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. 2018 Mar 9;4(3):eaap7970.
doi: 10.1126/sciadv.aap7970. eCollection 2018 Mar.

In situ formation of molecular Ni-Fe active sites on heteroatom-doped graphene as a heterogeneous electrocatalyst toward oxygen evolution

Jiong Wang  1 , Liyong Gan  2 , Wenyu Zhang  1 , Yuecheng Peng  1 , Hong Yu  3 , Qingyu Yan  3 , Xinghua Xia  4 , Xin Wang  1
Affiliations

In situ formation of molecular Ni-Fe active sites on heteroatom-doped graphene as a heterogeneous electrocatalyst toward oxygen evolution

Jiong Wang et al. Sci Adv. .

Abstract

Molecularly well-defined Ni sites at heterogeneous interfaces were derived from the incorporation of Ni2+ ions into heteroatom-doped graphene. The molecular Ni sites on graphene were redox-active. However, they showed poor activity toward oxygen evolution reaction (OER) in KOH aqueous solution. We demonstrated for the first time that the presence of Fe3+ ions in the solution could bond at the vicinity of the Ni sites with a distance of 2.7 Å, generating molecularly sized and heterogeneous Ni-Fe sites anchored on doped graphene. These Ni-Fe sites exhibited markedly improved OER activity. The Pourbaix diagram confirmed the formation of the Ni-Fe sites and revealed that the Ni-Fe sites adsorbed HO- ions with a bridge geometry, which facilitated the OER electrocatalysis.

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Figures

Fig. 1
Fig. 1. Characterizations of HG-Ni.
(A) TEM images of HG-Ni at different magnifications. (B) Normalized Ni K-edge XAS data of HG-Ni and Ni(acac)2. The inset presents the pre-edge region. a.u., arbitrary units. (C) Fourier-transformed EXAFS curves at Ni K-edge. (D) A DFT calculation–derived model to indicate the conjugation of Ni(acac)2 onto HG. The dopant of HG was a sulfoxide group as one representative interacting site. The gray, white, red, and yellow spheres represent C, H, O, and S atoms, respectively. The Ni atom is represented by a dark blue sphere.
Fig. 2
Fig. 2. Electrochemical tests of HG-Ni.
CV polarization curves of HG-Ni conducted in (A) a glass cell, 1 M KOH without any treatment; (B) a plastic vessel, 1 M KOH with removal of Fe impurity. (C) Steady CV curves of HG-Ni performed in a glass cell in 1 M KOH solution containing different contents of FeCl3 from 2.4 to 12 μM and (D) the corresponding TOF analysis after the potentials were compensated for iR drop. (E) Comparison of the initial and steady CV curves of HG-Ni conducted in a glass cell, 1 M KOH + 12 μM FeCl3. Scan rate, 50 mV s−1; rotation rate, 2000 rpm.
Fig. 3
Fig. 3. OER assessments of HG-NiFes.
(A) iR-compensated LSVs of HG-NiFe and HG-NiFex in 1 M KOH (5 mV s−1; 2000 rpm), corresponding Tafel slopes (B) and TOF analysis (C).
Fig. 4
Fig. 4. Formation of molecular Ni-Fe sites.
(A) Normalized Ni K-edge XAS data of HG-Ni and HG-NiFe. The inset presents the pre-edge region. (B) Fourier-transformed EXAFS curves at Ni K-edge. (C) Normalized Fe K-edge XAS data of HG-NiFe, FeOOH, and FeCl3. (D) Fourier-transformed EXAFS curves at Fe K-edge.
Fig. 5
Fig. 5. Adsorption of HO− ions at Ni-Fe sites.
(A) ks of metal redox in HG-Ni, HG-NiFex, and HG-NiFe. (B) Pourbaix diagram, formal potentials of redox versus pH values of KOH solutions. The inset shows the proposed configuration for the adsorption of HO− ions onto Ni-Fe sites before OER.
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