Energy generation from biomass represents a promising alternative to the use of fossil fuels and it will play a pivotal role in achieving the net-zero carbon target by 2050. Biomass refers to the organic material that is obtained from living organisms. The most common route for exploiting the energy contained in biomass is through the combustion of its components. This type of energy is also commonly referred to as “bioenergy”. Bioenergy is considered carbon neutral, since its use does not, in principle, lead to the accumulation of net carbon in the atmosphere, since biomass is generated through the fixation of CO₂ by plants. Moreover, the natural decomposition of agricultural or forestry residues releases CO₂, even if they are not used for energy generation [1]. Consequently, this energy source has the potential to reduce the use of underground fossil fuels. Although within the past few years new models and policies have been developed to take into account the release of greenhouse gases as a consequence of the use of fossil fuels for the production and refinement of biomass, the release of CO₂ by the burning of biomass itself is often disregarded [2]. The use of plant residues for the generation of energy still leads to a significant release of CO₂ to the atmosphere [3], which breaks with the assumption of neutrality from biomass resources [1]. In addition, the increased use of biomass resources for energy production through combustion will inevitably lead to a transformation of natural ecosystems [4]. Thus, alternative routes for energy generation that make use of biomass materials without the need for combustion and that can be reused for long periods of time are needed.

The development of alternative sources of energy, such as solar panels and wind energy, represents a promising alternative to mitigate CO₂ production from fossil fuel consumption, with a high-power yield. However, to allow for a truly renewable energy source, devices must not only involve materials and systems that can harvest energy in an efficient way, but also environmentally friendly materials that do not lead to significant environmental damage during production. The fabrication of some of the components used in renewable energy devices often involves the use of non-sustainable or toxic compounds. An example of these compounds is silicon tetrachloride, which has been shown to have a high environmental impact due to its toxicity [5]. On the contrary, a synthesis of plant-derived biomaterials could theoretically be achieved through “carbon negative” methods, which lead to a storage of CO₂ from the atmosphere given the natural ability of plants to fix carbon. Thus, the incorporation of biomass-derived materials in the fabrication of these renewable energy devices shows a promising approach that could further save environmental and power costs, while overcoming the limitations of biomass combustion.

One of the most common biomass-derived materials is cellulose [6], which is produced at a scale of 1.5 × 1012 tonnes per year [7]. This material is by far the most studied biomass compound in energy research given its low cost and chemical versatility, and its material shows advantages compared with traditional materials, which enables its incorporation in multiple applications. Cellulose gels show a high porosity and specific surface area, which can enhance energy generation through surface phenomena such as triboelectricity. Moreover, it is a flexible material, with a good ion conductivity when hydrated, which improves the use in portable [8], wearable devices compared with traditional materials. This material can be further modified chemically to enhance power generation through chemical oxidation or click chemistry among others.

Cellulose is an oligosaccharide, formed by D-glucose units, covalently attached by β(14)-bonds. This material is widely available, without the requirement for expensive synthesis or the use of hazardous chemicals. In addition, it can be extracted using low-cost and environmentally friendly methods, either from biomass ^[9][10][11][9,10,11] or by bacterial production ^[12][13][14][12,13,14]. Unlike most petroleum-based products, cellulose represents a renewable and biodegradable option, with a low environmental impact. Thus, in recent years, the applications of this biopolymer have expanded, and the number of scientific papers and patents being filed in this field have shown a high increase [15].

Cellulose is generally insoluble, with good mechanical properties, allowing for its use as a reinforcement material either in the form of nanocomposites ^[16][20] or hybrid materials, with the incorporation of other polymers ^[17][21]. In addition, it shows a high biocompatibility, enabling its use in biomedical applications such as wearable sensors ^[18][19][22,23] or implantable devices ^[20][21][24,25]. Natural cellulose can be found in the form of microfibres and presents crystalline domains due to the formation of strong hydrogen bonds between the molecular chains ^[22][26].

2. Energy Generation from Mechanical Movement

2.1. Piezoelectric Generators

Mechanical energy is one of the most ubiquitously available forms of energy in the environment. As such, it represents a large source of renewable power. The piezoelectric effect was first described in quartz crystals by the Curie brothers in 1880. Since then, multiple materials, such as barium titanate, tourmaline, and Rochelle, have been discovered and tested ^[23][58]. Piezoelectric generators can produce a change in the electrical polarisation of the active materials as a consequence of an induced mechanical stress. Cellulose nanocrystals can produce piezoelectricity due to their particular structure, presenting a noncentrosymmetry, with two different hydrogen bonding networks ^[24][25][60,61]. This effect was first reported in 1954 by Fukada ^[26][62], who observed the piezoelectric effects on the annual rings of wood. Since then, multiple devices have been designed to exploit this effect using cellulose-based materials. This material has been employed either as a piezoelectric generator or in the form of a nanocomposite, with nanoparticles embedded. The power yield of the devices based on natural cellulose alone tends to be low compared to commonly used organic materials in piezoelectric generators such as polylactic acid ^[27][63] or poly(vinylidene difluoride) (PVDF) ^[28][64]. In the case of PVDF, a power output of 112.8 μW has been reported by Song et al. ^[29][65], being similar to current ceramic-based harvesters. However, the poor yield of cellulose in comparison is reflected by a low piezoelectric coefficient, which is related to the strength of the piezoelectric effect on the material and indicates the polarisation on the electrodes obtained upon subjecting the material to a mechanical stress. The use of vertically aligned cellulose nanocrystals has been demonstrated to enhance this piezoelectricity generation ^[30][66]. The increase in energy generation is a consequence of the large dipole moment of cellulose within the cellulose chain direction ^[30][66]. Vertically aligned CNC films achieved a piezoelectric coefficient of 19.3 ± 2.9 pC/N, which is similar to the one observed in PVDF, in the range of 20–30 pC/N ^[31][67]. This piezoelectric coefficient was 50 times higher than the longitudinal piezoelectric coefficient observed in wood cellulose fibres, which has been reported to be 0.4 pC/N ^[32][68]. However, the formation of vertically aligned cellulose nanocrystals required the use of a DC voltage of 5 kV, reducing the scalability of the fabrication process industrially. The piezoelectric coefficient of cellulose can be further increased to up to 210 pm/V when used in the form of ultrathin films of aligned nanocrystals ^[33][69]. This value is similar to the one obtained by piezoelectric metal oxides and can be obtained by fabricating the films using a uniform electric field onto a mica substrate. Despite the high piezoelectric coefficient achievable by cellulose nanocrystals, especially by aligned nanocrystals, the expensive fabrication and processing involved make it an impracticable technology within a commercial setup. As such, alternative materials that require a lower degree of chemical processing are desired to reduce the production costs and improve the eco-friendliness of the devices. The power output of piezoelectric generators can be further increased by combination with triboelectricity, enabling a conversion of mechanical energy into electricity. This concept was applied by Shi et al. ^[34][80], who developed a cellulose/BaTiO₃ aerogel, increasing the power of the final device from 11.8 μW, when only piezoelectricity was harvested, to up to 85 μW. Thus, the harvesting of mechanical energy through piezoelectricity represents a promising alternative for the powering of devices. However, current piezoelectric generators are limited due to their relatively low power generation and the necessity for a mechanical pressure. As such, new approaches have been developed to enhance this generation of power. A summary of recent approaches in power generation using biomass-derived materials is shown in Table 1.

2.2. Triboelectric Nanogenerators

The concept of triboelectric generators was first reported by Prof. Zhonglin Wang in 2012 ^[45][85]. These devices can transform mechanical energy into electricity, even in small amounts of movement. The discovery of this form of energy opened up a new field in renewable energy harvesting and within 12 months of their discovery, the power output of the devices had been improved by 5 orders of magnitude ^[46][86]. Power generation through triboelectricity takes place by an electron exchange from an electron-donating material to a charge-trapping layer. As such, this method can combine the energy generated through contact electrification and electrostatic induction and is dependent on the total surface area ^[47][87]. When two materials with different surface potentials are put into contact, there is an electron movement that generates an electrical output ^[48][88]. Common strategies for increasing the power output of triboelectric generators focus on increasing the charges on the active materials or the surface contact between the electrodes. Materials such as modified fluorinated ethylene propylene through ionised gas injection ^[49][89] and plasma-treated PDMS ^[50][90] have been employed with high output yields. However, these chemical treatments are energy intensive and their scalability for industrial purposes is low. The incorporation of cellulose as an active material in energy generation through piezoelectricity or triboelectric nanogenerators represents a promising approach to sustainable power generators. As detailed in the previous section, cellulose presents a piezoelectric behaviour due to its crystalline structure. In addition, among all the available biomass-derived compounds, cellulose represents the most widespread material applied to triboelectric generators. Cellulose materials are positively charged in triboelectric terms due to the high amount of oxygen atoms within their molecular structure ^[51][91]. This chemical composition allows for the loss of electrons without requiring a high energy. This property of pure cellulose is reflected in a high frictional contact charge transfer, in the range of −130 μC m⁻², being higher than the average value for common polymers ^[52][19]. The first cellulose-based triboelectric generator was reported by Yao et al. in 2016. The cellulose was first oxidised with tetramethylpiperidine-1-oxy and paired with fluorinated ethylene propylene as the negative triboelectric layer ^[53][92]. The differences in the polarity between both the treated cellulose and the fluorinated ethylene propylene allowed for its use without the need for any additional material. In addition, an energy harvesting device combining the tribo- and piezoelectric capabilities of cellulose has been reported, with a power output as high as 10.6 μW·cm⁻² for the triboelectric generation and 1.21 μW·cm⁻² for the piezoelectric side ^[42][82]. This piezo- and triboelectric hybrid system could also be used as a pressure sensor with a low detection limit of 0.2 N cm⁻². This device combined a wood-derived nitrocellulose by modification using nitric acid and a piezoelectric nanocomposite containing BaTiO₃/MWCNTbs with bacterial cellulose. However, the power output of power generators can be further improved by focusing on strategies to increase the triboelectric output. Another biomass-derived material that has shown potential for its incorporation into triboelectric generators is lignin. Despite its poor electrical conductivity, the electron-transfer capabilities of lignin could be exploited by placing this material in contact with charge-trapping films such as Kapton ^[54][98]. Given the high stiffness and insolubility of lignin, this material had to be combined with starch, and led to a power density of 173.5 nW cm⁻². This value resulted in a lower value than the ones observed in cellulose, which explains the scarce amount of approaches exploring lignin as an energy material within the literature. Although the applications of biomass-derived materials, especially cellulose, are well established within the literature, the overexploitation of this resource could enhance deforestation rates ^[55][99]. Consequently, the market for alternative sources for cellulose generation, such as bacterial cellulose, have experienced a high growth within the last few years ^[56][100]. However, there is still great interest in the exploitation of whole plants for energy harvesting, which could improve the sustainability of the devices. As mentioned, power generation through triboelectricity is dependent on the surface area of the employed electrodes ^[47][87]. As such, to achieve a good performance, the materials have to be processed through lithography ^[57][101] or electrospinning ^[58][102]. These methods increase the production costs, limiting their commercialisation. Some plant organs, such as leaves and petals, present a large surface area due to their structure, with a hierarchical porosity and high roughness. In the case of leaves, the presence of cuticle and stoma cells can increase the surface area by up to 170 m² g⁻¹ for tobacco plants ^[59][103]. This surface area value is similar to and even larger than other engineered materials already incorporated as triboelectric generators such as cellulose hydrogels ^[60][61][62][97,104,105]. As such, plant leaves have been employed as templates for the fabrication of high surface area electrodes for the generation of triboelectricity using PDMS ^[63][106]. PDMS elastomer can be used as a negative friction layer in triboelectric generators and it can be patterned using laser ablation to enhance the surface area. Unmodified plant leaves have a low electron affinity, which can also be exploited to generate a current when it is placed into contact with materials with higher electron affinities such as poly-methyl methacrylate (PMMA). Jie et al. ^[64][110] compared the performance of different leaves from multiple tree species and obtained a maximum output power of 45 mW m⁻² in the case of Hosta leaves. This power output resulted in higher yields than in some of the nanocomposite-based approaches reported. The energy yield of these plants could be greatly improved by the modification of leaf powder with poly-L-lysine (PLL) ^[65][111]. By applying this method, a maximum of 17.9 mW was obtained, which was enough to power a group of 868 LEDs. Thus, the use of whole plants in energy generation has been proven to be a promising alternative for the development of triboelectric generators with a high yield. However, the performance of this approach still cannot meet the high power output of cellulose nanofibres and commercially available devices.

3. Osmotic Energy Harvesting with Cellulose

Whilst the generation of electricity through mechanical energy has been shown to be an efficient way to harvest electrical energy, especially for the powering of wearable devices through human motion, this approach cannot meet the current energetic needs that would enable a reduction in non-renewable energy sources. Thus, alternative renewable forms of energy are required. In recent years, osmotic energy has been developed as an alternative form of power generation. This type of energy is collected from the differences in concentration between two electrolyte solutions. Given the high abundancy of naturally occurring mixtures of fresh and salt water on Earth, especially at the estuaries, it has been estimated that, potentially, up to 2 trillion watts from the environment could be harvested using this technology ^[66][114]. The most common method used to generate electrical power through salinity changes is reverse electrodialysis. The first demonstration of this type of energy was reported by Pattle in 1955, using a stack of 94 polyethylene and polystyrene membranes, which generated an external power of 15 mW ^[67][115]. Since then, the power output of reverse electrodialysis has greatly improved, with up to 67 W m⁻² achieved by a single membrane by employing nanoporous and atomically thin carbon-based membranes ^[68][116]. Typical electrodialysis systems employ a stack of alternating cation and anion-selective membranes. As such, the performance of the devices is directly proportional to the properties of the ion-exchange membranes ^[69][117]. A highly concentrated salt solution is then used in contact with these ion membranes, separated from a low-concentrated water solution in contact with the membranes as well. The differences in potential between the low-concentrated solutions due to the diffusion of cations and anions, which is driven by the electrochemical gradient with the salt-concentrated solution, generate a voltage ^[70][118]. A first pilot plant exploiting this technology has already been implemented in the Netherlands by the REDStack company, being able to generate several watts per square metre of membrane ^[71][119]. Recent discoveries in the field of ion-exchange membranes have allowed for the development of high-performance reverse osmotic systems. However, the further improvement in reverse electrodialysis technologies relies on the discovery of affordable and sustainable materials that can be employed for energy generation. One of the main limitations of this technology, that hinders its full incorporation at an industrial scale, has been the relatively low power efficiency compared to other renewable energy sources. This power efficiency is directly related to the selectivity ^[66][114], the charge density, and the conductivity ^[72][120] of the ionic-exchange membranes. To improve the power conversion of current osmotic systems, nanoporous materials, such as MXenes ^[73][121], boron nitride ^[74][122], or graphene ^[75][123], have been employed. The porosity and permeation paths of these nanomaterials are located at the sub-nanometre range, allowing for a good selectivity on the cations ^[76][124]. However, the diffusion speed of water molecules and ions within such confined compartments is reduced, compromising the performance of the system ^[71][119]. In addition, the manufacturing costs of these materials is high, reducing the possibility of scale up. The incorporation of cellulose-based materials has been proven to be a promising alternative for the fabrication of porous membranes for osmotic energy generation. Specifically, the use of cellulose nanofibrils has demonstrated a high yield performance compared to traditional systems due to its high porosity ^[77][125] and the presence of highly polar hydroxyl groups at the surface of cellulose. These properties provide cellulose with a high ion conductivity, which can be further enhanced through chemical functionalisation, such as oxidation ^[78][126]. Consequently, cellulose has been employed as a sustainable alternative for ion-exchange membranes in fuel cell applications, among others ^[79][127]. Cellulose can be used either in its pure form, after chemical modification ^[80][128], or in the form of hybrid materials, with the incorporation of other nanostructured compounds. In the case of hybrid materials, the combination of graphene oxide nanoplatelets and cellulose nanofibrils in the form of assembled layers has been shown to improve the performance of current osmotic energy systems by enlarging the nanochannels while showing a large space charge ^[76][124]. Despite the low power output achieved in the case of lignin-based devices, which could be a consequence of the presence of less ionically conductive materials within wood such as lignin and hemicellulose, the price of each ion-exchange membrane was in the range of $10, which is considerably lower than the commercially available ones (about $350). However, the performance was significantly lower than the one obtained by the cellulose-based approaches. The reported values for lignin- and cellulose-based osmotic energy generators are shown on Table 23.