Synthesis and Electrochemical Characterization of Niobium Pentoxide/Carbon Nanotubes Composites

Synthesis and Electrochemical Characterization of Niobium Pentoxide/Carbon Nanotubes Composites Introduction In the last few years, a range of energy-storage device applications has expanded from portable electronics to large-scale energy storage systems, including renewable energy storage and electric transportation [1,2]. To fulfill the energy and power density requirements for energy applications, many researchers have given attention to the synthesis of nanomaterials based on oxides due to their chemical, physical, optical, and electronic properties [1–4]. In this way, niobium pentoxide (Nb2O5) is a promising candidate because of its semiconductor properties with a band gap of ~3.4 eV, n-type, low toxicity, surface acidity, and good chemical and thermal stabilities [3,5–8]. It has been demonstrated that Nb2O5 can deliver high power through a mainly pseudocapacitive reaction of Li-ion (Li+), which could occur not only at the surface but also in the bulk of the Nb2O5 nanocrystals in non-aqueous Li+ electrolyte [2,9]. Moreover, the intercalation pseudo-capacitive behavior was highly dependent upon the presence of a crystalline structure, where amorphous and pseudo-hexagonal (TT-Nb2O5) exhibited lower specific capacitance values than the orthorhombic (T-Nb2O5) phase, however, nanoparticles aggregation is inevitable due to the high calcination temperature (>600  °C) for the orthorhombic phase formation of Nb2O5 [10–17]. Nonetheless, the application of Nb2O5 has been hampered by low bulk electrical conductivity (~3.4Ãâ€"10-6 S.cm-1 at 300 K) and by the difficult control of the ideal crystal structure [3,10,11]. Therefore, when T-Nb2O5 nanocrystals were fabricated into a relatively thick electrode, the power performance would be limited due to the impaired mobility of electrons. A possible effective method to improve these adversities in Nb2O5 is through of surface modifications (e.g., carbon coating), which could expose more redox active nanoparticles to the electrolyte as well as greatly i mprove the electronic conductivity [1,3,11–13]. It must be noted that the introduction of multiwalled carbon nanotubes (MWCNTs) networks can enhance electron transport of the Nb2O5 and further improve the rate capability. Physical mixing of MWCNTs and Nb2O5 nanoparticles can fabricate composite with improved conductivity, but this type of mixing fails to fulfill a good interfacial interaction between the MWCNTs and Nb2O5 [3]. Wherefore, soft-chemistry methods like oxidant-peroxo method matched with hydrothermal treatment and heating by microwaves could be a promising alternative for niobium synthesis, mainly because it is performed at low temperatures which avoids the elimination of the hydroxyl groups in as-formed surfaces resulting in a material with high surface area and greater number of acid sites [6,18,19]. In addition, carbon materials have a good microwave radiation absorption, which facilitates the interaction with other particles [20]. The use of niobium oxides for energy storage devices is already well known and exploited [21] but its performance is only obtained by a hybrid supercapacitor based on lithium intercalation processes [21] where high values of energy and power density are obtained. These intercalation processes require the application of potentials between 1 to 3 V vs. Li/Li+, a careful and humidity-free assembly scheme and non-aqueous electrolytes [16]. In this work, niobium pentoxide with different structures and morphologies were grown onto the surface of carbon nanotubes were synthesized by MHS. Moreover, we evaluated some electrochemical properties of electrodes with high mass loading on Ni foam current collectors (Fig. 1) using aqueous electrolyte, which brings the material closer to real and practical applications. Methods The acid functionalization on the MWCNTs was performed to create functional groups through a controlled functionalization of the nanotube walls, which allowed the anchoring an d growth of the Nb2O5 nanoparticles. This functionalization is accomplished by exposing the MWCNTs in a concentrated HNO3/H2SO4 (1:3) as described by Goyanes, et al. 2007 [22]. Nb2O5 nanoparticles were synthesized by the microwave-assisted hydrothermal method followed by a heat treatment. The sample preparation started with the dispersion of the 50 mg of functionalized MWCNTs and 100 mg of SDS (1:2) in 100 mL of distilled H2O in an ultrasonic bath. After dispersing, 2 g of (NH4[NbO(C2O4)2(H2O)2] ·nH2O) and 1.6 mL of H2O2 were added under ultrasound frequency. The solution was ultrasonicated continuously for 30 min and then poured into a Teflon vessel reaching about 50% of its volume, the autoclave was sealed and heated in the microwave equipment (Electrolux, MEF41, Brazil). The synthesis was performed with amendments according to Marins, et al., 2017 [23], where the temperature was set at 160  °C during 30 min, under a constant pressure of approximately 6 kgf cm-2 using 2.45 GHz microwave radiation with a maximum output power of 1500 W. Temperature and pressure during synthesis were monitored and controlled with the aid of a shielded thermocouple inserted directly into the vessel and with a pressure transducer sensor connected to the vessel [5]. After cooling at r oom temperature, the obtained powders were washed and centrifuged at 5000 rpm six times with distilled H2O to remove unreacted reagents and surfactant. The washed powder was over dried with no air circulation at 50  °C for 48 h. This sample was identified as CNT+TT-Nb2O5. To obtain the samples under heat treatment, part of the powder was heated in a furnace (FT-1200 1Z, Fortelab, Brazil) at 400  °C for 120 min and 650  °C for 180 min with a heating rate of 10  °C/min under nitrogen atmosphere aiming to get the orthorhombic phase [12,22]. After cooling at room temperature, the powders were stored in an oven with no air circulation at 50  °C and identified as CNT+T-Nb2O5. PVB solution in ethanol was added to the mixture in the electrode preparation. The PVB content in the slurry was 3% of the total mass of Nb2O5 and MWCNTs to each sample. The obtained slurry was used for the impregnation of Ni foam current collectors with area of 1 cm2. The active mass loading was 32 mg cm-2.

Society and Culture in Contemporary China Essay

Society and Culture in Contemporary China - Essay Example At that time, Tibet was an independent country with its own military and spiritual leaders. However, 20th Century, China overthrew Tibet and this was where the violence began. In the year 1959, the Chinese government discharged Tibet's spiritual leader known as the Dalai Lama from the country as he is now exiled in India. (Japan Times, 2008) In the month of March, there were violent protests in the Tibet's Lhasa region. This meant that the entire world focused on China and Tibet in that Month. It is very interesting to note that the Chinese government decided to deal with other news occurring throughout the world and only left a small portion of the china daily for their report on the Tibet protests. (China Daily, 2008) The Chinese government asserted that there was a third force behind the recent incidences in Lhasa. They believe that the third force is the exiled Dalai Lama. They claimed that this spiritual leader organized group that had prepared way before time for the incidences that occurred in March., they also asserted that the manner of execution denoted the fact that there was indeed a mastermind behind the violent protests. Additionally, the report went on to describe the negative effects of the protests; they referred to the nature of destructions such as burning property and buildings, wanton destruction of properties, lootings and beatings.It is also interesting to note that the China Daily tried to bring out the fact that the Tibetans condemned the protests. This report mentions the ethnic groups in Tibet and asserts that they all rally behind the Chinese government's view. Analysis of the report made by the People's Daily called Xinhua indicates the fact that the Chinese government has granted all ethnic groups regional autonomy. In the report, the Director of a Committee called Ethnic Affirms Committee affirms that regional autonomy is one of the backbones of the Chinese political systems. The report brings out the following issues; There are 5 autonomous regions in China 155 autonomous areas 120 autonomous counties 44 autonomous areas deal; with minority tribes There are 55 minority tribes The daily reports that China has granted autonomy for over fifty years. The result of these actions was the rapid increase in social and economic progress in those areas. The report claims that autonomy covers sixty-four percent of the country's vast geography. (Xinhua, 2008) Additionally, the report puts forward the fact that there will be an exhibition for the Tibet people. This exhibition will be held by the Chinese governmement and seeks to bring out some of the latter's opinion about the Tibetan past. It will give material on the history of the Tibetan people from earlier times to present day. Purpose media intended to serve The Chinese government laid out the report in a way that favors them. The violent protests were an indication that the Tibetans were unhappy with Chinese rule. This was an issue that none of them highlighted. Additionally, the government was also quick to point an accusing finger at another party even without adequate evidence. For instance, they claimed that the Dalai Lama was behind the protests yet they had no tangible evidence to

Cities Essay Example | Topics and Well Written Essays - 2750 words

Cities - Essay Example Therefore to fulfill these requirements people come together and build a city where they facilitate every possible requirement of human nature. What makes a place a city are the people who live there, their interactions and activities. It’s not only perceived as a gathering of millions of people but also by the constructions and infrastructure designed for man’s feasibility and convenience. Kevin Lynch describes an environmental image in three components: identity, structure and the meaning. The identity for which the city is known and distinguished, the structure- how it is build and designed and then the meaning is the relation people have with it (Lynch, 1960). In 18000 only 3% of the world population lived in the urban areas; However, by 1900, 14% were urbanites. With time the percentage increased and today we have urbanized the maximum possible earth and with the rate of population increasing the entire world would be urbanized soon. Cities are known as a place of opportunities where money, services employment are centralized. In rural areas it is difficult for people to achieve the requirements the way they can in cities. Living in cities facilitates people with numerous opportunities, luxuries and diversities like transportation, education and jobs. It is an established place where one can actually promote his way towards success. Business generates the demand of labor which eventually results in employment to bag money. People in rural areas have low income and huge families to make both ends meet they need money for which they belief city is a better place. For instance a farmer or a fisherman might not pull that money in a rural area as compared to that he can in city (Scott, 1981). City provides them with the opportunity of variety of jobs to all calipers of people. A reason to this is also the population density in urban areas, more people have numerous demands