Role of Metal Ion Sites in Bivalent Cobalt Phosphorus Oxygen Systems toward Efficient Oxygen Evolution Reaction

Abstract

In the recent studies on various cobalt phosphates with different phosphorous oxyanion ligands for oxygen evolution reaction (OER), the influence of ligands in efficient proton abstraction has been highlighted for the observed higher activity. The role of CoO(OH) active sites has not been given sufficient importance to compare the activity in these systems. In order to explain the OER activity in relation to metal ion sites, we have synthesized three different cobalt phosphorous oxyanion systems such as NaCoPO 4 (CP-1), Co 11 (HPO 3 ) 8 (OH) 6 (CP-2), and Co 3 (OH) 2 (HPO 4 ) 2 (CP-3) and compared their OER activity with detailed analysis of cyclic voltammograms. Though CP-2 and CP-3 possess a higher number of proton abstracting ligands per metal site than CP-1, it is observed that CP-1 shows an OER activity similar to CP-3 in terms of onset potential and Tafel slope values. A careful estimation of the effective number of reduction sites in terms of Co 4+ /Co 3+ and Co 3+ / Co 2+ reduction reveals that CP-1 and CP-3 show higher values in accordance with the observed OER activity trend, CP-1 ∼ CP-3 > CP-2. The average number of CoO x (OH) y reduction sites in the Co 4+ ↔ Co 3+ ↔ Co 2+ sequence explains the OER performance better than the ratio of reduction current density or Co 3+ reducibility. The changes in morphology and chemical composition of the abovementioned systems after OER cycles support the formation of active species, cobalt oxyhydroxide during OER. The CP-1, CP-2, and CP-3 catalysts show stable current density at ∼10 mA/cm 2 up to 10 h.

Unveiled Performance of Iron-Based Phosphates as Precatalysts for Oxygen Evolution Electrocatalysis

“…The EDX and Raman analyses also indicated that a little amount of P still remained in the catalyst particles after the OER measurement, and it can presumably enhance OER activity compared with cases using originally pure Fe-based oxides. This result exhibited a good consistency with a paper by Alex et al, which presented that in situ-generated Co-based oxyhydroxide from a Co-based phosphate by P dissolution during OER measurement still contained 12% residual P and showed excellent OER activity . In addition, a reported Pourbaix diagram for the Fe–P–H 2 O system displayed that Fe-based hydroxide is more stable than Fe-based phosphate in an alkaline solution under high potential, which indicates that the dissolution of P from Fe 3 O 3 (PO 4 ) as PO 4 3– and/or PO 5 3– is preferable during OER measurement in 1 M KOH.…”

“…When we stirred Fe 3 O 3 (PO 4 ) in 1 M KOH overnight without applied potentials, its XRD patterns were unchangeable, as shown in Figure S19, indicating the high structural stability of this material in a harsh alkaline condition. Similar behavior was also reported for Co 3 (OH) 2 (HPO 4 ) 2 in alkaline OER . These results uncovered that Fe 3 O 3 (PO 4 ) was electrochemically transformed into Fe-based (oxyhydr)­oxides.…”

Effects of Alternating Magnetic Fields on the OER of Heterogeneous Core–Shell Structured NiFe2O4@(Ni, Fe)S/P

“…The overpotential of the P-NFO-Ms/C oxidation peak decreased gradually from 243 mV (0 mT) to 0 mV (4.320 mT) as the magnetic field increased. The oxidation peak was a transition process from the low-activity M 2+ to the high-activity M 3+ (M = Ni or Fe), , representing the surface reformation behavior in the OER process . AMF facilitated the transformation to a highly active phase at a lower potential, which contributed to the OER activity.…”

SILAR Synthesized Binder-Free, Hydrous Cobalt Phosphate Thin Film Electrocatalysts for OER Application: Annealing Effect on the Electrocatalytic Activity

“…Till now, different chemical methods have been adopted to synthesize cobalt phosphate-based catalysts with various microstructures ranging from nanoarray, spherical, flowerlike, flake-like, nanowires, 3D needle grass-like, and nanobelt to nanosheets for electrocatalysis [25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41]. For instance, several researchers used well-known hydrothermal strategies to synthesize cobalt phosphate catalysts, such as Zhao et al, who prepared flower-like CoPO on nickel foam (NF) [27], CoPPi nanowires synthesized by Du et al [31], Sun et al synthesized (3D) needle grass-like CoHPO/NF [32], Co 3 (PO 4 ) 2 -NC spherical nanoparticle synthesized by Ahamad et al using hydrothermal and postpyrolysis methods [33], Alex et al prepared bundled nanobelt-like NaCoPO 4 [35], Co 3 (PO 4 ) 2 @N-C nanoparticles synthesized by Yuan et al [37], micro flower like Co 3 (PO 4 ) 2 •4H 2 O synthesized by Katkar et al [40], and Yang et al prepared Ce-doped Co(PO 3 ) 2 nanosheets by hydrothermal and phosphorization method [41]. Likewise, other than the hydrothermal method, several wet chemical methods were adopted to prepare cobalt phosphate-based electrocatalysts, such as Xie et al synthesized Co-Pi/Ti 3D nanoarrays (NA) using an oxidative polarization strategy for electrocatalysis application [25].…”