Coordination tuning of cobalt phosphates towards efficient water oxidation catalyst

Abstract

The development of efficient and stable water oxidation catalysts is necessary for the realization of practically viable water-splitting systems. Although extensive studies have focused on the metal-oxide catalysts, the effect of metal coordination on the catalytic ability remains still elusive. Here we select four cobalt-based phosphate catalysts with various cobalt- and phosphate-group coordination as a platform to better understand the catalytic activity of cobalt-based materials. Although they exhibit various catalytic activities and stabilities during water oxidation, Na2CoP2O7 with distorted cobalt tetrahedral geometry shows high activity comparable to that of amorphous cobalt phosphate under neutral conditions, along with high structural stability. First-principles calculations suggest that the surface reorganization by the pyrophosphate ligand induces a highly distorted tetrahedral geometry, where water molecules can favourably bind, resulting in a low overpotential (∼0.42 eV). Our findings emphasize the importance of local cobalt coordination in the catalysis and suggest the possible effect of polyanions on the water oxidation chemistry.

Recent Progress on Multimetal Oxide Catalysts for the Oxygen Evolution Reaction

“…Qi et al also reported the improved OER performance of Fe-doped NiO. [189] The Fe-doped NiO annealed up to 200 °C presented the highest catalytic efficiency with η = 487 mV at 62 mA cm −2 for Ni 85 Fe 15 .…”

“…[4][5][6][7][8] However, the high cost of these scarce materials (Ru = $65 ozt −1 and Ir = $970 ozt −1 , Jul 2017) is a challenging obstacle for the largescale commercialization. [9][10][11] A number of studies have thus begun to focus on 3d-transition-metal-based catalysts, [12][13][14][15] with reports of high-efficiency catalysts such as amorphous Co-oxide, layered hydroxides, perovskites, and spinels. According to the works by Kanan and Nocera, an amorphous Co-oxide film catalyst was revealed to be an outstanding OER catalyst in neutral electrolytes, triggering extensive research on amorphous catalysts.…”

Bifunctional Electrocatalytic Behavior of Sodium Cobalt Phosphates in Alkaline Solution

“…[25] Very recently, sodium metal phosphates NaCoPO 4 and Na 2 CoP 2 O 7 were studied as electrocatalyst for OER. [26] The Na 2 CoP 2 O 7 with distorted cobalt tetrahedral geometry exhibited enhanced activity and stability relative to cobalt phosphate. This report demonstrates that surface reorganization by the pyrophosphate ligand induces distorted cobalt tetrahedral geometry.…”

“…This result favors binding of water molecules and it leads to low overpotential (~0.42 V) for water oxidation. [26] Particularly, polyanions affect the OER and ORR performance of the catalysts. [14,26,27] Recent report on OER catalytic performance of Mn 3 (PO 4 ) 2 in neutral solution showed the positive effect of polyanion.…”

“…[26] Particularly, polyanions affect the OER and ORR performance of the catalysts. [14,26,27] Recent report on OER catalytic performance of Mn 3 (PO 4 ) 2 in neutral solution showed the positive effect of polyanion. [14] The phosphate framework was found to stabilize the Mn 3 + active sites much better than in manganese oxides.…”