Internal Polarization Field Induced Hydroxyl Spillover Effect for Industrial Water Splitting Electrolyzers

Highlights

• Analyzed the function of internal polarization field in Ni₂P/FeP₂ via hydroxyl spillover effect.

• From theoretical design to experimental verification, to optimize adsorption energy of oxygen intermediates on Ni active site, and further boost the xygen evolution reaction process.

• A hydroxyl spillover effect driven by internal polarization field in Ni₂P/FeP₂ can be amplified in low concentration alkaline electrolyte environment, and facilitate the application in anion exchange membrane water electrolyzer systems.

Abstract

The formation of multiple oxygen intermediates supporting efficient oxygen evolution reaction (OER) are affinitive with hydroxyl adsorption. However, ability of the catalyst to capture hydroxyl and maintain the continuous supply at active sits remains a tremendous challenge. Herein, an affordable Ni₂P/FeP₂ heterostructure is presented to form the internal polarization field (IPF), arising hydroxyl spillover (HOSo) during OER. Facilitated by IPF, the oriented HOSo from FeP₂ to Ni₂P can activate the Ni site with a new hydroxyl transmission channel and build the optimized reaction path of oxygen intermediates for lower adsorption energy, boosting the OER activity (242 mV vs. RHE at 100 mA cm^–2) for least 100 h. More interestingly, for the anion exchange membrane water electrolyzer (AEMWE) with low concentration electrolyte, the advantage of HOSo effect is significantly amplified, delivering 1 A cm^–2 at a low cell voltage of 1.88 V with excellent stability for over 50 h.

1 Introduction

Due to the compact design and fast system response under widespread current density operation, proton exchange membrane water electrolyzer (PEMWE) received unprecedented attention for high purity hydrogen production, especially with intermittent hybrid wind-solar integrated energy system [1,2,3]. However, the harsh acidic environment forces the selection of precious metal-based catalysts as electrodes, which prevented commercial PEMWE from large-scale practical application [4]. On the contrast, transition metal (TM)-based catalysts with cost advantages and commercial prospects are favorable to show reasonable activity and stability in alkaline media [5,6,7]. In fact, two kinds of electrolyzer technologies using basic liquid electrolyte are alkaline water electrolyzer (AWE) and anion exchange membrane water electrolyzer (AEMWE) [8,9,10]. In addition to long start-up preparation and slow response to changes in electric power load, the development of AWE for further practical applications is dramatically hindered by the sluggish kinetics of oxygen evolution reaction (OER) with four-concerted proton-electron transfer (CPET) pathways [11,12,13].

From Sabatier’s principle, an ideal OER catalyst requires a moderate adsorption strength with oxygen intermediates, that is, the interaction should be neither too strong nor too weak [14]. As one of the well-acknowledged mechanisms of OER, adsorbate evolution mechanism (AEM) with metal bands serving as the redox center proceeds via multiple oxygen intermediates (OH*, O*, OOH*, and O₂) [28]. As illustrated in Fig. 1d, the work function of Ni₂P (4.867 eV) is obviously higher than FeP₂ phase (4.709 eV), suggesting the possible homogenization of multiple intermediates’ adsorption energy due to the strong electron interaction at the heterointerface [29]. Consequently, the potential differences will be generated at interface domain and the electrons will transfer from FeP₂ to Ni₂P spontaneously inside the heterostructure (Fig. 1e), which leads to the formation of IPF pointing from positively charged Ni₂P to the negatively charged FeP₂ region [30,31,32]. As a more intuitive evidence, planar average potential along the Z-direction of Ni₂P/FeP₂ system was calculated (Fig. S4). The electrostatic potential energy level of Ni₂P is much lower than that of FeP₂, corresponding to a higher work function. This is consistent with the results of the work function calculation in Fig. 1d. Thus, there exist IPF in the Ni₂P/FeP₂ system, in which electrons can spontaneously transfer from FeP₂ to Ni₂P. Such an IPF would provide extra hydroxyl supply channel via driving HOSo from FeP₂ to Ni₂P, boosting the OER. The work functions difference (ΔΦ) between two materials was 0.158 eV, and the IPF potential (ΔU) (= ΔΦ/e, e is the electron charge) was calculated by E = ΔU/d, in which the thickness of stacking layer (d) is 10 Å [33]. Thus, the IPF strength was roughly estimated to be 1.58 × 10⁸ V m⁻¹.

To reveal the origin of adsorption properties optimization, the projected density of states (PDOS) of Ni₂P, FeP₂ and Ni₂P/FeP₂ models (Fig. S5) were analyzed [34]. It was found that the d band center (ε_d) of Ni sites for Ni₂P is at −1.85 eV whereas the ε_d for heterostructure is downward shifted to −2.09 eV. Simultaneously, the ε_d of Fe sites for FeP₂ (−0.87 eV) is also downward shifted after forming the heterojunction (−1.08 eV). According to the d-band theory, a downward shift of the d states of Ni and Fe sites with respect to the Fermi level results in reduced occupancy of antibonding states with adsorbed oxygen intermediates, implying the optimal binding strength of the oxygen species and optimized Gibbs free energy [35].

The interface effect on OER kinetics was further investigated via well-established CPET pathway with Ni active site (act_Ni, Fig. S6) and Fe active site (act_Fe, Fig. S7), respectively [36]. The RDS of Ni₂P/FeP₂ with act_Ni possesses the lowest energy barrier of 2.38 eV (Fig. S8), compared with Ni₂P (2.69 eV) and FeP₂ (2.51 eV) in Fig. 1f, as well as the Ni₂P/FeP₂ with act_Fe (2.87 eV, Fig. S9). Thus, the OER kinetics of Ni₂P/FeP₂ would overcome the electron-transfer limitation and become hydroxyl-transfer-determining [25]. Correspondingly, the possible mechanism of the optimized Gibbs free energy was demonstrated in Fig. 1g. The non-spontaneous adsorption of hydroxyl (ΔG_OH* > 0) leads to higher energy consumption of pure Ni₂P (0.63 eV) in OER process. Moreover, in the subsequent step, O* at Ni site would also be more difficult to bind to the OH⁻ in electrolyte directly, resulting in a higher energy barrier (2.69 eV) for the formation of OOH* in RDS, which is unfavorable to the oxygen evolution process (Path A) [37]. By the contrast, cooperating with the ability of FeP₂ (−0.5 eV) to absorb spontaneously (ΔG_OH* < 0) and IPF at the heterojunction interface, hydroxyl captured at Fe site would migrate to the Ni site and combined with O* to form OOH* intermediate (Path B), in which the energy barrier (2.38 eV) is lower than Path A. Therefore, the hydroxyl supply at Ni site in Ni₂P/FeP₂ heterostructure comes from dual channel: one is from the electrolyte (CN I), and the other is from the OH⁻ captured by FeP₂ overflow under the function of IPF (CN II). The new pathway driven by IPF will provide extra hydroxyl supply for OOH* formation at the Ni active site, thereby reducing the energy barrier of the resolution step. The interfacial hydroxyl spillover routes of Ni₂P/FeP₂ were simulated to elucidate how HOSo effect contributes to the overall OER activity and how IPF affects the kinetics of the interfacial HOSo. Accordingly, Fig. S10 is the HOSo routes of Ni₂P (Fig. 10a) and Ni₂P/FeP₂ (Fig. 10b). Notably, the ΔG_OH(TS) at the heterojunction interface of Ni₂P/FeP₂ (0.96 eV) is much lower than that of Ni₂P (1.82 eV) in Fig. S11. Thus, the migration of OH to Ni_act on heterojunction interface of Ni₂P/FeP₂ is easier, which can facilitate the continuation of OER process.

In addition, the adsorption energy of H₂O on the catalyst surface is also regarded as an important index to evaluate the performance of OER [38]. As shown in Fig. S12, the water adsorption energy on FeP₂ is higher than Ni₂P, indicating that FeP₂ could adsorb H₂O easier. In a nutshell, DFT calculations certify that coupling FeP₂ with Ni₂P could drive the supply of hydroxyl by dual channel under the IPF and optimize the adsorption of OER intermediates, which can empower Ni₂P/FeP₂ to apply to high/low OH⁻ concentration electrolyte conditions in both AWE and AEMWE system.

2.2 Material Synthesis and Characterization

To test the hypothesis, Ni₂P was intimately combined with FeP₂ by hydrothermal and phosphating processes. The synthesis steps of Ni₂P/FeP₂/MN heterostructures are schemed in Fig. 2a, and the fabrication details are available in experimental procedures. As shown in Figs. 2b and S13, the scanning electron microscopy (SEM) image of MN-OH demonstrated interconnected nanosheets that uniformly distributed on the molybdenum nickel (MN) skeleton, and the MN can provide enough Ni source, high mechanical strength and good electrical conductivity. After redox reaction with K₃[Fe(CN)₆], a spherical Rubik’s cube framework (Figs. 2c and S14a–c) was constructed by stacking in-situ grown NiFe-PBA nanocubes (about 500–700 nm, Fig. 2d) on each other [39]. After phosphating, the Ni₂P/FeP₂/MN maintained with the spherical Rubik’s cube morphology, but the surface became rough and the corners of the cubes were passivated (Figs. 2e and S14d–f). As contrasts, the morphologies of FeP₂/MN and Ni₂P/MN were also investigated. As shown in Fig. S15a–c, the aggregated nanocubes of FeP₂/MN were unevenly distributed and collapsed, while the morphology of Ni₂P/MN was the nanoparticles grown on nanosheets (Fig. S15d–f). To verify the heterostructure of Ni₂P/FeP₂, analysis of high-resolution transmission electron microscopy (HRTEM) of Ni₂P/FeP₂/MN (Fig. 2f) was carried out, in which the fringe spacing of 0.234 nm (101) and 0.515 nm (100) can be ascribed to FeP₂ and Ni₂P, respectively. Notably, the decent heterojunction interface in Ni₂P/FeP₂ is the origin of constructing IPF. Figure 2g is the transmission electron microscopy (TEM) map** of target sample, in which Fe, Ni, O and P were homogeneous distributed.

Fig. 2

a Schematic illustration of the formation process of Ni₂P/FeP₂/MN, SEM images of b MN–OH, c, d NiFe-PBA/MN, e Ni₂P/FeP₂/MN; HRTEM image f and TEM map** g of Ni₂P/FeP₂/MN

Additionally, the crystal structure of the Ni₂P/FeP₂/MN and other contrast samples were verified by X-ray diffraction (XRD) in Figs. 3a and S16. The dominating diffraction peaks of Ni₂P/FeP₂/MN approximated at 23.8°, 36.5°, 37.6° and 52.3° belongs to the (111), (120), (101) and (211) crystal planes of FeP₂ (PDF No. 89–2261), while peaks approximated at 40.7°, 44.6°, 47.4° and 54.2° belongs to the (111), (201), (210) and (300) crystal planes of Ni₂P (PDF No. 74–1385), indicating the successful synthesis of heterostructure of Ni₂P and FeP₂, which is consistent with the result of HRTEM. The surface and chemical valence states of Ni₂P/FeP₂ were further examined by X-ray photoelectron spectroscopy (XPS). In the high-resolution Ni 2p spectrum (Fig. 3b), compared with pure Ni₂P, an obvious positive shift can be observed in the Ni 2p_3/2 (857.3 eV) and Ni 2p_1/2 (875.1 eV) peaks of Ni₂P/FeP₂/MN, resulting in a higher oxidation state of Ni atoms [38]. By contrast, there is no significant shift of Fe 2p in Fig. 3c, implying that the construction of heterojunction has weak regulation on the electronic structure of Fe. As depicted in Figs. 3d and S17, lower binding energy peaks located at 129.3 and 130.1 eV are ascribed to P 2p_3/2 and P 2p_1/2 in Ni₂P/FeP₂, indicating the bond between P and Ni/Fe, which coincident with the result form XRD [40]. Besides, there is a slightly negative shift compared with that of pure Ni₂P, indicating that the electron transfer from metal, especially Ni, to phosphorus. As a result, XPS certified that the construction of heterojunction interface can facilitate the redistribution of electron in Ni and P, which may further optimize the adsorption energy of intermediate in OER process. The electron transfer behavior was further investigated by the calculated charge density difference (Figs. 3e and S18), with the electron-poor (blue) and electron-rich (yellow) regions. Coupling Ni₂P/FeP₂ led to strong electron interaction at the interface and local electrophilic/nucleophilic region. The extracted 2D data plot (Fig. 3f) displayed the electron accumulation and depletion areas were mainly existed between Ni and P, which is consistent with the result of XPS [41].

Fig. 3

a XRD of Ni₂P/FeP₂/MN, FeP₂/MN and Ni₂P/MN; XPS of Ni₂P/FeP₂/MN, FeP₂/MN and Ni₂P/MN: b Ni 2p, c Fe 2p, d P 2p; e Electron density difference, f the extracted 2D data plot for Ni₂P/FeP₂ model

2.3 OER Performance in Alkaline Water Electrolyzer (AWE)

The OER performances of materials in AWE system were first characterized from linear scan voltammetry (LSV) with corresponding Tafel plots in 1 M KOH electrolyte. Ni₂P/FeP₂/MN exhibited superior OER activity and kinetics (Fig. 4a–c) with a low ƞ₁₀₀, 100 mA cm^–2 of 242 mV and Tafel slope of 79.4 mV dec⁻¹, as compared with those of NiFe-PBA/MN (356 mV, 111.9 mV dec⁻¹) and MN–OH (405 mV, 106.4 mV dec⁻¹). As depicted in Fig. S19, the Ni₂P/FeP₂/MN material was comparable with the state-of-the-art TM-based electrocatalysts. Besides, the Ni₂P/FeP₂/MN catalyst could achieve 500 mA cm^–2 only requiring the overpotential of 302 mV (Fig. 4b), demonstrating its potential for application in large current density. The reason for the improvement of intrinsic catalytic activity is inferred via contrast samples of FeP₂/MN and Ni₂P/MN. The specific activities of Ni₂P/FeP₂/MN and other contrast samples are compared in Table S1, which further proves that Ni₂P/FeP₂/MN possessed higher intrinsic activity. As illustrated in Fig. S20, the oxidation peak of nickel species in the Ni₂P/FeP₂/MN is greatly suppressed compared with that of Ni₂P/MN, indicating that the construction of heterojunction interface could regulate the electronic structure of Ni atoms and increase the content of high-oxidation-state nickel species, which is consistent with the results of XPS [

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Acknowledgements

This work is financially supported by National Natural Science Foundation of China (52174283 and 52274308) and Innovation Fund Project for Graduate Student of China University of Petroleum (East China) (22CX04023A) and the Fundamental Research Funds for the Central Universities.

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Open access funding provided by Shanghai Jiao Tong University.

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1. State Key Laboratory of Heavy Oil Processing, College of Chemistry and Chemical Engineering, China University of Petroleum (East China), Qingdao, 266580, People’s Republic of China

   **gyi **e, Fuli Wang, Yanan Zhou, Yiwen Dong, Yongming Chai & Bin Dong

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Correspondence to Yongming Chai or Bin Dong.

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**e, J., Wang, F., Zhou, Y. et al. Internal Polarization Field Induced Hydroxyl Spillover Effect for Industrial Water Splitting Electrolyzers. Nano-Micro Lett. 16, 39 (2024). https://doi.org/10.1007/s40820-023-01253-9

• Received: 30 June 2023

• Accepted: 24 October 2023

• Published: 30 November 2023

• DOI: https://doi.org/10.1007/s40820-023-01253-9

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