My research interests lie in electrochemical energy storage and conversion. In particular:


  • Energy storage: beyond Li-ion batteries (Na-ion and divalent-ion).


  • Energy conversion: power from salinity gradients (blue energy), seawater desalination and delithiation.


  • Electrocatalysis: oxygen and hydrogen evolution reactions.


I've recently focused my attention on grid-scale electrochemical energy storage technologies, working on a new class of materials with an open framework crystal structure that allows for a very long cycle life and high energy efficiency and power output.


This is a list of a few topics I’m currently working on:


Electrochemical Properties of Prussian Blue Analogues

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My group is currently working on a new family of insertion electrodes based on the Prussian Blue open-framework crystal structure. This structure is fundamentally different from other insertion electrode materials because of its large channels and interstices. This structure is composed of a face-centered cubic framework of transition metal cations where each cation is octahedrally coordinated to hexacyanometallate groups. Large interstitial “A Sites” within the structure can accommodate zeolitic water and hydrated alkali ions. This results in a general chemical formula of AxPR(CN)6·nH2O, where A is an alkali cation such as K+ or Na+, P is a transition metal cation such as Cu2+, Ni2+ or Fe3+, and R(CN)6 is a hexacyanometallate anion such as Fe(CN)63-, Mn(CN)63-, or Cr(CN)63-. Both the P-site transition metal cation and the R(CN)63- hexacyanometallate anion can be electrochemically active in this structure. The Prussian Blue framework structure has wide channels between the A sites, allowing rapid insertion and removal of Na+, K+, and other ions from aqueous solutions. In addition, there is little lattice strain during cycling because the A sites are larger than the hydrated ions that are inserted and removed from them.


Electrochemistry of salinity differences

The large-scale chemical energy stored as the salinity difference between seawater and freshwater is a renewable source that can be harvested. The entropic energy created by the difference in water salinities (also called “blue energy”) is normally dissipated when river water flows into the sea. This reduction in free energy due to mixing is estimated at 2.2 kJ per liter of fresh water.  We have previously developed a device called a “mixing entropy battery” that efficiently extracts this wasted energy. The device employed sodium manganese oxide and silver as sodium and chloride capturing electrodes respectively. While this device worked well, there are some limitations. Sodium manga has a limited specific capacity (i.e. number of sodium ions storable per gram of active material), while the silver-silver chloride electrode is much too expensive for this application. Moreover, both manganese and silver are heavy metals and their release in seawater is severely regulated. In addition, by operating the mixing entropy battery in reverse we demonstrated the possibility of efficiently desalinating seawater through a device I called a “desalination battery”. The development of the two devices will progress in parallel.


Phosphorus-based anode materials for Na-ion batteries

The increasing demand for portable devices and electric vehicles stimulated the research towards high energy and high voltage systems, leading to the development of Li-ion batteries (LIB). However, concerns about lithium’s raising price may limit their practical future implementation in large, grid-scale energy storage. Sodium represents a valid alternative to lithium, because of the similar chemistry, wider availability and lower cost. Unfortunately, most of the electrode materials used in Li-ion batteries, especially anodes, do not work with Na-ion because of the larger size of Na+ compared to Li+.

 

Phosphorus (in its amorphous, red phosphorus allotrope form) has recently emerged as a high capacity Na-ion anode material operating via an alloying-dealloying mechanism. Its high theoretical capacity of 2596 mAh g-1 and its low working potential of 0.2 V vs Na+/Na make it one of the most promising anode material candidates. However, its low electronic conductivity (10-14 S/cm) and large volume expansion (~500%) upon sodiation require the design of new phosphorous-based hybrid materials and electrode structures to optimize its electrochemical properties.