The science and technology of ultracapacitors are reviewed for a number of electrode materials, including carbon, mixed metal oxides, and conducting polymers. More work has been done using microporous carbons than with the other materials and most of the commercially available devices use carbon electrodes and an organic electrolytes. The energy density of these devices is 3¯5 Wh/kg with a power density of 300¯500 W/kg for high efficiency (90¯95%) charge/discharges. Projections of future developments using carbon indicate that energy densities of 10 Wh/kg or higher are likely with power densities of 1¯2 kW/kg. A key problem in the fabrication of these advanced devices is the bonding of the thin electrodes to a current collector such the contact resistance is less than 0.1 cm2. Special attention is given in the paper to comparing the power density characteristics of ultracapacitors and batteries. The comparisons should be made at the same charge/discharge efficiency.

Ultracapacitors: Why, How, and Where is the Technology

Photoreduction of CO2 into sustainable and green solar fuels is generally believed to be an appealing solution to simultaneously overcome both environmental problems and energy crisis. The low selectivity of challenging multi-electron CO2 photoreduction reactions makes it one of the holy grails in heterogeneous photocatalysis. This Review highlights the important roles of cocatalysts in selective photocatalytic CO2 reduction into solar fuels using semiconductor catalysts. A special emphasis in this review is placed on the key role, design considerations and modification strategies of cocatalysts for CO2 photoreduction. Various cocatalysts, such as the biomimetic, metal-based, metal-free, and multifunctional ones, and their selectivity for CO2 photoreduction are summarized and discussed, along with the recent advances in this area. This Review provides useful information for the design of highly selective cocatalysts for photo(electro)reduction and electroreduction of CO2 and complements the existing reviews on various semiconductor photocatalysts.

Cocatalysts for Selective Photoreduction of CO2 into Solar Fuels.

High-performance electrode materials are the key to advances in the areas of energy conversion and storage (e.g., fuel cells and batteries). In this Review, recent progress in the synthesis and electrochemical application of transition metal carbides (TMCs) and nitrides (TMNs) for energy storage and conversion is summarized. Their electrochemical properties in Li-ion and Na-ion batteries as well as in supercapacitors, and electrocatalytic reactions (oxygen evolution and reduction reactions, and hydrogen evolution reaction) are discussed in association with their crystal structure/morphology/composition. Advantages and benefits of nanostructuring (e.g., 2D MXenes) are highlighted. Prospects of future research trends in rational design of high-performance TMCs and TMNs electrodes are provided at the end.

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Transition Metal Carbides and Nitrides in Energy Storage and Conversion

Surface modification and enhanced photocatalytic CO2 reduction performance of TiO2: a review

Hydrogen is an ideal candidate for the replacement of fossil fuels in the future due to zero emission of carbonaceous species during its utilization. Water electrolysis is a dependable link of primary renewable energy and stable hydrogen energy. In this work, the fundamentals of water electrolysis, current popular electrocatalysts developed for cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER) in liquid electrolyte water electrolysis are reviewed. The main HER catalysts include noble metals, non-noble metals and composites, noble metal-free alloys, metal carbides, chalcogenides, phosphides and metal-free materials while the OER catalysts are focused on efficient Co-based, Ni-based materials and layered double hydroxide (LDH) materials. The strategies to improve catalytic activity, long-term durability and endurance to electrochemical erosion are introduced. The main challenges and future prospects for the further development of electrodes for water electrolysis are discussed. It is expected to give guidance for the development of novel low-cost nanostructured electrocatalysts for electrochemical water splitting.

Nanostructured catalysts for electrochemical water splitting: current state and prospects

In this study, lignin-based carbon nanofibers were prepared by electrospinning, followed by carbonization at four different temperatures (800, 1000, 1200, and 1400 °C). The surface and bulk elemental compositions were analyzed by energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy, respectively. In addition, the structure of the prepared carbon nanofibers was characterized by scanning electron microscopy, transmission electron microscopy, focused ion beam microscopy, and Raman spectroscopy. Results showed that all carbon nanofibers, irrespective of the carbonization temperature, had homogeneous structures. They were dense and no phase separation was observed. Moreover, the nanofibers carbonized at 800 °C or 1000 °C predominately contained amorphous carbon and some non-carbon elements. When the carbonization was performed at a higher temperature (1200 °C or 1400 °C), non-carbon elements were effectively removed and nanocrystalline graphite was formed, indicating that high temperature carbonization facilitated the formation of ordered carbon structures.

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Investigation of Structure and Chemical Composition of Carbon Nanofibers Developed From Renewable Precursor

Multi-walled carbon nanotubes (MWCNTs) grown directly on tetrahedral amorphous carbon (ta-C): An interfacial study

Three different metals (Pt, Pd and Ni) on multi-walled carbon nanotube and graphite supports were tested as catalysts in ethanol reforming experiments at low temperatures. The carbon nanotube based catalysts outperformed their counterparts supported on graphite in both ethanol conversion and hydrogen evolution. The palladium catalyst in both supports was rather inactive, whereas the nickel/nickel oxide catalyst showed excellent performance. The most active catalyst (Ni/CNT) showed ethanol conversion and hydrogen evolution already at 200 °C with complete conversion at 450 °C. The measured selectivity for H2 production was ∼42% with almost 100% conversion. Analysis of the spent catalyst samples revealed a considerable increase in carbon concentration caused by the formation of soot and coke on each sample but also carbon nanofibers and nanotubes were found to grow on the catalyst nanoparticles in the course of the steam reforming reactions.

Carbon supported catalysts in low temperature steam reforming of ethanol: study of catalyst performance

Transparent conductive films are used in a wide variety of devices. While solar cell top electrodes as well as tablet and mobile phone screens require high optical transparency and low sheet resistance (>80% and <10 Ω/□) to maximize power efficiency; other, less demanding applications, such as those in capacitive touch panels and antistatic coatings, in which only small currents are involved, can be managed with coatings of moderate conductivity. In this paper, we show that area-selective argon plasma treated polyethylene terephthalate surfaces are suitable for localized deposition of carbon nanotubes from their aqueous dispersions by a simple dip coating and subsequent drying processes. The as-deposited carbon nanotubes form entangled networks in microscopic patterns over the plasma-treated surface areas with sheet resistance of <1 kΩ/□ and optical transparency of ~75%. Based on this process, we demonstrate grid-type transparent conductive thin films of carbon nanotubes as capacitive touch sensors. Since each process step is robust, easy to up and downscale, and may be implemented even in roll-to-roll and sheet-to-sheet fabrication, the demonstrated technology is promising to produce grid-type structures even at an industrial scale in the future.

Olli Pitkänen

Papers

Investigation of Structure and Chemical Composition of Carbon Nanofibers Developed From Renewable Precursor

Multi-walled carbon nanotubes (MWCNTs) grown directly on tetrahedral amorphous carbon (ta-C): An interfacial study

Carbon supported catalysts in low temperature steam reforming of ethanol: study of catalyst performance

Grid-type transparent conductive thin films of carbon nanotubes as capacitive touch sensors.

Solder transfer of carbon nanotube microfin coolers to ceramic chips