Stabilization of active ultrathin amorphous ruthenium oxide via constructing electronically interacted heterostructure for acidic water oxidation

Abstract

Amorphous RuO_x (a-RuO_x) with disordered atomic arrangement and abundant coordinatively unsaturated Ru sites possesses high intrinsic electrocatalytic activity for oxygen evolution reaction (OER). However, the a-RuO_x is prone to fast corrosion during OER in strong acid. Here, we realized the stabilization of an ultrathin a-RuO_x layer via constructing heterointerface with crystalline a-MnO₂ nanorods array (MnO₂@a-RuO_x). Benefiting from the strong electronic interfacial interaction, the as-formed MnO₂@a-RuO_x electrocatalyst display an ultralow overpotential of 128 mV to reach 10 mA cm^-2 and stable operation for over 100 h in 0.1 mol L^-1 HClO₄. The assembled proton exchange membrane (PEM) water electrolyzer reach 1 A cm^-2 at applied cell voltage of 1.71 V. Extensive characterizations indicate the MnO₂ substrate work as an electron donor pool to prevent the overoxidation of Ru sites and the OER proceeds in adsorbent evolution mechanism process without involving lattice oxygen. Our work provides a promising route to construct robust amorphous phase electrocatalysts.

Headnote

Abstract

Amorphous RuO_x (a-RuO_x) with disordered atomic arrangement and abundant coordinatively unsaturated Ru sites possesses high intrinsic electrocatalytic activity for oxygen evolution reaction (OER). However, the a-RuO_x is prone to fast corrosion during OER in strong acid. Here, we realized the stabilization of an ultrathin a-RuO_x layer via constructing heterointerface with crystalline a-MnO₂ nanorods array (MnO₂@a-RuO_x). Benefiting from the strong electronic interfacial interaction, the as-formed MnO₂@a-RuO_x electrocatalyst display an ultralow overpotential of 128 mV to reach 10 mA cm^-2 and stable operation for over 100 h in 0.1 mol L^-1 HClO₄. The assembled proton exchange membrane (PEM) water electrolyzer reach 1 A cm^-2 at applied cell voltage of 1.71 V. Extensive characterizations indicate the MnO₂ substrate work as an electron donor pool to prevent the overoxidation of Ru sites and the OER proceeds in adsorbent evolution mechanism process without involving lattice oxygen. Our work provides a promising route to construct robust amorphous phase electrocatalysts.

Keywords: Amorphous ruthenium oxide; Manganese dioxide; Oxygen evolution reaction; PEM electrolyzer

1. Introduction

Hydrogen generation from electrochemical water splitting is a promising technology for sustainable green energy systems [1-6]. However, four-electron OER process with sluggish reaction kinetics hinders the overall efficiency of water splitting [7-11]. Compared with alkaline media, OER in acidic electrolytes presents faster response, higher current density, and fewer side reactions during electrochemical water splitting. Therefore, the development of OER electrocatalysts with high activity and durability in acidic media is of great significance [12-16]. At present, iridium (Ir)-based catalysts (IrO,) were often considered as the only practical OER electrocatalysts for proton exchange membrane water electrolysis due to their robust durability [17-19]. However, its high cost and scarcity limited its large-scale application. Compared with the Ir-based catalysts, Ru-based catalysts were superior to their lower price and higher intrinsic OER activity, which made them potential substitutes for Ir-based catalysts [11,20- 22].

The a-RuO_x inherently incorporated a higher density of unsaturated and randomly oriented bonds, fostering advantageous conditions for reactant adsorption and facilitating an accelerated electron transport rate [23,24]. Additionally, due to the presence of strong electron-lattice coupling effects within the amorphous phase, it can improve the intrinsic activity of the catalyst for OER [25,26]. Unfortunately, amorphous based were observed to be impaired in stability by rapid cation dissolution during acidic OER processes. Therefore, it is necessary to develop amorphous catalysts that strike a balance between stability and activity [23]. It had been shown that the deactivation of Ru-based materials was mainly due to excessive oxidation to the formation of Ru" > ". In particular, lattice oxygen processes were involved, producing Ru atoms exposed by oxygen vacancies, which were easily dissolved by oxidation and further leaded to the collapse of the crystal structure [11,27,28]. The electrocatalytic activity and stability of the catalyst were closely related to its electronic structure [21]. The electronic regulation of Ru by some donated electronic elements (Sr, La, Mn, Ce, etc.) can increase the proportion of low valence Ru and form a stable local structure of M-O-Ru [29-32], which can effectively inhibit excessive oxidation of Ru [33-36]. It can be predicted that the stability of a-RuO, can be enhanced by regulating the electronic structure.

Herein, we prepared a MnO, @a-RuO, heterogeneous catalysts, where manganese oxides act as an electron donor and promotes electron transfer to Ru. This orchestrated electron transfer mechanism effectively mitigated the dissolution of active Ru sites, presenting a resolution to the inherent stability challenges associated with the rapid dissolution of amorphous phase catalysts. Experiments showed that the representative 4MnO,@a-RuO0, in 0.1 mol L-' HCIO,, electrolyte delivered overpotentials as low as 128 mV at 10 mA cm ", and stable operation within 110 h. In-situ infrared (FTIR) analysis confirmed the adsorbate evolution mechanism (AEM) of RuO, catalysts without destabilizing the lattice of surface Ru and subsurface oxygen. Furthermore, X-ray photoelectron spectroscopy (XPS) and work function demonstrated the electron transfer from Mn to Ru. These were the reasons for its improved OER durability. The study provided a new strategy for constructing stable and highly active amorphous phase electrocatalysts.

2. Results and discussion

2.1. Structural characterization

Fig. 1a illustrated the synthesis process for MnO, @a-RuO, catalyst (Detailed information can be found in the experimental section in the supporting information). Firstly, MnO, were prepared via a hydrothermal method. Subsequently, a heterostructure of a-RuO, coating on MnO, was fabricated through a facile cation exchange process. The Ru content was regulated by varying RuCl; concentration, resulting in three distinct samples denoted as yMnO, @a-RuO, (y = 2, 3 and 4). The Ru loading capacities, determined by inductively coupled plasma-optical emission Spectrometry (ICP-OES), were 2.1, 34, and 4.2 wt.%, respectively. We further tuned the Ru content and conducted scanning electron microscope (SEM) analysis to characterize MnO, with different Ru loadings. MnO, (Fig. Sla, b and $2), 2MnO,@a-RuO, (Fig. Slc, d), 3MnO,@a-RuOy (Fig. Sle, f) and 4Mn0, Qa-RuO, (Fig. S1g, h) loaded on carbon paper presented nanorods with a diameter of about 50-100 nm. Upon closer inspection, the manganese dioxide showed negligible corrosion compared to the sample without ruthenium chloride immersion. X-ray diffraction (XRD) patterns of MnO, and the samples with different Ru loadings were shown in (Fig. lb-and S3) along with the standard pattern of a-MnO, (ref.31; JCPDS file no. 44-0141). Transmission electron microscopy (TEM) images (Fig. S4a-c and S5a-c) revealed a smooth nanorod structure with no apparent aggregation of ruthenium particles along the entire length of the rod. This observation emphasized the uniform dispersion of Ru on the surface of a-MnO>, nanorods. The interplanar lattice spacings were measured at 0.24 nm and 0.49 nm (Fig. 1c-d, S4d-f, and S5d-f), corresponding to the (211) and (200) crystal planes of a-MnO₂, respectively. A surrounding blurry region, devoid of visible lattice fringes, exists outside the nanorod structure, attributed to the amorphous region. Additionally, aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (AC HAADF-STEM) images of 4MnO₂@a-RuO_x (Fig. leh and S6a) revealed lattice fringes of only MnO, in the core of the nanorods, while a uniformly dispersed amorphous coating layer was present on the surface of the nanorods. This observation aligned with the energy-dispersive X-ray spectroscopy (EDS) mapping data (Fig. 1i-1 and S6b-e), confirming that a-RuO, was uniformly coated on the surface of MnO, nanorods and the thickness of the a-RuO, was about 2 nm. The results were in agreement with those obtained by 4MnO₂@aRuO_x in TEM lattice and XRD. In the crystalline region, atoms were arranged in long-range order, forming regular and clear lattice fringes, whereas in the amorphous region, atoms exhibited loose boundaries and random arrangements, lacking periodicity, thus displaying structural disorder. Consequently, lattice fringes cannot be observed. Therefore, a-RuO, coated a-MnO₂ nanorods were successfully prepared via cation exchange method.

2.2. Electronic structure of MnO₂@a-RuO_x

The electronic structure of MnO₂ @a-RuO_x heterostructure was examined with XPS (Fig. 2a, b and S7). In Fig. 2a, the sub-peaking C 1s and Ru 3d peaks were presented, encompassing 2MnO₂Qa-RuO_x, 3MnO₂@a-RuO_x, 4MnO₂@aRuO_x, and commercial RuO2 (Com-RuO2). Notably, due to the hydrophilic treatment of the carbon paper, the presence of the С==О peak was observed. Upon comparison of the data, it was evident that the binding energy of Ru 3ds, decreases with the increase in Ru loading. The binding energy values for Ru 3d5/2 were detailed in Table Sl, with the order being 4MnO₂Qa-RuO_x < 3MnO₂@a-RuO_x < 2MnO₂@aRuO_x, < Com-RuO2. This phenomenon can be attributed to the electronic interaction between ruthenium and manganese dioxide [13]. These findings indicated that the valence state of Ru in MnO₂@a-RuO_x was close to quadrivalent, but with the electronic regulation exerted by Mn-Ru interaction, the valence state of Ru gradually decreases. The average valence state of Mn in MnO₂@GRuO_x was determined by deconvolution of Mn 3s peaks through high-resolution XPS multiple divisions. In Fig. 2b, the Mn 3s peaks of 2MnO₂@a-RuO_x 3MnO₂@a-RuO_x and 4MnO₂ Qa-RuO_x can be deconvoluted into two peaks. This approach aligned with previous studies [37], linear relationship existed between the splitting energy of Mn 3s and the mean valence state: AE = 7.88-0.85n [36]. Therefore, for MnO₂, 2MnO₂ QC a-RuO_x 3MnO₂ @a-RuO_x and 4Mn0, @a-RuOy, the calculated average valence states of Mn atoms on the surface were 3.80, 3.82, 3.83, and 3.87, tively [38]. the valence state of Ru decreased while the Mn valence increased with an increase in Ru loading (Fig. 2c). The catalyst prepared using this method allowed for the controllable adjustment of the Ru loading.

The work function of the material can determine the direction of electron transfer. This meant that electrons moved from the material with the low work function to the material with the high work function [39]. MnO, had a lower average work function and RuO2 had a higher average work function (Fig. 2d and S8). The local work function difference causes interfacial electron transfer and promotes electron transfer from Mn to Ru. In other words, following the electron transfer process, the average work function of MnO, and the average valence state of Mn on the contact surface increased, while the average work function of a-RuO, and the average valence state of Ru decreased [40]. In acidic media, Ru deactivation primarily resulted from the formation of Ruthenium superoxides (Rux>4+), Through electron regulation, the synthesized 4MnO₂ @a-RuO_x exhibited medium work function and the low valence state of Ru characteristics. This low valence Ru effectively prevented the peroxide reaction of Ru and inhibits lattice oxygen from participating in the reaction, consequently demonstrating excellent stability [41].

2.3. Electrochemical properties of catalysts

The acidic OER performance of three MnO₂ Q a-RuO_x and Com-RuO2 (1 mg cm-2) were measured using a three-electrode system at room temperature (Fig. 59). In Fig. 3a, the iR compensated OER polarization curve demonstrated that 4MnO₂@a-RuO_x exhibited superior activity, with a reaction overpotential of only 128 mV at current density of 10 mA cm". Evaluation based on overpotential the activity order was as follows 4MnO₂@a-RuO_x (128 mV) < 3MnO₂@a-RuO_x (149 mV) <2MnO₂ @a-RuO_x (182 mV) < Com-RuO₂ (258 mV). Notably, the OER performance of the three MnO₂ Qa-RuO₂ catalysts surpassed that of Com-RuO₂. Fig. 3b illustrated the correlation between the catalyst's catalytic performance and the valence state of Ru. The data indicated that as the valence state of Ru decreased, the OER performance of the catalyst exhibited an upward trend. This suggested that electronic regulation of the catalyst, leading to a reduction in the valence state of Ru sites, was conducive to enhancing the catalytic activity of the material. To further assess the intrinsic activity of the prepared electrocatalyst, mass activity, roughness factors and electrochemically active surface area (ECSA) normalized linear sweep voltammetry (LSV) curves were calculated based on the total load mass (Fig. S10-S13 and Table S2, S3) of the precious metal (Ru), allowing for a comparative analysis with Com-RuO2. In Fig. 3c, it was obvious that both mass activity and specific activity of the 4MnO₂ @a-RuO_x significantly surpassed that of Com-RuO2. This signified that the heterostructure had intrinsic high activity. Additionally, Fig. S12b depicted Nyquist plots for the three MnO₂@a-RuO_x heterostructure and Com-RuO₂ exhibiting semi-circular patterns in the high-frequency range. These patterns reflected the charge-transfer resistance of the catalysts, providing insights into their electrochemical behavior [42]. The resistances of the self-synthesized MnO₂ @a-RuO_x catalysts are consistently lower than that of Com-RuO₂ with 4MnO₂@aRuO_x (9.5 n) < 3MnO₂@a-RuO_x (9.7 n) < 2MnO₂Qa-RuO_x (11.8 n). This observation suggested that 4MnO₂ Qa-RuO_x possessed inherent fast charge transfer, translating to superior catalytic activity in the OER. The calculated Tafel slope [43] (Fig. 3d) further supported this evaluation with 4MnO₂ @a-RuO_x displaying a low Tafel slope value of 51.2 mV dec 1, notably lower than 3MnO₂@a-RuO_x (140.6 mV dec 1), 2MnO₂@a- RuO_x (150.0mVdec 1), andCom-RuO2 (290.1mVdec 1).This indicated excellent OER kinetics for 4MnO₂@a-RuO_x. The acidic OER stability of 4MnO₂@a-RuO_x was evaluated at room temperature and pressure using a typical three-electrode system. The chronopotentiometric measurements at 10 mA cm-2 in a 0.1 mol L 1 HClO₄ solution showed that 4MnO₂@a-RuO_x catalyst maintains activity over 110 h, showcasing significantly higher stability compared to Com-RuO2 (Fig. 3e). As shown in Fig. S14, the electrocatalysts well-matained stability after running under the current density of 50 mA cm-2 for 50 h. Meanwhile, the dissolution of Ru and Mn during the reaction at different voltages was detected (Fig. S15). The concentrations of Mn and Ru ions showed a slight elevation during the initial 4-h period, subsequently stabilizing. This phenomenon suggested that the early dissolution phase primarily involves the elimination of unstable surface species, leading to the stabilization of the catalyst's reconstructed structure. Consequently, the integrity of the bulk structure is maintained, ensuring the preservation of the catalyst's electrochemical properties. The selectivity of the catalyst can be evaluated by Faraday efficiency, which is as high as 99% when tested on 4MnO₂@a-RuO_x at a current density of 15 mA cm-2 (Fig. S16), with excellent OER selectivity. These findings underscore the robust performance and stabilization of 4MnO₂@a-RuO_x in acidic OER. Fig. 3f and Table S4 compared the overpotential and stability of 4MnO₂@a-RuO_x catalyst with other previously reported Ru-based electrocatalysts in acidic OER. Encouragingly, 4MnO₂@a-RuO_x exhibited ultra-low overpotential and extremely long lifetime at a current density of 10mAcm-2, better thanmost previously reported results [11,44- 58].

2.4. Insight to the stability of a-RuO_x

To validate the catalytic mechanism, in-situ Fourier transform infrared (FTIR) spectroscopy was employed. With the increase of voltage, a characteristic peak appeared on the electrolyte surface at 1210 cm-1 (Fig. 4a). This peak corresponded to the characteristic peak of ·OOH, signifying the presence of ·OOH as the intermediate in the AEM pathway [59]. This observation provided valuable insights into the reaction intermediates and contributed to a comprehensive understanding of the catalytic process [60]. To delve into the reasons for the catalyst's high stability, a comparative analysis of the structural characterization before and after the OER was conducted. Fig. 4b presented SEM images, revealing that the catalysts nanorod structure remained unchanged, demonstrating robust morphology. Fig. 4b also provided TEM comparisons before and after the reaction, showcasing the persistence of the core-coated nanorods structure with only the lattice fringe of MnO, visible. These results indicated that the structure and properties of the catalyst were well-preserved. Furthermore, XPS tests were conducted on the catalysts before and after the OER to provide additional insights into the surface composition and chemical states. As represented in Fig. 4c, the binding energy of Ru before and after OER exhibited a minimal increase which is negligible. The change in the valence state of Mn was illustrated in Fig. 4d. A comparison of the binding energy of Mn 3s indicated a slight shift (0.02 eV) in the peak binding energy after the OER, suggesting relatively low impact on the electronic structure of Mn during the OER. The morphology and electronic structure analysis showed that the catalyst had stable morphology and electronic structure in the acidic OER process. In comparison to Com-RuO2 the 4MnO₂ @a-RuO_x exhibited superior performance and stability, underscoring the crucial role of electronic structure regulation in Mn-Ru for enhancing catalyst performance. As depicted in Fig. 4e, driven by the work function (Fig. 2d), MnO₂ in MnO₂ @a-RuO_x acted as an electron donor, transferring electrons to adjacent a-RuO_x sites. This process reduced the valence state of surface Ru, preventing peroxidation to soluble Ru>4+ during the reaction thereby enhancing catalyst stability. In summary, the OER process of MnO₂ @aRuO_x followed the AEM, and did not involve lattice oxygen processes, thus having no adverse effects on the internal structure or electronic states of the catalyst. Additionally, MnO₂ acted as an electron donor, effectively inhibiting the dissolution of surface Ru. Therefore, 4MnO₂Qa-RuO_x can have excellent stability during OER reaction.

2.5. PEM water electrolysis (PEMWE) device performance

We investigated the practical application feasibility of the catalyst as a water electrolytic anode catalyst by constructing a PEMWE cell. In this setup, 4MnO₂ Q a-RuO_x served as the anode catalyst, while commercialized Pt/C functioned as the cathode catalyst, with a PEMWE facilitating proton conduction. The membrane underwent acid and H2O2 treatment before use to activate the proton channels. The membrane electrode assembly (MEA) was prepared by spraying a catalyst slurry, with a cathode load of 1 mg cm-2 and an anode load of 2 mg cm-2. Fig. 5a illustrated the schematic illustration of the electrolytic cell device, with titanium felt used as porous transport layers (PTL) on both sides [49]. Using this device, we assessed the catalyst's performance, as depicted in Fig. 5b through E-I curves (without iR compensation). It was evident that 4MnO₂@a-RuO_x exhibited notable advantages over Com-RuO,, achieving 1 A cm-2 and 2 A cm-2 at 1.71 V and 1.87 v, respectively. Importantly, the device exhibited a low impedance of only -0.025 Q (Fig. S17). Subsequently, we evaluated the stability of the catalyst in deionized water at 60 °C at a current density of 500 mA cm-2, as shown in Fig. 5c. The 4MnO₂ @a-RuO_x demonstrated stable operation for 24 h Without significant performance degradation, emphasizing the promising application prospects of Ru-based catalysts as acidic OER catalyst for PEMWE.

3. Conclusions

In summary, we developed a heterogeneous a-RuO, catalyst supported by MnO, via a simple cation exchange method, which can be used in PEMWE devices. Extensive characterizations confirmed that MnO, as an electron donor, which effectively regulates the charge of a-RuO, and prevents excessive oxidation of Ru sites. Remarkably, the 4MnO, @a-RuO, OER catalyst only required a 168 mV overpotential to reach 10 mA ст"? accompanied with high stability of 110 h in 0.1 mol L' HCIO,. The PEMWE device assembled with 4MnO,@a-RuO, as OER catalyst has a cell voltage of only 1.71 Vat 1 A cm ". Therefore, this study provides an effective way to construct a stable amorphous phase electrocatalyst.

CRediT authorship contribution statement

Xiangxiang Pan: Writing - original draft, Validation, Formal analysis, Data curation. Huidong Qian: Investigation, Formal analysis. Jiansheng Xu: Data curation. Haifeng Wang: Investigation, Data curation. Han-Don Um: Investigation. Chao Lin: Writing - review & editing, Formal analysis, Data curation. Xiaopeng Li: Writing - review & editing, Project administration, Funding acquisition. Wei Luo: Writing - review & editing, Supervision.

Declaration of competing interest

The author Wei Luo is a Editorial Board Member for Green Energy & Environment and was not involved in the editorial review or the decision to publish this article. The other authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the Fundamental Research Funds for the Central Universities (2232024Y-01), the National Natural Science Foundation of China (52225204, 52272289, 52173233 and 52402231), the Innovation Program of Shanghai Municipal Education Commission (2021-01-07-00-03-E00109), the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning, Natural Science Foundation of Shanghai (23ZR 1479200), "Shuguang Program" supported by Shanghai Education Development Foundation and Shanghai Municipal Education Commission (20SG33) and the DHU Distinguished Young Professor Program (LZA2022001).

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.gee.2024.05.003.

Sidebar

Received 15 March 2024; revised 27 April 2024; accepted 11 May 2024

Available online 16 May 2024

© 2024 Institute of Process Engineering, Chinese Academy of Sciences. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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Footnote

* Corresponding authors.

E-mail addresses: [email protected] (C. Lin), [email protected] (X. Li), [email protected] (W. Luo).

1 These authors contributed equally to this work.