Potential‐ and Buffer‐Dependent Catalyst Decomposition during Nickel‐Based Water Oxidation Catalysis

Abstract

The production of hydrogen by water electrolysis benefits from the development of water oxidation catalysts. This development process can be aided by the postulation of design rules for catalytic systems. The analysis of the reactivity of molecular complexes can be complicated by their decomposition under catalytic conditions into nanoparticles that may also be active. Such a misinterpretation can lead to incorrect design rules. In this study, the nickel‐based water oxidation catalyst [NiII(meso‐L)](ClO4)2, which was previously thought to operate as a molecular catalyst, is found to decompose to form a NiOx layer in a pH 7.0 phosphate buffer under prolonged catalytic conditions, as indicated by controlled potential electrolysis, electrochemical quartz crystal microbalance, and X‐ray photoelectron spectroscopy measurements. Interestingly, the formed NiOx layer desorbs from the surface of the electrode under less anodic potentials. Therefore, no nickel species can be detected on the electrode after electrolysis. Catalyst decomposition is strongly dependent on the pH and buffer, as there is no indication of NiOx layer formation at pH 6.5 in phosphate buffer nor in a pH 7.0 acetate buffer. Under these conditions, the activity stems from a molecular species, but currents are much lower. This study demonstrates the importance of in situ characterization methods for catalyst decomposition and metal oxide layer formation, and previously proposed design elements for nickel‐based catalysts need to be revised.

Homogeneous Catalysts Based on First‐Row Transition‐Metals for Electrochemical Water Oxidation

“…Substitution of the buffer with bicarbonate demonstrated a decrease in redox potential for both the Ni IV /Ni III couple and for electrocatalytic water oxidation process, indicating that bicarbonate acts as a non‐innocent axial ligand, oxidized during the redox process. Furthermore, we recently found this catalyst goes through a pH and buffer dependent decomposition pathway: a layer of NiO x was formed in a pH 7 phosphate buffer verified by in situ characterization of electrochemical quartz crystal microbalance measurements, while no indication of NiO x layer formation at a pH of 6.5 in phosphate buffer nor in a pH 7.0 acetate buffer, albeit exhibiting low activity [32] . A similar Ni catalyst with complete N ‐methylation ([Ni(TMC)(CH 3 CN)](NO 3 ) 2 TMC=1,4,8,11‐tetramethyl‐1,4,8,11‐tetraazacyclotetradecane, Ni‐4 , Figure 14) was subsequently synthesized by Li et al [73] .…”

“…Recently, the formation of NiO x was confirmed with a homogeneous macrocyclic nickel(II) complex [Ni(meso-L)](ClO 4 ) 2 (L = 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane) as catalyst by our group. [32] The structure was originally reported as a homogeneous WOC at pH 7, due to instability of the NiO x formed during bulk electrolysis. The NiO x desorbed rapidly upon removal of applied bias, thus the metal-based nanoparticle species eluded detection.…”

“…Furthermore, we recently found this catalyst goes through a pH and buffer dependent decomposition pathway: a layer of NiO x was formed in a pH 7 phosphate buffer verified by in situ characterization of electrochemical quartz crystal microbalance measurements, while no indication of NiO x layer formation at a pH of 6.5 in phosphate buffer nor in a pH 7.0 acetate buffer, albeit exhibiting low activity. [32] A similar Ni catalyst with complete Nmethylation ([Ni(TMC)(CH 3 CN)](NO 3 ) 2 TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane, Ni-4, Figure 14) was subsequently synthesized by Li et al [73] It exhibited a moderate overpotential (~500 mV at pH 7) compared to preceding complexes (Ni-1, Ni-2 and Ni-3), but a record TOF (based on Ni) of 9.95 s À 1 was obtained which is attributed to the increased electron donation properties brought about by methylation of the macrocyclic ligand. Additionally, the catalytic current varies linearly with the proton-accepting ability (pK a ) of the added base, which plays the important role of regulating catalytic activity through participation in the key OÀ O bond-forming step (Scheme 1).…”

Homoleptic Ni(ii) dithiocarbamate complexes as pre-catalysts for the electrocatalytic oxygen evolution reaction

“…In the alkaline medium, a complete conversion of the molecular Ni complexes to the active catalyst Ni(OH) 2 /Ni(O)OH has been reported earlier whereas a partial conversion has also been detected for some of the complexes. [62][63][64] Looking at the previous results, we can consider three different active catalysts: (i) the molecular complex, (ii) the interface of the molecular complex and Ni(OH) 2 /Ni(O)OH, and (iii) Ni(OH) 2 /Ni(O) OH.…”

Application of a Nickel Complex for Water Oxidation under Neutral and Acidic Conditions

“…25 This layer desorbs from the electrode at fewer anodic potentials and prevents ex situ detection. 25 The group also showed that catalyst decomposition is a buffer-dependent reaction. 25 It was recently demonstrated that during OER and at very low overpotentials, a tetranuclear Ni complex is decomposed to Ni (hydr)oxide.…”