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| Fig. 1: Volumetric and gravimetric energy density of various fuels, with the mass of the container and apparatus needed for filling and dispensing the fuel factored in. (Source: K. Lee, after Crabtree, Dresselhaus, and Buchanan. [4]) |
Energy is a crucial component of the global society, sustaining economic growth and maintaining quality of our daily lives. [1] With the projected increase in the global population and economic growth in developing countries, it is no surprise to expect the worlds demand for energy to increase over time. [2,3] In fact, according to the U.S. Energy Information Administration, the world energy consumption is expected to be around 815 quadrillion Btu (8.59 × 1020 joules) by 2040, which is a 48% increase from that in 2012. [3] With such high global demand in energy, it is important that these large quantities of energy are accessible in an affordable, sustainable, and nondestructive way.
Historically and even up to recent times, fossil fuel has been the dominating source of world energy, comprising up to more than 80% of the world's energy consumption in 2011. [1] While fossil fuel has clear strengths in energy density and cost efficiency, it certainly holds clear problems as well, namely the uncertainties in sustainability and large environmental footprints. [4,5] These factors make hydrogen an appealing replacement for fossil fuel. The electrical energy harnessed from renewable energy sources such as solar and wind can be used to generate hydrogen gas from water (a process called "water splitting" or "water electrolysis"), with oxygen gas being the only byproduct. The reverse reaction can be used to convert hydrogen gas back to electricity, which is what hydrogen fuel cells do. With water, hydrogen, and oxygen being the only reactants in the process, hydrogen generation via water electrolysis resolves the disadvantages of fossil fuel. Although the energy density of hydrogen is worse than gasoline both volumetrically and gravimetrically (when factoring in the mass of the container and apparatus needed for filling and dispensing the fuel), it is better than batteries, making hydrogen an attractive alternative as an energy carrier (Fig. 1). [4] However, hydrogen as a fuel significantly lacks cost efficiency in comparison with fossil fuel; maximizing the efficiency and therefore reducing the cost of hydrogen production and hydrogen fuel cells is key to realizing the transition from fossil fuel to hydrogen fuel. [6]
A key reaction in water electrolysis which significantly brings down its efficiency is the oxygen evolution reaction (OER), in which water molecules split to generate oxygen gas under applied electrical potential (2H2O → O2 + 4H+ + 4e-). [7] This is a complex, multi-step chemical reaction involving transfer of four electrons, which explains the sluggish kinetics of the reaction. Therefore, the key to increasing the efficiency of water electrolysis lies dominantly in developing active OER catalysts for expediting the reaction. The activity of an OER catalyst is directly related to how much electric potential it is needed to operate the catalyst at a fixed current output. Often, this operating potential is subtracted by the theoretical minimum potential of 1.23 V required to drive the OER; this is termed the "overpotential" of the catalyst. [7] Consequently, making an active OER catalyst boils down to minimizing the overpotential of the catalyst.
Fueled by the increasing interest towards hydrogen as an energy carrier, OER catalyst development has been, and still is, an active field of scientific research. Many novel materials have been synthesized and tested for their electrochemical activity and stability as OER catalysts. Recently, transition metal oxides of the perovskite form ABO3, where B is a transition metal, have gained attention for their high OER catalytic activity. [7] A notable recent progress in this approach is the development of strontium iridate (SrIrO3) perovskite thin film OER catalyst, which was shown to achieve an overpotential of ~ 270 mV at current density of 10 mA/cm2 under acidic environment; this is the lowest overpotential so far reported for acid-stable OER catalysts. [8] Using nanoengineering to geometrically change the catalyst surface to increase the surface area of the catalyst and therefore its catalytic activity has been a well-explored path as well. [9] Theoretical investigations on catalyst surface chemistry and its effect on OER are also actively being done. [7,9] In parallel to maximizing the activity of OER catalysts, minimizing their cost is equally crucial to realize the commercialization of hydrogen generation via water electrolysis. Currently, OER catalysts with promising activity and stability involve rare, expensive metals such as iridium and ruthenium. [7] Acknowledging this, diverse research is being done to replace these expensive materials with other metals which are more affordable, such as nickel and cobalt, without degrading the catalytic activity. [7]
With its strengths in sustainability and pollution-free process, hydrogen generation via water electrolysis makes hydrogen an attractive alternative to fossil fuel. Commercial endeavors are already in progress, with electrolysis stacks for hydrogen generation available on the market. [7,10] To realize the transition from fossil fuel to hydrogen fuel, however, significant improvements in cost and efficiency are needed. OER catalyst development is an actively ongoing field of scientific research which directly engages the problem of efficiency and cost of hydrogen generation via water electrolysis. Developing OER catalysts with better catalytic activity and stability will be a crucial step towards making hydrogen a viable option as the alternative fuel.
© Kyuho Lee. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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[3] "International Energy Outlook 2016," U. S. Energy Information Administration, DOE/EIA-0484(2016), May 2016.
[4] G. W. Crabtree, M. S. Dresselhaus, and M. V. Buchanan, "The Hydrogen Economy," Phys. Today 57, No. 12, 39 (December 2004).
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[9] Z. W. Seh et al., "Combining Theory and Experiment in Electrocatalysis: Insights into Materials Design," Science 355, 146 (2017).
[10] A. Buttler and H. Spliethoff, "Current Status of Water Electrolysis for Energy Storage, Grid Balancing and Sector Coupling Via Power-to-Gas and Power-to-Liquids: A Review", Renew. Sustain. Energy Rev. 82, 2440 (2018).