Improved Electrochemical Phase Diagrams from Theory and Experiment: The Ni–Water System and Its Complex Compounds

Abstract

Electrode potential-pH (Pourbaix) diagrams provide a phase map of the most stable compounds of a metal, its corrosion products, and associated ions in solution. The utility of these phase diagrams is that they enable the assessment of electrochemical stabilities, for example, of Ni metal and its derived oxides, hydroxides, oxyhydroxides, against corrosion in aqueous environments. Remarkably, the Ni Pourbaix diagrams reported over the last 50 years are largely inconsistent with various electrochemical observations, which may be attributed to inaccurate experimental free energies of formation (∆ f G)for the complex Ni-based compounds used in producing the available diagrams. Here we show that state-of-the-art density-functional theory (DFT) can be used to obtain accurate ∆ f G values, which lead to Ni Pourbaix diagrams that are more consistent with direct electrochemical exeriments: Electrochemical impedance spectroscopy and surfaceenhanced Raman spectroscopy are used to characterize the electrochemical stabilities of NiO and Ni(OH) 2 formed on Ni, demonstrating the reliability in correction-free first-principles based Pourbaix diagrams. Our results show the importance in applying modern density functionals in combination with experimental advances in aqueous environment compound identification for assessing electrochemical phase stability of materials, which will be useful for the design, synthesis, and selection of corrosionresistant metals, photoabsorbers, and photocatalytic materials.

Carbon doping switching on the hydrogen adsorption activity of NiO for hydrogen evolution reaction

“…14 and Supplementary Note 4) confirmed that the conditioning did not change the chemical nature of C-Ni 1−x O. It is noteworthy that NiO is the thermodynamically stable phase in the potential window between 0 and −174 mV vs RHE according to the NiO pourbaix diagram 32 . The presence of overpotential of NiO reduction can further expand this potential window 8 .…”

Origin of Photoelectrochemical Generation of Dihydrogen by a Dye-Sensitized Photocathode without an Intentionally Introduced Catalyst

“…57,58 At the same time, the reduction potential of the dye anion is -0.70 V vs. NHE, 27,28 while the phase transition of the two-electron reduction of Ni 2+ to Ni 0 occurs at ca -0.4 V vs NHE. 58 Thus, a thermodynamic driving force of ca 0.3 V exists for nickel deposition, and Ni 0 is then expected to remain metastable ( Fig S10). This surface could then provide electrocatalytic sites for the observed generation of H2, similar to electrodeposited platinum metal in our previous work.…”

Reliable electrochemical phase diagrams of magnetic transition metals and related compounds from high-throughput ab initio calculations

“…In addition, there are many unavoidable technical or physical limitations imposed by experiment, for example, fierce combustion, defect contamination, uncontrollable/unmeasurable hydration, and solution filtering, which can result in large uncertainties (inaccuracies) in the extracted experimental Δ f G values. 31 Indeed, the Ti and Ni Pourbaix diagrams simulated using experimental Δ f G values are found to be inconsistent with various direct electrochemical phenomena, 31,32 which have been ascribed to the inaccuracies in the free energies of formation. Furthermore, some of the important compounds presenting potential and pH-dependent stabilities may exhibit structures that are poorly or difficult to characterize (e.g., NiOOH 31,33 ) or remain to be determined by experiment (e.g., Cr ).…”