprayed onto the solid substrates. In this technique, the thickness and uniformity of the films can be easily controlled by adjusting spraying conditions, including time, speed flow, and layer. Seo et al. [85] found that a thin and uniform multilayer film could be achieved through spin-assisted sputtering without affecting the wetting behavior of the electrode materials, as compared to dip-assisted films. Nevertheless, high cost and the equipment complexity are the main factors that hinder process applicability.

3D printing

3D printing or known as an additive manufacturing technique has been considered the latest technologies in the electrode fabrication. Herein, the printer works by building a 2-dimensional or 3-dimensional object in a layer-by-layer manner, and such process is reported to effectively reduce the processing difficulty and cost to manufacture complex structure. In addition, the researchers can freely design and customize the targeted shape of the electrode. On top of that, the type of material for 3D printing is extensive, which includes metal, plastic, or composite material [86]. Recently, Zhang et al. [87] had successfully fabricated a porous biochar electrode through a laser direct writing method that possessed an excellent electrocatalytic electrode performance. Nevertheless, due to the recentness of such technique, further work will be needed to improve the morphology of the electrode.

Conclusions and outlook

This paper provides a comprehensive review on the role of food waste–based biochar as an advanced electrode material in electrochemical energy storage. Specifically, an emphasis has been given to the food waste overview, globally as well as in Malaysia’s context, current progress in the food waste utilization as the electrode material, as well as the thermochemical technologies and electrodes’ fabrication method. Food waste has enormous prospective to be utilized as biochar due to its huge availability and biodegradability. Industrialist and environmentalist could benefit from the advancement in the processing technologies as it will assist in the waste mitigation by conversion to value-added biochar product. Due to the tunable properties of biochar, the desired characteristics of biochar for energy storage device application could be produced by an additional activation method, in order to modify carbons’ configuration and further promote ion exchange capacity. Besides, multiple pre-treatment techniques inclusive of particle size reduction, alkaline-, thermal-, and ultrasonic-processing method may be introduced to improve both porosity and specific surface area of the biochar. The pre-treatment process also aimed to reduce the energy required to process wet food waste and to increase the yield of the biochar products. Moreover, these pretreatment techniques are required to segregate various impurities in the food waste, which then increases the processing costs [57]. Due to the superior performance of food waste–based biochar in the supercapacitors’ field, continuous development in the energy applications field shall be further carried out, in parallel to Yaashikaa et al. [38] who reported that the biochars’ utilization as supercapacitors still require more attention. Future studies on the optimization of food waste as biochar and properties are imperative to address the existing knowledge gaps and limitations especially on the post-consumer food waste. Moreover, future work on the optimization of food waste is noteworthy, given that the food waste has a heterogeneous structure, which then leads to the biochar production with varying chemical structures and properties [56]. Indeed, each food waste conversion comes with its own associated process conditions as well. Though a wide range of biochar production methods has been introduced in literatures thus far, a proper classification is not available yet.

The following research directions include the following:

• Templating the post-consumer food waste to produce biochar with desirable properties for the energy storage application as well as strategy for the commercialization

• Surface modification of existing biochar with intrinsic textural properties that could further promote the capacitive value in energy storage application. In the context of synthesis and surface modification, the conceptual framework of green chemistry should be upheld in a way that green chemicals and process simplicity to be followed. Overall, it is significant to integrate green and sustainable design processes for an effective energy storage

• The techno-economic analysis at the pilot and industrial scale of food waste processing is important. Thereby, more efforts on economic analysis of the biochar production along with the electrode fabrication via different bio-substrates should be carried out

• Performing the full life cycle analysis; integrating the food waste biochar production, electrode fabrication, and in the electrochemical system. On top of that, emphasis on the reuse, stability, and post-treatment of biochar-based electrode should be carried out.

Author contribution

Intan Syafiqah Ismail: conceptualization, visualization, data curation, formal analysis, writing—original draft. Muhammad Farhan Haqeem Othman: writing—original draft, visualization, formal analysis, data curation. Nor Adilla Rashidi: conceptualization, writing—review & editing, validation, supervision. Suzana Yusup: conceptualization, validation, supervision.

Funding

This work has been financially supported by the Ministry of Higher Education Malaysia (grant no. 015MA0-052).

Data availability

All data generated or analyzed during this study are included in this review article.

Declarations

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Conflict of interests

The authors declare no competing interests.