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http://www.journals.elsevier.com/
journal-of-energy-chemistry/
Review
Microbe-derived carbon materials for electrical energy storage and
conversion✩
Li Wei a, H. Enis Karahan a, Shengli Zhai a, Yang Yuan a, Qihui Qian a, Kunli Goh a,
Andrew Keong Ng b, Yuan Chen a,c,∗
a
School of Chemical and Biomedical Engineering, Nanyang Technological University, 62 Nanyang Drive, 637459, Singapore
Singapore Institute of Technology, 10 Dover Drive, 138683, Singapore
c
School of Chemical and Biomolecular Engineering, The University of Sydney, NSW 2006, Australia
b
a r t i c l e
i n f o
Article history:
Received 28 September 2015
Revised 2 November 2015
Accepted 2 November 2015
Available online xxx
Keywords:
Microbe
Carbon materials
Supercapacitor
Lithium-ion battery
Oxygen reduction reaction
Hydrogen evolution reaction
a b s t r a c t
Microbes are microscopic living organisms that surround us which include bacteria, archaea, most protozoa, and some fungi and algae. In recent years, microbes have been explored as novel precursors to
synthesize carbon-based (nano)materials and as substrates or templates to produce carbon-containing
(nano)composites. Being greener and more affordable, microbe-derived carbons (MDCs) offer good potential for energy applications. In this review, we describe the unique advantages of MDCs and outline
the common procedures to prepare them. We also extensively discuss the energy applications of MDCs
including their use as electrodes in supercapacitors and lithium-ion batteries, and as electrocatalysts for
processes such as oxygen reduction, oxygen evolution, and hydrogen evolution reactions which are essential for fuel cell and water electrochemical splitting cells. Based on the literature trend and our group’s
expertise, we propose potential research directions for developing new types of MDCs. This review, therefore, provides the state-of-the-art of a new energy chemistry concept. We expect to stimulate future
research on the applications of MDCs that may address energy and environmental challenges that our
societies are facing.
© 2015 Science Press and Dalian Institute of Chemical Physics. All rights reserved.
1. Introduction
Modern human societies heavily rely on the consumption of
nonrenewable fossil fuels for powering engines and producing
electricity. However, the depletion of fossil fuels have triggered
great economic concerns, and the generation of huge amounts of
carbon dioxide from burning fossil fuels escalated environmental
crises like global warming [1,2]. A promising approach to address
these issues is the vast utilization of electrical energy converted
from renewable sources such as solar, wind, geothermal, hydroelectric, tidal energy, and biomass [3–6]. Although these sources
are already in use for decades, we still fail to maximize all their
potential due to a variety of issues. Among the technological and
economic issues associated with renewable energy sources, there
are two key issues worth mentioning.
✩
This work was supported by the Ministry of Education, Singapore (2013-T1-002132) and the iFood program of Nanyang Technological University. The corresponding author, Yuan Chen, also acknowledges The University of Sydney for financial
support.
∗
Corresponding author at: School of Chemical and Biomolecular Engineering, The
University of Sydney, NSW 2006, Australia. Tel: +61 2 8627 4620.
E-mail address: yuan.chen@sydney.edu.au (Y. Chen).
First, the electricity generated by renewable sources and the
daily energy demands often fluctuate over time [7]. Thus, in order to obtain a reliable power supply that can meet the demand
without continuously generating energy excess, we need affordable
energy storage devices that can effectively store the excess renewable energy output whenever available [8]. However, present energy storage systems are either ineffective or expensive to rationalize their common usage for such a purpose. Second, for most
engines, the consumption of fossil or chemical fuels with high specific energy density is still more favorable than the use of electrical energy [2,9]. Actually, being an efficient energy carrier with
one of the highest specific energy values among all chemical fuels (142 MJ/kg) and with an added benefit of zero-emission, hydrogen is a viable alternative [10]. However, as in other chemical
fuels, we still need to develop better processes for efficient conversion of the chemical energy of hydrogen into electrical energy,
which is typically conducted by fuel cells and electrochemical water splitting cells [1]. Unfortunately, similar to the problems of energy storage systems mentioned earlier, both fuel cells and water
splitting cells suffer from either low efficiency or high cost. To address both of these two key issues, a new family of carbon materials produced using microbes as precursors called microbe-derived
http://dx.doi.org/10.1016/j.jechem.2015.12.001
2095-4956/© 2015 Science Press and Dalian Institute of Chemical Physics. All rights reserved.
Please cite this article as: L. Wei et al., Microbe-derived carbon materials for electrical energy storage and conversion, Journal of Energy
Chemistry (2016), http://dx.doi.org/10.1016/j.jechem.2015.12.001
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Fig. 1. Possible roles of MDCs as electrodes for electrical energy storage and as electrocatalysts for electrical energy conversion systems.
carbons (MDCs) can be exploited either as electrode materials or
electrocatalysts (Fig. 1).
In this review, we first introduce the fundamentals of devices
and conventional devices and carbon materials involved in electrochemical energy storage and conversion. Next, we explain the
unique advantages of MDCs as new carbon materials. We subsequently describe the typical procedures for converting microbes
into MDCs. Then, we thoroughly discuss various latest applications
of MDCs as electrodes in supercapacitors and lithium-ion batteries and as electrocatalysts for electrochemical processes like oxygen reduction, oxygen evaluation, and hydrogen evolution reactions which are the basis of fuel cells and electrochemical water
splitting cells. Lastly, we propose a variety of potential research directions for MDCs based on the literature trend and our group’s
own expertise. We ultimately aim to provide the state-of-the-art
account of this emerging topic and to encourage future research on
the use of microbes as sustainable precursors or templates in producing carbon (nano)materials or (nano)composites beneficial for
the development of practically viable electrical energy storage and
conversion systems.
2. Devices and carbons for electrochemical energy storage and
conversion
2.1. Devices used for electrochemical energy storage and conversion
In order to overcome two key issues briefed in the Introduction, four types electrochemical devices are intensively studied: (1)
supercapacitors and (2) lithium-ion batteries for electrical energy
storage, (3) hydrogen fuel cells for converting hydrogen to electrical energy, and (4) electrochemical water splitting cells for transforming electrical energy into hydrogen.
As sketched in Fig. 2(a), supercapacitors store electrical energy by the physical adsorption of electrolyte ions on the surfaces
of electrode materials (a.k.a. electrochemical double layer capacitance, EDLC) and/or by reversible redox reactions, intercalation or
electrosorption at or near the surface of some electrode materials
(called pseudocapacitance) [11]. Lithium-ion batteries (Fig. 2b), on
the other hand, store electrical energy by moving lithium ions between an intercalated lithium compound in cathode and a carbon
material based anode. The electrolyte allows ionic movement during charge and discharge [12,13].
In hydrogen fuel cells (Fig. 2c), hydrogen is oxidized through
hydrogen oxidation reaction (HOR) on catalysts loaded on the anode to generate hydrogen ions and electrons. Subsequently, hydrogen ions go through the electrolyte towards the cathode, while
electrons travel through an external circuit producing direct current electricity. Oxygen, hydrogen ions and electrons then react by
Fig. 2. Schematic illustrations of working mechanism of (a) a supercapacitor, (b)
a lithium-ion battery, (c) a hydrogen fuel cell, and (d) an electrochemical water
splitting cell.
oxygen reduction reaction (ORR) to form water on the cathode side
of the cells [14]. Conversely, in an electrochemical water splitting
cells (Fig. 2d), an electrical power source is connected to two electrodes in water. Hydrogen appears at the cathode by hydrogen evolution reaction (HER), while oxygen is generated on the anode by
oxygen evaluation reaction (OER) [15,16].
2.2. Carbons used in electrochemical energy storage and conversion
devices
In the electrochemical energy storage and conversion platforms
explained earlier, the fundamental material used is carbon. Carbon
materials (in short, carbons) form a broad class of ordered and
disordered solid phase materials mostly composed of elemental
carbon (C). They can be either be synthetic or natural in origin,
including graphite, graphene and graphene oxide (GO), carbon
nanotubes, fullerenes, carbon fibers and filaments, porous carbons, pyrolytic carbon, glassy carbon, carbon black, diamond and
diamond-like carbon, and chars. All these carbons play many
important roles in energy storage and conversion systems. For example, graphene, GO, carbon nanotubes, and fullerenes can assist
the generation of electricity from solar energy [17–20]. Similar
carbons are also helpful in various biomass conversion reactions
both as catalyst supports and as active catalysts [21–24]. Carbons
like activated carbon and carbon black have been widely used for
electrical energy storage in supercapacitors as electrode materials
[25–29] and graphite is the dominant anode materials in lithiumion batteries [30,31]. Carbons are also utilized in electrochemical
energy conversion serving either as catalyst supports or as active
catalysts for ORR, OER, and HER in fuel cells and electrochemical
water splitting cells [18,32–34].
Carbons possess diverse and interconnected physical and
chemical properties. To achieve outstanding and reproducible
performance out of carbons in energy applications, their structures
should be controlled well [26–29,35,36]. For example, carbons
used for supercapacitor electrodes should have large specific surface area for ion adsorption, a suitable combination of micropores
and mesopores for fast ion mobility, good electrical conductivity
for electron transfer, and favorable surface functionalities for
Please cite this article as: L. Wei et al., Microbe-derived carbon materials for electrical energy storage and conversion, Journal of Energy
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pseudocapacitance and surface wettability [11,28,36,37]. Carbons
used as anodes of lithium-ion batteries, such as graphite and
hard carbons, need to have appropriate grain size, which allows
efficient intercalation of lithium, while minimizing the exfoliation
of graphene sheets. On the contrary, to form composite materials
for cathodes, conductive and porous carbons are utilized [38,39].
Heteroatoms or surface functional groups can enhance catalytic
activities of carbons for ORR, OER, and HER [37,40–43]. However,
the synthesis of such carbon materials usually requires fossilderived precursors. Furthermore, expensive sacrificial inorganic
templates are usually used to create suitable porosity in some
carbon materials [44–49]. Thus, it is of importance to develop sustainable processes in which carbons with desired properties that
can be obtained from renewable precursors economically. Indeed,
many different renewable sources have been exploited as precursors to yield carbons [50–53]. Among them, MDCs offer a good
opportunity for serving as new electrode materials and as new
electrocatalysts. The subsequent sections introduce the advantages
and preparation routines of MDCs, followed by their specific applications in various types of energy storage and conversion devices.
3. Microbe-derived carbon materials
3.1. Why microbe-derived carbon materials?
Microbes are microscopic living organisms that surround us;
they include bacteria, archaea, most protozoa, and some fungi and
algae [54]. As crucial players in our ecosystem, they not only take
a major role in the bottom of the food chain, but also serve as decomposers involved in the disintegration of dead organisms that
help sustain the life cycle [55]. In addition to their roles in ecosystem, their symbiotic presence in our bodies, as well as in other
higher organisms, is also highly essential [56]. Another great importance of microorganisms for human life is their involvement in
food and beverage preparation through fermentation since ancient
times. With the development of modern biotechnological techniques, microbes are now also being modified as living factories
to produce valuable chemicals and medicines [57].
There are several advantages of using microbes as precursors
for preparing carbons, MDCs. First, industrial fermentation processes produce microbial cells in large quantities for different purposes and resultant cellular biowastes may cause environmental
problems if not adequately reused [58,59]. Second, microbes, like
other cells, are naturally formed by different high-surface area cellular structures that can lead to a large surface area with potentially controllable porosity in the resultant carbons [60–62]. Third,
as all other living organisms, microbes are comprised mainly of
carbon, hydrogen, oxygen, nitrogen, phosphor and sulfur with trace
amounts of other elements. On top of their high carbon content which serve as the main structure in MDCs, their rich nitrogen, phosphor, and sulfur contents fruitfully provide abundant
heteroatoms as dopant [63,64]. It is important to highlight that
such heteroatom functionalities can serve as active catalytic sites
for electrochemical reactions like ORR, OER, and HER [64].
3.2. Common procedures for obtaining microbe-derived carbons
Despite the variation of exact parameters, the vast majority of
studies conducted on MDCs follow a general experimental flow.
In MDC preparation, microbial cells are first grown in aqueous
culture media (a.k.a. broth) until they reach a suitable cellular
yield. Then, cells are generally harvested via centrifugation (as a
fast alternative of filtration). Harvested cells are washed to remove
culture medium residues and cellular by-products formed during
growth. Subsequently, water content which accounts ∼60–80 wt%
of the total mass of cells is removed through a drying process like
oven-drying or freeze-drying. Dried cells are then carbonized under an inert atmosphere (e.g., Ar and N2 ) to prevent burning at
elevated temperatures, usually in between 300 and 1000 °C. Obtained carbons, MDCs, can be used for numerous applications as
mentioned earlier. However, additional activation procedures are
often performed to further enhance the porosity and to modify
surface chemistry of end products. For instance, micropores and
mesopores can be generated using some oxidizing agents, such as
water steam or carbon dioxide, which help remove volatile products from MDCs [65,66]. Alternatively, other activating agents like
H3 PO4 , KOH or ZnCl2 can be added to dry cells during their carbonization to assist the formation of pores and to create a larger
specific surface area [67,68]. Due to the chemical complexity of microbial cells, detailed mechanisms of these activation procedures
are not readily available; however, experimental results obtained
so far suggest that the pore size distribution and carbon yield can
be systematically controlled by tuning activation and carbonization
parameters [62,67].
Furthermore, various nanocomposite materials can be synthesized using MDCs together with either other carbon nanomaterials,
such as GO or metal nanoparticles like Pd, Co3 O4 , Ni3 S2 , MnO2 ,
and CoFe2 O4 . These additional nanocomponents may be directly
deposited on MDCs. Alternatively, carbon nanomaterials or metal
precursors can be added into culture media during cell growth or
dry cell powders before carbonization, resulting in nanocomposites
with hierarchical structures. Further details of these approaches,
along with the applications of MDCs, are discussed in the next section.
4. Applications of microbe-derived carbons
4.1. Supercapacitor electrodes
Although not based on microorganisms, it is worth mentioning
that, in a pioneering work by Zhu et al. in 2011, porous carbon
materials produced from edible fungi including Auricularia have
been successfully used as supercapacitor electrodes. Without
any activation step, the fungi-derived carbons reached a high
specific capacitance of 196 F/g [69]. In another study that took
the advantage of two activating agents, Sun et al. demonstrated
the preparation of a hierarchically porous MDC using baker yeast,
Saccharomyces cerevisiae (S. cerevisiae) [70]. As illustrated in Fig.
3(a), KOH was used not only to connect yeast cells but also to
introduce micropores. During the high temperature carbonization process, KOH would react with carbon to form H2 , K and
K2 CO3 , followed by other reactions and/or decomposition of K2 CO3
[70,71]. Furthermore, glutaraldehyde was added as a binder, which
helps form macropores by cross-linking cells. The resultant MDCs
showed an excellent specific capacitance of 330 F/g with good
charge/discharge cycling stability. It is worth mentioning that
such a specific capacitance value is higher than commonly used
activated carbons of ∼100–300 F/g in aqueous electrolytes [72]. In
general, the capacitive performance of all types of carbon-based
materials depends on their specific surface area and porosity. Thus,
utilization of activating agents can significantly influence the pore
formation and performance of MDCs. Zhu et al. also compared
the effects of two common activating agents, KOH and ZnCl2 ,
on the properties of MDCs prepared by a rod-like bacterium,
Bacillus subtilis (B. subtilis) [68]. They found that KOH introduces
micropores increasing nearly 50% of the total specific surface area
in the resultant MDC. In contrast, ZnCl2 produces more mesopores
leading to lower specific capacitances.
Alternatively, microbial cells can also serve as templates or
substrates for loading other types of carbon materials with high
specific surface area or pseudocapacitive materials. Due to their
large specific surface areas, graphene-family nanomaterials like
Please cite this article as: L. Wei et al., Microbe-derived carbon materials for electrical energy storage and conversion, Journal of Energy
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terial cells. The resultant MDCs exhibited a high specific capacitance of 327 F/g. Likewise, B. subtilis was adopted as substrates
for loading porous Ni3 S2 , as shown in Fig. 3(c) [76]. Bacteria were
mixed with nickel acetate and GO in aqueous solution. Ni2+ ions
helped to bind GO and bacteria each other. Thiourea (CH4 N2 S) was
added to reduce Ni2+ and GO to form composites of Ni3 S2 , reduced
GO (rGO) and bacteria. The composites were then electrostatically
sprayed on nickel foam. The yielding composite materials showed
extremely high specific capacitance of 1424 F/g. In another related
work, Shim et al. mixed sphere-shaped bacterium Micrococcus lylae
(M. lylae) with CoCl2 and a reducing agent (NaBH4 ). After the reduction process, Co3 O4 was deposited on cells as rendered in Fig.
4(d) [77]. This composite design can also achieve a high specific
capacitance of 1324 F/g. A summary of MDCs used as supercapacitor electrodes is listed in Table 1.
4.2. Lithium-ion battery electrodes
Fig. 3. Schematic illustrations of the synthesis routines (left) and scanning electron
microscope (SEM) images (right) of nanocomposite MDCs used as electrode materials in supercapacitors. (a) Formation of hierarchically porous MSCs using yeast
cells as templates and KOH as activating agent [70], (b) preparation of a GO-bacteria
composite using Fe3+ as a binder [75], (c) synthesis of porous Ni3 S2 /carbon composite using B. subtilis, GO and Ni2+ as precursors [76], (d) formation of Co3 O4 /carbon
composite by reducing Co2+ on M. lylae cells [77].
graphene and GO can be utilized to increase EDLC of resulting electrodes [73,74]. A hierarchically porous composite carbon was prepared with GO and a common bacterium, Escherichia coli (E. coli),
as sketched in Fig. 3(b) [75]. Fe3+ ions were used as “binder” to
connect negatively charged GO sheets and negatively-charged bac-
Several MDCs have been used to produce nanocomposite materials as anodes in lithium-ion batteries as summarized in Table 2.
As illustrated in Fig. 4(a), Wang et al. reported a composite of GO
and E. coli as an anode material, displaying a stable reversible
discharge capacity of 501.5 mAh/g [78]. This value is much higher
than that of commercial graphite at 372 mAh/g. In another work,
Shim et al. used B. subtilis cells as sacrificing templates to synthesize hollow MnO nanorods. A precursor solution containing MnCl2
was added to cell suspension with a reducing agent (NaBH4 ). The
bacteria/MnO composite rods were calcined at 200 or 300 °C in
air to produce hollow MnO2 nanostructures. 200 °C calcination led
to a larger surface area (130 m2 /g) than that obtained after 300 °C
calcination (116 m2 /g). The resultant composite material exhibited
a low reversible specific capacity of 200–300 mAh/g after 10
cycles [79]. Another approach employed Nannochloropsis oculata
(N. oculata) cells to produce monodisperse MnO/C microspheres.
KMnO4 was reduced to deposit MnO2 on N. oculata cells as shown
in Fig. 4(b). The resulting material composed of cells and MnO2
particles was further coated with a polystyrene film. During heat
treatment, both polystyrene and N. oculata cells were carbonized,
and MnO2 was reduced to MnO [80]. It was suggested that the
free volume present in the hollow interior and the porosity of
the shell may favorably accommodate the volume changes in
electrochemical reactions and improve the accessibility of MnO
hosts to lithium ions. Resultant MnO/C microspheres demonstrated a high reversible capacity of 705 mAh/g after 50 cycles of
Table 1. Summary of MDCs and MDC-involved nanocomposites used as supercapacitor electrodes.
Microbial precursor
Activating agent
Temperature (°C)
Specific surface
area (m2 /g)
Electrolyte
Specific
capacitance (F/g)
Ref.
S. cerevisiae
B. subtilis
KOH
N/A
ZnCl2
KOH
N/A
N/A
N/A
750
800
1227
96
985
1578
182
N/A
149
1 M KOH
6 M KOH
330
N/A
N/A
310
327
1424
1324
[70]
[68]
E. coli
B. subtilis
M. lylae
700
N/A
N/A
1 M H2 SO4
2 M KOH
3 M KOH
[75]
[76]
[77]
Table 2. Summary of MDCs and their nanocomposites used in lithium-ion battery electrodes.
Microbial precursor
Active material
Temperature (°C)
Specific surface
area (m2 /g)
Specific
capacitance
(mAh/g)
Ref.
E. coli
B. subtilis
N. oculata
B. subtilis
rGO
MnO
MnO
Co3 O4
900
300
500
300
288
116
77
N/A
502
300
705
903
[78]
[79]
[80]
[81]
Please cite this article as: L. Wei et al., Microbe-derived carbon materials for electrical energy storage and conversion, Journal of Energy
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Fig. 4. Schematic illustrations of the synthesis routines (left) and SEM images (right) of composite MDCs used as electrode materials in lithium-ion batteries. (a) The use
of a composite of GO and E. coli as carbon precursors [78], (b) monodispersed MnO/C microspheres prepared from N. oculata cells [80], (c) porous Co3 O4 nanostructures
synthesized using B. subtilis cells as templates [81].
Fig. 5. Schematic illustrations of the synthesis routines (left) and SEM images (right) of composite MDCs used as electrocatalysts in ORR. (a) Porous nitrogen-doped carbon
materials derived from waste water treatment sludge comprising a mixture of cells and mineral compounds [86], (b) carbons derived from whole yeast cells (S. cerevisiae)
and cell walls [88], (c) composite carbons loaded with Pd nanoparticles obtained from S. oneidensis cells [89].
Please cite this article as: L. Wei et al., Microbe-derived carbon materials for electrical energy storage and conversion, Journal of Energy
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Table 3. Performances of MDCs as electrocatalysts in ORR.
Microbial precursor
Activating agent
Mixture of cells
B. subtilis
N/A
ZnCl2
KOH
N/A
N/A
Co(NO3 )2
FeCl3
E. coli
E. coli
Yeast
Yeast
a
b
c
Temperature (°C)
700
800
900
1000
800
900
Specific surface
area (m2 /g)
Heteroatom loadinga (at%)
n
Onset potential (V)
Ref.
311
985
1578
288
636
575
912
N:6.5
N:4.66
N:0.68
N:2.42; P:1.25; S:0.42
N:3.59; P:3.48
N:5.16; P:1.94
N/A
3.70
3.93
3.12
3.95
3.83
3.80
3–4
–0.04 vs. Ag/AgCl
–0.06 vs. Ag/AgCl
–0.18
–0.08 vs. Ag/AgCl
–0.18 vs. SCEb
0.93 vs. RHEc
–0.11 vs. SCE
[86]
[68]
[78]
[62]
[87]
[88]
N for nitrogen, P for phosphor, and S for sulfur.
SCE: saturated calomel electrode.
RHE: reversible hydrogen electrode.
charge/discharge [80]. Co3 O4 nanorods were also prepared with
rod-shaped B. subtilis cells as a template (Fig. 5c) [81]. Co ions
were adsorbed on negatively charged bacterial surfaces and the
composite obtained after carbonization showed a specific capacity
of 903 mAh/g. Furthermore, several microbial species have been
found to accumulate metal species from their living environments
[60,82,83]. Metal nanoparticles enriched in such microbes were
also tested as lithium-ion battery electrodes [84,85].
4.3. Catalysts for energy conversion
Microbial cells contain significant amounts of nitrogen, phosphor, and sulfur which can be successfully incorporated into carbon frames under appropriate carbonization conditions as discussed above. These heteroatoms may dramatically alter the electronic structures of resulting carbons and create active catalytic
sites. A number of studies have consistently indicated that MDCs
are promising electrocatalysts or catalyst supports for ORR, HER,
or OER.
Sludge from waste water treatment is a mixture of inorganic
waste and a variety of microbes. As depicted in Fig. 5(a), Zhou et
al. adopted this mixture as a precursor to prepare porous nitrogen
doped carbon materials as carbon catalysts for ORR [86]. The incorporation of nitrogen in the graphitic matrix yielded pyridinic
and pyrrolic moieties. Carbonization temperature influenced the
surface area, heteroatom loading, and ORR performances of resulting carbon catalysts. Higher carbonization temperature improved
graphitic carbon structures but led to lower heteroatom loadings.
The optimal temperature was identified within the range of 600 to
800 °C, where the number of electron transfer involving ORR was
3.5 to 3.7 in an alkaline media.
Nitrogen and phosphor compounds naturally present in microbial cells lead to the formation of MDCs dually-doped with nitrogen and phosphor. Although the exact mechanism is still unclear,
the synergistic effects of nitrogen and phosphor doping clearly improve the catalytic activity of carbons for ORR. This observation
has been made by several researchers using both bacterial (E. coli)
and yeast (S. cerevisiae) cells [62,78,87,88]. In particular, as illustrated in Fig. 5(b), Huang et al. demonstrated that carbons obtained from whole yeast cells have higher electrocatalytic activity than those produced from cell walls because the intracellular
components, such as proteins, phospholipids, DNAs, and RNAs, provided more heteroatom dopants [88]. We would like to emphasize
that this finding is important because it suggests possible use of
engineered cells as a way to obtain different MDCs which we will
explain in the last section as a future direction.
Various activating agents employed in carbonization processes
can significantly change the composition, porosity, and morphology of MDCs, thereby leading to somewhat different ORR performances. For example, micropore-enriched MDCs with 0.68 at%
nitrogen obtained by KOH activation exhibited inferior ORR performances to mesopore-enriched MDCs with 4.66 at% nitrogen pre-
pared with ZnCl2 activation (the on-set potential: –0.18 V vs. –0.06
V; the number of electron transfer: 3.12 vs. 3.93) [68]. In another
work, Gong et al. used Co(NO3 )2 to form NH4 CoPO4 nanoparticles
on yeast cells [87]. Mesopores were formed after washing away
NH4 CoPO4 , which increased the number of accessible active sites
and facilitated the mass transfer inside MDCs. Furthermore, silica
shells were used to retain the spherical morphology of yeast cells
during carbonization, whereas FeCl3 was used to facilitate graphitization process [88]. Performances of MDCs used as ORR electrocatalysts are listed in Table 3.
Nanocomposites comprising MDC and other catalytic active
materials were also synthesized for ORR. Liu et al. reported a
porous CoFe2 O4 /carbon nanocomposite prepared using yeast cells
as carbon precursors and structural templates. They directly added
Co(NO3 )2 and Fe(NO3 )3 on yeast cells as precursors and formed
CoFe2 O4 particles in situ using ammonia (NH4 OH) as the precipitating agent. After the synthesis of particles, they dried the final
cell-particle mixture and carbonized under N2 at 400 °C for 2 h.
Attributed to strong coupling of CoFe2 O4 with nitrogen and phosphor, they obtained dually-doped composed MDCs with improved
ORR performance [90]. Their material also possessed catalytic activity in OER. In another work, as illustrated in Fig. 5(c), Xiong
et al. added Na2 PdCl4 to Shewanella oneidensis (S. oneidensis) dispersions to form Pd-MDC composites [91]. They employed KOH as
an activating agent during carbonization at 420 °C under Ar. Resultant composites were loaded with Pd nanoparticles with a specific surface area of 986 m2 /g. This Pd/MDC catalyst possessed 2.2
times larger specific mass catalytic activity (at 0.1 V vs. Ag/AgCl)
towards ORR compared to commercial Pt/C catalyst. Recently, Jiang
et al. used Shewanella loihica (S. loihica) cells to mediate the morphology of MnCO3 microcubes [89]. It was proposed that MnO4–
was reduced by electrons released during cellular respiration process under anaerobic conditions. They found that the concentration
of S. loihica cells play an important role in forming MnCO3 microcubes with well-defined crystal structures. Their electrochemical measurements demonstrated that, after calcination at 500 °C,
porous Mn2 O3 micro/nanocubes can exhibit good catalytic activity
in ORR in KOH solution.
Apart from ORR, MDCs were also employed as catalysts in HER.
Li et al. used a low temperature hydrothermal treatment at 180 °C
to convert Streptomyces sporoverrucosus (S. sporoverrucosus) cells
into carbon-based microtubes with a diameter of 400–500 nm
and a wall thickness of about 50 nm (Fig. 6a). Their MDCs supported by Ni foam substrate showed good catalytic activity in HER
in an acidic electrolyte (0.1 M H2 SO4 ) [92]. Since dual doping of
graphene with nitrogen and phosphor was demonstrated to be favorable for HER performance [42], MDC with nitrogen and phosphor functionalities was also evaluated as catalysts for HER [61].
We directly carbonized Staphylococcus aureus (S. aureus) cells at
900 °C to yield nitrogen and phosphor dually doped MDCs, as
displayed in Fig. 6(b) [61]. We noticed that these MDCs can provide HER performance similar to that of graphene materials dually
Please cite this article as: L. Wei et al., Microbe-derived carbon materials for electrical energy storage and conversion, Journal of Energy
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7
Fig. 6. Schematic illustrations of the synthesis routines (left) and SEM images (right) of composite MDCs used as electrocatalysts in HER. (a) Preparation of carbon-based
microtubes using hydrothermal treatment to obtain [92], (b) nitrogen and phosphor dually doped carbon materials obtained from S. aureus [61].
Table 4. A comparison of key physiochemical properties of various carbon materials.
MDCs
Graphene/GO
CNTs
Mesoporous carbons
Activated carbons
Surface area
Porosity
Electrical conductivity
Heteroatom doping
high
high
medium
high
high
high and tunable
medium-high
low-medium
high
high
medium-high
low-high
high
medium-high
low
easy and tunable
easy and tunable
difficult
easy
easy
doped with nitrogen and phosphor. The catalytic activity of those
MDCs can be further improved by using ZnCl2 and cathodic activation. ZnCl2 increased specific surface area, while cathodic activation created more oxygenated functional groups on MDC surfaces.
It should be emphasized that Tafel slope obtained was as low as
58 mV/dec, which is comparable to those of hitherto best metalfree and well-fabricated metallic HER catalysts [61,93,94].
5. Summary and future perspectives
To promote the use of electricity produced by renewable
sources instead of heavy consumption of fossil/chemical fuels, we
need to develop more efficient and affordable energy storage and
conversion systems. As sustainable precursors, microbes have been
demonstrated to be promising candidates for preparing useful carbons in energy applications. Comparing with other carbon materials, such as graphene, CNTs, mesoporous carbons and activated carbons, existing research constantly showed that MDCs can be prepared with large specific surface area, tunable micro/mesoporosity
and graphitic carbon framework enriched with high content of heteroatoms such as nitrogen, phosphor, and sulfur (Table 4). These
properties are crucial in achieving good performance in electrical
energy storage and conversion applications. Moreover, using microbes as substrates or templates, various types of nanocomposites
with interesting electrochemical properties can be prepared. In the
preparation of such nanocomposites, metal precursors can be converted to nanoparticles chemically. Or alternatively, microbial reduction may lead to the formation of nanoparticles.
We propose that the following research areas are of great importance for realizing the practical applications of MDCs. First,
we foresee that the selection of suitable microbial species and
the optimization of culture, activation, and carbonization condi-
tions can significantly help tailor the properties of MDCs for specific applications. Second, some of the cellular components extractable from whole cells can be employed in depth as they offer significantly different carbon products. For example, carbons
derived from bacterial cellulose, an organic polymeric compound
with the chemical formula of (C6 H10 O5 )n obtained from certain
types of bacteria, has shown good energy storage properties in fibrous shaped energy storage devices [95–99]. It might be beneficial to exploit different types of microbial cellulous components
and microbe-derived chemicals to identify other suitable candidates. Third, as mentioned earlier, some microbial species can accumulate metal species from their living environments to produce
metal nanoparticles [60,82,83]. Such natural process can be further
explored to yield nanocomposites with desired properties. Fourth,
modern molecular biology and genetic engineering provide powerful tools to extensively modify microbial cells. Hence, it may
create genetically modified microbial cells having unique physical
structures that are enriched with specific elements or can work
as cell factories to yield desired chemical compositions [60]. Last
but not least, cost-efficient and environmentally sustainable production processes will be needed to grow and convert microbial
cells into MDCs of desired properties. Apparently, there are many
exciting challenges awaiting us to address; nonetheless MDCs hold
great promise for tackling the emerging energy and environmental
issues.
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