58th Annual Report on Research 2013 Under Sponsorship of the ACS Petroleum Research Fund

Peter Khalifah, PhD, Chemistry, State University of New York at Stony Brook, Stony Brook, NY

Aim 1: Explore conductive pyrochlores 

We have migrated from exploring pyrochlore compounds with highly reduced early 3d transition metals into exploring other structures with corner-sharing octahedra which contain early 3d transition metals.  We produced the reduced titanate compounds La₉Ti₇O₂₇, La₅Ti₄O₁₅, and La₅Ti₅O₁₇, as well as the solid solution La₂Ti_2-xNb_xO₇.  These compounds were not observed to be electrocatalytically active.  We are wrapping up this study by refining the structures of these complex oxides by synchrotron and neutron diffraction, and have begun preparing two manuscripts on the structure and properties of these systems.

Aim 2: Explore hollandite compounds

No addition work on these compounds was carried out in the past year.

Aim 3: Produce large single crystals

The focus this year has been on electrochemical testing rather than on crystal growth, so we do not have substantial new progress to report in this area.

New directions – transition metal oxynitride electrocatalysts.

We have additionally followed up on some recent literature reports of ORR activity in transition metal nitride materials such as rock salt type Mo₂N and hexagonal MoN, and have been exploring some second generation nitride and oxynitride compounds to see if the activity and/or stability of these materials can be improved by carrying out substitutions within these frameworks.  The initial results have been very promising.  We have demonstrated very good electrocatalysis in Co-Mo-O-N rock salt compounds.  In basic conditions (0.1M KOH), an onset potential of 0.918 V vs. RHE (only about 0.1 V lower than that of Pt/C measured under similar conditions) has been achieved, and stable electrocatalysis in acidic solutions (0.1M HClO₄)  has also been demonstrated with an onset potential of 0.65 V.  In both cases, a primarily 4-electron mechanism is followed.  The complex structure and morphology of these semiconductors has been elucidated through SEM/EDX, TEM/EELS, XANES, EXAFS, XPS, and XRD experiments, and this work has been recently published in Angewantde Chemie.  Also, we have found that a compound with approximate composition of “CoMoN₂” is an excellent HER catalyst under acidic conditions (iR-corrected current of 10 mA/cm² at an overpotential of 0.2 V). Neutron diffraction and pair distribution function (PDF) studies have shown the composition to be Co_0.6Mo_1.4N₂, with a four-layered superstructure of the delta-MoN structure, which is simple tungsten carbide (WC) type structure.  A paper on the HER activity and structure of this compound has just been accepted by JACS.  An additional paper on the modest ORR activity of this and related compounds is currently being edited.