Highly Active LiNi1-XMxO2 (M = Mn, Co, Fe) Electrocatalysts for Oxygen Evolution Reaction in Alkaline Conditions

The efficient electrocatalysis of the oxygen evolution and reduction reactions (respectively, OER and ORR) remains a serious challenge for the development of electrochemical energy conversion and storage devices such as fuel cells, electrolyzers, and metal-air batteries.  In addition to the need for new non-noble metal catalysts is the need for improved understanding of their electrocatalytic mechanisms.  This presentation will describe a new class of alkaline OER electrocatalysts based on layered LiNi_1-xM_xO₂ (M = Mn, Co, Fe) where the Ni content is ≥ 0.85.  In this class of materials, the catalytic activity can be tuned over two orders of magnitude by engineering the bulk properties of the layered oxide.  Due to the rich compositional space afforded by the O3 layered structure of LiNiO₂, the M content and composition and the Li content can be varied while maintaining relatively similar surface areas.  First, Fe-doped materials exhibit the highest activities, with the maximum activity for 0 ≤ Fe ≤ 0.1.  The Li content can be varied during the solid-state synthesis as well as after the synthesis by chemical lithium extraction with an appropriate oxidizing agent.  During the solid-state synthesis, Li deficiency introduces Ni²⁺ into the structure which is correlated with poor electrocatalytic activity.  This indicates that Ni³⁺, with a single e_g orbital electron, may be important for high catalytic activity.  Indeed, NiO and Li-doped NiO synthesized under similar conditions exhibit low activities.  On the other hand, chemical delithiation leads to increased Ni⁴⁺ content as well as increased surface areas, resulting in an improvement in catalytic activity.  Lastly, appropriate heat treatment conditions result in materials with electrocatalytic activities on par with the best bulk electrocatalysts, on the order of 178 mA/mg for a 0.4 V overpotential.  In addition to the correlation of bulk properties with electrocatalytic activity, the surfaces of the materials were also characterized with XPS.  Further improvements in the activity of LiNi_1-xM_xO₂ should be possible with higher surface, nanostructured materials.