Contributed by Dr. Zhichao Hu, Materials Science and Engineering, Rutgers University
Many of us have heard of wind turbines, solar panels, electric vehicles, and energy-efficient lighting, but we rarely ask what is behind these clean energy applications. A group of critical materials including rare earth (RE) elements is a key component for clean energy innovation. The Critical Materials Institute (CMI) was created in response to the spike in RE prices and subsequent supply crisis a few years ago. Although RE prices have dropped since then, China still controls over 90% of the global RE production. Developing RE substitutes, establishing domestic supplies, and promoting reuse and recovery are essential to American competitiveness in clean energy technologies.
The thermodynamics (thermo) team led by Prof. Richard E. Riman, part of the enabling science branch of CMI, consists of researchers from Rutgers University, University of California Davis, and OLI Systems, Inc. Aiming to provide thermodynamic data for critical materials to other CMI partners, the team thrives on forging collaborations within this innovation hub and helping its partners reach their project goals. One of the team’s long-term objectives is to help recover RE from phosphate fertilizer production wastes. Global phosphate deposits contain 27 million tons of RE, equal to nearly 200 years of the current world demand, and 38% of these RE end up in the 150 million tons of phosphogypsum (PG) waste generated annually during global phosphate fertilizer production. PG piles in central Florida constitute some of the highest points in the state. Although ‘closed’ piles are often revegetated and monitored for acid and or metal releases to the immediate surroundings, they still occupy acres of potential farm land and pose potential environmental threats. Extracting RE from PG is a first step towards trash-to-treasure transformation. The thermo team is currently collaborating with researchers at the Oak Ridge National Laboratory (ORNL) led by David DePaoli and researchers at the Florida Industrial and Phosphate Research Institute led by Patrick Zhang to develop a water-based environmentally friendly process to extract RE from PG. This approach could potentially supply RE equivalent to 20% of annual world demand.
CMI researchers are also exploring various other green approaches to improve and increase RE recovery. David Reed at the Idaho National Laboratory (INL) and Yongqin Jiao at the Lawrence Livermore National Laboratory (LLNL) are using microorganisms to achieve this goal. These two groups of researchers are using bacteria which produce acids to leach RE from waste feedstocks such as lamp phosphor powders, RE-containing catalysts, and RE ores. The leached RE can subsequently be collected by a genetically engineered bacterium which expresses specific RE-binding peptides on its cell surface (Environ. Sci. Technol., 2016, 50, 2735). Membrane-based or electrochemical recovery methods are also being evaluated by other CMI research groups.
Complementary to reuse and recovery, developing RE-substitutes is also crucial to advancing clean energy technologies. CMI researchers are investigating the use of earth abundant elements with desirable emission properties to replace RE in the crystal lattices of phosphors. In the photo, Nerine Cherepy of LLNL is illuminating a number of lighting phosphors with a UV lamp, including a new manganese-doped aluminum nitride (AlN:Mn) red phosphor (at right, 4th from bottom) which is intended to serve as a RE-free replacement for the current yttrium europium oxide (YEO) phosphor widely deployed in fluorescent lighting. A replacement for the green terbium-doped lanthanum phosphate (LAP) phosphor is also pictured. This work was performed by scientists at LLNL, ORNL, General Electric and Ames Laboratory (Opt. Mater., 2016, 54, 14). The author of this feature article also has experience in designing RE-free phosphors. This technology (featured at an ACS press conference) immobilizes organic chromophores into rigid coordination polymers to fine-tune their emission and increase emission efficiency and thermostability. RE-free yellow phosphors created using this approach (Chem. Comm., 2015, 51, 3045 and J. Am. Chem. Soc., 2014, 136, 16724) have emission efficiency comparable to the commercially available cerium-doped yttrium aluminum garnet.
Developing new materials or new processes to reduce RE consumption, replace RE entirely, or recycle RE from waste products is just the first step; evaluating their potential impacts on the environment is also important. Yoshiko Fujita at INL is leading a study on the effects of wastewaters generated by new CMI processes on microorganisms with a focus on biological wastewater treatment systems (Environ. Sci. Technol., 2015, 49, 9460). Her team’s work can help guide the selection of components and processes that will improve the RE demand/supply balance while also protecting the environment.
RE elements are at the core of many clean energy technologies. CMI’s initiatives in diversifying and expanding critical materials production, promoting reuse and recovery, and developing substitutes to reduce and eliminate RE-dependence play a vital national role in achieving critical materials sustainability and U.S. energy security.
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