Fe₂NiSe₄ Nanowires Array for Highly Efficient Electrochemical H₂S Splitting and Simultaneous Energy-Saving H₂ Production

Abstract

The electrochemical removal of abundant and toxic H₂S from highly sour reservoirs has emerged as a promising method for hydrogen production and desulfurization. Nevertheless, the ineffectiveness and instability of current electrocatalysts have impeded further utilization of H₂S. In this communication, we introduce a robust array of Fe₂NiSe₄ nanowires synthesized in situ on a FeNi₃ foam (Fe₂NiSe₄/FeNi₃) via hydrothermal treatment. This array acts as an active electrocatalyst for ambient H₂S splitting. It offers numerous exposed active sites and a rapid electron transport channel, significantly enhancing charge transport rates. As an electrode material, Fe₂NiSe₄/FeNi₃ displays remarkable electrocatalytic efficiency for both sulfide oxidation and hydrogen evolution reactions. This bifunctional electrode achieves efficient electrochemical H₂S splitting at a low potential of 440 mV to reach a current density of 100 mA∙cm⁻², with a faradaic efficiency for hydrogen production of approximately 98%. These findings highlight its significant potential for desulfurization and energy-efficient hydrogen generation.

1. Introduction

With the continuous advancement of the energy revolution, energy consumption is evolving toward electrification, cleanliness, low carbon, and intelligence. At present, it is a critical period to accelerate energy transformation and promote “carbon peak” and “carbon neutrality”. Natural gas, as the cleanest and lowest-carbon fossil fuel, plays a crucial role in transitioning from high-carbon to low-carbon energy sources and has been extensively utilized for reducing carbon emissions in recent decades [1]. However, the focus on natural gas has shifted from conventional gas reservoirs to sour ones, in which the H₂S reached 10 million tons per year worldwide in 2019 [2]. The substantial amount of H₂S, with its strong toxicity, corrosivity, and malodor, has traditionally been deemed useless or even hazardous, requiring pretreatment before further processing. Efficient desulfurization through sulfide oxidation reactions is crucial for environmentally friendly development, as well as for enhancing human health and safety [3]. Traditional desulfurization methods, like chemical adsorption, biological oxidation, or catalytic conversion, demand substantial energy and chemical use, making them energy-intensive. The Claus process, the standard industrial method operating at high temperatures (900 °C), partially oxidizes H₂S to produce sulfur and water vapor. Yet, this process only recycles sulfur, with the potential energy of hydrogen being lost as water vapor, leading to the underutilization of H₂S resources.

Hydrogen energy, as a clean and sustainable new energy source, has a wide range of application scenarios. For example, it is utilized in distributed power generation or cogeneration to provide heat and power buildings [4,5], as hydrogen fuel for fuel cell vehicles [6], as reducing agents for industrial metallurgy, and as raw materials for the synthesis of methanol, ammonia, and other chemical products [7]. Electrochemical water splitting driven by sustainable energy has been investigated as a prospective alternative way to produce green hydrogen, especially with the rapid growth of clean energy power capacity [8,9]. However, commercial water electrolyzers typically operate at a much larger voltage of 1.8 to 2.0 V, especially the oxygen evolution reaction (OER), with high activation energy barriers for O-O bond formation, and sustain huge overall electricity consumption for the multiple proton and electron transfer steps [10]. Therefore, replacing the oxygen evolution reaction with a thermodynamically more readily oxidation reaction to reduce overall energy consumption offers a viable strategy for an energy-saving approach to facilitate H₂ production. Tang et al. developed a Ni₂P nanoarray as a high-performance non-noble-metal electrocatalyst for anodic hydrazine oxidation reaction and cathodic for low-energy hydrogen production [11]. Moreover, the anode oxidation reactions have the extra advantage of obtaining valuable chemicals or dealing with environmental pollution.

Electrochemical conversion of H₂S into hydrogen and sulfur products is an economically and environmentally favorable method, addressing significant industrial waste gas issues. Electrocatalytic H₂S offers a mild and efficient means of hydrogen production and sulfur recycling. From the thermodynamic point of view, H₂S splitting requires less energy compared to water splitting (33 kJ mol⁻¹ for H₂S, 237 kJ mol⁻¹ for H₂O), suggesting its feasibility. Nevertheless, its effectiveness has been limited by the inefficient activity and inadequate stability of electrocatalysts in sulfur-rich environments [12]. To solve the problem, an indirect electrochemical–chemical loop strategy has been developed using the intermediate redox media to oxidize sulfide anions. Although this method has achieved modest success in preventing direct contact of the electrocatalysts with sulfur-containing solutions, it faces challenges such as the need for complex redox reagents (Fe²⁺/Fe³⁺, VO₂⁺/VO²⁺, I⁻/I₃⁻) and poor long-term stability in acidic conditions. Alternatively, direct electrocatalytic H₂S splitting methods have also been developed. For example, a series of metal sulfide catalysts have shown sulfur resistance under harsh electrolyte conditions, making them potential electrocatalysts [13,14,15,16,17,18,19]. However, its potential problems, such as poor electrical conductivity, restrict its further development [20,21]. The metal selenides also demonstrate excellent stability, great conductivity, and high activity in sulfur-rich conditions. Duan et al. reported an anti-sulfuretted NiSe/NF array catalyst for sulfur oxidation reaction with low-energy H₂ production, exhibiting 500 h outstanding stability. However, further improvement in its catalytic activity remains necessary [22].

Metal selenides and nanostructure materials have received extensive attention due to their great electrochemical properties [20,23,24]. Motivated by these advancements, we developed a Fe₂NiSe₄ nanowires array grown in situ on conductive FeNi₃ foam substrates (Fe₂NiSe₄/FeNi₃). This configuration exposes numerous active sites and facilitates rapid electron transport, enabling high-efficiency electrocatalytic decomposition of H₂S coupled with hydrogen production. Remarkably, the Fe₂NiSe₄/FeNi₃ serves as an attractive bifunctional catalyst for hydrogen generation and sulfide oxidation reaction (SOR), outperforming previously reported anti-sulfur NiSe/NF catalysts. It achieves an impressive performance in alkaline environments, requiring a low potential of 440 mV to achieve 100 mA∙cm⁻². Additionally, it demonstrates robust durability for more than 800 min and approximately 98% faradaic efficiency toward H₂ production. Furthermore, the product of anodic sulfide oxidation follows the chain-growing mechanism of S²⁻/HS⁻ to polysulfides in alkaline solutions by combining UV–vis analysis. Our research provides guidance for designing efficient electrocatalysts of metal selenide catalysts based on highly efficient electrochemical H₂S splitting and simultaneous energy-saving H₂ production.

2. Materials and Methods

2.1. Chemicals

The commercial FeNi₃ foam was purchased from Kunshan Long Sheng Bao electronic material form Su Zhou city. Reagents used in the experiment were purchased from Shanghai Aladdin Industrial Inc. (China) with the following purities: NaBH₄ (97%), Se powder (99.9%), HCl (36–38%), ethanol (75%), Na₂S·9H₂O (98%), and NaOH (98%). All reagents were of analytical pure grade and were without any treatment before use. All experimental water was used ultrapure water through a Millipore system.

2.2. Synthesis of Fe₂NiSe₄/FeNi₃

An Fe₂NiSe₄ nanowires array was synthesized on FeNi₃ foam through a one-step hydrothermal method. The synthesis process is shown in Figure S1. First, prepare 2 × 3 cm² of FeNi₃ foam and wash it several times with 3 M HCl, ethanol, and deionized water to ensure that the surface of FeNi₃ foam was adequately cleaned before use. Firstly, to prepare the NaHSe solution, 0.118 g Se powder was added to 3 mL ultrapure water along with 0.130 g NaBH₄. The mixture was stirred continuously until it became clear. Next, the prepared NaHSe solution was added to a 30 mL ethanol solution. The resulting mixture was then transferred into a 50 mL Teflon-lined stainless steel autoclave with previously treated FeNi₃ inside. The autoclave was sealed tightly and placed in an oven at 180 °C for 12 h. After the reaction, we allowed the autoclave to slowly cool to room temperature. The post-reaction sample was rinsed with ultrapure water and ethanol and, finally, placed in a vacuum drying oven at 60 °C for 4 h. As a control material, NiSe was synthesized according to the previous literature [22].

2.3. Characterization

X-ray diffraction (XRD) data were acquired on a Philips X′Pert diffractometer with Cu Kα radiation (λ = 1.5418 Å). Scanning electron microscopy (SEM) measurements were carried out on a ZEISS Sigma 300 scanning electron microscope at an accelerating voltage of 20 kV. Transmission electron microscopy (TEM) measurements were performed on an FEI TECNAI G20 electron microscope (FEI, American) with an accelerating voltage of 200 kV. Energy dispersive spectroscopy (EDS) mapping was conducted by FEI TF20. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo Fisher Scientific K-Alpha X-ray photoelectron spectrometer using Mg as the exciting source. UV-vis absorption spectra were recorded with a Shimadzu UV-2600 spectrophotometer (Shimadzu, Japan)

2.4. Electrochemical Measurement

All electrochemical measurements were performed with a CHI 660D electrochemical analyzer (CH660 Instruments, Inc., Shanghai, China) in a standard three-electrode system using Fe₂NiSe₄/FeNi₃ as the working electrode, a graphite rod as the counter electrode, and a Hg/HgO electrode as the reference electrode. All electrochemical tests were carried out in a 50 mL H-cell. The electrolyte solution of 1.0 M NaOH with 1.0 M Na₂S, as a simulated hydrogen sulfide absorption solution, served as an electrolyte for the half-cell SOR study, while the electrolyte solution of 1.0 M NaOH served for the half-cell hydrogen evolution reaction and half-cell oxygen evolution reaction. All potentials measured were calibrated to RHE using the following equation: E (RHE) = E (Hg/HgO) + 0.098 V + 0.0591 × pH. Polarization curves were obtained using linear sweep voltammetry (LSV) with a scan rate of 5 mV s⁻¹, and no activation was used before recording the polarization curves. Electrochemical impedance spectra (EIS) were carried out from 1 Hz to 1000 KHz at the open circuit voltage. Electrochemical active surface area (ECSA) was calculated by capacitance (C_dl) measurements. The long-term durability was performed using chronoamperometry measurements.

3. Results and Discussion

3.1. Characterization of Fe₂NiSe₄/FeNi₃

The catalytic material of Fe₂NiSe₄/FeNi₃ was successfully prepared by the hydrothermal synthesis method. Figure 1a displays the XRD patterns of Fe₂NiSe₄/FeNi₃ and FeNi₃. Three strong diffraction peaks at 44.5°, 51.8°, and 76.1° belong to the (111), (200), and (220) crystal faces of FeNi₃, respectively (JCPDS no. 38-0419) [25]. After selenization of the FeNi₃ foam, a new series of diffraction peaks at 28.9°, 33.1°, 59.6°, and 70.6° corresponded to (220), (−112), (116), and (404) crystal faces of Fe₂NiSe₄ solid solution materials, respectively (JCPDS no. 65-2338) [26]. The surface morphologies of Fe₂NiSe₄/FeNi₃ were characterized by scanning electron microscope and transmission electron microscopy images. From Figure 1b–d and Figure S2, it is evident that the Fe₂NiSe₄/FeNi₃ exhibits the shape of the nanowires array. Figure 1e shows the high-resolution transmission electron microscopy (HRTEM) image, which displays well-defined lattice fringes with interplanar distances of 2.6 Å and 2.7 Å, respectively. The angle between the two crystal faces is 73°. This corresponds to the Fe₂NiSe₄ (213) and (−303) crystal planes, respectively. At the same time, the EDS mapping images of Fe₂NiSe₄ nanowire further reveal the uniform distribution of Fe, Ni, and Se.

The valence state of Fe₂NiSe₄/FeNi₃ was determined using X-ray photoelectron spectroscopy. The survey spectrum shown in Figure S3 confirms the characteristic peaks for Ni, Fe, and Se, and the C and O signals may be attributed to air contamination. Figure 2a displays the high-resolution XPS spectrum of the Ni 2p region. Two spin-orbit doublets characteristic of peaks around 853 eV and 871 eV are attributed to Ni 2p_3/2 and Ni 2p_1/2, respectively. The binding energies of 876.9 eV and 858.9 eV are ascribed to two shakeup satellites from superficial oxidation of Fe₂NiSe₄ owing to air contact. The other binding energies of 870.9 eV and 853.2 eV are attributed to Ni²⁺ [27]. Figure 2b shows the high-resolution Fe 2p spectrum. The peak at 710.2 eV is indexed to Fe 2p_3/2, and the peak at 723.8 eV is distributed to Fe 2p_1/2 [28,29]. As for the Se 3d spectrum in Figure 2c, the binding energies of 54.72 and 53.71 eV are assigned to Se 3d_3/2 and Se 3d_5/2, respectively, which are assigned to the Se bonded to Ni or Fe in the form of metal selenide [28,29]. The peak at 58.32 eV is associated with the surface oxidation state of Se species. All these observations confirm the successful formation of Fe₂NiSe₄ on FeNi₃ foam.

3.2. Electrocatalytic Performance

The electrocatalytic performance of Fe₂NiSe₄/FeNi₃ for electrochemical H₂S splitting was examined in 1.0 M NaOH solution with 1.0 M Na₂S, employing a three-electrode system: a Hg/HgO as the reference electrode, a graphite rod as the counter electrode, and a square Fe₂NiSe₄/FeNi₃ with a side length of 0.5 * 0.5 cm as the work electrode. An H-type electrolytic cell with a separate cathode and anode was used for the electrocatalytic activity study. The image of the laboratory electrochemical cell is shown in Figure S4. Figure 3a illustrates the LSV polarization curves of Fe₂NiSe₄/FeNi₃, NiSe/NF, and bare FeNi₃ foam with a scan rate of 5 mv s⁻¹ for SOR. It can be found that bare FeNi₃ foam exhibits poor SOR activity. In sharp contrast, the Fe₂NiSe₄/FeNi₃ catalyst demonstrates approximately twice the catalytic activity compared to NiSe/NF in the SOR process, achieving a current density of 100 mA cm⁻² at a potential of 0.44 V vs. RHE. This potential is 50 mV lower than that of NiSe/NF and compares favorably to the behaviors of transition metal sulfide catalysts (as shown in Table S1). These findings suggest that the Fe₂NiSe₄/FeNi₃ electrode exhibits high efficiency in catalyzing the SOR, which is well verified under the comparison of different voltages (0.4 V, 0.5 V, and 0.6 V vs. RHE) in Figure 3b. At the same potential, the Fe₂NiSe₄/FeNi₃ electrode demonstrates a higher current density, indicating a faster rate of sulfide oxidation reaction. Subsequently, the electrocatalytic performances of HER for Fe₂NiSe₄/FeNi₃, NiSe/NF, Pt plate, and bare FeNi₃ foam were also evaluated. As illustrated in Figure 3c, Fe₂NiSe₄/FeNi₃ achieves a low overpotential of 0.14 V to achieve a current density of 20 mA cm⁻², the best performance among several catalysts, superior to NiSe/NF and commercial Pt plate electrodes. Strikingly, Fe₂NiSe₄/FeNi₃ provides a current density of 100 mA cm⁻² at an overpotential of 0.31 V. The above electrochemical performance suggests that Fe₂NiSe₄/FeNi₃ is an active catalyst toward both SOR and HER. Owing to the outstanding activity toward both HER and SOR, we used Fe₂NiSe₄/FeNi₃ as both anode and cathode for hydrogen sulfide and water electrolysis in an H-cell, respectively. As shown in Figure 3d, it can be found that when the current density reaches 100 mA cm⁻², the electrolytic splitting of H₂S saves 1.1 V compared to water splitting.

To understand the possible origins of the superior performance of the Fe₂NiSe₄/FeNi₃, the effective surface areas (ECSAs) of Fe₂NiSe₄/FeNi, NiSe/NF, and FeNi₃ foam were estimated by measuring the capacitances of the double layer at the solid–liquid interface of these electrodes. ECSA can be estimated from the electrochemical bilayer capacitance (C_dl). The cyclic voltammetric curves for various catalysts with scan rates of 20, 40, 80, 120, 160, and 200 mV s⁻¹ in the range of 0.27 to 0.37 V vs. RHE with non-faraday interval were tested. As shown in Figure S5, the capacitances of Fe₂NiSe₄/FeNi₃, NiSe/NF, and FeNi₃ foam are evaluated at 37.96 mF cm⁻², 28.41 mF cm⁻², and 3.15 mF cm⁻², respectively, implying that Fe₂NiSe₄/FeNi₃ exhibits enlarged electrochemically active surface area with the possibilities of exposing more active sites. Moreover, the impedance characteristics of different materials were investigated. A smaller semicircle in the high-frequency region reflects lower charge transfer resistance (R_ct). As depicted in Figure 3e, the Nyquist plots indicate that Fe₂NiSe₄/FeNi₃ exhibits the smallest semicircle radius, demonstrating the lowest charge transfer impedance. Therefore, it has better charge transport capability and faster catalytic dynamics.

It is essential to evaluate the long-term stability of a catalyst electrode to assess its quality for practical applications. We performed chronoamperometric measurements at 0.4 V constant potential for 800 min of Fe₂NiSe₄/FeNi₃ catalyst. Figure 3f illustrates that the current density of the Fe₂NiSe₄/FeNi₃ electrode remained relatively stable at about 28 mA cm⁻² in NaOH with Na₂S electrolyte. To further assess the stability, we also compared the LSV curves before and after the long SOR, as shown in Figure S6a. We observed a slight degradation in electrocatalytic performance after prolonged reaction, indicating that the formation of polysulfides can affect the performance of the electrocatalyst during long-term operation. In contrast, the catalytic performance was restored by stirring the electrolyte at 100 rpm to enhance mass transfer, as shown in Figure S6b. These findings indicate that polysulfides can affect the long-term performance of electrocatalysts. When polysulfides are redistributed into the electrolyte, the catalyst resumes high performance. The morphology of the catalyst post-reaction was examined. As shown in Figure S7, the nanowires array structure maintained its integrity. However, previous studies have reported that the electrode material surface is easily passivated by electro-oxidized product of elemental sulfur and thereby stops the electrocatalytic reaction. We explored the contact angle of different materials with liquid sulfur to verify their sulfurophilic or sulfurophobic properties. The results show that the contact angles of Fe₂NiSe₄/FeNi₃, NiSe/NF, and FeNi₃ for liquid sulfur are 126°, 105°, and 68°, respectively (Figure S8). The Fe₂NiSe₄/FeNi₃ shows a larger contact angle, proving that Fe₂NiSe₄/FeNi₃ is a sulfurphobic material. So, the Fe₂NiSe₄/FeNi₃ nanowires array retained steady activity within the process of SOR.

3.3. Products Tests

To investigate the actual yield and reaction process of electrocatalytic decomposition of H₂S, the post-electrolytic practical product was studied. Firstly, we used the external standard method to calibrate the reaction cell to quantify and then carried out a galvanostatic test at a current density of 100 mA cm⁻². The gas produced at the cathode was confirmed by gas chromatography to be H₂. As the reaction proceeded, the H₂ production rate was monitored in real time by gas chromatography every 10 min. The calculation result indicates that the H₂ production rate is maintained at about 0.68 mL min⁻¹ cm⁻², and the faradaic efficiency of H₂ production achieves 98% (Figure 4a,b). For the anode, during the progress of the reaction, the color of the electrolyte changed from clear to yellow. The products were studied in the electrolyte by a UV–vis spectrophotometry system. The peak at 293 nm can be reduced to S_n²⁻ (Figure 4c). With the longer electrolysis time, the signal assigned to S_n²⁻ is a more obvious strength, revealing that S²⁻/HS⁻ has been selectively converted to S_n²⁻. Furthermore, we acidified the anode electrolyte to obtain a yellow powdered product (Figure 4d). The product characterization by XRD identified sulfur. Therefore, electrochemical splitting of H₂S not only decreases energy consumption for desulfuration but also produces valuable H₂ and sulfur.

4. Conclusions

In summary, electrocatalytic decomposition of H₂S into high-value H₂ and sulfur is a promising approach for the utilization of H₂S resources. We successfully prepared a Fe₂NiSe₄/FeNi₃ electrocatalyst that was demonstrated to be a highly active bifunctional catalyst for energy-saving electrolytic H₂ generation for H₂S electrolysis. For the anodic SOR, the electrode provides an extremely low onset potential of 0.33 V vs. RHE, and the current density reaches 100 mA cm⁻² at 0.44 V vs. RHE. At the cathode, we measured the H₂ generation rate of about 0.68 mL min⁻¹ cm⁻² at a current density of 100 mA cm⁻², and the faradaic efficiency of H₂ was around 98%. This study demonstrates that the Fe₂NiSe₄/FeNi₃ electrocatalyst can facilitate highly efficient electrochemical splitting of H₂S, yielding valuable chemical sulfur and simultaneous production of energy-saving H₂ production, offering promising features for potential use as an electrode in technological devices.