Electrochemical capacitors, also called supercapacitors, store energy using either ion adsorption (electrochemical double layer capacitors) or fast surface redox reactions (pseudo-capacitors). They can complement or replace batteries in electrical energy storage and harvesting applications, when high power delivery or uptake is needed. A notable improvement in performance has been achieved through recent advances in understanding charge storage mechanisms and the development of advanced nanostructured materials. The discovery that ion desolvation occurs in pores smaller than the solvated ions has led to higher capacitance for electrochemical double layer capacitors using carbon electrodes with subnanometre pores, and opened the door to designing high-energy density devices using a variety of electrolytes. Combination of pseudo-capacitive nanomaterials, including oxides, nitrides and polymers, with the latest generation of nanostructured lithium electrodes has brought the energy density of electrochemical capacitors closer to that of batteries. The use of carbon nanotubes has further advanced micro-electrochemical capacitors, enabling flexible and adaptable devices to be made. Mathematical modelling and simulation will be the key to success in designing tomorrow's high-energy and high-power devices.

https://hal.archives-ouvertes.fr/hal-02417326/document

Materials for electrochemical capacitors

https://www.chem.ccu.edu.tw/~joyce/referance/gump/IL/Chem.%20Rev.%201999,%2099,%202071.pdf

Room-Temperature Ionic Liquids. Solvents for Synthesis and Catalysis

In this critical review, metal oxides-based materials for electrochemical supercapacitor (ES) electrodes are reviewed in detail together with a brief review of carbon materials and conducting polymers. Their advantages, disadvantages, and performance in ES electrodes are discussed through extensive analysis of the literature, and new trends in material development are also reviewed. Two important future research directions are indicated and summarized, based on results published in the literature: the development of composite and nanostructured ES materials to overcome the major challenge posed by the low energy density of ES (476 references).

/pdf/a-review-of-electrode-materials-for-electrochemical-2cok5em0bo.pdf

A review of electrode materials for electrochemical supercapacitors

Fuel cells convert chemical energy directly into electrical energy with high efficiency and low emission of pollutants. However, before fuel-cell technology can gain a significant share of the electrical power market, important issues have to be addressed. These issues include optimal choice of fuel, and the development of alternative materials in the fuel-cell stack. Present fuel-cell prototypes often use materials selected more than 25 years ago. Commercialization aspects, including cost and durability, have revealed inadequacies in some of these materials. Here we summarize recent progress in the search and development of innovative alternative materials.

Materials for fuel-cell technologies

Supercapacitors, also called ultracapacitors or electrochemical capacitors, store electrical charge on high-surface-area conducting materials. Their widespread use is limited by their low energy storage density and relatively high effective series resistance. Using chemical activation of exfoliated graphite oxide, we synthesized a porous carbon with a Brunauer-Emmett-Teller surface area of up to 3100 square meters per gram, a high electrical conductivity, and a low oxygen and hydrogen content. This sp 2 -bonded carbon has a continuous three-dimensional network of highly curved, atom-thick walls that form primarily 0.6- to 5-nanometer-width pores. Two-electrode supercapacitor cells constructed with this carbon yielded high values of gravimetric capacitance and energy density with organic and ionic liquid electrolytes. The processes used to make this carbon are readily scalable to industrial levels.

/pdf/carbon-based-supercapacitors-produced-by-activation-of-1wqrpq3luo.pdf

Carbon-based Supercapacitors Produced by Activation of Graphene

One promising approach to reduce the cost of fuel cell systems is to develop hydroxide exchange membrane fuel cells (HEMFCs), which open up the possibility of platinum-group-metal-free catalysts and low-cost bipolar plates. However, scalable alkaline polyelectrolytes (hydroxide exchange membranes and hydroxide exchange ionomers), a key component of HEMFCs, with desired properties are currently unavailable, which presents a major barrier to the development of HEMFCs. Here we show hydroxide exchange membranes and hydroxide exchange ionomers based on poly(aryl piperidinium) (PAP) that simultaneously possess adequate ionic conductivity, chemical stability, mechanical robustness, gas separation and selective solubility. These properties originate from the combination of the piperidinium cation and the rigid ether-bond-free aryl backbone. A low-Pt membrane electrode assembly with a Ag-based cathode using PAP materials showed an excellent peak power density of 920 mW cm−2 and operated stably at a constant current density of 500 mA cm−2 for 300 h with H2/CO2-free air at 95 °C. A key challenge for hydroxide exchange membrane fuel cells is the development of membranes with both high ionic conductivity and mechanical strength. Here the authors report a high-performance family of poly(aryl piperidinium) membranes enabling promising durability and power density.

Poly(aryl piperidinium) membranes and ionomers for hydroxide exchange membrane fuel cells

Polymer electrolyte fuel cells.

Life tests of polymer-electrolyte fuel cells using supported Pt catalyst in thin film catalyst layers are run for up to 4,000 h at maximum power. Particle ripening is readily evident using these types of electrodes in which the high catalyst utilization efficiency apparently subjects in majority of the platinum to conditions that sustain particle growth. X-ray diffraction analyses indicate that the initial platinum specific surface areas of 100 m[sup 2]/g Pt eventually stabilize to about 40 to 50 m[sup 2]/g in the cathode and 60 to 70 m[sup 2]/g in the anode. Interestingly, this loss in surface area does not affect the apparent catalytic activity of these fuel cell electrodes. A crystallite migration particle growth mechanism is suggested by the shape of the particle size distribution curves. Since the presence of liquids is known to lower the activation energy for particle growth, the particle size difference between the two electrodes may possibly be attributed to the different hydration levels at the anode and the cathode in operating polymer electrolyte fuel cells.

https://iopscience.iop.org/article/10.1149/1.2220925/pdf

Surface Area Loss of Supported Platinum in Polymer Electrolyte Fuel Cells

This paper compares direct methanol fuel cells (DMFCs) employing two types of Nafion{reg{underscore}sign} (E.I.DuPont de Nemours and Company) membranes of different equivalent weight (EW). Methanol and water uptakes in 1,100 and 1,200 EW Nafion membranes were determined by weighing P{sub 2}O{sub 5}-dried and methanol solution-equilibrated membranes. Both methanol and water uptakes in the 1,200 EW membrane were about 70--74% of those in the 1,100 EW membrane. The methanol crossover rate corresponding to that in a DMFC at open circuit was measured using a voltammetric method in the DMFC configuration and under the same cell operating conditions. After accounting for the thickness difference between the membrane samples, the methanol crossover rate through a 1,200 EW membrane was 52% of that through an 1,100 EW membrane. To resolve the cathode and anode performances in an operating DMFC, a dynamic hydrogen electrode was used as a reference electrode. Results show that in an operating DMFC the cathode can be easily flooded, as shown in a DMFC using 1,100 EW membrane. An increase in methanol crossover rate decreases the DMFC cathode potential at open circuit. At a high cell current density, the DMFC cathode potential can approach that of a H{sub 2}/air cell.

Shimshon Gottesfeld

Papers

Poly(aryl piperidinium) membranes and ionomers for hydroxide exchange membrane fuel cells

Polymer electrolyte fuel cells.

Surface Area Loss of Supported Platinum in Polymer Electrolyte Fuel Cells

Water and Methanol Uptakes in Nafion Membranes and Membrane Effects on Direct Methanol Cell Performance

A study of the electrochemical properties of conducting polymers for application in electrochemical capacitors