Investigating the pH-Dependent Activity, Selectivity, and Dynamics of Metal Nitride Electrocatalysts for Fuel Cells and Electrolyzers

Abstract

To mitigate climate change caused by the use of fossil fuel energy sources, there is an urgent need to decarbonize our energy systems. Developing fuel cell and water electrolysis technologies to support a green hydrogen infrastructure is a promising strategy for this transition. In both hydrogen fuel cells and water electrolyzers, platinum-group metal (PGM) materials such as Pt and IrOx are benchmark electrocatalysts, exhibiting high activity and stability for kinetically-hindered reactions including the oxygen reduction reaction (ORR), the oxygen evolution reaction (OER), and alcohol oxidation reactions (AOR; alternative anodic reactions to OER). Despite their remarkable performance, the high cost and low abundance of PGMs is a key factor limiting the widespread adoption of these technologies.Cobalt- and nickel-based metal and oxide catalysts have emerged in the literature as a promising PGM-alternative class of low-cost, high-performing ORR, OER and AOR catalysts. Substituting oxides with nitrides or other chalcogenides could be a favorable method to improve activity, selectivity, and stability; however, the material stability of these non-oxides as a function of reaction, potential window, and pH is not well-understood. Furthermore, the most active of these materials tend to be carbon-supported nanoparticles with complicated and heterogeneous active sites, which makes deconvoluting the role of the cobalt and secondary element(s) (e.g. N, C, etc.) difficult.With the goal of developing fundamental activity-stability relationships for non-precious metal-based electrocatalysts, we have developed a synthesis procedure for well-defined Co- and Ni-based thin film materials. Focusing on Co, using physical vapor deposition followed by thermal treatments with ammonia and an Sb-based precursor, we have synthesized CoxSbyNz thin films of varying compositions. These synthesis methods can be extended to synthesize a broad class of metallic (Co, Ni, Fe, etc.) or bimetallic X-ide (X = N, S, etc.) thin film materials. We evaluate ORR, OER, and AOR activity with cyclic voltammetry, demonstrating that activity depends on composition and electrolyte pH. Specifically, for Co-based thin films for ORR, we see improvement in activity with the incorporation of N, and improvement in acid-stability with the incorporation of Sb. In addition to ex situ post-characterization with x-ray photoelectron spectroscopy, we utilize in situ techniques to evaluate catalyst stability in real time. We monitor time- and potential-dependent cobalt dissolution with an on-line inductively coupled plasma mass spectrometer (ICP-MS) as well as nitride degradation with electrochemical mass spectrometry (EC-MS). In acidic media, the stability of CoxN is limited by Co dissolution, while in alkaline media stability appears to be limited by nitride degradation. With highly active non-oxide materials it is typically too complicated to disentangle the role of each individual element within an electrocatalyst. With our stepwise material synthesis platform and multifaceted characterization techniques we are able to individually probe the material stability and product-specific activity of each element. These results provide new insights to guide the strategic design of electrocatalysts across a range of reactions and applications.