The Origin of Catalytic Activity of Nickel Phosphate for Oxygen Evolution in Alkaline Solution and its Further Enhancement by Iron Substitution

Abstract

Nickel(II) phosphate (NPO) can be an effective catalyst for oxygen evolution reaction (OER) in alkaline solution. Unlike other transition metal phosphates, the OER is preceded by a prominent reversible redox reaction. The current study is aimed at understanding the origin of NPO‐catalyzed OER, in particular the role of the pre‐OER redox reaction. We detected the progressive formation of Ni(OH)2/NiOOH on the NPO surface during potential cycling in the anodic region similar to previous observations from Ni oxides, and found that it is associated with the change in the Ni/phosphate stoichiometry from 1.5 to 1.0, resulting in the loss of phosphate to the alkaline solution. Despite the conversion, NPO still has a good and sustained OER performance due to the activity similarity between NiPO4 (and possibly NPO) and NiOOH (the activated form of Ni oxide and hydroxide catalysts). NPO is however an easier catalyst to use than Ni oxides and hydroxides, which require extensive electrode conditioning to reach a stable performance. We also mixed NPO with Fe in view of recent findings that Ni–Fe double hydroxides are more active than simple Ni oxides in oxygen catalysis. The experimental results corroborate the positive outcome of this modification.

A New Member of Electrocatalysts Based on Nickel Metaphosphate Nanocrystals for Efficient Water Oxidation

“…The only peak in P 2p XPS spectra further demonstrates the more oxidizable surface structure, consistent with the higher Ni oxidation peak in polarization curves. Such surface reconstruction has been verified favorable for OER performance enhancement in previous literatures …”

Amorphous Ni0.75Fe0.25(OH)2‐Decorated Layered Double Perovskite Pr0.5Ba0.5CoO3‐δ for Highly Efficient and Stable Water Oxidation

“…Additional peaks are observed in the potential range of 1.3–1.5 V versus RHE for Ni 0.75 Fe 0.25 (OH) 2 decorated catalysts which is attributed to Ni (II)/Ni (III) redox pair. These peaks signify the formation of NiOOH as featured in the previous studies on nickel borate and nickel phosphate . This redox pair has been pointed out as the catalytic active sites for OER.…”

Bifunctional Electrocatalytic Behavior of Sodium Cobalt Phosphates in Alkaline Solution

“…The Jahn-Teller distortion affects the ORR activity as it alters the electronic structure of the catalysts, which further cannot bind with the oxygen species. [27,35] Considering above mentioned results (Figure 3), it is clear that the Na 2 CoP 2 O 7 pyrophosphate offers superior electrocatalytic activity than the NaCoPO 4 phosphate. Although the electrocatalytic performance of these cobalt phosphates are not superior to Pt/C, they can be advantageous for the economic large-scale application.…”

“…[19][20][21] Recent reports demonstrate the phosphate based materials could be very competent catalysts for OER and ORR. [22][23][24] For example, cobalt phosphate nanoparticles decorated with nitrogen-doped carbon (Co 3 (PO 4 ) 2 @NÀC) exhibited efficient and stable OER activity with low onset potential in strong alkaline medium. The electrocatalytic OER performance of the Co 3 (PO 4 ) 2 @NÀC is superior compared to state-of-the-art noble metal catalysts such as IrO 2 .…”