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Transition between double-layer and Faradaic charge storage in porous carbon nano-material

In electrochemical cells, such as fuel cells or electrolyzers, electric double-layer (EDL) formation occurs on their electrode surfaces. These EDL act as both, capacitors and resistors and impact therefore the performance of electrochemical cells. Understanding the structure and dynamics of EDL formation could significantly improve the performance of, electrochemical systems, for example in energy storage and conversion, including supercapacitors, water desalination, sensors and so forth.

On a planar electrode, electrolyte ions and the solvent are adsorbed at the electrode surface. The resulting capacitance depends on charge, solvation state and concentration. Traditionally, the capacitance of electrochemical interfaces can be divided into two types:

  1. Double-layer capacitance: ions are adsorbed based on their charge. Ion adsorption is non-specific.
  2. Faradaic pseudocapacitance: specific ions are adsorbed, for example through chemical interactions the electrode surface. This may involve charge transfer.

The electrode interface in the most energy application-based technology is, however, not planar but porous. Layer materials in such situations have various degrees of electrolyte confinement and thus different capacitive adsorption mechanisms. Understanding electrosorption in such materials requires a refined view of electrochemical capacitance and charge storage.

A team of researchers from the North Carolina State University, the Paul Sabatier University in Toulouse and the Karlsruhe Institute of Technology reported new insights in electrolyte confinement at the non-planar interfaces in the journal Nature Energy.

Electric double-layer at planar electrodes

The degree of ion solvation (the process of reorganizing solvent and solute molecules) at ideal (planar) electrochemical interfaces determines the ions interaction with the electrodes. There are two distinct cases:

  1. Ions are non-specifically adsorbed: this is the case with strong ion solvation. The electrode’s interactions are primarily electrostatic. This type of interactions can be considered as the induction – charge is induced but not transferred.
  2. Ions are specifically adsorbed: in this case, ions are not solvated and can undergo specific adsorption and chemical bonding to the electrode. This process can be described as charge transfer reaction between the electrode and the adsorbed ion. However, the charge transfer reaction depends on the bonding between the ion and the electrode. This correlates with the state of ion solvation.  Thus, it can be expected that the ion solvation is crucial for understanding the ion-electrode interactions in a nano-confined environment such as porous materials.

Carbon based EDL capacitor – the confinement effect

There is a great interest for understanding the relationship between the porosity of carbon nano-materials and their specific capacitance.

When electric double-layer formation occurs in a nano-confined micro-environment, the EDL capacitor in porous carbon materials deviates from the classic EDL model on flat interfaces. The degree of the ion solvation under confinement is determined by the pore size in nano-porous materials and by the inter-layer distance in layered materials that is, 2D-layer materials.

Confinement of ions in sub-nanometer pores results in their desolvation, leading to the capacitance increase and deviation from the typical linear behavior on the surface area. During negative polarization of porous carbon materials with the pore sizes <1 nm, a decrease of capacitance  is observed. This is due to the ion selection limiting ion transport.

These insights are important for effectively tailoring carbon pore structures and for increasing their specific capacitance. Since carbon material is not an ideal conductor, it is important to consider its specific electric structure. For graphite materials, the availability of the charge carriers increases during the polarization which leads to increased conductivity.

Unified model of electrochemical charge storage under confinement

Since the electrochemical interface in the most technological application is non-planar, the researchers proposed a detailed evaluation and different concept of electrochemical capacitance on such non-ideal interfaces. The team evaluated electrosorption on 2D surfaces and 3D porous carbon surfaces with a continuous reduction in pore size in a step-by-step approach of increasing complexity.

The example provided relates to the charge storage characteristics of lithium ions (Li+) in the graphene sheets of organic lithium-containing electrolytes depending on the number of graphene layers. In a single graphene layer, the capacitive response is potential independent due to the absence of specific adsorption. However, with an increase of graphene sheets, redox peaks emerged that are associated with the intercalation of desolvated lithium ions. Lithium intercalation is responsible for battery wear. The team’s hypothesis was that the transition of solvated lithium ion adsorption on a single graphene sheet into subsequent intercalation of desolvated lithium ions occurs with a continuous charge storage behavior. There can be a seamless transition based on the increased charge transfer between an electrolyte ion and host associated with the extent of desolvation and confinement.

In the presented research, a unified approach was proposed that involves the continuous transition between double-layer capacitance and Faradaic intercalation under confinement. This approach excludes the traditional “single” view of electrochemical charge storage in nano-materials regarded as purely electrostatic or purely Faradaic phenomenon.

The increasing degree of ion confinement is followed by decreasing degree of ion solvation thus the increase ion-host intercalation. This results in a continuum from EDL formation through transitioning state to Faradaic intercalation, typical for EDLC nanomaterial.

Image: Pixabay

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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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Highly durable platinum-palladium-based alloy electrocatalyst for PEM fuel cells

To decrease the consumption of fossil fuel-derived energy for transportation, proton exchange membrane fuel cells (PEMFCs) are one of the most promising clean power sources. Their performance, however, strongly depends on the efficiency and durability of the electrocatalyst used for the hydrogen and oxygen reactions occurring at the electrodes. Noble metals such as platinum and gold are still considered as the most efficient catalysts. At the same time, their high cost and scarcity are major road blocks for scale commercialization of these energy devices.

Various solutions of catalyst design are intensively investigated in order to make this technology economically successful. Searching for high catalyst activity and durability for fuel cells is in focus of current research and development. To date, state-of-the-art electrocatalysts are based on carbon materials with varying platinum loadings.

Ultra-high active platinum group metal (PGM) alloy catalyst

Although, recent research reported ultra-high activity of some metal alloy catalysts, problems still remain. Some of these issues are related to utilization of high atomic percentages of PGM (sometimes up to 75% Pt), poor durability and performance under industrial conditions. In search for new solutions, researchers of the State University of New York at Binghamton, USA, and their collaborators reported a new design in journal Nature Communication: a highly-durable alloy catalyst was obtained by alloying platinum and palladium at less than 50% with 3d-transition metals (Cu, Ni or Co) in ternary compositions.

They addressed the problem of severe de-alloying of conventional alloy catalysts under the operating conditions, resulting in declining performance. For the first time, dynamic re-alloying as a way to self-healing catalysts under realistic operating conditions has been demonstrated to improve fuel cell durability.

Alloy combination and composition

The wet-chemical method was used for synthesis of Pt20PdnCu80−n alloy nanoparticles with the desired platinum, palladium and copper percentages. The selected set of ternary alloy nanoparticles with tunable alloy combinations and compositions, contained a total content of platinum and palladium of less than 50%, keeping it lower than current PGM-based alloy catalysts. The incorporation of palladium into platinum nanomaterials enabled a lower degree of de-alloying and therefore better stability. Additionally, palladium is a good metal partner to platinum due to their catalytic synergy and their resistance to acid corrosion.

To reduce the need for platinum and palladium core catalysts, a third, non-noble transition metal played a central role in the catalytic synergy of alloying formation. Non-noble metals such as copper, cobalt, nickel or similar were used. The platinum-palladium alloy with base metals allowed the researchers to fine tune the thermodynamic stability of the catalysts.

Morphology and phase structure

The thermochemical treatment of carbon-supported nanoparticles was crucial for the structural optimization. The metal atoms in the catalytic nanoparticles were loosely packed with an expanded lattice constant. The oxidative and reductive treatments of the platinum-palladium alloy (PGM <50%) allowed a thermodynamically stable state in terms of alloying, re-alloying and lattice strains. The re-alloying process not only homogenized the inhomogeneous composition by inter-diffusion upon calcination of nanoparticles, but also provided an effective pathway for self-healing following de-alloying.

Single face-centered cubic type structures were observed in Pt20PdnCu80–n nanoparticles (n = 20, 40, 60, 80) nanoalloys. Copper-doping of platinum-palladium alloys reduced the lattice constant effectively, as shown by high energy X-ray diffraction. Maximized compressive strain and maximized activity of the Pt20Pd20Cu60 catalyst confirmed strong correlation between the lattice constants and the oxygen reduction activity.

The researchers demonstrated that the thermodynamically-stable Pt20Pd20Cu60/C catalyst exhibited not only the largest compressive strain after 20,000 cycles, but also high activity and high durability. The discovery that the alloy catalyst remains alloyed under fuel cell operating condition is in sharp contrast to the fully de-alloyed or phase-segregated platinum skin or platinum shell catalysts reported in almost all current literature.

The significance in understanding of the thermodynamic stability of the catalyst system is a potential paradigm shift of design, preparation, and processing of alloy electrocatalysts.

(Photo: Pixabay)


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Production of Green Hydrogen through exposure of nano particles to sunlight

The demand for energy is increasing and raw material for the fossil fuel economy is diminishing. Moreover, the emission of gases from fossil fuel usage significantly degrades air quality. The carbon by-products produced from these fossil fuels severely affect the climate.

Hence, there is a need to find a renewable energy resource, that can be produced, stored, and used easily as per requirement. Hydrogen can be a promising energy resource as it is an abundantly available, non-toxic resource, and can be readily used to store excess electrical energy.

Hydrogen when combined with oxygen in a fuel cell produces electricity and the by-products obtained are water and heat. Based on the method of production of hydrogen it is categorized as blue hydrogen and green hydrogen. Blue hydrogen is produced from fossil fuels such as methane, gasoline, coal while green hydrogen is produced from non-fossil fuels / water. The cleanest way to produce eco-friendly hydrogen is via electrolysis of water where water is electrolyzed to separate hydrogen and oxygen. Renewable energy can be used as a power electrolyzer to produce hydrogen from water. Solar driven photo electrochemical (PEC) water splitting is one of the common method used these days. In photo electrochemical (PEC) water splitting, hydrogen is produced from water using sunlight.

PEC cells comprise of a working photoelectrode and a counter electrode. The photoelectrode consists of semiconductor material with a band gap to absorb solar light and generate an electron-hole pair. The photo-generated charges are responsible for the oxidation of water and its reduction into hydrogen. The PEC suffer devices from low stability and efficiency.

The research team from the  Institut national de la recherche scientifique (INRS) along with researchers from the Institute of Chemistry and Processes for Energy, Environment and Health (ICPEES) , a CNRS-University of Strasbourg joint research lab published a way to significantly improve the efficiency of water dissociation to produce hydrogen by the development of sunlight photosensitive-nanostructured electrodes.

A comparative study between cobalt and nickel oxide nanoparticles deposited onto TiO 2 nanotubes prepared through anodization was carried out. The TiO 2 nanotubes were decorated with CoO (cobalt oxide) and NiO (nickel oxide) nanoparticles using the reactive pulsed laser deposition method. The surface loadings of CoO or NiO nanoparticles were controlled by the number of laser ablation pulses. The efficiency of CoO and NiO nanoparticles as co-catalysts for photo-electrochemical water splitting was studied by cyclic voltammetry, under both simulated sunlight and visible light illuminations and by external quantum efficiency measurements

The entire research work was carried out in the following steps:

Catalyzed Green Hydrogen synthesis
Steps followed to improve the efficiency of hydrogen production

(Source: Favet et al ., Solar Energy Materials and Solar Cells , 2020)

In this study Cobalt (CoO) and Nickel (NiO) oxides were considered as effective co-catalysts for splitting water molecules. Both co-catalysts improved photo-electrochemical conversion of ultra violet as well visible light photons.

However, CoO nanoparticles were found to be the best co-catalyst under visible light illumination, with a Photo Conversion Efficiency almost 10 times higher than for TiO 2 . The performance of CoO nanoparticles got enhanced in the visible spectral region (λ> 400 nm). The possible reason can be a consequence of their visible bandgap which enables them to harvest more photon in the 400-500 nm range and transferring effectively the photo-generated electrons to TiO 2 nanotubes.

At Frontis Energy we are exited about these new discovery to improve hydrogen production from sunlight and hope to see an industrial application soon.

(Image: Engineersforum)

Reference: Favet et al ., Solar Energy Materials and Solar Cells , 2020