Origin of the Universal Potential-Dependent Organic Oxidation on Nickel Oxyhydroxide

Abstract

The paired electrolysis via anodic organic oxidation and cathodic hydrogen evolution has been an emerging hydrogen production technology with high efficiency and high values. Nickel oxyhydroxide is a special electrocatalyst that is capable of selectively oxidizing various alcohols, aldehydes, and amines to the corresponding carboxylates and nitriles at more favorable potentials compared to the oxygen evolution reaction. However, its detailed molecular-level mechanism is still in debate, especially for the potential-dependent behavior of some organic substrates. In this study, we revealed that the nickel oxyhydroxide can be dissected into two functional regions with a more facilely oxidized surface for facilitated substrate adsorption and a relatively inert bulk phase for accelerated electron transfer via probe-assisted kinetics and in situ surface-enhanced Raman spectroscopy. The prerequisite of the two regions conceals a universal potential-dependent oxidation behavior for almost all organic substrates. Further combining with the computational investigation unravels the origin of this potential dependence from the exothermic O/N-centered adsorption process on the surface and the rate-limiting proton-coupled electron-transfer (PCET) steps for the C−H bond breaking that demands bulk electron conductivity. This provides a rationale for designing more conductive underlayers to break the intrinsic limitations of nickel oxyhydroxide toward more efficient organic electrooxidation processes.

Direct Electrocatalytic Methanol Oxidation on MoO3/Ni(OH)2: Exploiting Synergetic Effect of Adjacent Mo and Ni

“…The Ni­(OH) 2 shows the Raman characteristic peaks of NiOOH at 483.3 and 561.7 cm –1 , , which become stronger as the electrolysis continues (Figure c). This is consistent with the observation of Gong et al and Wang et al, in which Ni in the catalyst bulk must be transformed into NiOOH to make the alcohol oxidation catalysis sustainable. As a consequence, the in situ observation of NiOOH Raman characteristic peaks confirms the indirect reaction mechanism on Ni­(OH) 2 .…”

“…The Ni(OH) 2 shows the Raman characteristic peaks of NiOOH at 483.3 and 561.7 cm −1 , 20,21 which become stronger as the electrolysis continues (Figure 2c). This is consistent with the observation of Gong et al 3 and Wang et al, 16 3c). 23 Therefore, the Mo 6+ doping induces Ni in Ni(OH) 2 to form an unsaturated coordination state.…”

“…This explains why NiOOH is not found on MoO 3 /Ni­(OH) 2 surfaces through the in situ Raman test. In addition, since the rate-determining step of the MOR process on Ni­(OH) 2 is Ni­(OH) 2 → NiOOH, , the required energy barrier is higher than the maximum energy barrier required for the MOR process on MoO 3 /Ni­(OH) 2 . Consequently, MoO 3 /Ni­(OH) 2 exhibits a significantly higher MOR performance than Ni­(OH) 2 at potentials below 1.48 V vs. RHE.…”

“…Yet, the pathway proposed by Choi et al is also based on the formation of NiOOH and the regeneration of the Ni 4+ /Ni 2+ mediator as a precondition, which highly limits the overall MOR efficiency. The indirect pathway confronts two main challenges: (1) As the electrocatalytic MOR is proton coupled electron transfer (PCET) process, , the deprotonated CH x O – ( x = 1, 2, 3) and H + are supposed to be adsorbed on two adjacent oxygen sites connected to the same Ni atom, in which at least one process should be inhibited. Specifically on NiOOH, the effective abstraction of α-hydrogen is important but rarely investigated.…”

Mechanistic Investigations of Electrochemical Ethanol Oxidation Reaction by In Situ Raman Spectroscopy

“…Furthermore, Hao et al . proposed that the bulk Ni redox was not the reaction center for EOR but the surface NiO x (OH) 2‐x species was, [56] as revealed by the in situ time‐resolved SERS in Figure 6. According to the authors, Ni(OH) 2 would exhibit a predominant oxidative peak at approximately 1.34 V vs. RHE, as evidenced by the notably increased current density in Figure 6a.…”