Ni–P–PTFE cathode with low surface energy for enhancing electrochemical water softening performance

Despite the numerous advantages of the electrochemical water softening technique, practical commercial applications face challenges such as low water softening rates and difficulties in regenerating electrodes (Hasson et al. 2012). Researchers are exploring electrode materials to improve precipitation rates. On one hand, a 3D stainless steel (SS) wool cathode with a high specific surface area is conducive to electrochemical water softening (Sanjuán et al. 2019). On the other hand, enhancing the hydrogen evolution performance of the cathode can lead to improved precipitation rates. Transition metals, such as Ni, Mo, Fe, and Co (Qian et al. 2022; Yuan et al. 2022), are abundant and transition metal-based compounds formed with P, B, S, and N have excellent electrolytic water hydrogen evolution performance (Wu et al. 2023). Among these, Ni–P electrodes have been extensively studied due to their abundant active sites and long-term stability. The synthesis of NiMoP catalyst on NF (nickel foam) has demonstrated exceptional hydrogen evolution stability during prolonged electrolysis (Luo et al. 2023).

Another important issue to address is the performance of electrode regeneration. Initially, manual cleaning is time-consuming and labor-intensive. To improve this process, a regenerative device for automatic cleaning was developed, incorporating a scraper, air purging, polarity reversal, and pickling (Kraft et al. 2002; Yu et al. 2018; Jin et al. 2019). However, external separation devices did not effectively remove strongly adhering deposits, necessitating manual cleaning over time. Previous research introduced an electrolytic device with a three-dimensional SS mesh that demonstrated improved regeneration performance. At the same time, the hydrogen bubbles generated on the cathode surface accessorily scour the electrode for the purpose of cleaning during the electrolysis process (Luan et al. 2019; Li et al. 2020). However, hydrogen bubbles are not completely ideal for flushing, and the layer of scale thickens over time. Recently, Wang et al. (2024) introduces a vibration device to assist in electrode regeneration, but stubborn scale layers remained strongly bonded to the electrode surface, affecting the regeneration process. To enhance the removal rate of scale layers and decrease their adhesive force on the electrode surface, reducing the surface-free energy of the solid is crucial. In general, the lower the surface free energy of a solid, the weaker the binding force of the scale layer on it is, which is more favorable for electrode regeneration (Müller-Steinhagen & Zhao 1997). The cathode with low surface energy is one of the solutions to the electrochemical water softening cathode regeneration problem.

In this work, a Ni–P–PTFE electrode is used as the cathode along with shear force to improve water softening performance. The regeneration performance is also explored by electrochemical testing and material characterization. The aim of this work is to utilize the low surface energy performance to improve cathode regeneration and thus maintain the cathode water softening rate. Further, the calcium and magnesium ions in the circulating water are kept in a stable range.

Nickel sulfate hexahydrate as the main salt (NiSO₄·6H₂O), sodium hypophosphite monohydrate as the reducing agent (NaH₂PO₂·H₂O), acetate anhydrous (CH₃COONa) as the buffer, ammonia water for adjusting pH (NH₃·H₂O), acetone for pretreatment (C₃H₆O), hydrochloric acid for activating the electrodes (HCl), and ethyl alcohol (C₂H₅OH) were supplied by Tianjin Kemiou Chemical Reagent Co., Ltd. Citric acid monohydrate as complexing agent (C₆H₈O₇) was supplied by Shanghai Xilong Chemical Co., Ltd. Polytetrafluoroethylene concentrated dispersion (PTFE 60% suspension) was supplied by Shanghai Maclin Biochemical Technology Co., Ltd. #20 carbon steel sheet was supplied by Changzhou Qingli Energy and Chemical Co., Ltd. 304 SS plate was supplied by Hai Kuang Yang Steel Trading Co., Ltd

The electrochemical tests were performed on the SP-300 electrochemical workstation (SP-300 110-240 V 50/60 Hz 350 W Bio-logic, France). A standard three-electrode system was selected for testing, with saturated calomel electrode (SCE), platinum electrode and synthesized electrode as reference electrode, counter electrode, and working electrode, respectively. The electrolyte was a mixed solution of NaCl and NaHCO₃ (pH = 8.2) solution, and the test temperature was 25°C. The area of the electrode used for electrochemical testing is 1 × 1 cm². All prepared electrodes were pre-scanned by cyclic voltammetry (CV) until the curve was stable before the linear sweep voltammetry (LSV) test. The LSV was used to exhibit the hydrogen evolution reaction (HER) performance from E_ocp ∼ E_ocp −2.0 V vs. SCE at a scanning rate of 5 mV/s and the potential was corrected by iR (85%) compensated (Luo et al. 2023). The electrochemical impedance spectroscopy (EIS) was tested in the range of 0.1 Hz–100 kHz at a hydrogen evolution potential of −0.12 V vs. SCE.

As shown in Figure 1(b), the electrolytic device consisted of a DC source (MS305DS 0-30V 0-5A, MAISNG Electronics Co.) an electrochemical water softening cathode, a dimension stable anode (DSA) and a water tank. The tank was equipped with a simulated industrial circulating cooling solution including 0.5 and 2 L. While the device was in operation, perturbations were induced by stirring to simulate the state of circulating water flow. The two DSA anodes were aligned parallel and separated by 1.5–2 cm. The simulated solution was made of CaCl₂, MgSO₄, NaHCO₃, and NaCl dissolved in deionized water with analytical grade chemical reagents (Tianjin Comer Chemical Reagent Co., Ltd, China). The composition of the circulating cooling water is shown in Table S3. The simulated solution with total hardness was 700 mg/L as CaCO₃, with 350 mg/L each of calcium and magnesium hardness and alkalinity of 350 mg/L as CaCO₃(Shandong Jingbo Petrochemical Co.).The pH value was 8.2 ± 0.1, and the electrical conductivity was about 2,500 μS/cm. The cathode covered with a large area of scale was regenerated by using the device, as shown in Figure 1(c). Rotationally generated shear forces were used to simulate fluid flushing for electrode regeneration. The parameters of the device are shown in Table S4.

Total alkalinity was determined by ASTM standard method D1067 by titration with hydrochloric acid to an endpoint of pH = 4.3, and total hardness was determined by EDTA titration. Conductivity and pH were measured using a conductivity meter (WTW Multi 9310, Germany) and a pH/ISE meter (Lei Magnet PHS-3C, China). The performance of electrochemical water-softened cathodes was evaluated by softening rate, and the softening rate and energy consumption were calculated by bibliography (Wang et al. 2024). Cleaning efficiency is defined as the difference in the mass of fouling deposited by the two electrodes as a percentage of the mass of fouling deposited by one of the electrodes.

Elemental compositions and surface morphology were analyzed using an energy scattering X-ray spectroscopy (EDS) accompanying a field emission scanning electron microscope (FE-SEM, JEOL JSM-7900F, Japan). The contact angle (WCA) was measured by Contact Angle Meter (DSA25, Kruss, Germany). The crystalline structures were characterized by X-ray diffraction (Shimadzu Lab XRD-7000s) in 2θ range from 20° to 80° by 0.02° s⁻¹ steps, operated at 40 kV accelerating voltage and 40 mA current using Cu Kα radiation source.

The presence of PTFE particles embedded in the Ni–P plating is confirmed by a small peak near 2θ of 18° as shown in Figure 2(e) and 2(f) (Sheu et al. 2017). A noticeable spike at 45° indicates a diffraction peak associated with the substrate iron atoms, likely due to the thin deposit scale layers. The crystal structure of the composite plating remains unaffected by the incorporation of PTFE particles, consistent with previous literature reports (Sheu et al. 2017). Surface-free energy, an essential physical–chemical property for solid surfaces, significantly influences wettability. The hydrophobic angle of the Ni–P electrode is roughly about 87°, while the Ni–P–PTFE electrode reaches around 138°, demonstrating super hydrophobic characteristics. The rough morphology of the superhydrophobic electrode surface and the waxy, non-polar coating, similar to that of a lotus leaf, form a layered double layer that minimizes the adhesion of water droplets on the surface (Lee et al. 2022). Consequently, Ni–P–PTFE effectively delays the crystallization induction period and reduces the scale formation rate, showcasing good anti-scaling performance due to its low surface energy (Malayeri et al. 2009; Cheng et al. 2016).

The instantaneous potential of each electrode is evaluated in Figure 3(c) during the water softening process. The potential of the SS electrode ranges between −1.95 and −2.1 V whereas the Ni–P–PTFE electrode ranges between −1.65 and −1.75 V. Clearly, the Ni–P–PTFE electrode exhibits a more positive potential than the SS electrode, positioning it between the SS and Ni–P electrodes. This suggests that the SS electrode requires a more negative potential to reach the set current, while the Ni–P–PTFE electrode requires a more positive potential. At the same time, the SS and Ni–P electrode exhibit modest potential variations, while the Ni–P–PTFE electrode displays large potential oscillations during the electrochemical water softening process. When the potential is shifted negatively, a scale layer is deposited on the electrode surface; conversely, when the potential is shifted positively, the electrode surface peels off the scale layer. The more pronounced the potential oscillations, the better the electrode self-cleaning performance. Consequently, scales on the Ni–P–PTFE electrode surface easily drop off, maintaining a high effective area during the electrochemical water softening process (Wang et al. 2024). The Ni–P–PTFE electrode, with oscillations in the curves, demonstrates a wonderful self-cleaning performance compared to the SS and Ni–P electrodes.

The alkalinity index of the circulating water is also a crucial factor in maintaining consistent water quality (Michałowski & Asuero 2012). The changes in hardness and alkalinity in the circulating water for three types of electrodes under varying current densities are characterized in Figure 4(b)–4(e). The hardness and alkalinity line segments show a steeper incline for the Ni–P–PTFE electrode, whereas the Ni–P and SS electrodes gradually level off at the 4 h period. It indicates that the Ni–P–PTFE electrode still has a high precipitation rate after 4 h. In Figure 4(b)–4(e), it is evident that the hardness and alkalinity levels in the water decrease significantly for SS and Ni–P electrodes at the conditions of 30 and 50 A/m², indicating a higher precipitation rate on the electrodes compared to other current densities, Figure 4(f) shows a quicker decrease in hardness in the solution for the Ni–P–PTFE electrode as the current density increases from 20 to 50 A/m², leading to an enhanced precipitation rate. The amount of change in hardness varies from 52.64 to 84.3 mg/L. The amount of change in hardness and alkalinity at different current densities for each electrode is shown in Table S5. Additionally, it illustrates from Figure 4(g) that minimal variation in alkalinity in the solution between 30 and 40 A/m² for the Ni–P–PTFE electrode, from 69.66 to 71.55 mg/L, suggesting that increased current density does not significantly alter alkalinity levels. Considering these findings collectively, a current density of 30 A/m² is chosen for further research.

The Ni–P–PTFE electrode exhibits the highest K value, indicating superior self-cleaning performance in long-term experiments. At the same time, the K value of the Ni–P–PTFE electrode differs significantly from other electrodes within the first 4 h, with values of approximately 3.8 for Ni–P–PTFE, 3.0 for Ni–P, and 2.0 for SS. This highlights the superior self-cleaning performance of the Ni–P–PTFE electrode in the initial 4 h. Although the self-cleaning performance of each electrode diminishes over time, the Ni–P–PTFE electrode maintains a certain advantage. Therefore, the Ni–P–PTFE electrode, with its low surface energy, demonstrates remarkable self-cleaning performance for long-term operation. The modified superhydrophobic electrode will affect the mass transfer process at the electrode interface to a certain extent. The hydrophobic PTFE on the surface of the superhydrophobic electrode does not have electrical conductivity, and therefore it will reduce the hydrogen precipitation performance of the electrode, which will affect the cathodic OH⁻ production during electrolysis and thus reduce the electrode precipitation rate, but this paper focuses on the regeneration performance of the electrode and maintains the active area of the electrode to further maintain the ability of the electrode to operate for a long period of time by means of better regeneration performance.

Figure 5(c) and 5(d) displays the precipitation rate on Ni–P–PTFE electrode under various rotation speeds and the scale weight change after seven cycles at 30 A/m². From Figure 5(c), the precipitation rate on the Ni–P–PTFE electrode fluctuates over the seven cycles due to different levels of force applied during electrode scouring, ultimately maintaining a relatively stable precipitation rate. It can be seen from Figure 5(d) that the scale layer deposits on the Ni–P–PTFE electrode surface are thinner at 500 r/min, with a minor scale weight change of only 9.51 mg/cm². Conversely, scale weight changes of 14.75, 14.25, and 11.01 mg/cm² are observed at 300, 400, and 600 r/min, respectively, indicating that the Ni–P–PTFE electrode exhibits optimal cleaning efficiency at 500 r/min. Therefore, the recommended rotation speed is 500 r/min, corresponding to a linear velocity of 2.62 m/s.

Macroscopic images of three-electrode surfaces after the first, third, fifth, and seventh rotation cleanings are shown in Table 5. Ni–P–PTFE electrode possesses better self-cleaning performance than other electrodes. Most of the scales on the surface are peeled off with the rotation. Especially, the electrode surface is exposed to a large active region after the fifth rotation.

Table 5

The appearance of the electrode after the first, third, fifth, and seventh rotation

Cycle index .	SS .		Ni–P .		Ni–P–PTFE .

Cycle index .	SS .		Ni–P .		Ni–P–PTFE .

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The self-cleaning performance of the Ni–P–PTFE electrode is enhanced by its low surface energy, which also helps maintain a balance between water softening and regeneration. The electrode demonstrates a more pronounced self-cleaning effect during long cycles and at high current densities. Regeneration is improved through a combination of hydrogen bubble flushing and fluid shearing. In this study, the water softening performance of the electrode is strengthened by the regeneration performance of the electrode.

In this work, the electrochemical water softening and regeneration performances of low surface energy Ni–P–PTFE electrodes are studied. The results indicate that the Ni–P–PTFE electrode enhances water softening performance by improving regeneration performance. Self-cleaning is particularly effective at high current densities. The Ni–P–PTFE electrode shows high precipitation rate and relatively low energy consumption at 30 A/m². Regeneration performance of the Ni–P–PTFE electrode with low surface energy is further improved through assisted hydrogen bubble rinsing and shear force. During the last multiple cycles, scale layers on the electrode surface are easily removed during the electrochemical water softening process. Only 9.51 mg/cm² of scales are deposited on the electrode surface after seven cycles. The precipitation rate on the Ni–P–PTFE electrode gradually increases, stabilizing around 18 g/m²·h after several cycles, resulting in lower energy consumption during long-term operation. Therefore, it offers a novel idea to design electrochemical water softening cathode.

This work was supported by the Open Competition Mechanism to Select the Best Candidates in Xinjiang (NO. 2023JBGS01), the Sixth Division Wujiaqu City Science and Technology Plan Project (No. 2329), the National Natural Science Foundation of China (No. 52301078,NO. 21978036), the Major Science and Technology Special Project of Xinjiang Uygur Autonomous Region (No. 2022A01002-4, No. 20223101904).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.