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.

Full text

Catalysts 2023, 13, 80. https://doi.org/10.3390/catal13010080 www.mdpi.com/journal/catalysts
Review
ORR Catalysts Derived from Biopolymers
Jelena Rupar 1, Danijela Tekić 2, Aleksandra Janošević Ležaić 1 and Kush K. Upadhyay 3,*
1 Department of Physical Chemistry and Instrumental Methods, University of Belgrade—Faculty of
Pharmacy, Vojvode Stepe 450, P.O. Box 146, 11221 Belgrade, Serbia
2 University of Belgrade—Faculty of Physical Chemistry, 11158 Belgrade, Serbia
3 Centro de Química Estrutural-CQE, Departamento de Engenharia Química, Instituto Superior Técnico,
Universidade de Lisboa, 1049-001 Lisboa, Portugal
* Correspondence: kush.upadhyay@c2cnewcap.com or kushkumar434@gmail.com
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 phenom-
enon, its insufficient durability, and its susceptibility to gas poisoning are the reasons for the con-
stant 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 ac-
tivity 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 ad-
vances regarding biopolymer-derived ORR catalysts are summarized, with a focus on alginate as a
source.
Keywords: oxygen reduction reaction; biopolymers; alginate; carbon porous catalysts
1. Introduction
In recent years, the awareness of the human influence on climate change has in-
creased. More and more attention is being paid to factors leading to global warming, es-
pecially 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 envi-
ronmentally friendly and can be used for small, portable electronic devices and military
and space devices [3–5]. 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 elec-
trons 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 [10–12].
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
Citation:
Rupar, J.; Tekić, D.;
Janošević Ležaić, A.;
Upadhyay, K.K.
ORR Catalysts
Derived from Biopolymers.
Catalysts
2023, 13, 80. https://doi.org/
10.3390/catal13010080
Academic Editor
: Nicolas
Alonso
-Vante
Received:
30 November 2022
Revised:
17 December 2022
Accepted:
25 December 2022
Published:
30 December 2022
Copyright:
© 2022 by the authors. Li-
censee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and con-
ditions of the Creative Commons At-
tribution (CC BY) license
(https://cre-
ativecommons.org/licenses/by/4.0/).

Catalysts 2023, 13, 80 2 of 24
serious anode crossover, and it is easily poisoned by CO (an intermediate product of elec-
tro-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 conductivi-
ties, 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 O2, 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 per-
formances 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 [17–20]. 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 [21–23]. 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 addition-
ally 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. Com-
pared 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 [27–30]. 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 facilitat-
ing the O-O bond breaking in OOH* leads to the inhibition of H2O2 formation, this mate-
rial 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 co-
ordinated by N in the carbon network are considered as the most promising catalysts for
ORR [13].

Catalysts 2023, 13, 80 3 of 24
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–
39].
The carbonization temperature presents another important factor that affects the ac-
tivity 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 in-
creases [40,41]. In addition, the temperature increase leads to porosity development [36].
Additionally, the activity of the carbon-based catalyst depends on the relationship be-
tween 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 activ-
ity increases. The performance improvement is primarily due to the presence of free-flow-
ing 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 het-
eroatoms doping can enhance the catalytic activity given that the doped heteroatom gen-
erally 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 conse-
quently 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 sub-
stantial 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 polyfer-

Catalysts 2023, 13, 80 4 of 24
ric flocs by Shewanella oneidensis MR-1, the obtained material showed an excellent cata-
lytic 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 ob-
taining heteroatom-doped carbon material through the process that uses no hazardous
reagents [53].
Biopolymers, substances consisting of numerous repeating monomer units, are pre-
sent in natural sources. In Table 1, 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 biopol-
ymers are used as edible films, emulsions, packaging materials in the food industry, drug
transport materials, medical implants, tissue scaffolds, dressing materials in pharmaceu-
tical industries, etc. [55].
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
In this review, we will discuss the use of some biopolymers-based materials as prom-
ising 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 [56–61].
Figure 1 represents the biopolymer categorization [55] with respect to the biopoly-
mers considered in this review paper.
Figure 1. A pictorial depiction of several natural, renewable biopolymers categorized according to
their source [55].

Catalysts 2023, 13, 80 5 of 24
In Table 2, the structures of the biopolymers reviewed in this paper are presented.
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

Catalysts 2023, 13, 80 6 of 24
lignin
lignin monomer
p-coumaryl alcohol
coniferyl alcohol
synapil alcohol
Major challenges during the synthesis of ORR catalysts include the efforts to extract
biopolymers from the biomass, along with methods for cleaning and decomposition, sim-
plifying 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].

Catalysts 2023, 13, 80 7 of 24
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 se-
quence 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 us-
ing 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 temper-
atures (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 influ-
enced 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 perfor-
mance. Based on the Koutecky–Levich (K-L) plots, the electron transfer number was cal-
culated. At 0.4 V, the n-value was 3.95, indicating that the catalyst ensures that the reduc-
tion mechanism takes place as a four-electron mechanism. Voltametric studies were em-
ployed, 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 elec-
trode (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 toler-
ance, whereby N-PCNF-600 had a better fuel selectivity toward ORR than Pt/C consider-
ing 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.

Catalysts 2023, 13, 80 8 of 24
Figure 2. Schematic illustration of the synthesis process of N-doped porous graphitic carbon nano-
fibers (N-PCNFs). Reprinted from [63] with permission. Copyright 2015 American Chemical Soci-
ety.
The high charge and discharge performance, based on the specific structure and bi-
modal shape of the pore size distribution, is one of the important advantages of this cata-
lyst.
Zhao et al. used calcium ions to form a network with alginate, after which wet spin-
ning 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 [71–73]. 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 nitro-
gen 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

Catalysts 2023, 13, 80 9 of 24
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 con-
sidered as active sites. Additionally, it was suggested that pyridinic nitrogen could coor-
dinate with cobalt ion. The Co nanoparticles influenced the aggregation process and cor-
rosion 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 meas-
urement, 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 com-
mercial 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 at-
tributed to the ternary Ni/NiO/NiCO2O4 active site for electron transfer and the 3D hier-
archical 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].

Catalysts 2023, 13, 80 10 of 24
Figure 3. Schematic synthesis of Ni/NiO/NiCo2O4/N-CNT-As electrocatalysts; (a,d) shematic rep-
resentation of SA with CNs, (b,e) hydrogels formed upon addition of aqueous solution containing
Co2+ and Ni2+ ions; (c,f) freeze-dryied hydrogels formed aerogels. Reprinted with permission from
[78]. Copyright 2015 The Royal Society of Chemistry.
According to the K–L equation, as in other reported materials in this review, the num-
ber 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 cur-
rent 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 ma-
trix 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 [81–85]. 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 [86–89].
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 at-
tracted 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 [92–94]. 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 solu-
tion, Co3O4/Co-NPC had a peak at 0.738 V, which was at more positive potentials com-
pared 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 neg-
ligible response after the addition of methanol and thus showed better tolerance. O2-satu-
rated 0.1 M KOH at 0.5 V (vs. RHE), with a rotating speed of 1600 rpm, was used to de-
termine 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

Catalysts 2023, 13, 80 11 of 24
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 accelerat-
ing the process of mass transfer and the transmission of electrons [96]. As ZIFs were com-
posed 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 au-
thors confirmed that the reduction reaction occurred through a four-electron pathway ac-
cording 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 mate-
rial 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 electrocata-
lytic 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-dry-
ing 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 sup-
posed/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 conse-
quence 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 ad-
sorption.
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 alka-
line 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

Catalysts 2023, 13, 80 12 of 24
method for synthesis, these authors have significantly contributed to the development of
SACs. In addition, they avoided physical and traditional chemical approaches which usu-
ally 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 syn-
thesized 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 ma-
terial 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.
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 elimi-
nated upon an-
nealing
N N N N N N
Dopant / / Co
Co,
Ni Co Co Fe Cu
Half-wave
potential of
cata-
lyst/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 cat-
alyst
N-doped
porous gra-
phitic car-
bon nano-
fibers
defective car-
bon fibers
3D Co/N-
doped hier-
archical po-
rous carbon
micro-
spheres
carbon
nanotubes
nanoaero-
gels
Co3O4/Co
species incor-
porated into
the N-doped
carbon matrix
carbon fiber-
coated
Co@N-
doped po-
rous carbon-
doped
3D Fe-algi-
nate/gra-
phene aer-
ogels
single-
atom
catalyst
Synthesis
technique
electrospun
wet spinning
electrospin-
ning
freeze-dry-
ing
hydrothermal
reaction
chemical re-
action
freeze-dry-
ing
chemical re-
action
3. Other Widely Used Biopolymers
Cellulose, lignin, and chitin are widely distributed polymer materials that are in-
creasingly used as sustainable precursors for the synthesis of carbonaceous materials since
they are non-toxic, biodegradable, and biocompatible [103,104].

Catalysts 2023, 13, 80 13 of 24
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 struc-
ture, 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 hier-
archical 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 ma-
terial was carbonized at different temperatures. The authors explained in detail the influ-
ence of the preparation parameters and contributed to the improvement of the methodol-
ogy 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 encour-
aged pyrolysis at low temperatures and the rapid exfoliation of cellulose fibers. In addi-
tion, 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). Conse-
quently, 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 hierar-
chical open pore structure contributed to the increase in the exposed active sites, thus en-
abling 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 po-
tential 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 car-
bonization 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 de-
creased [108].

Catalysts 2023, 13, 80 14 of 24
Figure 4. Schematic diagram of the preparation of N-doped porous carbon particles with a hierar-
chical pore structure from cellulose. Reprinted with permission from [108]. Copyright 2019 Ameri-
can Chemical Society.
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 ex-
clude 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 treat-
ment 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 num-
ber 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].

Catalysts 2023, 13, 80 15 of 24
Lignin presents an amorphous aromatic polymer [58] which consists of p-hydro-
phenyl (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, nan-
ofibers, 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 produc-
tion. Accordingly, the development of an environmentally friendly process for the pro-
duction 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 activ-
ity is not absolutely understood yet.
The reported carbon materials derived from lignin include nitrogen-doped and ni-
trogen/sulfur- and nitrogen-phosphorus-co-doped catalysts [119–121]. 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 FeNx and FePx species had an impact on the prevention of the
agglomeration of active sites and thus provided great durability. Finally, the obtained cat-
alyst 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 con-
sisted 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 pro-
cesses, 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 car-
bon contents and low ash residues [123]. Borghei et al. [124] developed an environmen-
tally friendly method, without using hazardous chemicals to synthesize N/P-doped po-
rous 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.

Catalysts 2023, 13, 80 16 of 24
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-dimen-
sional structure of a high porosity. The long-term stability was evaluated by chronoam-
perometry at 0.7 V and at 900 rpm after 50,000 s, and the results showed that the function-
alization 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 to-
ward 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 ena-
bles 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 co-
conut 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 accom-
plished 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 pres-
ence of nitrogen, it had an enhanced electrocatalytic performance, a higher current den-
sity, 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-elec-
tron reduction pathway was obtained for N-doped activated carbon [123]. The LSV anal-
ysis 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 opti-
mal 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-dimen-
sional 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 [132–134]. The onset potential in 0.1 M KOH satu-
rated 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].

Catalysts 2023, 13, 80 17 of 24
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 [135–137]. The solution to
this problem may involve the immobilization of carbon nanodots onto conductive carbon
with the aim of overcoming the falling-off process [135–137]. Additionally, the low graph-
itization 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 effi-
cient 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 car-
bons 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 tem-
perature of 900 °C was proved to be the best for optimal ORR characteristics. The combi-
nation 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 meso-
pores 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 ex-
cellent 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 fabrica-
tion 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 activ-
ity is comparable to that of Pt/C. Apart from alginate, cellulose, chitin, and lignin are ex-
cellent 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 ex-
cellent starting compound for obtaining ORR catalysts with better durability, an im-

Catalysts 2023, 13, 80 18 of 24
proved tolerance to methanol, and better electrocatalytic properties compared to commer-
cial ORR catalysts. Various biopolymer-derived, metal-free ORR catalysts have been stud-
ied 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-de-
rived carbons can ensure efficient oxygen transport and even alter the reaction mecha-
nism, making them a viable component in PEMFCs. Despite the tremendous advance-
ments 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-cen-
tered 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.
Author Contributions: J.R.: Conceptualization, Writing−Original Draft, Visualization, Writing−Re-
view and Editing. D.T.: Visualization, Writing−Review and Editing, A.J.L.: Writing-Original Draft,
Writing−Review and Editing. K.K.U.: Conceptualization, Writing−Original Draft, Supervision,
Writing−Review and Editing. All authors have read and agreed to the published version of the man-
uscript.
Funding: This research was funded by the Ministry of Education, Science, and Technological De-
velopment, the Republic of Serbia, through a Grant Agreement with the University of Belgrade—
Faculty of Pharmacy No. 451-03-68/2022-14/200161 and the University of Belgrade—Faculty of
Physical Chemistry No. 451-03-68/2022-14/200146.
Data Availability Statement: Not applicable.
Conflicts of Interest: The authors declare no conflict of interest.
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