Surfactant Improved Interface Morphology and Mass Transfer for Electrochemical Oxygen-Evolving Reaction

Abstract

The surface microstructure of a catalyst coating layer directly affects the active area, hydrophilicity and hydrophobicity, and the high porosity is desirable especially for solid–liquid–gas three-phase catalytic reactions. However, it remains challenging to customize catalyst distribution during the coating process. Here, we report a simple strategy for achieving ultrafine nanocatalyst deposition in a porous structure via introducing the surfactant into coating inks. For a proof-of-concept demonstration, we spin-coated the nanoscale IrO₂ sol with a surfactant of sodium dodecyl sulfate (SDS) onto the glassy carbon (GC) electrode for oxygen evolution reaction (OER). Due to the surfactant action, the deposited IrO₂ nanocatalyst is evenly distributed and interconnected into a highly porous overlayer, which facilitates electrolyte permeation, gas bubble elimination and active-site accessibility, thus affording high-performance OER in alkaline media. Particularly, the SDS-modified electrodes enable the industrial-level high-current-density performance via enhanced mass transfer kinetics. Such manipulation is effective to improve the coating electrodes’ catalytic activity and stability, and scalable for practical applications and suggestive for other gas-evolving electrodes.

1. Introduction

Energy and environmental problems are increasingly threatening the development of global human society, which call for a wave of revolutionary new energy technology [1]. Compared to fossil fuels, hydrogen with zero emission and high energy density is considered as an ideal energy carrier for the provision of end-use services [2,3]. Electrochemical water splitting combined with renewable electricity promises an eco-friendly technology for sustainable green hydrogen production [4]. Over the years, numerous studies have focused on the catalyst developments for hydrogen evolution reactions (HER) and oxygen evolution reactions (OER) [5,6,7]. These gas evolution reactions at the three-phase interface of solid, liquid and gas are uniquely complex, where gas bubbles may adhere to the electrode surface and lower the catalysis efficiency because of the dynamic blocking of active sites and mass transport [8,9]. Thus, regardless of the catalysts themselves, architectural engineering to facilitate electrolyte penetration and bubble removal in electrochemical gas evolution reactions is an important but rarely explored are [10]. Recently, researchers have found that surface porosity could endow the electrodes with superhydrophilicity (i.e., extreme electrolyte permeability) and superaerophobicity (i.e., extreme bubble repellency) [11], thus exhibiting superior catalytic activity and stability. In addition, the porous structure can definitely bestow the electrodes with a larger surface area with more accessible active sites. Along this line, the nano- and microfabrication approaches have been employed to design highly porous building blocks for superhydrophilic/superaerophobic electrodes, such as PtNiMo nanoarrays for HER [12], IrW nanochannels for OER [13], NiFe-layered double hydroxide (LDH) nanosheet arrays for OER [14] and Cu₃P microsheets [15] for overall water splitting. However, these approaches are difficult to scale up for practical application because they are material-specific and procedurally complex. In contrast, the spin-coating method to fabricate catalyst-loading electrodes is easy, low-cost, scalable and universally applicable to various types of nanocatalysts, no matter what the materials. However, for the spin-coating method, there is yet no effective strategy to control the catalyst landing architecture that especially features uniform porosity, and thus more attempts to manage overlayer microstructures are urgently needed for nanocatalyst-coated electrodes.

Herein, we developed a simple strategy for the realization of a high porosity nanocatalyst coating overlayer via the intervention of a surfactant in coating sols. As the model system, we used a commercial IrO₂ nanocatalyst to prepare four sols with a surfactant of sodium dodecyl sulfate (SDS) from 0 to 7 mg and coat them on glassy carbon (GC) electrodes as contrast electrodes for electrochemical OER. By comparison, the 5 mg surfactant of SDS makes the catalyst particles ultrafine in size and uniformly dispersed, inducing an ultrasmall catalyst unit with an interconnected porous and hydrophilic structure. This structure features ultrafine grains rich in active edge sites and high porosity favorable to electrolyte permeation and gas bubble elimination. On such highly porous and hydrophilic electrodes, oxygen readily evolved and quickly escaped in tiny bubbles, resulting in the significantly enhanced OER activity and stability especially at high current densities, compared with the other contrast electrodes coated with one conventional sol without SDS and two modified sols with other amounts of SDS. In short, this study describes a simple means of customizing an ultrasmall catalyst-unit-interconnected porous overlayer via spin-coating technology, amendable to universal and scalable schemes for nanocatalyst electrode designs.

2. Results and Discussion

To determine an optimal SDS dosage, we first examined the electrochemical polarization performance of IrO₂ electrodes prepared with different SDS amounts. As shown in Figure 1a, the effect of SDS intervention to improve performance is universal and especially obvious at higher current densities. Moreover, with the SDS dosage increasing, the electrode performance first enhances and then declines, whereby the best-performing ED5 represents an optimum 5 mg of SDS dosage. For example, the overpotentials at the current density of 200 mA cm⁻² are 505.1 mV, 504.5 mV, 469.8 mV and 476.8 mV, respectively.

Figure 2a–d show the SEM images of the IrO₂ catalyst films coating on ED0, ED3, ED5 and ED7. The XRD pattern of ED5 is shown in Figure 2e, where the peaks are consistent with the IrO₂ pattern (PDF#43-1019). ED5 displays more superfine and porous catalyst components than ED3, while catalysts on ED0 and ED 7 accumulate to combine into large block units. Additionally, the BET specific surface area of ED5 is 207.293 m² g⁻¹, while that of ED0, ED3 and ED7 is 50.262 m² g⁻¹, 103.472 m² g⁻¹ and 124.512 m² g⁻¹. These changes in morphology were due to the involvement of SDS, but the involvement of SDS did not change the structure of the IrO₂ catalyst. Figure 3a reveals the coexistence of O, Ir and S elements in the IrO₂ catalyst film coatings on ED3. Figure 3b,c reveal the coexistence of O, Ir, S and Na elements in the IrO₂ catalyst film coatings on ED5 and ED7. The existence of SDS at the electrodes of ED3, ED5 and ED7 was confirmed.

During the traditional film-making process, IrO2 catalysts generally accumulate to combine into large block units. By contrast, the catalyst components become superfine due to the involvement of 5 mg SDS surfactant, and they are interconnected to form porous networks. Obviously, such an ultrafine unit-interconnected porous structure can preferentially expose the boundary active sites and favor electrolyte infiltration and gas product overflow. Here, the modified involvement of the surfactant can keep the nanocatalysts independently dispersed during the spin-coating process, which is crucial for the formation of this characteristic structure. Therefore, we conducted further comparative research over ED0 and ED5 to shed light on the role of SDS in improving electrochemical OER performance.

The OER activities of ED0 and ED5 were evaluated by LSV measurements. As shown in Figure 4a, ED5 shows the earlier and faster polarization properties compared with ED0. The overpotentials required by ED5 to deliver current densities of 10 mA cm⁻² and 200 mA cm⁻² were 337 mV and 469.8 mV, respectively, which are much lower than that of ED0 (346 mV 505.1 mV). It is noteworthy that the OER performance improvement of ED5 becomes larger at higher current densities. Moreover, when the current density exceeds 100 mA cm⁻², the polarization curve of ED0 depicts obvious fluctuations, suggesting performance instability and attenuation upon strong catalytic bubbling. This is because more bubbles are created as the current increases, and the adhesion of evolving bubbles to the electrode dynamically shields the active sites, interfering with the catalytic dynamics, and violent bubbling can also cause the mechanical shedding of catalytic species leading to activity degradation. In contrast, ED5 exhibits stable polarization performance even at large current densities, indicating that its catalytic dynamics are not affected by strong bubbling. Accordingly, in Figure 4b, ED5 shows a smaller Tafel slope of 43 mV dec⁻¹ than that (48 mV dec⁻¹) of ED0, indicative of faster catalytic OER kinetics on ED5. The Nyquist plots of EIS spectra in Figure 4c show that ED5 possesses smaller semi-arcs compared to ED0, which reveals better charge transfer capability in ED5. From the photos of electrodes evolving bubbles (Figure 4d), it can be clearly seen that gas bubbles stick to the ED0 surface until a relatively large size, which seriously interferes with the re-exposure of active sites, whereas much smaller gas bubbles rapidly move away from the ED5 surface, guaranteeing the catalytic process. According to the mechanism of bubble evolution [16], the bubble formation and separation is the balance relationship between bubble buoyancy and adhesion force. The bubbles stick to the electrode surface until the buoyancy force overcomes the adhesion force, which is proportional to the third power of the bubble radius. So, the smaller the bubbles are, the lower the adhesion force caused bubble detachment. We hypothesized that the porous structure of ED5 with the involvement of SDS increased the curvature of the adhesion surface, meanwhile, SDS itself is a hydrophilic active agent, which also increased the hydrophilicity of the adhesion surface.

To demonstrate our hypothesis, we conducted the water contact angles test of ED0 and ED5. As expected, Figure 4e exhibits an ultra-fast water wetting process within 12 ms with vacuole diminution on the surface of the ED5 electrode while one vacuole remains on the surface of ED0 over 24 ms, which indicates the super-hydrophilicity of ED5 electrode favorable for electrolyte permeability.

Based on the solid–liquid–gas interface theory [17], a porous surface generally induces strong capillary force featured with twinned hydrophilicity and aerophobicity due to the interfacial energy balance [18], facilitating electrolyte diffusion and gas overflow. Hence, the ultrafine-catalyst-unit-interconnected porous surface, that is tailored by the SDS surfactant, is an important feature responsible for enhanced OER performance on ED5, especially at large current densities.

The stability for catalytic OER was evaluated by chronopotentiometry with a constant current density of 50 mA cm⁻² for ED0 and ED5 under the same conditions. As shown in Figure 5a, the overpotential of ED0 fluctuates obviously and increases rapidly after running for 5 h, while the overpotential of ED5 almost remains relatively stable for 25 h. Furthermore, after stability tests, the SEM imaging reveals the severe deformation of the ED0 surface caused by the shedding of coated catalysts. By sharp contrast, the porous structure constructed by ultrafine catalyst units is almost unchanged and SDS still existed on ED5, indicating an excellent durability of ED5 for long-term operation. From the last analysis, it can be inferred that under the continuous high current, a large number of large bubbles are generated on ED0, which need to overcome the high surface tension to separate from the electrode surface. This continuous process will cause damage to the surface structure of ED0. Therefore, we can summarize that ED5 has higher activity and better stability, benefiting from the surfactant in the coating sol regulating the nanocatalyst dispersion to form a porous and hydrophilic texture, which not only maximizes active-site exposure but also modulates surface hydrophilic and aerophobic characteristics.

3. Experimental Method

3.1. Materials and Chemicals

Sodium dodecyl sulfate, acetylene black powder, ethanol and Nafion solution were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). IrO₂ powder was purchased from Aladdin Industrial Co., Ltd. (Shanghai, China). All of these reagents were of analytical grade purity and used as received.

3.2. Preparation of Electrode

An amount of 5 mg of IrO₂ powder, 1 mg of acetylene black powder and 5 mg of SDS powder were added to a graduated cylinder, and then 300 µL of ethanol, 30 µL of 5 wt.% Nafion solution and 300 µL of deionized water (DIW) were successively injected with sonication for at least 30 min to form a homogeneous ink. The glassy carbon (GC) electrode was polished into a mirror finish and thoroughly cleaned before loading catalysts. Then, 6 µL of the catalyst ink was loaded onto a cleaned GC electrode (2 mm in radius, 0.1256 cm² of area) via spin-coating process, with a constant catalyst loading of 0.379 mg cm⁻². The as-fabricated catalyst film supported on GC electrode was dried at room temperature for 12 h in a vacuum oven.

The same procedure was used to fabricate the reference electrodes with different SDS dosages (x) varying from 0 to 7 mg, and thus the obtained different electrodes were denoted as EDx (x = 0, 3, 5, 7).

3.3. Structural Characterization

The morphology and elemental composition of samples were examined using scanning electron microscopy (SEM, S-4800II, Hitachi, Kyoto, Japan) with an accelerating voltage of 15 kV. X-ray diffraction (XRD) measurements were conducted on XRD diffractometer (XRD-7000, Shimadzu, Kyoto, Japan) with Cu Kα radiation (λ = 0.15406 nm) and a scan speed of a 5 °/min to study the crystal structure and the phase composition. The Brunauer–Emmett–Teller (BET)-specific surface areas were calculated from adsorption data with(ASAP 2420, Micromeritics, Norcross Ga, America). The surface wettability of the electrode was examined through a vacuolar contact manner tracked by a high-speed camera (Theta Lite, Biolin, Frölunda, Sweden).

3.4. Electrochemical Measurement

The electrochemical measurements of OER were performed on an electrochemical workstation (CHI 660E, CH Instruments, Inc., Shanghai, China) with a standard three-electrode configuration equipped, using 1 M KOH (pH = 13.6) as the electrolyte.

The catalyst-coated GC electrodes (ED0, ED3, ED5 and ED7) were employed as the working electrodes, an Ag/AgCl electrode and a graphite rod (Alfa Aesar, 99.9995%, CH Instruments, Inc., Shanghai, China) were used as the reference and counter electrodes, respectively. The linear sweep voltammetry (LSV) curves were recorded at a scan rate of 5.0 mV s⁻¹. The Tafel plots were depicted with the linear portion fitting the Tafel equation to determine the Tafel slopes. Electrochemical impedance spectra (CHI 660E, CH Instruments, Inc., Shanghai, China) were measured with frequency scan range from 100 kHz to 0.1 Hz at open-circuit voltage with 5.0 mV amplitude. The overall water-splitting measurements were carried out with the two electrodes (ED0 and ED5) via conventional three-electrode model. The durability tests were conducted in long-term chronopotentiometry (CHI 660E, CH Instruments, Inc., Shanghai, China) model with a constant current density of 50 mA cm⁻² for ED0 and ED5 under the same conditions. All the potentials were reported against the reversible hydrogen electrode (RHE) via a standard conversion formula: E_RHE = E_Ag/AgCl + 0.059 × pH + 0.1976.

4. Conclusions

In summary, we report a simple method to realize a highly porous nanocatalyst coating layer via the surfactant action in coating sols. The ED5 fabricated with an SDS surfactant of 5 mg features an ultrafine catalyst-unit-interconnected porous and hydrophilic structure. Such a unique structure is very beneficial to electrochemical reactions, especially for gas evolving catalysis, with the desirable advantages including preferred exposure of active sites, easy electrolyte penetration and gas product overflow. As a result, ED5 exhibits enhanced OER activity and stability especially at high current densities, compared with the contrast ED0 fabricated using conventional sol. This study describes a facile means of customizing ultrafine catalyst-unit-interconnected porous layer, which is universal and scalable for nanocatalyst electrode fabrication for practical applications in water electrolysis.