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Improving direct ethanol fuel cells by fluorine doping

Direct ethanol fuel cells (DEFCs) are fuel cells that run on ethanol to directly produce electrical power. Despite having much to offer they have not been forayed into. Ethanol can be made from biomass by yeasts and its oxidation products – CO2 and H2O – are hence environmentally friendly. The application of DEFCs could be a lucrative solution for vehicles due to the energy efficiency if mass-produced. Our current infrastructure for combustion fuels is ready for ethanol. DEFC usage would therefore be a sustainable and environment-friendly alternative to current internal combustion engines. Moreover, ethanol is liquid, which facilitates distribution, storage and use.

According to studies sponsored by  International Energy Agency (IEA), DEFCs deliver high power densities, culminating between 50 to 185 mW / cm2. Currently, DEFCs face multiple challenges such as slow redox kinetics, limited performance, and the high cost of electrocatalysts needed for DEFCs.

In a DEFC, the two key reactions are:

  1. Ethanol Oxidation Reaction (EOR)
  2. Oxygen Reduction Reaction (ORR)

Their sluggish rates have prevented widespread adoption of this technology. State-of-the-art DEFCs require expensive platinum-based materials to catalyze these reactions. Yet, they do not completely oxidize ethanol to CO2 to complete the EOR reaction, limiting the energy efficiency. One way to fix this issue is to separate and re-inject the unreacted ethanol. Since this adds more engineering to the fuel cell, a better solution is to find more efficient catalysts. Hence, to realize the true potential of DEFCs, is to find cheaper and more active catalysts for the two reactions in DEFCs.

The researchers at the University of Central Florida and their colleagues experimented on Pd–N–C catalyst and attempted to improve catalyst performance by introducing fluorine atoms. The team used alkaline membranes and platinum-free catalysts. Not only were these more cost-effective but also produced a high power output.

Previous research on electrocatalytic systems revealed that the local coordination environment (LCE) of the electrode surface is pivotal in tuning the activity of electrocatalysts made of carbon-supported metal nanoparticles. The study showed that introducing fluorine atoms in Pd–N–C catalysts regulated the LCE around the Pd, improving both activity and durability for the two key reactions. This improved the catalytic performance, and ultimately the fuel cell’s performance.

The new study demonstrated that fluorine doping rearranged the electron structure of the fuel cell catalyst. This substantially improved power density and ultimately the performance of the DEFC when compared with present-day benchmark catalysts. The experimental results on long-term stability demonstrated promising advancements towards practical applications of such catalysts in DEFCs.


Upon experimental analysis, it was found that the fluorine atoms in the catalyst weakened carbon-nitrogen bond and pushed the N atoms towards palladium. This electron translocation efficiently regulated the LCE of palladium by forming palladium-nitrogen active sites for catalytic reactions.

The N-rich palladium surface promoted carbon-carbon bond cleavage and enabled complete ethanol oxidation. During the ORR, the N-rich palladium surface surface not only weakened CO2 adsorption but also created more accessible catalytic sites for rapid O2 adsorption.

According to the authors, a commonly occurring problem in DEFCs – the inability to complete the two key reactions – has been resolved. Their catalyst enhanced the overall performance of the fuel cell. The addition of fluorine also enhanced the durability of the catalyst by reducing the corrosion of carbon materials as well as inhibiting palladium migration and aggregation.

When the novel catalyst was tested in a DEFC, an output maximum power density of 0.57 W/cm2 was obtained. The fuel cell was stable for more than 5,900 hours. The proposed strategy, when experimented with using other carbon-supported metal catalysts, also gave improved results in activity and stability.


The main shortcoming of DEFCs running in the alkaline condition is their durability. Currently, it is not sufficient for practical applications. Moreover, the anion-exchange membranes in use have two issues:

  • Structural stability of membrane is insufficient for long-term use
  • Carbonation occurs in presence of CO2 due to its reaction with hydroxide ions, ultimately degrading the catalyst.

Albeit stable for remarkable 5,900 hours, the membrane was replaced after 1,200 hours in the presented study. Since replacing membranes require complete disassembly of the cell, this is not a long-term practical solution.

Hence, there must be further research on increasing ionic conductivity and stability of anionic membranes for practical use of DEFC in alkaline conditions. Ideally, the hydroxide solution used to increase ionic conductivity is avoided to preserve energy density and reduce the complexity of the device. Solid oxide fuel cells offer a solution for these problems since the fuel is oxidized in gaseous form but their ceramic membrane are too fragile for mobile applications.

The current experiment makes significant strides in improving power density in DEFCs much more than any state-of-the-art DEFCs. The way ahead is further research to overcome these smaller obstacles in the long-term use of anionic membranes.

Experimental analysis

Materials used

Commercial Pd/C (10%, 8 nm Pd particles on activated carbon), as well as Pt/C (20%, 3 nm Pt particles on carbon black), were used as baseline catalysts. Also, Nafion™ solution (5%), carbon paper (TGP-H-060), and anion-exchange membranes (Fumasep FAS-PET-75)

Synthesis of heteroatom X-doped carbon (X–C, X=N, P, S, B, F)

Carbon black with abundant oxygen functional groups and melamine (C3H6N6) were mixed and ground, and finally pyrolyzed. After cooling to room temperature, N–C was obtained by washing with ethanol and water. The same method was used to synthesize P–C, S–C, B–C, and F–C from sodium hypophosphite anhydrous, sulfur powder, boric acid, and polyvinylidene difluoride.

Synthesis of hetero-atom fluorine-doped carbon catalysts

N–C and polyvinylidene difluoride were mixed and ground before adding them into a solution of acetone and water. After ultra sound treatment, the mixture was refluxed in an oil bath until fully dried. The mixture was then pyrolyzed and after cooling to room temperature, the samples were washed with ethanol and ultrapure water, followed by a vacuum to obtain the fluorinated catalyst support. The same method was used for the other precursors.

A microwave reduction method was used to synthesize palladium catalyst on the catalyst support. The content of palladium in all samples was kept at 1.0%, which was determined and double-confirmed by X-ray spectroscopy and inductively coupled plasma.

Electrochemical characterizations

For the electrical measurements, either a glassy carbon ring-disc electrode or rotating ring-disc electrode were used. The Fumasep membrane was used as an anion-exchange membrane, modified to change it to a hydroxide environment.


Chang et al., 2021, Improving Pd–N–C fuel cell electrocatalysts through fluorination-driven rearrangements of local coordination environment. Nature Energy 6, 1144–1153

Image Source: P_Wei, Pixabay

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Remarkable performance of Fe–N–C cathode electrocatalysts in anion-exchange membrane fuel cells (AEMFC)

Catalysts for low-temperature fuel cells are permanently improved to overcome high costs. Only when low-temperature fuel cells are competitive with internal combustion engines will they be an alternative power source for transportation or even portable devices. The US Department of Energy’s (DOE) milestones for the cost of a light-duty vehicle fuel cell system is $30 per kWnet. However current costs of a proton-exchange membrane (PEM) fuel cells ranges between $45 and $51 kWnet.

Challenged to reduce fuel cell production cost, researchers have suggested changing the fuel cell operating environment from acidic pH to alkaline. This will require to replace PEM by anion-exchange membranes (AEM) in fuel cells. The true advantage of AEM over PEM fuel cells is the cost reduction through cheaper membranes. Additionally, a broader spectrum of materials could be used and the oxygen reduction reaction (ORR) kinetics would be improved. Yet, acidic conditions corrode non-precious metals quickly while at the same time the high loading of platinum group metals (PGM) catalysts  need to be reduced as well.

Synthesis of Fe-N-C electrocatalyst and it structure

Researchers from the University of South Carolina, Columbia (USA) together with their partners recently reported in Nature Energy the remarkable performance of inexpensive Fe-N-C cathode catalysts with single-atom Fe-Nx active sites in AEM fuel cell. The Fe-N-C catalyst was constructed in respect to two important aspects: increase the average pore size (ranging from 5-40 nm, 1 µm) as well as the level of graphitization. Both measures reduce the hydrophobicity of the catalyst layer. To optimize their catalyst’s performance, the researchers went through an iterative process using various material characterization techniques. Energy Dispersive Spectroscopy mapping was used to ensure the catalyst composition was homogeneous. Iron atoms in the catalyst were present as single atoms, which was confirmed by Scanning Transmission Electron Microscopy imaging.

Catalyst performance and integration in AEM fuel cells

The electrochemical analyses carried out by the scientists showed that their Fe-N-C catalyst achieved high ORR activity via four-electron O2 reduction. In this reduction reaction, oxygen is directly reduced to water without the intermediate hydrogen peroxide step. The yield of hydrogen peroxide as function of potential over the entire experimental range was less than 1% – a good result for a non-precious metal catalyst. The current density of the reaction was of 7 mA / cm2.

The Fe-N-C catalyst was used on the cathode of a hydrogen-oxygen AEM fuel cell. An high peak power density of 2 W / cm2 was reported. This performance is the highest reported value for polymer membrane fuel cells (AEM and PEM) using a non-precious metal catalyst. Especially the 4x lower loading of Fe-N-C catalyst compared with previous reports makes this type of fuel cell economically interesting. Moreover, the electrocatalyst was stable at voltages of 0.6 V for more than 100 hours.

To evaluate feasibility of Fe-N-C cathode for more practical application, the fuel cell was tested in the air flow as cathode oxidant. The achieved current density was 3.6 mA / cm2 at 0.1 V with a peak power density of over 1 W / cm2. These results again show the highest reported values in the literature up to date compared to other hydrogen-air AEM fuel cell.

Fuel cell test target DOE-criteria

The cell configuration simulating more realistic operation was intended to benchmark against the DOE targets and the DOE2022 milestones. Cathodes with 0.6 mg Pt / cm2 and a 1 mg Fe-N-C per cm2 were compared. The paired cell was operated under conditions similar to the DOE-defined protocol: 0.9 V iR-free, cell temperature 80°C and 100 kPa partial pressure of O2 and H2. A steady-state current density reached at 0.9 V (iR-free) was approx. 100 mA / cm2. This was more than twice the DOE target.

Finally, the next configuration was designed using the DOE2022 milestones protocol postulating that the total precious metal loading should be less than 0.2 mg Pt / cm2. This was achieved by integrating Fe-N-C cathode with low-loading PtRu/C anodes (0.125 mg PtRu per cm2). This cell reached a peak power density of 1.3 W / cm2 under hydrogen-oxygen operation. Recalculating this value to a specific power output of 16 W per mg Pt results in the highest value of any AEM fuel cell ever reported in the literature.

It has been demonstrated that the Fe-N-C electrocatalyst can compete with noble metal-based catalysts for AEM fuel cells. The reported cell configuration provided remarkable performance in terms of activity and durability under fuel cell condition.

Methodology and electrode preparation

  • Rotating ring disc system – RRDE, was used for evaluation of electrochemical performance for ORR of Fe-N-C catalyst.
  • Fe-N-C catalyst was prepared with higher density of Fe-Nx centers since it has been reported that a higher carbon proportion also results in a higher number of positions in the graphene sheets available for insertion of active sites.
  • For the comparison Pt/C electrode was analyzed.
  • In the electrochemical cell the electrodes were: working electrode – catalyst was cast on the GC disk and stabilized with 5% Nafion® ;
  • Platinum mesh was used as counter electrode and Ag/AgCl as reference electrode, 0.1 M KOH was used as electrolyte.
  • For the tests in anion-exchange membrane fuel cell, gas diffusion electrodes were used: Anode was prepared with low-loading PtRu/C material (0.125 mg PtRu per cm2, 0.08 mg Pt per cm2), while for the cathode Fe-N-C catalyst was used – both were prepared by spraying catalyst ink onto a gas diffusion layer.

Image: iStock

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Water desalination and fluoride ions removal from water using electrodialysis

Clean freshwater is of the utmost importance for our health. Despite its central role for our lives, progressing global industrialization threatens freshwater resources around the world. Albeit a vital trace element, fluoride is a serious public health threat. Absorbed in larger quantities for a long time, fluoride causes fluorosis, a form permanent poising responsible for irreparable bone damage.

Fluoride bearing rocks are particularly common in India. Fluoride is leached into adjacent aquifers and contaminates the soil. Sometimes, the concentration of fluoride ions in Indian aquifers exceeds 30 mg/L. Toxic concentrations of 20-80 mg / day over a period of 10 to 20 years cause irreparable damage to the human body.

Fluoride ions in groundwater are removed for water treatment using membranes. However, such membranes foul easily, for example by bacteria present in wastewater or other deposits.  Fouling can become a serious threat to public health. Therefore, a particular focus in membrane research is on the development of fluoride removing membranes that prevent fouling. It can be accomplished when bacterial growth is slowed down or inhibited entirely. For water treatment, antimicrobial surface modifications are used in high-quality membranes for ultrafiltration, nanofiltration, reverse osmosis and electrodialysis.

Electrodialysis is often used to remove water contamination, because only little energy is needed for the process. For electrodialysis membranes, salt deposits are an economic risk that is to be avoided. Precipitates can occur when the concentration of bivalent ions in the water is too high. Added to precipitates comes the risk of biofouling caused by microbial growth. Both affect the performance of electrodialysis membranes, causing economic losses as the membranes must be cleaned or replaced. For efficient water treatment, it is therefore important to improve the thermal and mechanical properties of the membranes.

A group of scientists have synthesized a composite anion exchange membrane for water-salt altitude and fluoride ion removal by electrodialysis that has improved antimicrobial properties. She published her results in the journal ACS ES&T Water. The consortium consisted of researchers of the Academy of Scientific and Innovative Research in Ghaziabad, India and the University of Tokyo.

Their anion exchange membranes are based on cross-linked terpolymers with built-in silver nanoparticles to slow microbial growth. The membranes are suitable for water desalination and fluoride ion removal by electrodialysis. The preparation of the terpolymers and polyacrylonitrile copolymers was carried out by N-alkylation using various alkyl halides. N-alkylation of the terpolymer through various alkyl groups affected the water absorption, hydrophobicity, ion transport and ionic conductivity of the membrane. Long alkyl groups increased the effectiveness of fluoride removal as well as the oxidative and physical stability of the membranes. The suitability of the composite membranes was verified by testing removal efficiency of fluoride ions (5.5 and 11 mg/L) from a sodium chloride solution (2 g/L) by electrodialysis at an applied voltage of 2 V.

The incorporation of 0.03% silver nanoparticles in the quaternized polymer caused the desired antimicrobial effect. The uniform distribution of silver nanoparticles in the liquid and solid phases was detected by transmission electron microscopy and atomic force microscopy. The attachment of bacteria was quantified counting colony forming units and 100x lower when silver nanoparticles were present in the membrane. The reduced microbial attachment to the membrane surface is therefore due to the antimicrobial effect of the silver nanoparticles. The small amount of 0.03% silver nanoparticles was sufficient to achieve desired antimicrobial effect in the membrane.

After 15 days and at a water temperature of 50°C, no detectable silver leaching occurred. The novel membranes are thus an improved anion exchange solution with antimicrobial properties for efficient removal of fluorine and desalination by electrodialysis.


The entire synthesis was carried out in four steps:

  • Step 1: Silver nitrate was diluted with deionized water to produce a 30 mm solution
  • Step 2: Terpolymer and quaternized terpolymers were prepared by free radical polymerization
  • Step 3: Composite additives were prepared by the reduction of silver nitrate with sodium borohydrite in the presence of dimethylformamide
  • Step 4: The membrane was networked with the silver nanoparticles

Characterization of the anion exchange membrane

The membrane was characterized using several analytical methods:

  • UV-VIS and IR spectroscopy
  • Incorporation of silver nanoparticles by scanning electron microscopy, atomic force microscopy and transmission electron microscopy
  • Thermal stability, tensile properties, solubility and further physicochemical and electrochemical properties of the silver nanoparticle composite
  • Desalination and fluoride removal
  • The effectiveness of silver nanoparticles on microbial attachment
  • Energy consumption and efficiency during water desalination and fluoride removal by the composite membrane
  • Membrane stability with respect to pH, temperature and Fenton’s Reagent was evaluated


Pal et al. 2021 “Composite Anion Exchange Membranes with Antibacterial Properties for Desalination and Fluoride Ion Removal” ACS ES&T Water 1 (10), 2206-2216,

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Reverse electrodialysis using Nafion™ membranes to produce renewable energy

In the order to address the global need for renewable and clean energy sources, salinity-gradient energy harvested by reverse electrodialysis (RED) is attracting significant interest in recent years. In addition, brine solution coming from seawater desalination is currently considered as a waste; however thanks to its high salinity it can be exploited as a valuable resource to feed RED. RED is an engineered adaptation of nature’s osmotic energy production where ions flow pass the cell membrane in order to produce the universal biological currency ATP. This energy is also harvested by the RED technology.

Now, more than ever there is need for sustainable and environmentally friendly technological solutions in order to keep up with ever growing demand for clean water and energy. The traditional linear way “produce and throw away” does no longer serve the society anymore and the new approach of circular economy has take a place, where any waste can be considered as a valuable resource for another process. In this respect, reverse electrodialysis is a promising electromembrane-based technology to generate power from concentrated solutions by harvesting the Gibbs free energy of mixing the solutions with different salinity. In particular, brine solutions produced in desalination plants, which is currently considered as a waste, can be used as concentrated streams in RED stack.

Avci et al. of the University of Calabria, Italy, have recently published their solution for brine disposal using RED-stack. They have realized that in order to maximize generated power, the high permselectivity and ion conductivity of membrane components in RED are essential. Although Nafion™ membranes are among the most prominent commercial cation exchange membrane solutions for electrochemical applications, no study has been done in its utilization toward RED processes. This was the first reported RED stack using Nafion™ membranes.

A typical RED unit is similar to an electrodialysis (ED) unit, which is a commercialized technology. ED uses a feed solution and the electrical energy, while producing concentrate and dilute, separately. On the other side, RED uses concentrated and dilute solutions that are mixed together in a controlled manner in order to produce spontaneously electrical energy. In a RED stack, repeating cells comprised of alternating cation and anion exchange membranes that are selective for anions and cations. The salinity gradient over each ion exchange membrane creates a voltage difference which is the driving force for the process. The ion exchange membranes are one of the most important components of a RED stack.

The performance of Nafion™ membranes (Nafion™ 117 and Nafion™ 115) have been evaluated under a high salinity gradient conditions for the possible application in RED. In order to simulate the natural environments of RED operation, NaCl solution as well as multicomponent NaCl + MgCl2 have been tested.

Gross power density under high salinity gradient and the effect of Mg2+ on the efficiency in energy conversion have been evaluated in single cell RED using Nafion™ 117, Nafion™ 115, CMX and Fuji-CEM-80050 as cation exchange membranes. Two commercial cation exchange membranes – CMX and Fuji-CEM 80050, frequently used for RED applications, have served as benchmark.

The results show that under the condition of 0.5 M / 4.0 M NaCl solutions, the highest Pd,max was achieved using Nafion™ membrane. This result is attributed to their outstanding permselectivity compared to other CEMs. In the presence of Mg2+ ions, Pd,max reduction of 17 and 20% for Nafion™ 115 and Nafion™ 117 were recorded, respectively. Both membranes maintained their low resistance; however a loss in permselectivity was measured under this condition. Even though, it was reported that Nafion™ membranes outperformed other commercial membranes such as CMX and Fuji-CEM-80050 for RED application.

(Photo: Wikipedia)