Understanding the Dynamics of Cobalt Nitride-Based Thin Films for Electrocatalytic Oxygen Reduction
Authors: Crago Colin Flynn , Aleman Ashton M. , Burke Stevens Michaela , Jaramillo Thomas F. 
Published: December 2024

In both acidic and alkaline hydrogen fuel cells, platinum is the benchmark cathode catalyst, exhibiting high activity for the oxygen reduction reaction (ORR). Despite platinum’s exceptional performance, its low abundance and high cost are major factors limiting the widespread adoption of hydrogen fuel cells—a critical technology for the decarbonization of our energy systems. Cobalt nitrides (CoNx) along with cobalt-nitrogen-carbon macrocyclic compounds (Co-N-C) have emerged in literature as a promising subset of active, low-cost ORR catalysts in both acidic and alkaline media. However, the most active of these materials are carbon-supported nanoparticles or single atom catalysts with complex, non-uniform active sites, making it difficult to disentangle the impact of each element within the catalyst (Co, N, C, etc.) on performance. Additionally, the pH- and potential-dependent material stability of these materials is not well-understood.
To broaden our fundamental understanding of the activity and dynamics of non-precious ORR electrocatalysts, we have devised a synthesis procedure for well-defined metallic and bimetallic X-ide (nitride, oxide, sulfide, etc.) thin film catalysts. Following a two-step procedure, we have used physical vapor deposition followed by thermal ammonia treatments to form CoxSbyNz films of various compositions. To develop a holistic understanding of the factors influencing ORR electrocatalysis on these materials, we utilize ex situ characterization along with in situ techniques to probe time- and potential-dependent material stability and selectivity during catalysis. With depth-profiling x-ray photoelectron spectroscopy, we observe uniform incorporation of N at approximately 10 atomic percent throughout the NH3-treated films below a native oxide layer. X-ray diffraction indicates that untreated films have a hexagonal close packed structure matching a Co standard, and the NH3-treated films have a hexagonal structure similar to Co3N.
To understand catalyst dynamics in situ, we use on-line inductively coupled plasma mass spectrometry (ICP-MS) to measure Co and Sb dissolution, electrochemical mass spectrometry (EC-MS) to probe nitride degradation, and a rotating ring disk electrode (RRDE) to monitor peroxide selectivity. With each of these in situ techniques, we couple cyclic voltammetry experiments, demonstrating that activity and stability are dependent on catalyst composition, electrolyte pH, and the upper and lower potential limits. We find that the stability of CoxSbyNz is limited by Co dissolution in acidic electrolyte and nitride degradation in alkaline electrolyte. The individual incorporation of Sb and N help to prevent the anodic dissolution of cobalt in acidic media, and the metastable incorporation of N improves activity at both pH extremes. Although it is often too difficult to deconvolute the role of individual elements within highly active non-oxide electrocatalysts, our multistep synthesis procedure and comprehensive in situ and ex situ characterization techniques allow us to probe the impact of each element in various CoxSbyNz films on material stability and product-specific activity. Our results impart wisdom that will be useful in the development of low-cost, high-performance electrocatalysts across a broad set of reactions and applications.