Electrochemical Activation of Atomic Layer-Deposited Cobalt Phosphate Electrocatalysts for Water Oxidation

Abstract

The development of efficient and stable earth-abundant water oxidation catalysts is vital for economically feasible water-splitting systems. Cobalt phosphate (CoPi)-based catalysts belong to the relevant class of nonprecious electrocatalysts studied for the oxygen evolution reaction (OER). In this work, an in-depth investigation of the electrochemical activation of CoPi-based electrocatalysts by cyclic voltammetry (CV) is presented. Atomic layer deposition (ALD) is adopted because it enables the synthesis of CoPi films with cobalt-to-phosphorous ratios between 1.4 and 1.9. It is shown that the pristine chemical composition of the CoPi film strongly influences its OER activity in the early stages of the activation process as well as after prolonged exposure to the electrolyte. The best performing CoPi catalyst, displaying a current density of 3.9 mA cm –2 at 1.8 V versus reversible hydrogen electrode and a Tafel slope of 155 mV/dec at pH 8.0, is selected for an in-depth study of the evolution of its electrochemical properties, chemical composition, and electrochemical active surface area (ECSA) during the activation process. Upon the increase of the number of CV cycles, the OER performance increases, in parallel with the development of a noncatalytic wave in the CV scan, which points out to the reversible oxidation of Co 2+ species to Co 3+ species. X-ray photoelectron spectroscopy and Rutherford backscattering measurements indicate that phosphorous progressively leaches out the CoPi film bulk upon prolonged exposure to the electrolyte. In parallel, the ECSA of the films increases by up to a factor of 40, depending on the initial stoichiometry. The ECSA of the activated CoPi films shows a universal linear correlation with the OER activity for the whole range of CoPi chemical composition. It can be concluded that the adoption of ALD in CoPi-based electrocatalysis enables, next to the well-established control over film growth and properties, to disclose the mechanisms behind the CoPi electrocatalyst activation.

Understanding the Surface Reconstruction on Ternary WxCoBx for Water Oxidation and Zinc–Air Battery Applications

“…This result suggests the higher concentration of OH − in the solution accelerates the surface transformation to generate oxides/hydroxides actives species. [10] Therefore, based on the above discussion, the superior surface reconstruction of W 2 CoB 2 was related with the OH − ions to leach the tungsten atoms from the lattice. The process is schematically represented in Figure 3e ).…”

Phytic Acid‐Based FeCo Bimetallic Metal‐Organic Gels for Electrocatalytic Oxygen Evolution Reaction

“…Metal phosphides and phosphate‐based materials have recently been considered to be competitive candidates as efficient electrocatalysts due to high electrocatalytic activity, adjustable structure composition and stability [11–13] . There are two main methods for the preparation of phosphorus‐containing electrocatalysts.…”

“…There are two main methods for the preparation of phosphorus‐containing electrocatalysts. One is to introduce additional inorganic phosphorus sources during high‐temperature carbonization process, such as NaH 2 PO 2 ⋅ H 2 O, [11,12,14,15] red phosphorus, [12] Na 2 HPO 4 , [13] and NH 4 H 2 PO 4 [16] . For example, Ni 12 P 5 and Ni 2 P can be prepared by a hydrothermal method using red phosphorus as phosphorous source [12] .…”

“…Phytic acid (PA) is a natural organic phosphorus resource containing six phosphoric acid groups, which can be obtained from plant seeds and grains, and has a strong chelating ability. Currently, PA‐based polymer gels have been developed, [11–14,25–27] and PA‐based materials are usually employed as precursors for further pyrolysis for electrocatalysis [28–30] . For example, Han and co‐workers synthesized cobalt phosphide nanocrystals encapsulated by P‐doped carbon (PC) and married with P‐doped graphene (PG) nanohybrids CoP‐Co 2 P@PC/PG through controllable thermal conversion of pre‐synthesized supramolecular gels, which showed the overpotential of 39 mV at 10 mA cm −2 for HER and 272 mV at 20 mA cm −2 for OER [11] .…”

“…Currently, PA‐based polymer gels have been developed, [11–14,25–27] and PA‐based materials are usually employed as precursors for further pyrolysis for electrocatalysis [28–30] . For example, Han and co‐workers synthesized cobalt phosphide nanocrystals encapsulated by P‐doped carbon (PC) and married with P‐doped graphene (PG) nanohybrids CoP‐Co 2 P@PC/PG through controllable thermal conversion of pre‐synthesized supramolecular gels, which showed the overpotential of 39 mV at 10 mA cm −2 for HER and 272 mV at 20 mA cm −2 for OER [11] . This electrocatalyst only needed a cell voltage of 1.57 V for water splitting to reach the current density at 10 mA cm −2 .…”