Transition Metal—Carbon Bond Enthalpies as Descriptor for the Electrochemical Stability of Transition Metal Carbides in Electrocatalytic Applications

Abstract

Transition metal carbides are used for various applications such as hard coating, heterogeneous catalysis, catalyst support material or coatings in fuel cell applications. However, little is known about the stability of their electrochemically active surface in aqueous electrolytes. Herein, the transition metal—carbon bond enthalpy is proposed as stability criterion for various transition metal carbides. The basis is an oxidation mechanism where the rate determining step is the metal—carbon bond cleavage under acidic conditions which was supported by a detailed corrosion study on hexagonal tungsten carbide. In situ flow cell measurements that were coupled to an inductively coupled plasma mass spectrometer corroborated experimentally the linear dependency of the oxidation overpotential on the transition metal—carbon bond enthalpy. The proposed model allows the estimation of the activation overpotential for electrochemical carbide oxidation resulting in a maximized stabilization for carbides in the 4th group (Ti, Zr, Hf). Together with the calculated thermodynamic oxidation potentials, TiC and VC exhibit the highest experimental oxidation potentials (0.85 VRHE). The model can be used for preselecting possible carbide materials for various electrochemical reactions.

Stable and Active Oxygen Reduction Catalysts with Reduced Noble Metal Loadings through Potential Triggered Support Passivation

“…Early transition metal carbides were reported to exhibit metallic conductivity, high corrosion resistance and are able to strongly bind to noble metals such as Pt anchoring it in place . Our own research revealed stabilization of tungsten carbide compared to bare tungsten, and we were able to identify the metal‐carbon bond enthalpy as stabilization criterion for various metal carbides . Titanium carbide (TiC) shows a maximum formation and bonding enthalpy compared to other transition metal carbides resulting in enhanced thermodynamic stabilization .…”

“…Our own research revealed stabilization of tungsten carbide compared to bare tungsten, and we were able to identify the metal‐carbon bond enthalpy as stabilization criterion for various metal carbides . Titanium carbide (TiC) shows a maximum formation and bonding enthalpy compared to other transition metal carbides resulting in enhanced thermodynamic stabilization . We used exemplarily readily available TiC and deposited multiple Pt‐layers on a 510 nm thick magnetron sputtered TiC thin film via atomic layer deposition (ALD) offering high control over the final metal film .…”

Catalyst Stability in Aqueous Electrochemistry

“…The increased covalent bonding character is ascribed to the observed stability enhancement and a positive shift in the oxidation onset potential, i.e., a higher electrochemical nobility was observed. 44 For some metal carbides, metal dissolution takes place at similar potentials (∼0.85 V RHE for TiC) where usually only noble metal oxidation, such as that on Pt (∼0.95 V RHE ), takes place and demonstrates the potential of these materials. 5 Besides carbides, the stability of a variety of phosphides and oxides can be magnitudes higher compared to that of their bare metal congeners depending on the material and operation conditions.…”

“…While Pourbaix diagrams mostly focus on bare metals and the formed hydroxides or oxides, thermodynamic stabilization has been found, for instance, for metal carbides. 5,44 Not only does the thermodynamic stability increase but also the experimentally observed oxidation can differ drastically compared to that of the bare metal. The covalent character of the metal−carbon bond and the difference in electronegativity create a polar surface, leading to electron density transfer from the metal sites to the neighboring heteroatom sites.…”

Recent developments of nanocarbon based supports for PEMFCs electrocatalysts

“…Moreover, because metal is introduced on the carbon support, the oxidation potential of the catalysts and the binding energy with oxygen will change [219], resulting in an unstable ORR catalytic performance. The ultimate goal is for the catalyst to concurrently have an excellent catalytic activity and stability [220] and the introduction of multiple components (both activity-promoting components and stability-promoting components) may be a feasible idea for preparing functional carbon supports, however, more research and exploration is needed.…”