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Analysis of the current state and development of direct carbon fuel cells with an alkaline electrolyte


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Among the numerous modern, high-efficiency energy technologies allowing for the conversion of chemical energy of coal into electricity and heat, the Direct Carbon Fuel Cells (DCFC) deserve special attention. These are devices that allow, as the only one among all types of fuel cells, to directly convert the chemical energy contained in solid fuel (coal) into electricity. In addition, they are characterized by high efficiency and low emission of pollutants. The paper reviews and discusses previous research and development works, both around the world and in Poland, into the technology of direct carbon fuel cells with an alkaline (hydroxide) electrolyte.
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2018 Volume 21 Issue 4 87–102
DOI: 10.24425/124509
1 Częstochowa University of Technology, Faculty of Infrastructure and Environment, Department of Energy
Engineering, Częstochowa; e-mail:
Andrzej Kacprzak1
Analysis of the current state and development of direct
carbon fuel cells with an alkaline electrolyte
abstract: Among the numerous modern, high-efciency energy technologies allowing for the conversion
of chemical energy of coal into electricity and heat, the Direct Carbon Fuel Cells (DCFC) deserve
special attention. These are devices that allow, as the only one among all types of fuel cells, to direc-
tly convert the chemical energy contained in solid fuel (coal) into electricity. In addition, they are
characterized by high efciency and low emission of pollutants. The paper reviews and discusses
previous research and development works, both around the world and in Poland, into the technolo-
gy of direct carbon fuel cells with an alkaline (hydroxide) electrolyte.
keywords: direct carbon fuel cell, alkaline electrolyte, high efciency energy technologies, fossil carbons,
Electrical energy is currently a condition for the development of the economic and civiliza-
tional world. The dynamics of electrical energy consumption in individual countries or regions
of the world depends primarily on the number of inhabitants, the level of economic, social and
civilization development as well as on the structure and efciency of energy use. For example,
the report of the International Energy Agency (Report... 2017) shows that in order to meet the
growing demand for electrical energy, by 2040, China needs to add the equivalent of today’s
US energy system to its power infrastructure, while India needs the equivalent of the Europe-
an Union’s energy system. In turn, the report of the BP group (Report... 2016a) shows that by
2035 there should not run out of energy despite the fact that the demand will grow by 34% at an
average rate of 1.4% per year. According to BP analysts, fossil fuels will remain the dominant
energy source in this period. It is forecast that they will satisfy 60% of the anticipated increase in
demand and will constitute almost 80% of global energy supplies, despite the rapid growth in the
importance of other energy sources. In our country, the strategic issue is the use of hard coal and
brown coal, which form the basis of the Polish energy system. In 2015, coal provided over 50%
of primary energy supplies, the second largest share among the countries of the Organization for
Economic Cooperation and Development (OECD) and 81% of total electrical energy generation
(Report... 2016b).
Generation of electrical energy in a conventional manner, mainly as a result of combustion
of fossil fuels, exerts a negative impact on the natural environment. During the conversion of
chemical energy of coal in power plants and combined heat and power plants, unwanted gas
products (CO2, CO, nitrogen oxides, sulfur oxides, heavy metals, etc.) and solid (y ash, slag)
are formed, which when introduced into the environment cause air pollution, surface water pol-
lution, and soil degradation.
The increase in energy demand and the problem of environmental pollution force us to look
for new low-emission and highly-efcient energy sources, among which fuel cells should be paid
special attention.
Direct carbon fuel cells are modern, alternative sources of electrical energy in which direct
conversion of chemical energy of fuel into electrical energy takes place without the need to
implement a long chain of changes which must be implemented in the thermodynamic cycle of
a typical thermal power plants. When energy is supplied to the fuel cell in the form of chemical
energy of fuel, it is immediately converted into electrical energy. The operation of fuel cells,
if continuity of fuel and oxidant supply is ensured, may theoretically last endlessly, whilst
their construction and operating principle is the same as in all galvanic cells. They consist of
two electrodes (an anode and cathode), separated by a type II conductor, on which electro-
chemical oxidation reactions (anode) and reduction (cathode) take place. The connection of
the electrodes with the electric circuit causes the current to ow. In contrast to the circuits of
thermal engines, chemical energy in direct carbon fuel cells is directly converted into electri-
cal energy, bypassing the heat generation stage. The efciency of the cell does not depend on
the temperature difference in the device and is not limited by the maximum efciency of the
Carnot cycle, which is why it often exceeds 50%, which in addition to the simplicity of this
system surpasses even the most modern conventional coal burning fossil power plants. These
devices also generate less pollution in comparison with traditional sources of electrical energy,
because the product of electrochemical reactions is water or a pure CO2 stream (depending on
the type of cell and the fuel used).
Fuel cells may be powered with different types of fuels: gas (H2, CO, CH4), liquid (metha-
nol) and solid fuel (coal), thanks to which they may be considered as universal generators of
electrical energy. In the case of hydrogen fuel cells, the problem is the production of fuel, there
are also difculties associated with its storage and transport, which is expensive and dangerous.
This problem does not affect direct carbon fuel cells that can be directly supplied with “carbon”.
The fuel for this type of cell may be almost any substance containing the carbon element, inclu-
ding fossil carbons.
This paper reviews and analyzes the current research and development works, both in the
world and in Poland, into the technology of direct carbon fuel cells with alkaline (hydroxide)
1. Direct carbon fuel cells
A direct carbon fuel cell is an electrochemical device which directly converts chemical ener-
gy of elemental carbon into electrical energy. The substrates supplied to this type of fuel cell are
the carbon element (contained, among others, in hard coals, brown coal, carbonized biomass,
graphite, soot, coke, etc.) and oxygen (pure or contained in atmospheric air), while the products
are: electrical energy, pure stream of carbon dioxide, and mineral residue. The coal fuel is intro-
duced into the anode space of the cell and in the electrochemical reaction, carried out at elevated
temperature, oxidizes to CO2, generating electric current (Fig. 1).
Fig. 1. The basic scheme of construction and operating principle of a direct carbon fuel cell
Rys. 1. Ogólny schemat budowy i funkcjonowania węglowego ogniwa paliwowego
Currently used around the world are the DCFC-type cells which differ one another primarily
by the type of electrolyte used. The type of electrolyte determines both the conguration of the
device itself and the temperature of its operation. There are used four basic types of electrolytes:
molten carbonates, oxygen-stable ceramic materials (most often zirconium oxide ZrO2 stabilized
with yttrium oxide Y2O3), aqueous solutions of hydroxides and molten hydroxides. Recently,
cells using mixed electrolytes (so-called hybrid) have also been developed.
As a result of the use of different types of electrolytes in DCFC, different electrochemi-
cal processes occur in the electrodes. The effect of these processes is the potential difference
between the electrodes and the product of the carbon dioxide stream generated in the general
reaction (1). By connecting the electrodes with an external circuit, the cell becomes a source of
electrical energy.
C + O2 = CO2 (1)
The voltage under electrostatic conditions (electromotive force SEM or otherwise the
cell’s reversible potential – E) is an important parameter characterizing direct carbon fuel
cells. The reversible potential of the cell for the general reaction (1) running in the DCFC at
700 K is obtained by dividing the Gibbs free energy change (ΔG700K = −395,37 kJ mol–1)
by the product of the Faraday constant (F = 96485,3 C mol–1) and the number of electrons
transferred in a single redox reaction (n = 4). The value of this potential calculated from equa-
tion (2) is 1.025 V.
E = −ΔG/nF = 1,025 V (2)
Importantly, the reversible potential of cells directly fed with coal practically does not change
with the cell’s operating temperature. In addition, direct carbon fuel cells enjoy a number of other
advantages among which one should mention: the use of solid fuel – elementary coal, which can
be obtained from many different sources (including fossil carbons), high theoretical (100%) and
real (50–80%) efciency.) (Kacprzak et al. 2016; Basu ed. 2007), low emission of pollutants
(SO2, NOx, dusts) and about 50% reduction of CO2 emissions per unit of generated electrical
energy (compared to a conventional thermal power plant).Due to high efciency, the DCFC are
able to help to reduce the rate of depletion of fossil fuels, since there is needed more fuel to pro-
duce the same amount of energy in a power plant than in fuel cells. In addition, the DCFC may
form an element of distributed cogeneration systems producing electrical energy and heat from
a few kWe to several dozen/several hundred kWe. Thanks to the above-mentioned advantages,
the coal-powered cells may becomethe key technology in the future for “clean” production of
electrical energy and heat, especially in distributed energy systems.
2. Direct carbon fuel cells with an alkaline electrolyte
In direct carbon fuel cells with an alkaline electrolyte, water solutions of hydroxides or mol-
ten hydroxides are used as electrolyte. In individual electrodes of such cells there take place the
following reactions:
C + 4OH → CO2+ 2H2O + 4e (3)
2 + 2e → O22– (4)
22– + 2H2O + 2e → 4OH (5)
These types of cells enjoy a number of advantages, among which rst mentioned should be
(Zecevic et al. 2005): high ionic conductivity, high activity of electrochemical oxidation of coal,
which allows to obtain a high degree of its use, and a relatively low operating temperature, which
in turn allows to apply cheaper construction materials and avoid the formation of undesirable CO
as a result of the Boudouard reaction.
The disadvantage of direct carbon fuel cells with an alkaline electrolyte is the risk of car-
bonate formation in the reaction of the CO2 formed on the anode with the electrolyte. This un-
favourable phenomenon causes electrolyte degradation. Carbonate formation may be limited by
increasing the amount of water in the electrolyte, e.g. by supplying humidied air to the cell or
by modifying the structure of the device itself (Zecevic et al. 2005).
The rst DCFC with alkaline electrolyte with power of 1.5 kW and consisting of 100 indivi-
dual cells (the target) forming the stack (Fig. 2a) was developed in 1896 by the American engine-
er William W. Jacques (Jacques 1986). Each cell (Fig. 2b) was made of a steel crucible (cathode)
and a carbon rod (anode) containing a small amount of ash. A single fuel cell was characterized
by a voltage of about 1 V and a current density of 100 mA cm–2. The Jacques cell was working at
temperatures ranging from 673–773 K using molten sodium hydroxide as the electrolyte.
The main research and development centres currently involved in the development of direct
carbon fuel cell with an alkaline electrolyte technologies are:
University of Hawaii, Hawaii Natural Energy Institute – HNEI, USA,
Scientic Applications & Research Associates, SARA, USA,
University of West Virginia, USA,
Brown University, USA,
Czestochowa University of Technology, Department of Energy Engineering, Poland.
2.1. Aqueous solutions of hydroxides
Early research on direct carbon fuel cells with electrolyte in the form of aqueous hydroxide
solutions was carried out in autoclaves at 473 K and 3 MPa pressure (Lowry 1945). The autoc-
lave container was an anode, while an iron rod was used as the cathode. The fuel was raw brown
coal dispersed in the electrolyte. As a result of the experiments, a voltage of about 0.55 V was
obtained from the cell in noncurrent conditions, which, however, quickly dropped to zero.
Later on, the research on this type of cell was started at the Hawaii Natural Energy Insti-
tute – HNEI (University of Hawaii, USA) (Nunoura et al. 2007; Antal and Nihous 2008). The
diagram of the structure of the tested cell is shown in Figure 3. In the presented model, aqueous
solutions of potassium, sodium, lithium, cesium, and magnesium hydroxides were used as the
electrolyte. The cell was working at a pressure of 3.5 MPa and a temperature in the range of
353–518 K using compacted biochar as fuel.
During the tests, there were examined various types of materials and various cathode con-
struction solutions (Fig. 4), of which solutions (a) and (f) proved to be the best.
The highest values of the basic electrical parameters were achieved for the electrolyte in
the form of a mixture of aqueous solution of potassium and lithium hydroxide (6 M KOH/1
M LiOH) obtaining voltage under electried conditions equal to 0.574 V and maximum current
and power densities of 43.6 mA cm–2 and 6.5 mW cm–2 respectively at 518 K and pressure of
3.58 MPa (Nunoura et al. 2007). According to the authors, the electrodes should work at diffe-
rent temperatures: a cathode below 500 K, and an anode above 510 K. The two-temperature cell
Fig. 2. Drawings of William W. Jacques’ carbon fuel cell; a) fuel cell stack, b) a single fuel cell
Source: gures redrawn based on reference Jacques 1986
Rys. 2. Szkice konstrukcji ogniwa węglowego Williama W. Jacques’a: a) stos ogniw, b) pojedyncza cela ogniwa
structure is designed to eliminate some of the problems encountered during the research, inclu-
ding evaporation of water from the cathode area. The authors have already developed the design
of the new link, and its construction is being planned. In addition, the challenges facing the
Fig. 3. Schematic drawing of an aqueous hydroxide electrolyte direct carbon fuel cell tested in HNEI
Source: gure redrawn based on reference Wolk et al. 2007
Rys. 3. Schemat ogniwa węglowego z elektrolitem w postaci wodnego roztworu wodorotlenków badanego w HNEI
Fig. 4. Variation of cathode congurations used in carbon fuel cell constructed in HNEI; 1 − nickel perforated tube
sparger, 2 − high nickel alloy porous sparger, 3 − silver foil, 4 − silver screen mesh
Source: gures redrawn based on reference Nunoura et al. 2007
Rys. 4. Rozwiązania konstrukcyjne katody stosowane w ogniwie paliwowym skonstruowanym przez HNEI:
1 – układ rozprowadzania powietrza wykonany z niklowej perforowanej rurki, 2 – porowaty areator wykonany
ze stopu wysokoniklowego, 3 – srebrna folia, 4 – srebrna siateczka
HNEI team are primarily the improvement of the voltage and current parameters obtained from
the cell, the development of a continuous fuel supply method, and the assessment of scalability.
2.2. Molten hydroxides
2.2.1. Scientic applications & research associates (SARA)
On the basis of the design and principle of operation of the Jacques fuel cell, the American
company Scientic Applications & Research Associates (SARA) has designed and patented its
own cell of this type. During many years’ worth of research, the research team has developed two
structural solutions for the cell:
A. With one electrolyte chamber,
B. With two electrolyte chambers.
In the rst solution (Fig. 5a), a cylindrical graphite rod serving at the same time as a fuel
and current collector of the anode was immersed in molten sodium hydroxide located in a cy-
lindrical or cuboidal container, which also was a cathode current collector. The humidied
air was supplied to the bottom of the container with electrolyte and distributed over its walls
by means of a special perforated system made of low carbon steel. The cell construction was
simple and used inexpensive construction materials, e.g. titanium doped carbon steel (Zecevic
et al. 2003, 2005).
Fig. 5. Schematic of SARA’s direct carbon fuel cell design with one electrolyte chamber; a) fuel cell scheme, b) view
of the prototype with an anode surface of 26 cm2 and c) 300 cm2
Source: gures redrawn based on reference Zecevic 2003
Rys. 5. Konstrukcje ogniwa węglowego rmy SARA z jedną komorą elektrolitową: a) schemat ogniwa, b) widok
prototypu o powierzchni anody wynoszącej 26 cm2 i c) 300 cm2
During the research works conducted with a fuel cell with one chamber, there were ma-
nufactured several prototypes. The Mark II-D prototype (Fig. 5b) had an anode surface equal to
26 cm2 (the distance between the electrodes was 1.3 cm) and allowed to obtain a current of 7–8 A
(≈270 mA cm–2) and a maximum power density of 57 mA cm–2. The cell prototype marked as
Mark III-A (Fig. 5c) had an anode of 300 cm2 (the distance between the electrodes was 3 cm)
and the current intensity exceeding 40 A (≈150 mA cm–2). The Mark III-A model enabled obta-
ining an average power of 12–20 W during 540 h of operation (instantaneous power surges were
even 35–50 W). This model generated maximum current and power densities of 150 mA cm–2
and 40 mW cm–2, respectively. The efciency of the non-optimized Mark III-A cell operating
at 50 mA cm–2 has been estimated at around 60%. However, the calculations indicate that in the
case of a power plant with a tested cell, there may be achieved the efciency of 70–75% (Patton
2003). In addition, it was found that the work of the tested cell depends on the cathode material
used, the intensity of aeration, the operating temperature, and the size of the device itself.
In the second construction solution (Fig. 6), a special porous separator made of perforated
nickel foil wrapped around a steel pipe constituting the supporting structure was placed between
the anode and cathode. The aim of this solution was to eliminate the problem of electrolyte
degradation caused by carbonates produced during the cell’s operation. The developed cell con-
struction causes that the electrolyte composition in the proximity of the anode and cathode is
different and therefore they do not mix. The porous separator separating the electrode spaces
may ultimately also be made of ceramic materials or corrosion-resistant porous metals, where
the thickness thereof should be small enough to limit the ion ow resistance and ensure minimal
mechanical strength (Patton 2003; Zecevic et al. 2003).
Tests using a prototype with two electrode chambers included a test with a separator made of
porous zirconium oxide. The cell was working for 120 hours without degradation of the voltage
and current parameters. Analysis of the electrolyte composition in the cathode space carried
outfollowing the test showed the absence of carbonates, which initially proved the correctness
Fig. 6. Schematic of SARA’s direct carbon fuel cell design with two electrolyte chambers
Source: gures redrawn based on reference Patton 2005
Rys. 6. Konstrukcja ogniwa węglowego rmy SARA z dwiema komorami elektrodowymi: a) schemat ogniwa,
b) widok poglądowy
of the adopted concept (Patton 2005). The intended future application of the developed cell is in
distributed generation installations, also as part of hybrid systems which additionally use wind
turbines or photovoltaic cells.
2.2.2. University of West Virginia
After 2004, the SARA research team began cooperation with the University of West Virginia
(USA) (Patton 2005) within the scope of development of methods for manufacturing solid cylin-
drical carbon rods which were fuel for the tested cell. The solid carbon electrodes were produced
from various amounts of petroleum coke, coal packs as a binder and one or two carbonaceous
fuels. Hackett and his collaborators from the University of West Virginia (Hackett 2007) also
carried out numerous test in order to determine the cell characteristics (the developed prototype
shown in Fig. 7 structurally and functionally similar to the SARA cell shown in Fig. 5) depen-
ding on the properties of the developed fuel. The cell was working at temperatures in the range
of 873–973 K with electrolyte in the form of molten NaOH. During the tests, both graphite rods
and carbon electrodes were used to power the cells. Using graphite, there was obtained a current
density of 230 mA cm–2, while the maximum voltage of unloaded cell was 0.788 V. Carbon
electrodes made it possible to achieve a higher voltage (1.044 V), however, the obtained current
densities were up to only 35 mA cm–2. Power densities generated in a graphite rod powered
cell did not exceed 84 mW cm–2, while in the case of coal, values no higher than 33 mW cm–2
were observed. Differences in the obtained electrical parameters, according to the authors, were
associated with a higher resistance of carbon electrodes in comparison with graphite electrodes.
Fig. 7. Schematic drawing of West Virginia University direct carbon fuel cell
Source: gure redrawn based on reference Hackett 2007
Rys. 7. Schemat węglowego ogniwa paliwowego skonstruowanego na Uniwersytecie West Virginia
The cell using carbon rods, unlike a cell powered by graphite rods, was also characterized by
unstable performance. During some tests, the carbon rods began to crack and break. According
to the researchers, this was due to the use of coal packs as binder, which turned out to be more
reactive than the actual fuel.
2.2.3. Brown University
Recently, studies on carbon cells with electrolyte in the form of molten hydroxides were also
started at the University of Brown (USA) (Guo et al. 2013, 2014). Experiments were conducted
for several design variants of the cell. The anode was made in two congurations: A1) nickel net
with a mesh size of 149 μm fastened in a frame made of chromium-nickel wire, A2) nickel-sized
rectangular shaped container (25 mm×25 mm×5 mm) with holes drilled in one of the side walls
that have been covered with nickel mesh. The cathode was also made in two different construc-
tion variants: K1) nickel tube with a diameter of 6.35 mm placed inside a larger tube with a di-
ameter of 12.7 mm. Outside the smaller tube a nickel mesh was created which was the surface
where the oxygen reduction reaction took place, whereas the cathode K2) was built similarly to
the A2 anode, the difference being that inside there was an air distribution system allowing to
obtain ne gas bubbles.
The diagram of cell structure with anode A1 and cathode K1 is shown in Fig. 8. The elec-
trolyte used during the tests was molten sodium hydroxide or eutectic mixture of NaOH-KOH
(54–36 mol%). As the fuel, there was used C-3014 activated carbon. The determined character-
istics of cells with two different electrolytes at 773 K indicated that the voltage of the unloaded
cell working with NaOH electrolyte was higher than in the case of electrolyte as a mixture of
NaOH-KOH, on the other hand, the maximum values of current density were higher for the sec-
ond tested electrolyte composition.
Fig. 8. Schematic drawing of Brown University direct carbon fuel cell
Source: gure redrawn based on reference Guo et al. 2013
Rys. 8. Schemat węglowego ogniwa paliwowego skonstruowanego na Uniwersytecie Browna
Replacements in the anode construction have not resulted in major differences in the perfor-
mance characteristics of the cell, while the use of the K2 cathode caused an increase in the power
density of the cell by about 50%. In addition, the use of a mixture of NaOH and KOH hydroxides
allowed the cell to work at lower temperatures than when using NaOH alone, thanks to which
the corrosion rate of materials used in the construction of the device may ultimately be reduced.
2.2.4. The Częstochowa University of Technology
The only Polish, and at the same time European, research centre dealing with the theme of
direct carbon fuel cells with hydroxide electrolyte is the Department of Energy Engineering
(KIE) which is part of the Faculty of Infrastructure and Environment of the Częstochowa Uni-
versity of Technology.
The KIE research team in the course of preliminary experiments, during which three proto-
types of direct carbon fuel cells were designed and manufactured (Fig. 9), made the selection
and chose the appropriate materials for the individual structural elements of the cell (Kacprzak
et al. 2013a).
Finally, as a result of the carried out preliminary tests and modications, there was manufac-
tured a nickel model (prototype III, g. 10), characterized by work stability and repeatability of
electrical parameters measurements (unaffected by corrosive processes) under the same con-
ditions at successive time intervals.
Fig. 9. View of the direct carbon fuel cell prototypes constructed in KIE; A) prototype I − manufactured
from the carbon steel, B) prototype II − manufactured from the stainless steel and C) prototype III − manufactured
from the nickel and high nickel alloys
Rys. 9. Widok poglądowy modeli węglowych ogniw paliwowych opracowanych w KIE: A) prototyp I – wykonany
ze stali węglowej, B) prototyp II – wykonany stali stopowych oraz C) prototyp III –
wykonany z niklu i stopów wysokoniklowych
During the works carried out so far, the research team has focused its efforts, among others,
on checking the possibility of using different forms and forms of coal to power the cell. The rst
successfully completed tests using solid graphite and carbon electrodes encouraged the authors
to take the opportunity to use as fuel the crushed hard coal and biochar, being a product of car-
bonisation of various types of biomass.
The conducted research indicated that the disordered structure of hard coals and biochars
resulted in their higher reactivity and susceptibility to electrochemical oxidation in the cell than
would be the case in the ordered graphite structure. In addition, there was observed a correlation
between the oxygen content in individual fuels and the maximum power density obtained from
the cell. The higher the oxygen content in individual fuels (with which correlated was the relative
amount of oxygen function groups on the surface of the fuel grains), the higher the maximum
power density. In order for this observation to be conrmed, however, there is required further
research, already being planned by the authors (Kacprzak et al. 2014).
During subsequent tests, there were determined the inuence of individual process parame-
ters (Kacprzak et al. 2013b) as well as the chemical composition of the electrolyte (Kacprzak
et al. 2013c) on the electrical parameters obtained from the cell. As part of the work, there was
carried out research on the impact of, among others, fuel fragmentation, the amount of air fed
to the cathode, electrode surface, chemical composition and electrolyte temperature on such
electrical parameters as: current and power density, electromotive force, internal resistance, etc.
were carried ouv.
Depending on the type of fuel, values of individual process parameters and of the electrolyte
composition, there were obtained power densities in the range from 18 to 42 mW cm–2. The
highest values were recorded for a cell powered by biochar with a grain size of 0.18–0.25 mm
Fig. 10. Construction of a prototype III of direct carbon fuel cell constructed in KIE; a) scheme, b) view
Rys 10. Budowa prototypowego modelu III ogniwa węglowego wykonanego w KIE; a) schemat, b) widok
obtained through pyrolysis of apple wood chips, the electrolyte temperature (NaOH-KOH, 50:50
mol%) of 673 K and the value of the air stream supplied to the cathode equal to 0,5 dmn3 min–1.
The estimated energy efciency of the tested cell was 41% (in reference to the biochar ca-
loric value), which is a very good and promising result in comparison with other technologies
of converting chemical energy of biomass into electrical energy. In turn, the determined elec-
trochemical efciency was 59% (at the voltage of 0.65 V) and was only 4% lower than the one
theoretically possible to be obtained under the tested conditions (Kacprzak et al. 2016).
Direct carbon fuel cells is a technology enabling direct conversion of chemical energy of
carbon-based fuels through electrochemical reactions into electrical energy, thanks to which it is
possible to achieve high efciency. The results of research carried out on direct carbon fuel cells
with hydroxide electrolyte conrm the possibility of powering them with many types of fuels:
from solid graphite and carbon electrodes through crushed hard coal, to the carbonated biomass
of various origin (bio-coal). The high-efciency direct carbon fuel cells presented in the paper
are a technology which in the long term,mayoffer a response to the challenges currently facing
the energy sector, including to the growing demand for electrical energy, depletion of fossil fuel
resources, and increased pollution of the natural environmenv. Among the numerous technical
problems stalling further development of the discussed technology, the following should be di-
Preparation of coal-based fuel and development of an effective way of its introduction into
the cell.
Selection of materials and development of a structure which ensures adequate availability
and durability of the cell.
Selection of physical (thermodynamic) and chemical conditions allowing to maintainstability
of electrolyte and/or to develop an effective and energy-saving method of electrolyte rege-
Optimization of unit costs and the technological system efciency with a direct carbon fuel
Conducting long-term tests aimed at verifying technical and technological assumptions and
conrming the ability of the cell to long-term performance in the conditions of the target
industrial energy supply system.
The direct coal fuel cells with an alkaline electrolyte discussed in the papermay be powered
not only with hard coal, but also with carbonized plant and waste biomass, which makes them
alternative sources of electrical energy using commonly available renewable fuel, characterized
by zero CO2 emission. Therefore, they may ultimately be an element of distributed systems
with power from several kWe to several hundred kWe generating electrical energy and heat from
biomass, the effective use of which contributes to the implementation of basic principles of the
state’s energy policy, including mainly:
Energy independence.
Diversication of primary energy sources and reduction of fossil fuel consumption.
Increased efciency of energy use.
Reducing negative impact of the energy sector on the environment and implementing the
principles of sustainable development.
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Andrzej Kacprzak
Analiza obecnego stanu rozwoju technologii węglowych ogniw
paliwowych zelektrolitem alkalicznym
Wśród wielu nowoczesnych, wysokosprawnych technologii energetycznych pozwalających na prze-
twarzanie energii chemicznej węgla w energię elektryczną i ciepło na szczególną uwagę zasługują węglowe
ogniwa paliwowe (ang. Direct Carbon Fuel Cells – DCFC). Są to urządzenia, które umożliwiają, jako
jedyne spośród wszystkich typów ogniw paliwowych, bezpośrednią konwersję energii chemicznej zawar-
tej w paliwie stałym (węglu) w energię elektryczną. Ponadto charakteryzują się one wysoką sprawnością
i niską emisją zanieczyszczeń. W artykule dokonano przeglądu i omówienia dotychczasowych prac badaw-
czo-rozwojowych, prowadzonych zarówno na świecie, jak i w Polsce, nad technologią węglowych ogniw
paliwowych z elektrolitem alkalicznym (wodorotlenkowym).
słoWa KluczoWe: węglowe ogniwo paliwowe, elektrolit alkaliczny, wysokosprawne technologie energe-
tyczne, węgle kopalne
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Full-text available
Historically, despite its compelling cost and performance advantages, the use of a molten metal hydroxide electrolyte has been ignored by direct carbon fuel cell (DCFC) researchers, primarily due to the potential for formation of carbonate salt in the cell. This article describes the electrochemistry of a patented medium-temperature DCFC based on a molten hydroxide electrolyte, which overcomes the historical carbonate formation. An important technique discovered for significantl y reducing carbonate formation in the DCFC is to ensure a high water content of the electrolyte. To date, four successive generations of DCFC prototypes have been built and tested to demonstrate the technology - all using graphite rods as their fuel source. These cells all used a simple design in which the cell containers served as the air cathodes and successfully demonstrated the ability to deliver more than 40 A with the current density exceeding 250 mA/cm2. Conversion efficiency greater than 60% was achieved.
The direct carbon fuel cells (DCFCs) belong to new generation of energy conversion devices that are characterized by much higher efficiencies and lower emission of pollutants than conventional coal-fired power plants. In this paper the DCFC with molten hydroxide electrolyte is considered as the most promising type of the direct carbon fuel cells. Binary alkali hydroxide mixture (NaOH-LiOH, 90–10 mol%) is used as electrolyte and the biochar of apple tree origin carbonized at 873 K is applied as fuel. The performance of a lab-scale DCFC with molten alkaline electrolyte is investigated and theoretical, practical, voltage, and fuel utilization efficiencies of the cell are calculated and discussed. The practical efficiency is assessed on the basis of fuel HHV and LHV and the values are estimated at 40% and 41%, respectively. The average voltage efficiency is calculated as roughly 59% (at 0.65 V) and it is in a relatively good agreement with the values obtained by other researchers. The calculated efficiency of fuel utilization exceeds 95% thus indicating a high degree of carbon conversion into the electric power.
Effects of the operating conditions, such as environmental temperature(ET), methanol feeding temperature(MFT), methanol concentration(MC), and methanol flow rates(MFR) on the performance and cell temperature(CT) of a 5-stacked direct methanol fuel cell (DMFC) have been investigated. The ET, MFT, MC, and MFR were varied from -10°C to +40°C, 50°C to 90°C, 0.5M to 2.0M and 11.7 mL/min to 46.8 mL/min, respectively to measure the performance of the fuel cell. The performance with various operating conditions was measured in the I-V polarization curve The CT was increased significantly with increasing the ET. The effect of MFT and MFR on CT was moderate, and the effect of MC was marginal. A multiple linear regression (MLR) analysis was also carried out in order to find a relationship of CT with ET, MFT, MC, and MFR.
Of the hydrogen-oxygen fuel cell systems the most mature is the phosphoric acid fuel cell (PAFC). It operates at 150–190°C and pressure ranging from ambient to 5 atm. PAFC systems use primarily Pt as catalyst both for hydrogen and oxygen electrodes. The operating temperature range of PAFC allows it to take up hydrogen directly from hydrogen sources like reformer gases. Less than one percent of CO present in the reformer gases are not adsorbed on Pt sites owing to high operating temperature. The other components used in PAFC are mainly made of graphite and carbon. All these factors make PAFC a versatile member of the hydrogen-oxygen fuel cell family.
Results are presented of an analysis of the nature of the reaction zone in the anode of a direct carbon fuel cell (DCFC). Five different types of particulate carbonaceous fuels were investigated, including nonconductive as-received coals, and more conductive pyrolyzed coal chars and an activated charcoal. All the fuels exhibited linear voltage–current density behavior indicative of ohmic-controlled polarization. The two as-received coals (Pittsburgh No. 8 bituminous coal and Beulah-Zap lignite) exhibited greater open-circuit voltages (OCV) of ∼1.2 V than their corresponding pyrolyzed forms and the activated charcoal, the latter of which were all ca. 1.0 V. It was also found that differences in electrochemical reactivity of the as-received and pyrolyzed coal fuels correlated with their thermal heating values. Even so, maximum power and current densities were comparable for all the particulate fuels investigated, irrespective of the conductivity of the fuel particles. Based on fuel characterization and performance data, it is concluded that the electrochemical reaction zone in packed-bed anodes of the type examined here is limited to the three-phase solid fuel-anode-molten electrolyte contact zone. This intrinsic characteristic represents a limitation on the electrochemical performance of these types of DCFCs, in comparison to other fuel cells with fluid fuels.
This paper describes the performance of a direct carbon fuel cell. Particular attention was made to investigate the effects of the composition and temperature of the electrolyte on cell operation and parameters. Various binary and ternary molten alkali hydroxides have been used as electrolytes. The performance of the fuel cell was investigated at various temperatures for four compositions of the electrolytes. Graphite rod and biochar (apple tree chips carbonized at 873 K) were used as fuels.The experiments indicated that the composition of the electrolyte and temperature significantly affected the performance of the fuel cell. The best results were obtained for the fuel cell operated with biochar fuel and NaOH–KOH electrolyte (50–50 mol%) at 673 K.
The direct carbon fuel cell (DCFC) employs a process by which carbon is converted to electricity, without the need for combustion or gasification. The operation of the DCFC is investigated with a variety of solid carbons from several sources including some derived from coal. The highly organized carbon form, graphite, is used as the benchmark because of its availability and stability. Another carbon form, which is produced at West Virginia University (WVU), uses different mixtures of solvent extracted carbon ore (SECO) and petroleum coke. The SECO is derived from coal and both this and the petroleum coke are low in ash, sulfur, and volatiles. Compared to graphite, the SECO is a less-ordered form of carbon. In addition, GrafTech, Inc. (Cleveland, OH) supplied a well-fabricated baked carbon rod derived from petroleum coke and conventional coal–tar binder. The open-circuit voltage of the SECO rod reaches a maximum of 1.044V while the baked and graphite rods only reach 0.972V and 0.788V, respectively. With this particular cell design, typical power densities were in the range of 0.02–0.08Wcm−2, while current densities were between 30 and 230mAcm−2. It was found that the graphite rod provided stable operation and remained intact during multi-hour test runs. However, the baked (i.e., non-graphitized) rods failed after a few hours due to selective attack and reaction of the binder component.
The performance and characterization of a batch, direct carbon fuel cell (DCFC), employing molten hydroxide electrolytes, is presented. Particular attention was focused on reducing the operating temperature of the system to minimize corrosion of both fuel cell materials and fuel carbon. Constant power was achieved over a current density range of 37 to 92 mA/cm2 (200 to 500 mA) with NaOH electrolyte at 550 °C, and up to 170 mA/cm2 with a NaOH/KOH (54/46 mol %) eutectic. The voltage decreased with temperature, becoming unstable at ≤400 °C. Electrochemical impedance spectroscopy (EIS) measurements showed that charge-transfer resistance, Rp, is more temperature-dependent than the ohmic resistance, Ro, with Rp decreasing by 55% for the anode and 82% for the cathode with an increase in temperature from 370 °C to 500 °C. Stable operation was achieved at temperatures as low as 400 °C with the hydroxide eutectic electrolyte. This was attributed to the higher concentration of superoxide ions in the eutectic, as identified with cyclic voltammetry. Using a three-electrode system, it was found that the anode overpotential was significantly greater than that of the cathode at low temperatures during galvanostatic polarization.
Thermodynamics permits the carbon fuel cell, which generates electrical power via the electrochemical combustion of its carbon fuel, to realize a theoretical efficiency of 100%. A recent paper [Nunoura et al. Ind. Eng. Chem. Res. 2007, 46, 734-744] reported promising results that were obtained from a moderate-temperature, aqueous-alkaline biocarbon fuel cell. In view of the fact that aqueous-alkaline hydrogen fuel cells have been used to power an Austin car and a commercial Black Cab in London, these recent results suggest the potential use of aqueous-alkaline carbon fuel cells for vehicular transportation. Usually, the practicality of an aqueous-alkaline carbon fuel cell is discounted, because the carbon dioxide product of carbon oxidation reacts with and consumes hydroxyl ions in the aqueous-alkaline electrolyte, thereby forming carbonate ions. As a result of this reaction, the performance of an aqueous-alkaline carbon fuel cell is expected to deteriorate over time. Contrary to this expectation, in this paper, we show that the aqueous-carbonate ion can be as effective as the hydroxyl ion as a charge carrier when the temperature of the cell approaches 300°C. Thermodynamic estimates of die Gibbs free energy of formation (Δ fG o) of the aqueous-carbonate ion indicate that the change in Gibbs free energy of the relevant anodic carbon oxidation reaction by carbonate ion equals that of carbon oxidation by hydroxyl ion at temperatures that approach 300°C. Also, consideration of the temperature dependence of the standard hydrogen electrode reveals that aqueous-hydroxyl ion production on the cathode should be favored at temperatures as high as 300°C. These findings are a cause for optimism, concerning the performance of an aqueous alkaline/carbonate biocarbon fuel cell designed to operate at 300°C, and they should encourage further work at temperatures that approach 300°C.