Electrolysis of water

Abstract

Hydrogen energy systems, based on renewable energy (RE) sources, are being proposed as a means to increase energy independence, improve domestic economies, and reduce greenhouse gas emissions from stationary and mobile fossil-fueled sources. In 2003, the United States consumed roughly 84.3 billion m3 (7.6 billion kilograms) of hydrogen, the majority of which was produced via the widely established thermal process known as steam methane reforming (SMR). The electrolytic production of hydrogen, while not economically competitive today with SMR, is positioned to become the preferred method due to the inevitable price increase of natural gas and as environmental, social, and economic factors are weighed. SMR constitutes roughly 50% of the 450-500 billion m3 yr-1 (38-42 billion kg yr-1) of global production of the gas. SMR, like hydrogen production from all fossil fuels, suffers from supply issues and climate-altering carbon-based pollution. The reforming process generates CO 2 as well as carbon monoxide (CO), which is poisonous to humans because the oxygen-transporting hemoglobin has 200 times the affinity to CO than O Electrolysis currently supplies roughly 4% of the world's hydrogen. If hydrogen is to be used as a transportation fuel, the United State could conceivably replace the 140 billion gallons per year (gal yr-1) of gasoline consumed in 2004 with domestically produced hydrogen. The energy equivalent of this much gasoline is 17.3 × 1015 BTU, assuming approximately 5.2 million BTU bbl-1 of motor gasoline.4 The environmental gains hoped for by the transition to a hydrogen economy can only be achieved when renewable sources are ramped up to produce an increasing amount of the hydrogen gas. From the early 1800s to the mid 1900s town gas was comprised of roughly 50% hydrogen that brought light and heat to much of America and Europe and can still be found in some parts of Europe, China and Asia. Due to hydrogen's thermal conductivity and low density the gas is being used to cool many large thermal electrical power generators. Hydrogen is used in a wide variety of applications: • Chemicals-Ammonia and fertilizer manufacture-Synthesis of methanol-Sorbitol production-General pharmaceuticals and vitamins • Electronics-Polysilicon production-Epitaxial deposition-Fiber optics • Metals-Annealing/heat treating-Powder metallurgy • Fuels-Petroleum refinement-Liquid rocket fuel-Some use in fuel cells • Food and float glass-Fats/fatty acids-Blanketing Renewable sources of electricity and off-peak hydroelectric can be used to produce a sustainable supply of hydrogen for transportation, peak-shaving applications and in some special cases to smooth the variability in the renewable source. Powering millions of hydrogen internal combustion engines and/or fuel cell vehicles with hydrogen generated with traditional fossil fuel sources (without carbon dioxide (CO2) capture and storage or geological sequestration) is merely transferring the pollution from the tailpipe to the stack pipe. In the case of SMR, liquid natural gas imports would increase to replace today's 12.9 million bbl day-1 of oil imports here in the U.S.4. As developing countries fall in love with motorized transportation, much like the developed countries already have, transportation's contribution to greenhouse gas emissions will grow from the 25% it holds today. Still today, the electrolytic production of hydrogen using renewable sources is the only way to produce large quantities of hydrogen without emitting the traditional byproducts associated with fossil-fuels. The electrolysis of water is an electrochemical reaction requiring no moving parts and a direct electric current, making it one of the simplest ways to produce hydrogen. The electrochemical decomposition of water into its two constituent parts has been shown to be reliable, clean and with the removal of water vapor from the product capable of producing ultra-pure hydrogen (> 99.999%). The primary disadvantage of electrolysis is the requirement of high-quality of electrical energy needed to disassociate the gas. Electricity is a convenient energy carrier as it can be transported to loads relatively easily. However, locating and constructing new transmission and distribution power lines is challenging and expensive. The cost of transporting electricity along power lines can constitute greater than 50% of the total cost at the point of end-use. Historically, hydrogen production via electrolysis has only been viable where large amounts of inexpensive electricity have been available or the high purity product gas was necessary in a downstream process. The potential environmental benefit of a hydrogen-based economy is hinged to a large degree on the ability to generate the gas from renewable resources in a costeffective manner. An apparently ideal solution is to use wind-generated electricity to electrolyze water. Today, hydrogen production via electrolysis only meets the U.S. Department of Energy (DOE) goals of $2-$3 per kilogram (kg) in large installations where electrolyzer capital costs are low, less than $800 per kilowatt (kW), and those having access to inexpensive electricity, less than $0.04 per kilowatt-hour (kWh). Electricity from large-scale wind farms in Class 4 or better resource can be generated in the range of $0.05-$0.08 kWh-1, not including today's $0.019 kWh-1 Federal production tax credit. The out-of-pocket cost of fossil-fuels, whether for electricity production or as transportation fuels, has remained relatively low; limiting the expansion of renewable forms of energy. For example, if the external costs of production were taken into account the cost of coal-generated electricity would rise an additional $0.03-$0.06 kWh-1. Further limiting market penetration of renewable sources is that fossil fuels continue to receive the bulk of tax incentives here in the U.S. The term renewable defines these technologies as driven by natural and sustainable processes which are inherently variable, not intermittent. Natural processes vary over time but are not subject to the on-off switching that, for example, a light bulb connected to a switch is subjected to. Advocates may want to begin training themselves to describe RE as variable, not intermittent, to better describe their naturally occurring behavior. RE sources of energy can provide cost-effective, emission-free electricity with zero-or low-carbon impact making it one of the preferred methods for supplying energy to society. The large-scale wind energy facilities being installed throughout the world are a testament to the growing demand, environmentally preferred and cost-effectiveness of this RE technology.