Source of energy
Dependence of mankind on non-renewable source of energy such as coal, oil and gases is increasing worldwide. In India, the main source of energy is coal, which accounts for 44 % of total energy consumption. Our country is now facing the shortage of coal supplies despite being the third largest coal producer in the world. According to energy information administration (EIA), our dependency on imported fossil fuels has risen to 38 % (USEIA 2014). Because of the limited availability of coal, an easily available, economical as well as environment friendly renewable source of energy is required. According to the United Nations Food and Agriculture Organisation (FAO), the animal waste on this planet produces around 55–65 % methane, which upon release in the atmosphere can affect global warming 21 times higher than the rate CO2does. Biogas, a mixture of different gases produced by anaerobic fermentation of organic matter from methanogenic bacteria, mainly constitutes methane (50–65 %) and CO2 (25–45 %) (Sharma 2011). One kilogram of cow manure can produce 35–40 l of biogas when mixed with equal amount of water with hydraulic retention time (HRT) of 55–60 days maintained at an ambient temperature of 24–26 °C (Kalia and Singh 2004). Li et al. (2009) reported 67 ml/g methane yield from anaerobic digestion of cow manure, whose total and volatile solids were 23.4 and 13.8 g/l, respectively. Green bacteria such as Pseudomonas sp., Azotobacter sp. and other purple sulphur or purple non-sulphur bacteria are known to produce maximum amount of methane gas in comparison to other photosynthetic bacteria present in cow dung (Rana et al. 2014). The optimum production of biogas depends upon mesophilic (32–38 °C) and thermophilic (50–55 °C) temperature range (Kashyap et al. 2003). The inability of mesophilic microorganisms to survive in psychrophilic temperature range results in 70 % reduced production of biogas during winters in hilly areas (Kanwar and Guleri 1994). This may be due to the collapse of cell energy, outflow of intracellular substances or cell lysis of mesophiles at lower temperature (Gounot 1986). But many researchers reported a fare amount of biogas production under psychrophilic range of temperature using some modifications (Safley and Westerman 1990; Kanwar and Guleri 1994).
Cow dung is the major source of biogas or gobar gas production in India. The total population of female cows in India is 190.90 million out of which 151 million are indigenous whilst 39 million are crossbreed (Livestock Census 2012). Cow dung generated from 3–5 cattle/day can run a simple 8–10 m3 biogas plant which is able to produce 1.5–2 m3biogas per day which is sufficient for the family 6–8 persons, can cook meal for 2 or 3 times or may light two lamps for 3 h or run a refrigerator for all day and can also operate a 3-KW motor generator for 1 h (Werner et al. 1989). A 1-m3 biogas plant has produced 28.78 l/kg (0.028 m3) and 32.76 l/kg (0.032 m3) of biogas respectively when daily feed with 22 kg of dung/m3 which is mixed with equal amount of water with 9–10 % of total solids. The maximum production of biogas from that plant is 39.00 l/kg (0.039 m3) and 40.04 l/kg (0.04 m3) respectively when operated at the temperature of 23.5 °C (Kalia and Singh 2004). On the other hand, farmer also gains 13.87 metric tons of organic fertiliser per year from the biogas plant. This co-production of biofertiliser also allow farmer to recover the initial investment for setting up of a biogas plant (Sharma 2011).
Though cow dung is solely used as the prime source for biogas production, but research continues to verify the potential of other sources for instance, addition of pig dung was found to have an enhanced effect. Mixture of cow and pig dung (60:40) showed 10 % increase in methane production as investigated by Li et al. (2014). Use of potato pulp and cow manure in the ratio of 20:80 also produced fair amount of methane in comparison to pure cow dung (Sanaei-Moghadam et al. 2014). Besides this, there are reports on comparative studies for biogas production where various feedstocks such as kitchen waste, corn waste and spent tea waste have been used along with cow dung in a ratio of 1:1 producing less average biogas after 25–30 days; however, cow dung alone produced approximately 50 % more biogas than these mixtures (Munda et al. 2012), thereby suggesting that other organic sources may produce biogas but cow dung still remains a potential source. In the light of above-discussed facts, biogas production can also be considered as an effective way of treating organic waste which may produce green house gases if remain untreated.
Supercapacitors are the in-between arrangement in electrochemical batteries which can store a large amount of energy that can be delivered with high power for few milliseconds (Gamby et al. 2001). They have high power density (103–104 W/kg), long cycle life (>106 cycles), pulse power supply, low maintenance cost, simplicity and better safety compared to secondary batteries. The use of porous carbon as electrode material is widespread in supercapacitors. This porous carbon is synthesised by many different methods such as using silica or surfactant, aerogels, organometallic compounds, chemical activation and physical activation. All these processes are costly and consume expensive precursors and time (Lee et al. 2006; Fang et al. 2009; Kim et al. 2012; Yang et al. 2012; Bhattacharjya et al. 2013; Inamdar et al. 2013; Bhattacharjya and Sung 2014; Yang et al. 2014). Now focus is shifting towards natural biomass as a potential source for carbon precursors. Several natural biomasses have been explored for production of activated carbon (Demiral and Demiral 2008; Hu et al. 2010; Li et al. 2010, 2011; Wei et al. 2011; Xu et al. 2012; Biswal et al. 2013; Falco et al. 2013; Huang et al. 2013; Wang et al. 2013; Bhattacharjya and Sung 2014). Activated carbon has recently been synthesised from cow dung by a modified chemical activation method, in which partially carbonised cow dung was treated with potassium hydroxide in the ratio of 2:1. The synthesised activated carbon when tested as supercapacitor electrodes in practical showed specific capacitance of 124 F/g at 0.1 A/g and retained up to 117 F/g at 1.0 A/g current density. It is also durable for long-term operations . The synthesis of activated carbon having high surface area along with optimum micropore and mesopore volume reflects excellent electrochemical application of cow dung for supercapacitors. The literature also suggest that biological waste like cow dung can be converted into a electrode material for other energy storage and conversion systems such as Li-ion batteries and fuel cells.
