ORR Catalysts Derived from Biopolymers

Abstract

Due to the limited reaction rate of the oxygen reduction reaction (ORR), it is considered as a limiting factor in the performance of fuel cells and metal-air batteries. Platinum is considered the benchmark catalyst for ORR; however, the scarcity of platinum, its high price, the drift phenomenon, its insufficient durability, and its susceptibility to gas poisoning are the reasons for the constant search for new ORR catalysts. Carbon-based catalysts show exceptional promise in this respect considering economic profitability and activity, and, in addition, they have favorable conductivity and often a large specific surface area. The use of chitin, cellulose, lignin, coconut shell particles, shrimp shells, and even hair for this purpose was reported, as they had similar electrochemical activity regarding Pt. Alginate, a natural polymer and a constituent of brown algae, can be successfully used to obtain carbon materials that catalyze ORR. In addition, metal atomic-level catalysts and metal N-doped porous carbon materials, obtained from sodium alginate as a precursor, have been proposed as efficient electrocatalysts for ORR. Except for alginate, other biopolymers have been reported to play an important role in the preparation of ORR catalysts. In this review, recent advances regarding biopolymer-derived ORR catalysts are summarized, with a focus on alginate as a source.

1. Introduction

In recent years, the awareness of the human influence on climate change has increased. More and more attention is being paid to factors leading to global warming, especially CO₂ emissions. As fossil fuel usage leads to CO₂ release into the atmosphere [1], in addition to their limited availability [2], research groups worldwide are dedicated to finding alternatives to the carbon cycle. Fuel cells are of extraordinary importance in this respect, as they can participate in the hydrogen cycle. Fuel cells, as devices that convert oxygen and chemical energy into electricity with a high efficiency, are considered environmentally friendly and can be used for small, portable electronic devices and military and space devices [3,4,5]. Metal-air batteries represent cost-effective devices that require an air atmosphere [6], and the most promising are Zn-air batteries, with a high theoretical energy density, solid rechargeability, and flat discharge voltage [7,8]. During discharge, the reduction of oxygen at the cathode occurs while the metal oxidizes and releases electrons which pass through the external circuit in metal-air batteries. In the fuel cells, the H₂ dissociation occurs at the anode, while at the cathode, O₂ reduction occurs, transforming chemical energy into electricity [9]; thus, oxygen reduction reaction (ORR) is the most important cathode reaction for both types of devices [10,11,12].

The ORR catalyst is the main descriptor for the performance of these devices, with conventional platinum still regarded as the best ORR catalyst in both acid and alkaline electrolytes. However, Pt has a high price, a susceptibility to time-dependent drift, and serious anode crossover, and it is easily poisoned by CO (an intermediate product of electro-oxidizing alcohol fuels), which limits its usage. The limited availability of Pt is another important fact and constrains the possibility of using devices whose operation is based on ORR [13,14]. Therefore, platinum has to be replaced with appropriate alternatives that have a similar catalytic activity and can be produced massively and cost-effectively from Earth’s abundant resources [15]. Electrocatalysts should have high electrical conductivities, large specific surface areas, and electroactive properties [11,16]. Conductive polymers (CPs) and carbon materials (grapheme, carbon nanotubes (CNTs), amorphous carbon, and carbon nanofibers) can fulfil these requirements, and they have been widely explored for oxygen reduction in fuel cells, among other applications [13,17].

CPs that have been found to be useful as ORR catalysts can be classified into three groups: inherent CPs, CP-derived heteroatom-doped carbons, and CP composites. The activity of inherent CPs is a consequence of their structure, so the transfer of electrons with oxygen in molecular form occurs due to the characteristic that the neutral CP is an electron donor for O_2, while O₂ is an electron acceptor. Adsorbed oxygen on the CP surface accepts an electron, and its molecular symmetry decreases while the length of the O = O bond increases. In addition, due to the reaction with adsorbed oxygen molecules, the CPs that are mildly oxidized transform to a higher oxidation state, while the oxygen molecules are reduced into O²⁻ anions [17,18]. As many research groups directed their research interest towards the development of ORR catalysts from CPs, it was established that catalytic performances can be improved if a metal is incorporated into the CP [19] or if the CP is doped with a heteroatom. When Dai et al., in 2009, reported the strong ORR catalytic activity of carbon nanotubes doped with nitrogen, a new research field for N-doped carbon catalysts investigation was opened [17,18,19,20]. In addition to monoatomic doping, co-doping with other heteroatoms (B, O, S, P) proved to be an important direction for the synthesis of ORR catalysts, taking into account the increase in the asymmetrical spin density obtained in that case [21,22,23]. While B- and P-doped carbon without N-doping mainly enabled ORR via a two-electron process [24,25], B,N-doped carbon was proposed as an ORR catalyst, indicating a four-electron ORR in alkaline media [26]. In acidic media, the co-doping of N-doped material with only B influenced the increase in H₂O₂ production, and additionally doping it with P decreased the production of H₂O₂, consequently leading to it favoring the four-electron process [21]. Zhang et al. [22] designed N and P co-doped material by using polyaniline pyrolysis and developed the ORR OER dual catalyst, which enables the reduction to occur via the four-electron pathway.

For tridoped nanoporous carbon material with N-, O-, and S-, it was suggested that the O₂ molecule was reduced via a four-electron process in 0.1 M KOH solution. Compared to metal-based catalysts, this material had improved the catalytic activity; thus, it influenced the growth of research in this area [23].

Additionally, it was reported that the incorporation of transition-metal active centers had an influence on the electrocatalytic activity [27,28,29,30]. Transition metals such as Mn, Fe, Co, Ni, etc. have empty 3d orbitals; thus, they can accept an electron and have the ability to reduce the bonding energy between OOH*,O*/OH* intermediates [31,32]. As facilitating the O-O bond breaking in OOH* leads to the inhibition of H₂O₂ formation, this material quality is important for improving the four-electron ORR pathway selectivity [33].

Published materials [34] containing Fe [35], which had an electrocatalytic activity comparable to Pt/C in alkaline as well as in neutral and acidic medium, with an electron transfer number of about 4. Additionally, there is evidence that the introduction of two transition metals instead of one enhances the four-electron pathway selectivity [31].

Yang et al. [31] synthesized a binuclear catalyst containing Fe and Mn with excellent four-electron pathway selectivity in acidic media [34].

In accordance with the above, the heteroatom-doped carbon and transition metal coordinated by N in the carbon network are considered as the most promising catalysts for ORR [13].

The porous structure is an important parameter for the platinum group metal (PGM)-free ORR catalysts. Micropores host the majority of the active sites, while the mesopores primarily have the role of the mass transfer channel. Thus, the electrocatalytic activity is determined by micro- and mesoporosity [13,36].

The materials derived from biomass can inherit the macro-, meso-, and microporosity of the original biological precursors well after appropriate conversion processes [13,37,38,39].

The carbonization temperature presents another important factor that affects the activity of the bio-based carbon catalyst. As the temperature increases, the graphitization degree increases from about 75% to about 85%, and the electrical conductivity also increases [40,41]. In addition, the temperature increase leads to porosity development [36]. Additionally, the activity of the carbon-based catalyst depends on the relationship between the electron conductivity and the specific surface area. However, many works do not contain information about the specific surface area. The future is certainly represented by carbon foams with a system of open pores that enable the highest availability of active sites for oxygen and thus improve catalytic activity.

The defects of different sorts had a great influence on the ORR mechanism in the alkaline medium, while in the acidic medium, the influence was less pronounced [42]. Additionally, there is evidence that the rate of heteroatom influenced the catalytic activity; a higher content of heteroatom leads to an increase in structural defects and improves catalytic activity [43]. The researchers agree on the fact that the level of nitrogen influences the electrocatalytic activity. While increasing the level of nitrogen content, the ORR activity increases. The performance improvement is primarily due to the presence of free-flowing sp2-hybridized π electrons. In addition, the presence of free electron pairs of nitrogen atoms contributes to the bonding of the delocalized carbon matrix system, thereby further improving the electrocatalytic activity of ORR to a large extent. It is also known that heteroatoms doping can enhance the catalytic activity given that the doped heteroatom generally has the greater electronegativity compared to carbon atoms; this induces a partial positive charge near the carbon atoms and enhances the ORR [44,45]. The problem arises when the influence of the nitrogen type is discussed, since it is not entirely clear which nitrogen functional groups had a direct effect on the catalytic activity.

Various loadings of carbon-based materials in the range between 0.10 and 1.00 mg/cm² [46,47] were tried, and its influence on ORR was assessed. The results showed that low-carbon loadings were not sufficient in covering the electrode surface and consequently led to lower current densities and, predominantly, the 2e⁻ mechanism. With an increase in loading up to around 0.25 mg/cm², higher currents were measured, with a shift in the mechanism towards 4e⁻ reduction. A further increase in loading was detrimental to the ORR performance of carbon-based materials. An explanation for this occurrence is the excessively long electron part caused by the catalyst layer thickness, which results in substantial electrical resistance and mass transport losses. Both phenomena limited the access to some of the electroactive sites, independent of the sites’ actual activity towards ORR. Film thickness also affects the total available surface area as well as the materials’ bulk density, so there exists a trade-off between all these different effects when considering the overall ORR performance. Lastly, layer thickness seems to have an effect on the ORR mechanism itself by increasing the number of apparently exchanged electrons with the increase in loading. Namely, thicker layer intermediates, such as OOH-, remain near the carbon surface for longer periods of time and participate in the second 2e⁻ reduction to OH- [46,48], pushing the number of electrons towards four. This phenomenon is more pronounced for materials with sites of higher activity.

It is of particular importance to mention that ORR catalysts can also be synthesized using biogenic precursors. The authors Ye et al. [49] presented a synthesis process of ORR catalysts containing Fe-P active centers, After the carbonization of the biogenic precursor, i.e., the bacteria@vivianite composite, which was obtained due to the reduction of polyferric flocs by Shewanella oneidensis MR-1, the obtained material showed an excellent catalytic activity. Chitin, as the most abundant natural nitrogen-containing compound, is an ideal precursor for the synthesis of ORR catalysts using only a high temperature, without additional dopants [50,51]. Human hair, consisting of keratin, with contents of about 15–16% nitrogen and 4.5–5.5% sulfur [52], may also be used as a biogenic precursor for obtaining heteroatom-doped carbon material through the process that uses no hazardous reagents [53].

Biopolymers, substances consisting of numerous repeating monomer units, are present in natural sources. In

Table 1. The positive and negative aspects of natural biopolymers.

Positive Aspects	Negative Aspects
biologically renewable	low melting point
biodegradable	less stable
biocompatible	structurally more complex
non-toxic	high surface tension
biofunctional

, the principal advantages and disadvantages of natural biopolymers are presented [54,55]. These are promising candidates for different spheres of medicine and industry due to their biocompatibility and biodegradability. The biopolymers are used as edible films, emulsions, packaging materials in the food industry, drug transport materials, medical implants, tissue scaffolds, dressing materials in pharmaceutical industries, etc. [55].

In this review, we will discuss the use of some biopolymers-based materials as promising candidates for ORR catalysis, whose performance was most often tested in alkaline conditions. Those that are applied most often are alginate, cellulose, chitin, and lignin due to their broad availability and low price [56,57,58,59,60,61].

Figure 1 represents the biopolymer categorization [55] with respect to the biopolymers considered in this review paper.

In

Table 2. Main biopolymers and their chemical structures [62].

alginate
alginate monomer	α-D guluronic acid	β-D mannuronic acid
cellulose
cellulose monomer	glucose
chitin	N-acetylglucosamine
lignin
lignin monomer	p-coumaryl alcohol	coniferyl alcohol	synapil alcohol

, the structures of the biopolymers reviewed in this paper are presented.

Major challenges during the synthesis of ORR catalysts include the efforts to extract biopolymers from the biomass, along with methods for cleaning and decomposition, simplifying the conversion processes with the aim of ensuring the low cost of synthesis and production on a bigger scale. Knowledge of active sites and the relationship between the structure and mass transfer can facilitate the selection of materials and the methodology for ORR synthesis. Additionally, it is necessary to intensively transfer the half-cells tests of synthesized ORR catalysts to corresponding devices, such as fuel cells and metal-air batteries [13].

2. Alginate as a Source of Carbon Material

Biopolymers, as renewable and non-toxic precursors, are important for the synthesis of carbon materials. Abundant sea resources represent one of the most important sources, considering that the ocean occupies three-quarters of the Earth’s surface [63].

The wide alginates availability, with the simplicity of their extraction and synthesis via “green” processes, makes them exceptionally favorable materials, both commercially and environmentally. The field of their applications and significance is expanding, and one of the main challenges is their use as precursors of carbon materials production on a large scale.

Brown algae are a valuable source of alginate, a natural polymer composed of β-D-mannuronate and α-L guluronate. Due to the structure, as alginate consists of a large number of carboxyl and hydroxyl groups in the polymer chain, it is possible to obtain porous carbon material after alginate carbonization [64,65]. However, the different sequence and structure that vary depending on the source may be a disadvantage when this biopolymer is used [66]. Alginate interacts with metal ions and can chelate divalent and trivalent ions such as Ca²⁺, Co²⁺, Ni²⁺ Zn²⁺, and Fe³⁺, consequently forming an “egg-box” structure that can be used for the synthesis of metal-doped/free three-dimensional carbon nanomaterials with multimodal pores. This represents one of the main advantages of using the alginate as a precursor considering that the 1D structure has a very pronounced limited mass transfer of electrolyte ions. Multimodal pores formation was reported as a result of the material acid rinsing after thermal treatment in an inert gas atmosphere [67,68]. Large pores corresponded to the elimination of metal chelated into alginate, while smaller pores were the result of the release of H₂O and CO₂ during the thermal treatment.

Li et al. developed N-doped porous graphitic carbon nanofibers (N-PCNFs) with multimodal pores by the pyrolysis of electrospun alginate nanofibers at different temperatures (Figure 2). The result of the annealing treatment at 600 °C in ammonia was the formation of a metal-free catalyst for ORR with a higher stability and methanol tolerance for ORR in comparison to Pt/C. The mechanism revealed the four-electron pathway in an alkaline solution, with a capacity of 625 mAh g⁻¹ [63]. Porous graphitic carbon nanofibers without N were also prepared via the pyrolysis of alginate. However, N-doping influenced the formation of defective structures in the carbon framework with a specific area of 283 m² g⁻¹, implying increased active sites and an enhanced electrochemical performance. Based on the Koutecky–Levich (K-L) plots, the electron transfer number was calculated. At 0.4 V, the n-value was 3.95, indicating that the catalyst ensures that the reduction mechanism takes place as a four-electron mechanism. Voltametric studies were employed, and cyclic voltammograms of N-PCNFs showed a well-defined reduction peak (at about 0.76 V vs. RHE), which indicated oxygen reduction. Linear sweep voltammetry (LSV) was also applied in an O₂-saturated 0.1 M KOH solution with a rotating disk electrode (RDE), and the ORR onset potential and half-wave potential of N-PCNFs-600 were 0.953 and 0.810 V, respectively. In 0.5 M H₂SO₄, the onset potential was 0.55 V, while the half-wave potential was 0.35 V. Chronoamperometric measurements were conducted at 0.70 V with a rotating speed of 1600 rpm with the aim of evaluating the methanol tolerance, whereby N-PCNF-600 had a better fuel selectivity toward ORR than Pt/C considering that the current density was stable after the addition of methanol. The durability was examined at a constant voltage of 0.3 V for 15 h in an 0.1 M KOH solution at a rotation rate of 1600 rpm. The synthesized catalyst had a slower decrease of 16% than Pt/C (25%) and, thus, a better durability.

The high charge and discharge performance, based on the specific structure and bimodal shape of the pore size distribution, is one of the important advantages of this catalyst.

Zhao et al. used calcium ions to form a network with alginate, after which wet spinning was applied with the aim of obtaining calcium alginate fibers (CAFs). CAFs were treated with an acid solution to remove metal ions [69,70], and the remaining carbon fibers were used for further experiments. N and S atoms from thioacetamide were doped into carbon fibers during pyrolysis, and after annealing at 1000 °C, the recombinant carbon atoms formed a metal-free structure without S and N atoms with defects (D-CFs), which aimed to improve the catalytic activity for ORR [71,72,73]. D-CFs had micro- and mesopores which enabled the availability of active sites and facilitated the transport of the reaction participants with a specific surface area of 485.2 m² g⁻¹ and a 795 mAh g⁻¹ capacity [74]. The initial potential was 0.92 V vs. RHE in 0.1 KOH, and the limiting current density was 5.38 mA cm⁻², which were comparable to those of Pt/C. The half-wave potential was 0.84 V vs. RHE, equivalent to that of Pt/C in 0.1 M KOH. The reaction process catalyzed by D-CFs was determined to take place as a four-electron pathway at 0.25, 0.35, and 0.45 V vs. RHE. By comparing D-CFs and Pt/C, the chronoamperometric results confirmed the better methanol tolerance that was expected. After 10 h, the current density for D-CFs was 94%, compared to 87% for Pt/C [70].

Considering the characteristics mentioned in the paper and the low cost, the obtained material could find its usage in large-scale applications of zinc-air batteries.

Alginate-Based Materials Doped with Metal Ions

Shu et al. [75] reported a novel strategy, thus broadening the methodology for the synthesis of Co/N-doped hierarchical porous carbon microspheres (Co/NHPCMS) with ZIF-67 (zeolite imidazolate framework), which were grown on the carbon frameworks. During the synthesis, ZIF-67 nanocrystals were grown on a carbon framework, and they improved the catalytic activity considering that they represent an in situ source of nitrogen for the material doping [76]. Sodium alginate/NaHCO₃ (SA/NaHCO₃) solution as a precursor and the electrospinning technique were used to form Co-SA/NaHCO₃ hydrogel microspheres. The specific structure of the alginate enabled the formation of a 3D network which improved the mass/charge transfer and increased the density of active sites. In such a structure, Co²⁺ formed an “egg-box” model and acted as the metal source of ZIF-67. ZIF-67@SA/NaHCO₃ microspheres were pyrolyzed in the nitrogen atmosphere, and Co/NHPCMS was obtained, with a specific surface area of 252 m² g⁻¹. For comparison, the same procedure was performed without NaHCO₃, and the ORR activity test showed that NaHCO₃ contributes to better ORR catalytic activity considering that the decomposition of NaHCO₃ results in the formation of more pores and defects, which increase the density of active sites. According to the XPS spectrum, the pyridinic and graphitic-N were considered as active sites. Additionally, it was suggested that pyridinic nitrogen could coordinate with cobalt ion. The Co nanoparticles influenced the aggregation process and corrosion and improved the stability of the catalyst [75,77].

The LSV-obtained data included the half-wave potential, which was 0.827 V, close to that of commercial Pt/C (0.851 V). Such positive values indicated a great advantage of the hierarchical porous structure. The K-L plots gave a linear dependence, and the electron transfer number was greater than 4, probably as a result of the adopted values for the K-L equation being different from the ideal. According to the chronoamperometric measurement, in O₂-saturated 0.1 M KOH at a potential of 0.6 V (vs. RHE), at a rotation rate of 1600 rpm, the relative current decreased to 79.10% after 18,000 s. Thus, compared to Pt/C, the newly synthesized catalyst showed greater ORR stability. Co/NHPCMS was also more tolerant to methanol compared to commercial Pt/C, as no change in the current density was observed after 200 s, when 1 mL of methanol was added [75].

Ma et al. [78] developed a novel hybrid material, a three-dimensional nanostructured electrocatalyst, for ORR and OER by the pyrolysis of (Ni,Co)/carbon nanotubes (CNTs) alginate hydrogels. SA/CNTs aqueous solution (Figure 3a,d) was exposed to a solution containing Co²⁺ and Ni²⁺ ions to form an ”egg-box structure” (Figure 3b,e). Upon obtaining hydrogels, the freeze-drying process was used to form (Ni, Co)-alginate/CNT aerogels (Figure 3c,f). Carbonization in the NH₃ atmosphere was conducted in order to obtain the final form of the catalyst, whereby the catalyst Ni/NiO/NiCO₂O₄-N-CNT-As (As-aerogels) showed a significant advantage in terms of catalytic properties in comparison with commercial Pt/C, with a surface area of 222 m² g−¹. The high activity of CNTs was explained as a result of the possibility of pyridinic and graphitic nitrogen exerting an electron on the encapsulated Co nanoparticles [14,65,79]. Accordingly, the catalytic activity was attributed to the ternary Ni/NiO/NiCO₂O₄ active site for electron transfer and the 3D hierarchical mesoporous hybrid network for mass transport. The onset potential and half-wave potential of Ni/NiO/NiCO₂O₄-N-CNT-As (obtained by LSV) were 0.89 V and 0.74 V vs. RHE, respectively, compared to 0.92 V and 0.78 V for Pt/C. Based on the reviewed results, newly synthesized catalysts were recommended as ORR catalysts, especially as bifunctional for ORR and OER. According to the N₂ adsorption-desorption isotherms and BJH pore size distribution, the synthesized material had a bimodal shape in the pore size distribution, large mesopores as a consequence of interconnected voids from tangled CNTs in the aerogels, and small mesopores as a consequence of H₂O and CO₂ release [79].

According to the K–L equation, as in other reported materials in this review, the number of electrons was 3.8, a value desirable for ORR. The durability was tested at a constant voltage of 0.69 V vs. RHE for 22,000 s at a rotation rate of 1600 rpm, where the current loss was 9%, proving the improved durability compared to commercial Pt/C. Additionally, the tolerance to methanol was better for the Ni/NiO/NiCO₂O₄-N-CNT-As, as the negative current appeared after the introduction of O₂ in the KOH saturated with N₂ at 1000 s, while subsequently, after the addition of 3M methanol, the newly synthesized catalyst’s current had no changes [78].

The proposed method may be easily large-scaled, so it presents a valuable reference for the formation of dual ORR and OER catalysts.

Zhang et al. [80] developed a catalyst consisting of a 3D N-doped porous carbon matrix with Co₃O₄/Co active species–Co₃O₄/Co-NPC and a uniform porous nanostructure with a surface area of 496.4 m² g⁻¹. The great advantage of the developed method was certainly the simple and low-cost synthesis, as they used hydrothermal reaction, unlike methods that use an electrospun technique or complex procedures. This method is one of the most promising methods for the synthesis of ORR catalysts [81,82,83,84,85]. In addition to catalyzing the ORR, the catalyst can be used for anodic oxygen evolution reaction (OER) as well; thus, it possibly has practical applications for energy conversion.

Carbon materials contributed to a better stability and conductivity and an increased surface area compared to the nanostructures of cobalt oxides and/or hydroxides [86,87,88,89]. Nevertheless, hybrid material consisting of metal at the nanoscale and heteroatom co-doped carbon showed better performances as ORR catalysts and bifunctional ORR/OER catalysts [90,91]. As biopolymers provide a cost-effective and renewable source, they attracted the attention of these authors as well [80]. After the chemical reaction of SA with Co ions, SA formed a network such that, after carbonization, a catalyst containing both Co and Co₃O₂ species-Co₃O₄/Co-NPC was obtained. Melamine was used as a source of nitrogen, and after the carbonization at different temperatures, the product obtained at 800 °C had the optimal characteristics [92,93,94]. The synergistic effect of the Co₃O₄/Co active species and NPC was responsible for the desired mass transport and charge transfer and the specific dual activity towards ORR and OER.

CV measurements were conducted to evaluate the ORR activity. In 0.1 M KOH solution, Co₃O₄/Co-NPC had a peak at 0.738 V, which was at more positive potentials compared to Pt/C. The onset potential obtained by LSV was 0.91 V due to its value being smaller than that for Pt/C (0.987 V) and its half-wave potential being 0.806 V vs. RHE (less than the half-wave potential for Pt/C 0.8875 V). This result encourages the application of the synthesized catalyst for ORR catalysis. According to K–L plots, the average electron transfer number at the potential from 0.25 to 0.45 V was 3.9, indicating a four-electron mechanism for ORR. The chronoamperometric curves of Co₃O₄/Co-NPC and Pt/C were compared to define the tolerance to methanol, where the synthesized catalyst had a negligible response after the addition of methanol and thus showed better tolerance. O₂-saturated 0.1 M KOH at 0.5 V (vs. RHE), with a rotating speed of 1600 rpm, was used to determine the long-term stability, where, after 7 h, Co₃O₄/Co-NPC had a current value at 94.5% compared to Pt/C (86.5%). Thus, the porous N-doped carbon hybrid with Co and Co₃O₄ was suggested as the promising ORR and OER agent [80].

Alginate fibers (AF) which formed an “egg-box” structure with Co²⁺ ions were used as growing sites for the coating of Co-based ZIF (Zeolitic imidazolate frameworks)-MOF (metal organic framework), and by applying the solvothermal process, the AF@ZIF-67 was obtained [95]. Synthesized material was carbonized at different temperatures, and optimal characteristics were shown by the Co-CF obtained at temperatures of 900 °C in an N₂ atmosphere. Due to the specific structure formed, Co and N, which were uniformly distributed on the one-dimensional CFs, were active sites and had the effect of accelerating the process of mass transfer and the transmission of electrons [96]. As ZIFs were composed of molecules with a large number of N atoms, after carbonization, the CoNC-CF material was obtained, which was shown to be an efficient catalyst for the ORR reaction. In 0.1 KOH, the half-wave potential was 0.833 V vs. RHE, and the onset potential was 0.926 V (vs. RHE) compared to 0.827 V and 0.942 V vs. RHE, respectively, for 20 wt.% Pt/C. The limiting current density was −5.56 mA⁻², and, compared with catalysts without N and Co active sites, the aforementioned catalyst had obvious advantages, as stated. The authors confirmed that the reduction reaction occurred through a four-electron pathway according to K–L plots. After 20,000 s, at 1600 rpm, CoNC-CF pyrolyzed at 900 °C had 94.8% of relative current in contrast to Pt/C, with a retention of 80.9% [95].

Compared to previously reported materials, the greatest disadvantage of this material would be its 1D structure. Despite that, the obtained material has been shown to be an excellent ORR catalyst.

The synthesis of Fe₂N/C ORR catalysts via a simple, eco-friendly route, where Fe³⁺ cations formed a novel structure of an “egg box” with alginate in the excellent yields, was described in work of Liu et al. [14]. Fe₂N nanoaerogels had outstanding ORR electrocatalytic activity, stability, and methanol tolerance in alkaline as well as in an acidic medium.

The (SA) was mixed with graphene; thus, the dispersion was obtained. The mixture was added to the FeCl₃ solution, and hydrogel was formed [63,65,97]. Upon a freeze-drying process, 3D Fe-alginate/graphene aerogels (GAs) were made, and as the size of the aerogel was only limited by the size of the container where the ion-exchange process took place, the proposed method may be a facile strategy for scaling up the production of Fe₂N/N-GAs from brown algae. After calcination at different temperatures, the material obtained at 700 °C in the NH₃ atmosphere, i.e., the optimal 3D Fe₂N/N-GAs, was formed. The 3D Fe₂N/N-GAs had a high-density N-doped amorphous carbon shell (N-AC) which encapsulated Fe₂/N NPs. This characteristic presents the great advantage of this material, considering that some Fe/N/C electrocatalysts for ORR did not have the controlled porous structure; thus, active sites were not exposed adequately, and transport properties were poor. The resulting aerogel was bimodal, as 3D mesoporous networks of about 30 nm and small mesopores of 3 nm were achieved, where the specific surface area was 465 m² g⁻¹ [14]. The molecular transport throughout the entire 3D architecture was supposed/achieved, and small mesopores were beneficial for the O₂ adsorption in the ORR [98]. The rate-determining step in the ORR, i.e., the splitting of O-O bonds as a consequence of the transfer of two electrons from active sites to adsorbed O₂, was influenced by quaternary-type and pyridine-like nitrogen, while the Fe was probably the site for O₂ adsorption.

The Fe₂N-based nanoaerogels had electrocatalytic activity for ORR in 0.1 M KOH and 1 M HClO₄ solution. In an alkaline medium, the onset potential and half-wave potentials were 1.02 V and 0.93 V, respectively, while in an acidic medium, those values were 0.82 V for the onset and 0.71 V for the half-wave potential vs. RHE. The electron transfer number was four, and the current density was −4.5 mA cm⁻². By comparing the durability of Fe₂N/N-GAs with that of Pt/C, the authors showed that the durability in acidic and alkaline media was better for Fe₂N/N-GAs. After 9 h, there was only a 10% loss of the initial current density in KOH (for Pt/C, 20%), while in HClO₄, there was a 19% loss after 10.5 h (for Pt/C, 45%). Fe₂N/N-GAs also had a good ability to resist the crossover effect; thus, there was no noticeable change in the current density after the addition of methanol [14]. This excellent catalytic activity was probably a consequence of an optimal balance of the surface area, the density of the active sites, and the electrical conductivity [99].

Yu. et al. [100] developed a simple and universal method for obtaining single-atom catalysts (SACs) with a surface area of 1551 m² g⁻¹ using Cu ion as a representative of transition metals that can form a network with alginate (i.e., “egg-box” structure). As one of the main problems for the investigation of SACs applications is the lack of a universal method for synthesis, these authors have significantly contributed to the development of SACs. In addition, they avoided physical and traditional chemical approaches which usually result in low production, complex equipment, a cumbersome process, and a high cost [101,102].

After the calcium from Ca-alginate (derived from sodium alginate) was replaced with H⁺ ions, the chemical reaction with CuCl₂ was enabled. Hydrogel consisting of Cu and alginate was dried and carbonized at 900 °C under an NH₃ atmosphere. With the aim of eliminating the metal ions, the obtained material was washed with an acidic solution. The obtained product, Cu-SAC/N, was examined for ORR activity, as well as SACs synthesized using the same procedure but with different transition metals, such as Fe-, Co-, and Mn. For catalysts obtained with Cu, in 0.1 M KOH, the onset potential was 0.90 V, and the half-wave potential was 0.80 V vs. RHE. Compared to the onset and half-wave potentials for the Pt/C catalyst (0.93 and 0.83 V, respectively), the newly synthesized material had an improved electrocatalytic activity, primarily owing to the catalytic site, which was Cu-N₄. As in the majority of experimentally obtained materials, only surface atoms acted as active sites, and the SACs have a great atom utilization efficiency. The experimentally obtained transfer electron number was about four. After the addition of methanol, the current remained stable. The same trend was achieved by analyzing the current density after 3000 cycles of the CV, where the decrease was not more than 10%.

To summarize, a comparative table with the indicated method for obtaining the type of carbon material and the characteristics of the ORR catalysts derived from alginate is given in

Table 3. ORR catalysts derived from alginate; characteristics, methods for obtention, and type of carbon materials.

Reference	[63]	[70]	[75]	[78]	[80]	[95]	[14]	[100]
Specific area	283 m2 g−1	485.2 m2 g−1	252 m2 g−1	222 m2 g−1	496.4 m2 g−1	/	465 m2 g−1	1551 m2 g−1
Heteroatom	N	N and S eliminated upon annealing	N	N	N	N	N	N
Dopant	/	/	Co	Co, Ni	Co	Co	Fe	Cu
Half-wave potential of catalyst/Pt/C	0.810V vs. RHEl	0.84 V vs. RHE	0.827 V vs. RHE	0.74 V vs. RHE	0.806 V vs. RHE	0.833 V vs. RHE	0.93 V vs. RHE	0.80 V vs. RHE
Optimal preparation temperature	600 °C	1000 °C	1000 °C	400 °C	800 °C	900 °C	700 °C	900 °C
Derived catalyst	N-doped porous graphitic carbon nanofibers	defective carbon fibers	3D Co/N-doped hierarchical porous carbon microspheres	carbon nanotubes nanoaerogels	Co3O4/Co species incorporated into the N-doped carbon matrix	carbon fiber-coated Co@N-doped porous carbondoped	3D Fe-alginate/graphene aerogels	single-atom catalyst
Synthesis technique	electrospun	wet spinning	electrospinning	freeze-drying	hydrothermal reaction	chemical reaction	freeze-drying	chemical reaction

.

3. Other Widely Used Biopolymers

Cellulose, lignin, and chitin are widely distributed polymer materials that are increasingly used as sustainable precursors for the synthesis of carbonaceous materials since they are non-toxic, biodegradable, and biocompatible [103,104].

3.1. Cellulose

Cellulose is a homopolymer consisting of glucopyranose [105]; therefore, to obtain materials with good ORR electrocatalytic activity, doping or co-doping with heteroatoms is desirable [51]. The chains of cellulose are connected in parallel, and the crystalline structure, via hydrogen and van der Waals interactions, forming microfibrils [51]. The degree of polymerization varies according to its source [106] Besides many advantages, such as the large amounts, biodegradability, low cost, etc., they also have some disadvantages; the low moisture resistance may be the most important [107].

Kim et al. used cellulose as the biomaterial for the high-value-added N-doped hierarchical porous carbon (NHPC) [108].

They developed a simple, inexpensive, and efficient process in which they mixed cotton cellulose with magnesium nitrate hexahydrate or magnesium acetate tetrahydrate and urea in different proportions. As previously reported methods included high-cost materials and low outputs, this method presents a great improvement. Upon drying, the mixture was pre-pyrolyzed at 500 °C in an Ar atmosphere. Afterwards, the obtained material was carbonized at different temperatures. The authors explained in detail the influence of the preparation parameters and contributed to the improvement of the methodology of making carbon-based ORR catalysts. Pyrolysis and carbonization were separated in order to characterize the material after pyrolysis and to avoid the pollution of high-temperature furnaces due to a large amount of gas emission. The treatment with an acidic solution was applied after the carbonization with the aim of obtaining a final material without metal ions.

The novelty of the proposed method was that the authors used accelerated pyrolysis, where the exothermic reactions were caused by the application of nitrates, which encouraged pyrolysis at low temperatures and the rapid exfoliation of cellulose fibers. In addition, urea was used as an additional source of nitrogen but also as a reactant that supports the total exfoliation of cellulose when applied in the optimal content (Figure 4). Consequently, the sample synthesized with magnesium nitrate and urea had a highly 3D porous structure with macropores, mesopores, and abundant micropores and a specific surface area of 1173 m² g⁻¹. Graphitic and pyridinic N, i.e., the N-C parts of the catalysts, were considered as active sites, and the high specific surface area combined with the hierarchical open pore structure contributed to the increase in the exposed active sites, thus enabling efficient mass transport and fast ion transport. According to the electrochemical measurements, including CV and LSV, the onset potential was 0.94 V, the half-wave potential was 0.83 V, and the estimated number of transferred electrons was in the range of 3.5–4 for the sample carbonized at a temperature of 1000 °C. As the temperature of carbonization influences the doping amount and doping species of N, as the graphitization degree of the carbon, according to the stated results, the optimal temperature was 1000 °C. The durability of the optimal material was better compared to that of the Pt/C catalyst, as the current obtained by the chronoamperometric measurements was 88% after 10 h in KOH, while for the Pt/C, it was 83% at 0.6 V vs. RHE in the O₂-saturated KOH solution at a rotating speed of 1600 rpm. The material was methanol-tolerant, as the current had no significant changes after the addition of 2% methanol, while for the Pt/C, the current decreased [108].

3.2. Chitin

Chitin is, besides cellulose, the most widespread biopolymer [109,110]. It has a high content of nitrogen. Before using from the biomass, the chitin has to be extracted. This process includes mechanical grinding, chemical demineralization, and deprotonation, techniques that may be time-demanding. In addition, the direct carbonization of chitin could not provide a defined morphology and porosity of the newly obtained material [111]. Chitosan, the deacetylated form of chitin with a different degree of deacetylation, may also be used for different purposes [110,112]. As chitosan contains about 7% of N, it presents a valuable resource of material containing carbon and nitrogen from amine and acetamide functional groups [57,103,113,114], which, upon thermal treatment, may exclude an additional step of N-doping in the synthesis process of functionalized carbons.

The specific surface area and the type of functional groups [115], both dependent on the carbonization temperature, are the main factors favoring an increase in the catalytic activity for N-doped porous carbons obtained from chitin and chitosan. The urea treatment of these materials had a great benefit for their ORR performance, since it further increased the specific surface area and enhanced the N-doping [116].

Wang et al. designed cobalt- and nitrogen-co-doped carbon material (CoNC), using a mixture of chitin and cobalt as a precursor, through a one-step pyrolysis process at 800 °C [117]. The obtained CoNC showed a specific area of 165 m² g⁻¹, and it was reported as a good material for ORR electrocatalysts in alkaline media for Al-air batteries. The onset potential was 0.86 V vs. RHE, and the high-limiting current density of 4.91 mA cm⁻² was comparable to those obtained for Pt/C 0.9 V and 5.67 mA cm⁻². The electron-transfer number was 3.73. The long-time durability was examined at 900 rpm in 0.1 M KOH saturated with O₂, and after 15,000 s, the current density was 94.82%.

3.3. Lignin

Lignin is the by-product obtained in the process of pulping. It is the constituent of grass, trees, and plants [118].

Lignin presents an amorphous aromatic polymer [58] which consists of p-hydrophenyl (H), syringyl (S), and guaiacyl (G) components, which form a three-dimensional structure. As lignin has a high content of benzene rings and phenolic functional groups, it is widely used as a precursor for the synthesis of carbon material (carbon spheres, nanofibers, nanosheets, 3D-porous carbon, carbon composites, etc.). The availability, low cost, the unique designability and controllability of the lignin structure are certainly its great advantages for its usage in different spheres [58]. The disadvantages of lignin include the fact that its usage may be a complicated procedure that disables the large-scale production. Accordingly, the development of an environmentally friendly process for the production of carbon materials from lignin is mandatory, with special attention given to the fact that the relationship between the structure of carbonized lignin and its catalytic activity is not absolutely understood yet.

The reported carbon materials derived from lignin include nitrogen-doped and nitrogen/sulfur- and nitrogen-phosphorus-co-doped catalysts [119,120,121]. Li et al. [122] used lignosulfonate and synthesized a robust multifunctional carbon catalyst for ORR, OER, and hydrogen evolution reaction (HER), which contained iron, nitrogen, phosphorus, and sulfur. Although different methods for carbon-doping were reported, the catalytic activity for the ORR, OER, and HER of such materials was not satisfactory. The improvement was achieved by the synthesis of the catalyst with a larger specific surface area, where active sites were exposed to the surface. The highly porous structure improved the rate of mass/electron transfer, the doping with heteroatoms upgraded the electroconductibility and charge transfer, and the FeN_x and FeP_x species had an impact on the prevention of the agglomeration of active sites and thus provided great durability. Finally, the obtained catalyst had the intrinsic activity of each active site and, consequently, great activity toward these three different reactions. The material was synthesized during the process that consisted of the preparation of lignin-Fe, and then the obtained substance was mixed with hypophosphite (inorganic molten salt template), annealed at 500 °C, and carbonized at 800 °C. After treatment with HCl solution, the material was again annealed at 800 °C and tested for catalytic activity [122]. The obtained catalyst had a specific surface area of 782 m² g⁻¹, which is larger than that of some reported catalysts [121]. The half-wave potential was 0.9 V, and the electron transfer number was 3.9. After injecting 3 M methanol, the current density had a negligible change, indicating a robust tolerance to methanol [122]. After 86,400 s, the current changed by 9.1%, while for the Pt/C, the change was 17.1%. Shen et al. [121] used lignin with melamine to form lignin carbon nanosheets of a surface area of 1208 m² g⁻¹ co-doped with nitrogen and sulfur. The material may be used in an acid medium as well as in alkaline due to its better performances compared to Pt/C (a more positive half-wave potential and (nearly) current density and a high graphitic N ratio including four electron mechanisms for ORR).

4. Biomass Used for the ORR Catalysts Preparation

The carbonization of waste biomass, including adequate pre- and post-treatment processes, can be used for the preparation of ORR catalysts. Biobased N-doped carbons have been synthetized from coconut shell particles, shrimp shell, and even hair.

Due to the specificity of the structure formed by cross-linked cellulose, hemicellulose, and lignin molecules, coconut shells have a high density and low porosity; they therefore represent a good precursor material whose carbonization produces catalysts of high carbon contents and low ash residues [123]. Borghei et al. [124] developed an environmentally friendly method, without using hazardous chemicals to synthesize N/P-doped porous carbon, where, after the single-step activation of coconut shells with phosphoric acid, urea was the source of nitrogen doping. New material obtained after carbonization at 1000 °C with a specific surface area of 1260 m² g⁻¹ had a comparable electrocatalytic activity with Pt/C, a better tolerance to methanol crossover, and an improved long-term durability in alkaline media.

Although it is shown that the tailoring of the pore size distribution is better achieved through physical activation [125], the authors performed the activation chemically using phosphoric acid, which led to the high-yield synthesis of carbons with a three-dimensional structure of a high porosity. The long-term stability was evaluated by chronoamperometry at 0.7 V and at 900 rpm after 50,000 s, and the results showed that the functionalization with urea improved the stability by reaching a plateau of about 75% after 13 h. The tolerance to methanol was examined, where the current intensity was the same before and after the addition of methanol, which indicated a good selectivity of the material toward the ORR. The excellent electrocatalytic activity of this material may be the result of different factors, including the contribution of a large fraction of mesopores, which enables enhanced electrolyte mass transport and the easy access of oxygen molecules to the active sites due to the 3D open pore structure, the synergic effect of N and P doping, and the large proportion of N-graphitic and N-pyridinic species. The authors suggested that the O₂ adsorption probably occurred on N-graphitic sites, while N-pyridinic sites enabled fast decomposition or an electroreduction of hydroperoxide, resulting in a fast conversion of oxygen molecules [126]. Nevertheless, the influence of the N-doping level and ORR activity still remains absolutely defined [127,128]

Jahran et al. [123] also reported the synthesis of N-doped activated carbon from coconut shells with bifunctional electrocatalytic activities towards ORR and OER, showing a good stability in alkaline electrolytes. First, they pulverized the coconut shells, soaked them overnight with 50 wt.% HNO₃, and exposed them to a temperature of 550 °C under the N₂ atmosphere. After the functionalization of the material, the N-doping was accomplished with urea, and the obtained product was subsequently pyrolyzed at 900 °C.

N-doped material had a three-dimensional structure with pores, and upon the presence of nitrogen, it had an enhanced electrocatalytic performance, a higher current density, and a more positive onset potential compared to raw material. The authors used a K–L plot to determine the number of electrons transferred in ORR, where a direct four-electron reduction pathway was obtained for N-doped activated carbon [123]. The LSV analysis of the material confirmed that the presence of nitrogen enhances the electrocatalytic activity, i.e., enables a more positive onset potential and a higher current density. The durability of the prepared material was better compared to that of Pt/C since there was a slow decrease in the current after 1000 s in 0.1 M KOH. The authors claimed that the method was optimal for ORR catalyst synthesis, but no data for the onset and half-wave potential were presented.

Liu et al. presented a simple template-assisted method for ORR catalyst development using shrimp-shell-based carbons [129]. Shrimp waste is seafood production waste [110,130], and chitin is one of the most important parts of a shrimp shell, as it makes up as much as 40% of the shrimp mass [110,131]. Shrimp shells were ground to powder and exposed to a hydrothermal reaction such that N-doped carbon nanodots (N-CN) were formed. N-CN reacted with SiO₂ spheres under experimental conditions that were optimal for obtaining N-CNs@SiO₂, which can be pyrolyzed in an N₂ atmosphere at 800 °C. At the end of the process, SiO₂ was removed, and the material with an intact three-dimensional N-doped porous carbon network was obtained. A three-modal-pore structure was noticed, with micro-, meso-, and macropores; thus, the material contained many defects in the carbon structure and, consequently, active sites for electrocatalysis predominantly consisting of pyridinic and graphitic-N [132,133,134]. The onset potential in 0.1 M KOH saturated with O₂ was 0.905 V, which was close to that of Pt/C. The limiting current was 5.3 mA cm⁻² at 0.6 V vs. RHE. The electron transfer number was between 3.75 and 3.95, over the potential range from 0.56 V to 0.32 V. The NPC material had no crossover effect, as the current change after the addition of 3.0 M methanol was negligible.

The authors also examined the durability, and after 20,000 s, the NPC exhibited a 12% decrease in current at a rotation speed of 1600 rpm and under an applied potential of 0.62 V, which certainly recommends the obtained material for use as an ORR catalyst [129].

Some studies suggested that carbon nanodots have high water adsorption properties, especially in an alkaline solution, due to the presence of O- and N-containing hydrophilic functional groups; thus, they could not be used as catalysts owing to the low uniformity and stability of the film made of the catalyst on the electrode [135,136,137]. The solution to this problem may involve the immobilization of carbon nanodots onto conductive carbon with the aim of overcoming the falling-off process [135,136,137]. Additionally, the low graphitization degree as a result of the low-temperature hydrothermal process, as one of the problems, may be solved by increasing the pyrolysis temperature, but agglomeration with a low surface area and an unacceptably low electrical conductivity may still be a problem [138]. Taking all of this into account, Li et al. contributed significantly to the field of efficient catalysts synthesis using biomass as a source material.

As human and animal hair can be classified as organic waste and represents the source of carbon, it can be used for the synthesis of carbon-based materials with different characteristics, including porous structures with a great surface area [53,139,140]. Chaudhari et al. [53] synthesized porous carbon material by using hair as a source of carbons and heteroatoms.

Hair consists of the α-keratin protein and thus contains nitrogen and sulfur from amino acids which are interconnected by peptide bonds [52]. Keratin is known as a protein with high content of cysteine, an amino acid rich in sulfur, making hair a good precursor material for heteroatom-doped carbon material synthesis. The specific structure of hair makes it an excellent single precursor for N- and S-doped material, consequently avoiding the usage of hazardous chemicals. Chaudhari et al. used carbonization, mild activation, and graphitization with the aim of obtaining the high-porosity material that consists of N- and S- doped carbons, which may be used as ORR catalysts. After carbonization at 250 °C in the N₂ atmosphere, the obtained char was activated at 600 °C using NaOH under a constant N₂ flow. As the graphitization was conducted at different temperatures, the temperature of 900 °C was proved to be the best for optimal ORR characteristics. The combination of heteroatom doping, a high surface area, and electrical conductivity makes the hair a promising precursor for the synthesis of ORR catalysts. The micropores and mesopores ranging from 0.5 to 4.0 nm were dominant. The specific surface area was 1813.95 m² g⁻¹. The electrochemical parameters were as follows: the onset potential was 0.949 V vs. RHE, and the reaction current was −4.92 mA cm⁻² at 0.4 V. The electron transfer number was between 3.80 and 3.90. According to the chronoamperometry results, there was no noticeable change in the current response after 300 s of the addition of 3.0 M methanol. The durability and stability were tested at a constant potential of 0.7 V for 14 h, and after 50,000 s, the current density in 0.1 M KOH saturated with O₂ decreased by 14% at 1600 rpm.

5. Conclusions

Biopolymers contribute inestimable value towards the preparation of ORR catalysts. After appropriate treatment (mainly carbonization at an optimized temperature), these compounds give highly porous carbon material with/without dopants, making them excellent candidates for ORR catalysts. The porous carbon materials obtained from alginate are of great interest due to their low cost, wide availability, and relatively simple fabrication procedure. The inherent ability of alginate to form the “egg-box” structure after cross-linking with metal ions makes it possible to obtain materials whose electrocatalytic activity is comparable to that of Pt/C. Apart from alginate, cellulose, chitin, and lignin are excellent precursors for the preparation and production of large quantities of catalysts. As the application of Pt/C catalysts is limited by its high production costs, poor tolerance to methanol, and low durability, the limits of biopolymer applications are expanding.

Undoubtedly, natural polymers represent a wide range of compounds, which, apart from being available and affordable, have a specific structure which makes them an excellent starting compound for obtaining ORR catalysts with better durability, an improved tolerance to methanol, and better electrocatalytic properties compared to commercial ORR catalysts. Various biopolymer-derived, metal-free ORR catalysts have been studied and shown to possess excellent electrocatalytic activity, a remarkable tolerance to CO poisoning, fuel crossover effects, and a concomitant, good operational stability that is comparable to the best among non-precious metal catalysts. Porosity in biopolymer-derived carbons can ensure efficient oxygen transport and even alter the reaction mechanism, making them a viable component in PEMFCs. Despite the tremendous advancements made in the development, with heteroatom doping, a pore structure design, and surface modification, some inherent drawbacks still remain. Changes in the conductivity with higher doping levels, the lack of catalytic activity in acid media, and the difficulty of the preparation of catalysts with an exact structure and surface groups are still some of the issues that remain unresolved. The exact active site for ORR is still under debate among many researchers, many of whom turned to theoretical calculation, mainly DFT, to explore the main descriptor guiding the catalysis. Predictions of the electronic structure, active sites, reaction pathways, and intermediates provided by DFT present a part of the solution, with further research being needed to guide and connect the predictions from theoretical studies to the results from the laboratory. Biopolymer-derived, carbon-centered electrocatalysts have proved very versatile, with a great potential for improvement through doping, the pore structure, and active site design, making them an unavoidable part of the solution for many energy conversion and storage devices.