BIOHYDROGEN PRODUCTION USING ELECTRODEPOSITED CATHODES IN MICROBIAL ELECTROLYSIS CELLS

Abstract

Industrial revolution and population growth have led to a surge in the global energy demand as well as generation of huge amount of wastewater in recent years. Global primary energy demand (~0.55 quadrillion MJ) is increasing at a rate of 2.1 % per year with a projected total increase of 56 % by 2040. At present, around 85 % of world energy demand is fulfilled by non-sustainable resources (fossil fuels) such as petroleum (34 %), coal (27 %), and natural gas (24 %), which create significant emission of polluting gases as well as generate a huge amount of wastewater. The increasing rate of pollutants and wastewater generations have adverse impacts on global climate, human health and ecosystems. To mitigate these challenges renewable energy resources such as hydrogen production from wastewater are getting strong research interest around the globe because hydrogen from wastewater will not run out ever while other sources of energy are finite and may be depleted someday. Indeed, one of the most promising approaches to these critical problems is the conversion of wastewater into renewable energy such as hydrogen gas. It is also realized that the hydrogen production from the wastewater should be economical and sustainable. Thus, the world energy production research scenario has shifted from fossil fuels to sustainable or renewable resources such as economic hydrogen production from wastewater. The volume of wastewater generated per year in Asia is ~160 cubic kilometers and in India ~53 cubic kilometers, in which more than 80% of wastewater flows back into the ecosystem without being treated or reused. The major wastewater generating industries are the sugar industries, paper industries, beverage industries and food- processing industries. The wastewater of these industries consists of many organic nutrients, dyes, lignin, complex compounds, and heavy metals that are low biodegradable in nature. Amongst various industries the sugar and paper industries are water-intensive and daily generate large quantity (~ 65-75 m3 of wastewater per ton of paper/sugar produced) of wastewater containing very high amount of organics (COD 4,000– 20,000 mg/L, BOD: 2,000–12,000 mg/L) and toxic compounds. Organics present in these industrial wastewaters reduce dissolved oxygen and pollutants mostly enter into water stream through various industrial processes that create problem to the aquatic living system for their survival. The characteristics of the effluent generated in different steps of paper production such as debarking, pulping, bleaching and alkali extraction, washing varies significantly, while the nature of the organic compounds presents in the sugar industry effluents vary significantly with end products, process conditions, and raw material being utilized. Recent literature suggests that MEC technology is one of those technologies that can improve the treatment efficiency of these industrial effluents along with waste reduction in the aquatic system. Hydrogen is the clean energy carrier and fuel for the future, which has high energy density (122 142 MJ/Kg) than gasoline (46 MJ). Currently, the world produces around 70 million tonnes of hydrogen per year (6 % of global natural gas consumption) from the processes such as electrolysis of water, steam reforming of hydrocarbons, and auto-thermal processes. Biological (bio-photolysis, photo-fermentation, dark fermentation) or bio-electrochemical process, i.e. microbial electrolysis cells, for of hydrogen production from wastewater offers several advantages, such as metals recovery, renewable incentives, and higher energy revenues. Among different processes, the bio-electrochemical system i.e. microbial electrolysis cell (MEC) is getting immense research interest in recent years because of their potential for sustainability and diversity. Some recently published literature reported that the commercialized hydrogen production could be acclaimed using the biotechnology and bioprocess in the engineering of certain processes such as MEC. In order to enhance the hydrogen production and treatment efficiency of the industrial wastewater in the MEC requires efficient cathode, which possesses high electrocatalytic activity and low over-potential. Platinum (Pt.) is widely explored in the MEC due to low-over potential. However, precious metal Pt cannot be utilized as cathode in the wider practical operation of MECs, due to its high cost and poisoning by sulphides. Selection of suitable cathode material in the MEC is challenging in nature due to the presence of a three-phase interface: water (protons), air (oxygen), and solid/electrode (electron), which increases the over-potential losses. Literature report says that first-row transition metals, stainless steel and nickel alloys are promising alternative cathodes utilized in the MEC, due to their stability, low cost and low over potential with high intrinsic catalytic activity. Some SS-based cathode such as SS316 contains a high percentage of nickel (8–14%), which enhances the hydrogen production as compared to Pt. In MEC literature various alternative cathodes such as Fe, Zr, Mo, NiMo, NiFe and NiW were reported as cathode in the MEC. Among these alloys Nickel and Cobalt or their combination alloys could be considered as more promising electrode materials due to their electrocatalytic properties, durability of electrodeposits, reduction in over potential losses, enhanced the rate of hydrogen production due a synergetic effect of their individual catalytic properties. It is also demonstrated in literature that the presence of some weak acids such as phosphate species in the cathode materials can increase the hydrogen production in the MEC; presence of charged species reduces the over potential of cathode and increases the conductivity of the wastewater. Similar to SS, the ductile metal copper has very high thermal and electrical conductivity with a good conductivity of ions as well. However, Cu as cathode material in MEC has rarely been explored in the literature till date. MECs concurrently reduce the toxic/complex effluent with higher than 60% energy recovery at neutral conditions, however, the recovery of hydrogen can be enhanced by using engineering methodology to produced effective cathodes. Economical and efficient cathodes with high electrocatalytic activity play a vital role for increasing hydrogen recovery from the wastewater. Under this backdrop, the present investigation has been undertaken to develop an alternate wastewater treatment method using MEC. In this study, viability of MECs with synthetic acetate wastewater, real sugar and paper industries wastewater, using electrodeposited cathodes (Ni, Ni-Co, Ni-Co-P), for the hydrogen recovery as well as waste reduction (COD removal) has been explored. SS and Cu have been selected as the base cathode material and electroplating of Ni, Ni–Co and Ni–Co–P on SS316 and Cu cylindrical rod were carried out to produce effective cathodes. Sugar and paper industry wastewater were collected from local industries nearby Roorkee, Uttarakhand, India. Synthetic acetate wastewater has also been considered because of the fact that the acetate is present in most industries wastewater as well as it is the end/intermediate product of the anaerobic fermentation process. The sugar and paper industry effluents(wastewaters) have been characterized to assess their potential for energy production under the optimum conditions determined by the batch MEC process. Concurrent treatment of wastewater in terms of COD (chemical oxygen demand) removal and hydrogen recovery from synthetic wastewater using acetate as model substrate, sugar and paper industry real effluents have been carried out in the MEC unit using electrodeposited cathodes. Eight different cathodes were developed by the electrodeposition of Ni, Ni–Co and Ni–Co–P on the surface of SS316 and Cu cylindrical rod (10 cm, 1.5 cm diameter) individually by electroplating techniques. The electrodeposition of Ni, Ni-Co and Ni-Co-P on stainless steel (SS316) and Cu experiments was optimized to get uniform coating layer of cathode catalyst. The optimized condition of electrodeposition was 40 min time, 25 cm2 exposed surface area of electrode at the applied current density of 125 Am-2 in the temperature range of 90-100oC at pH 9. The SS and Cu rod were used as cathode and nickel plate was as anode and it was placed at a distance of 4 cm.