Charge Self‐Regulation of Metallic Heterostructure Ni2P@Co9S8 for Alkaline Water Electrolysis with Ultralow Overpotential at Large Current Density

Abstract Designing cost‐effective alkaline water‐splitting electrocatalysts is essential for large‐scale hydrogen production. However, nonprecious catalysts face challenges in achieving high activity and durability at a large current density. An effective strategy for designing high‐performance electrocatalysts is regulating the active electronic states near the Fermi‐level, which can improve the intrinsic activity and increase the number of active sites. As a proof‐of‐concept, it proposes a one‐step self‐assembly approach to fabricate a novel metallic heterostructure based on nickel phosphide and cobalt sulfide (Ni2P@Co9S8) composite. The charge transfer between active Ni sites of Ni2P and Co─Co bonds of Co9S8 efficiently enhances the active electronic states of Ni sites, and consequently, Ni2P@Co9S8 exhibits remarkably low overpotentials of 188 and 253 mV to reach the current density of 100 mA cm−2 for the hydrogen evolution reaction and oxygen evolution reaction, respectively. This leads to the Ni2P@Co9S8 incorporated water electrolyzer possessing an ultralow cell voltage of 1.66 V@100 mA cm−2 with ≈100% retention over 100 h, surpassing the commercial Pt/C║RuO2 catalyst (1.9 V@100 mA cm−2). This work provides a promising methodology to boost the activity of overall water splitting with ultralow overpotentials at large current density by shedding light on the charge self‐regulation of metallic heterostructure.


Figure
Figure S3 XRD patterns of Ni2P and Co9S8.

Figure S7
Figure S7 Plots used to evaluate the double-layer capacitances of Ni2P, Co9S8 and

Figure S8
Figure S8 Polarization curves with the current density normalized to ECSA of Ni2P,

Figure S10
Figure S10The long-time current density vs time curve (i-t curve) of Ni2P@Co9S8 at

Figure S14
Figure S14 Plots used to evaluate the double-layer capacitances of Ni2P, Co9S8 and

Figure S15
Figure S15 Polarization curves with the current density normalized to ECSA of Ni2P,

Figure S17
Figure S17The i-t curve of Ni2P@Co9S8 at the potential of 1.485 V versus RHE, and

Figure
Figure S18 (a) XRD patterns of Ni2P@Co9S8 before and after HER and OER reactions; (b) In situ Raman spectra of OER on Ni2P@Co9S8 in 1 M KOH.

Figure S20
Figure S20The energy barrier and reaction pathway of water dissociation on Ni2P,

Figure S21
Figure S21The energy barrier and reaction pathway of water dissociation on Co9S8,

Figure S23
Figure S23 Bader charge analyses of (a) Ni2P@Co9S8 and (b) Ni2P including the

Figure S24
Figure S24 Bader charge analysis of topmost Co and S atoms in (a) Ni2P@Co9S8 and (b) Co9S8.Target Co and S atoms are shown by red circles, and blue and yellow numbers are the average Bader charge values of three target Co and S atoms, respectively.

Table S1
Calculated double layer capacitance and corresponding RF values (HER).

Table S3
Calculated double layer capacitance and corresponding RF values (OER).

Table S5
Comparison of overpotentials at 100 mA cm -2 and Tafel slopes of different heterogeneous electrocatalysts recently reported for HER in 1.0 M KOH solution TableS6Comparison of overpotentials at 100 mA cm -2 and Tafel slopes of different heterogeneous electrocatalysts recently reported for OER in 1.0 M KOH solution

Table S7
Comparison of cell voltages at 10 and 100 mA cm -2 and stabilities of different bifunctional non-noble metal electrocatalysts recently reported for overall water

Table S8
Comparison of the overpotentials at 200 mA cm -2 toward the HER in 1 M KOH of the Ni2P@Co9S8 with other reported high-performance bifunctional catalysts.

Table S9
Comparison of the overpotentials at 200 mA cm -2 toward the OER in 1 M KOH of the Ni2P@Co9S8 with other reported high-performance bifunctional catalysts.Table S10Comparison of the overpotentials at 200 mA cm -2 toward the overall water splitting in 1 M KOH of the Ni2P@Co9S8 with other reported high-performance