Superaerophilic/superaerophobic cooperative electrode for efficient hydrogen evolution reaction via enhanced mass transfer

Abstract

Hydrogen evolution reaction (HER), as an effective method to produce green hydrogen, is greatly impeded by inefficient mass transfer, i.e., bubble adhesion on electrode, bubble dispersion in the vicinity of electrode, and poor dissolved H₂ diffusion, which results in blocked electrocatalytic area and large H₂ concentration overpotential. Here, we report a superaerophilic/superaerophobic (SAL/SAB) cooperative electrode to efficiently promote bubble transfer by asymmetric Laplace pressure and accelerate dissolved H₂ diffusion through reducing diffusion distance. Benefiting from the enhanced mass transfer, the overpotential for the SAL/SAB cooperative electrode at −10 mA cm⁻² is only −19 mV, compared to −61 mV on the flat Pt electrode. By optimizing H₂SO₄ concentration, the SAL/SAB cooperative electrode can achieve ultrahigh current density (−1867 mA cm⁻²) at an overpotential of −500 mV. We can envision that the SAL/SAB cooperative strategy is an effective method to improve HER efficiency and stimulate the understanding of various gas-involved processes.

A superaerophilic/superaerophobic cooperative strategy is shown to boost hydrogen evolution reaction via enhanced mass transfer.

INTRODUCTION

Electrocatalytic hydrogen evolution reaction (HER) is one of the hottest research fields due to the global efforts in exploring sustainable energy (1–10), of which efficiency is greatly impeded by mass transfer (11), especially at high current density. For HER in acid media (2H⁺ + 2e⁻ → H₂), three critical steps are sequentially coupled with each other and strongly affect HER performance (12, 13): (1) H⁺ transfer from the electrolyte to the interface of the electrocatalyst; (2) electrocatalytic reaction at the electrode interface, including reactant adsorption, interfacial electron transfer, and intermediate/product desorption; and (3) mass transfer of products from the electrode interface to the electrolyte or atmosphere, including gaseous bubble transfer and dissolved hydrogen diffusion (14, 15). Great efforts usually focused on step 2, and numerous elegant electrocatalysts with optimized electronic structures and surface morphologies have been developed to improve HER efficiency (16–26). Nevertheless, HER performance is still greatly affected by mass transfer processes in steps 1 and 3 (27–29), which is gradually becoming the bottleneck in further improving HER performance. The negative influences of mass transfer in step 3 can be generally divided into three categories (30): (i) Bubble adhesion on the electrode surface will isolate electrocatalytic sites from the electrolyte and result in dead areas (29); (ii) bubble dispersion in the vicinity of the electrode will generate volume fraction in electrolyte and lower H⁺ transfer efficiency in step 1 (31); and (iii) oversaturated dissolved H₂ close to the electrode surface, caused by inefficient H₂ diffusion, will result in large concentration overpotential (28, 32). There are several methods for accelerating the detachment of bubbles on electrodes, such as electrolyte-flow cell (27, 33), magnetic field (34), ultrasonic field (35), and supergravity field (36). Regardless of complex equipment and additional energy consumption, detached H₂ bubbles will result in bubble dispersion in the vicinity of the electrode, and the issue of oversaturated H₂ at the electrode still exists, both of which need to be efficiently addressed. In recent years, wettability-based interface engineering has been developed and applied in gas-involved electrodes (11). For example, aerophilicity-based electrodes have been reported to improve the performance of gas consumption reactions, which can increase the concentration of gas reactants at electrode interface and thus improve electrode efficiency (37–39). As for gas evolution reaction, aerophilic electrodes are no longer suitable, owing to the “dead area” caused by serious bubble adhesion, while aerophobic electrodes can efficiently accelerate bubble detachment and improve electrode performance owing to its low adhesive force to gas bubble (40–42). Although achieving fast detachment, bubbles are directly released from the aerophobic electrode to the electrolyte and cannot be timely removed from the reaction system. Meanwhile, owing to the low diffusion coefficient and the large diffusion distance from the electrode to the atmosphere, dissolved gas molecules at the electrode interface can easily accumulate and cause large concentration overpotential. Consequently, efficient strategies to enhance mass transfer, i.e., removing H₂ bubbles directly from the reaction system and lowering H₂ concentration at the electrode interface, are still urgently needed to promote HER.

Here, we have developed a superaerophilic/superaerophobic (SAL/SAB) cooperative electrode composed of SAL stripes and SAB electrocatalytic regions, which can efficiently enhance mass transfer, involving fast bubble transfer and efficient dissolved H₂ diffusion. The SAL stripes, which will be covered by gas cushion underwater, were designed as a gas channel to absorb H₂ bubbles and directly transport them out of the reaction system. Because of the interconnection between SAL stripes and ambient air, the oversaturated H₂ molecules could also timely diffuse into air through the SAL stripes, which can greatly lower the concentration of dissolved H₂ adjacent to the electrode. The SAB electrocatalytic region was where HER took place and favorable for the detachment of H₂ bubbles. With the cooperation of superaerophilicity and superaerophobicity, H₂ bubbles that were generated from the electrocatalytic region and H₂ molecules that dissolved in the electrolyte could be timely removed out of the reaction system through the adjacent SAL stripes, and H⁺ transfer from bulk electrolyte to the electrocatalytic region was thus improved. As a result, the SAL/SAB cooperative electrode achieved −10 and −100 mA cm⁻² at an overpotential of −19 and −80 mV in 0.5 M H₂SO₄, compared to −61 and −511 mV on the flat platinum (Pt) electrode. In 4 M H₂SO₄, the SAL/SAB electrode achieved ultrahigh current density (−1867 mA cm⁻²) at an overpotential of −500 mV. The present study provides an efficient approach to boost mass transfer and improve HER performance, which offers a universal guideline for the design of various gas evolution electrodes. The mechanism study in this work could also stimulate the understanding of other gas-involved processes, e.g., boiling heat transfer, electrochemical CO₂ reduction, microfluidics, and so on.

RESULTS

The SAL/flat Pt electrode with intensified HER through enhancing mass transfer

Owing to the relatively clear catalytic mechanism for HER (43, 44), we selected Pt to conduct the proof-of-concept experiments. Figure 1A is the optical image of the SAL/flat Pt electrode composed of SAL stripes and a flat Pt electrode, which was partially immersed in 0.5 M H₂SO₄ to make sure that the SAL stripes connect with atmosphere. A glass vessel was placed above the electrode as an H₂ collector. The scanning electron microscopy (SEM) image in Fig. 1B also reveals the composition of the SAL/flat Pt electrode. The corresponding wettability characterizations show that the flat Pt is aerophobic with an H₂ bubble contact angle of 121.4° ± 5.0°, and the SAL stripe is SAL with an H₂ bubble contact angle of ~0°. The adhesive force of the H₂ bubble on the flat Pt surface is ~143 μN (fig. S1A), showing high adhesive force to H₂ bubbles, while the SAL stripes are composed of numerous SAL nanoparticles (Fig. 1C), which endow the SAL stripes with a much higher bubble adhesive force than that of the flat Pt surface (fig. S1B). The laser confocal scanning microscopic (LCSM) images can clearly reveal the existence of gas cushion on the SAL stripes (fig. S2). In air, the average height at the SAL region is smaller than ~75 μm (fig. S2A). Under water, the average height at the SAL region is clearly increased to ~100 μm, indicating the generation of gas cushion (fig. S2B). As shown in Fig. 1D, the H₂ bubbles are generated at the aerophobic flat Pt region and can be transported to an H₂ collector through SAL stripes. For noncontacted bubbles, the gas cushion at the SAL stripe can greatly reduce H₂ diffusion distance and promote dissolved H₂ diffusion (Fig. 1E). As they grow, H₂ bubbles will contact the SAL stripes and be timely transferred through SAL stripes under the drive of asymmetric Laplace pressure between bubble and gas cushion (Fig. 1F). Because of efficient bubble transfer and dissolved H₂ diffusion, inactive electrocatalytic sites could be reactivated for HER, and the concentration overpotential caused by dissolved H₂ could also be greatly decreased. The bubble transfer process on the SAL/flat Pt electrode can be clearly observed in Fig. 1G and movie S1. At first, H₂ bubble 1 makes contact with the left SAL stripe (i) and starts to merge into it (ii). Then, the gas channel at the SAL stripe turns to an ON state (iii), whereby the H₂ bubble can be transferred into a gas collector (iv). After that, the gas channel turns off (v), and the SAL/flat Pt electrode revives the Pt region to continue HER (vi). With a similar process, H₂ bubble 2 is also successfully transferred into the gas collector through the right SAL stripe (iii and vi). After the efficient transportation of H₂ bubbles, the flat Pt region revives the fresh electrode surface to continue HER, which was beneficial for improving HER efficiency.

Fig. 1. The SAL/flat Pt electrode with intensified HER through mass transfer enhancement.

(A) Optical image of the SAL/flat Pt electrode composed of SAL stripes and Pt. A glass vessel was placed above the electrode as an H₂ collector. (B) SEM image and wettability characterization of the SAL/flat Pt electrode, showing superaerophilicity of the SAL stripe with a bubble contact angle of ~0° and aerophobicity of Pt with a bubble contact angle of 121.4° ± 5.0°. (C) High-magnification SEM image of the SAL stripe, which is composed of numerous SAL SiO₂ nanoparticles. (D) Schematic of the SAL/flat Pt electrode with enhanced mass transfer. The H₂ bubbles generating on Pt region can be transported to the H₂ collector (indicated by yellow arrows) through SAL stripes. (E) First, H₂ bubbles are not making contact with the SAL stripe, and the dissolved H₂ can diffuse out the reaction system (indicated by green arrows) through gas cushion on the SAL stripe. (F) As they grow, H₂ bubbles will contact the SAL stripes and be timely transferred through SAL stripes. (G) In situ optical observation of bubble transfer (indicated by red arrows) on the SAL/flat Pt electrode. First, H₂ bubble 1 makes contact with the left SAL stripe (i) and starts to merge into the SAL stripe (ii). Then, the gas channel at the SAL stripe turns to an ON state (iii), whereby the H₂ bubble can be transferred into a gas collector (iv). After that, the gas channel turns off (v) and the SAL/flat Pt electrode revives the Pt region to continue HER (vi). With a similar process, H₂ bubble 2 is also successfully transferred into the gas collector through the right SAL stripe (iii to vi).

Influence of the SAL stripe width and Pt electrode width on bubble transfer and H₂ diffusion

The SAL stripe width (SW) has an important influence on the transfer process of H₂ bubbles (Fig. 2A). For example, an H₂ bubble with a diameter of ~500 μm failed to be transferred to an electrode with an SW of 250 μm but was successfully transferred to an electrode with an SW of 750 μm in 14 ms (Fig. 2B). In addition to SW, the diameter of the H₂ bubble also determines its transfer efficiency, both of which are carefully revealed in Fig. 2C. When SW is about 250 μm, only bubbles with diameters less than 500 μm can be quickly transferred (≤50 ms), while bubbles with diameters larger than 500 μm tend to be slowly transferred (>50 ms) or pinned on the SAL stripe. For an SW larger than 500 μm, bubbles with diameters ranging from ~125 to ~1000 μm can be timely transferred. The asymmetric Laplace pressure Δp between a gas bubble adhered to the Pt electrode region and gas cushion trapped on the SAL stripe is the driving force for bubble transfer, which can be learned as follows (fig. S3 and note S1)

$$\mathrm{\Delta }p=\frac{2\gamma }{R_{\mathrm{bubble}}}-\frac{\gamma }{\frac{\delta }{2}+\frac{w^2}{8\delta }}$$ (1)

where γ is the surface tension of water, δ is the thickness of gas cushion, R_bubble is the radius of bubble, and w is SW. According to Eq. 1, a smaller bubble volume and a larger SW result in a larger Δp to promote bubble transfer. However, the SAL stripe with a larger width will consume more surface area of the electrode. In consideration of bubble transfer capability and electrode surface utilization, a SAL stripe with a width of 500 μm was selected in the further design of the SAL/SAB cooperative electrode for HER.

Fig. 2. Influence of SAL width (SW) and Pt electrode width (EW) on bubble transfer and H₂ diffusion.

(A) Schematic of the influence of SW on bubble transfer from the Pt electrode to the SAL stripe. (B) Optical images of bubble pinning on the electrode with an SW of 250 μm and bubble transferring on the electrode with an SW of 750 μm (indicated by red dashed circles and red arrows). (C) Statistics of the transfer time of H₂ bubbles with different diameters on the SAL stripes with various SWs. On the SAL stripe with an SW of 250 μm, lots of H₂ bubbles will spend much time (>50 ms) to transfer or pin on SAL stripes. (D) Schematic of the influence of EW on bubble growing and transferring. (E) Optical images of H₂ bubbles on the blank electrode (without SAL stripe) and the SAL/flat Pt electrode with an EW of 500 μm. H₂ bubbles cannot be timely removed on the blank electrode, while the SAL/flat Pt electrode is capable of transferring H₂ bubble timely (indicated by red dashed circles and red arrows). (F) Variation of the current density of the SAL/flat Pt electrodes with various EWs at an overpotential of −350 mV. (G) Simulation of H₂ diffusion on the blank Pt electrode. The maximal H₂ concentration at the Pt interface can reach as high as ~0.66 M. (H) Simulation of H₂ diffusion on the SAL/flat Pt electrode. The H₂ concentration can be lowered to ~0.29 M, showing enhanced H₂ diffusion. (I) Simulation of H₂ diffusion on the SAL/flat Pt electrodes with various EWs. The SAL/flat Pt electrode with minor EW is favorable for dissolved H₂ diffusion.

The electrode width (EW) also plays a critical role in bubble transfer (Fig. 2D). As shown in Fig. 2E, H₂ bubbles grow up and adhere to the blank electrode, i.e., flat Pt without SAL stripes. After introducing the SAL stripes, the size of the generated H₂ bubble is greatly restricted, which is beneficial for its coalescence with gas cushion at the SAL stripes. Consequently, H₂ bubbles generated on the SAL/flat Pt electrode can be timely transported out of the reaction system, which can revive the electrode surface and promote HER. In situ bubble incubation and transfer behaviors were recorded as a time-dependent current curve with obvious current fluctuation, which corresponds to bubble transfer behaviors, indicating that HER performance is greatly influenced by bubble transfer (fig. S4). On the blank electrode (flat Pt), H₂ bubbles are not confined and apt to adhere to the electrode surface, which resulted in a dead area on the electrode and consequently achieved the lowest current density of ~−82.8 mA cm⁻² (Fig. 2F and fig. S5). After introducing the SAL stripes, the current densities of the electrodes are first increased with the decrease of EW, i.e., ~−140.1, ~−153.1, ~−212.9, and ~−288.7 mA cm⁻² for 1000, 750, 500, and 250 μm, respectively. This phenomenon arises from the fact that a smaller EW can confine bubble size more efficiently to promote H₂ bubble transfer in their minor sizes, which can greatly alleviate the issue of bubble adhesion to electrode surfaces. However, when the EW is decreased to 150 μm, the current density decreases to −210.8 mA cm⁻² because of the partial coalescence of gas cushion (fig. S6), which blocks the direct contact of the electrode with the electrolyte (detailed discussion can be found in figs. S7 to S9 and note S2). Consequently, 250 μm is the optimal EW for the further design of the SAL/SAB cooperative electrode.

In addition to the timely removal of H₂ bubbles from the reaction system, the SAL stripes are also supposed to decrease the dissolved H₂ concentration near the electrode interface. Here, finite element simulation was used to investigate the diffusion of dissolved H₂ molecules (see details in Materials and Methods). Figure 2G shows H₂ concentration distribution near a flat Pt electrode with a width of 250 μm, where the generated H₂ molecules can freely diffuse into the bulk liquid. The simulated H₂ concentration at the electrode interface can reach as high as ~0.66 M. Owing to the existence of gas cushion at the introduced SAL stripes, H₂ diffusion distance on the SAL/flat Pt electrode is dramatically decreased, which can enhance H₂ diffusion rate according to Fick’s first law and result in low H₂ concentration (0.29 M) at the Pt electrode interface (Fig. 2H and fig. S10). Consequently, the concentration overpotential (η_c) caused by dissolved H₂ could be reduced according to Eq. 2 (28, 32)

$${\eta }_c=\frac{RT}{nF}\mathrm{In}\frac{C_{{\mathrm{H}}_2}}{C_{{\mathrm{H}}_2}^{\mathrm{sat}}}$$ (2)

where R is the molar gas constant, T is the temperature, n is the number of electrons participating in the electrode reaction, F is the Faraday constant, C_H2 is the dissolved H₂ concentration at the electrode interface, and $\$C_{{\mathrm{H}}_2}^{\mathrm{sat}}\$$ is the saturation concentration of H₂. The peak H₂ concentration was decreased with the reduction of EW (Fig. 2I), indicating that minor EW is favorable for the diffusion of dissolved H₂. However, an EW of less than 250 μm was not considered because of the coalescence of adjacent gas cushion, which would impede the contact of the electrode with the electrolyte and result in a negative influence on HER (Fig. 2F and fig. S6).

Influence of electrode region’s wettability on HER

The wettability of the electrode region also has a great influence on bubble behavior and HER performance. Figure 3 (A to D) shows the bubble behavior on the flat Pt electrode, the SAB Pt electrode, the SAL/flat Pt electrode, and the SAL/SAB Pt electrode, respectively. Owing to the large adhesive force on H₂ bubbles, serious bubble adhesion to the flat Pt is clearly observed, which results in a large dead area, i.e., bubble coverage (Fig. 3A). Meanwhile, H₂ bubble timely detaches from the nanostructured SAB Pt electrode, which is attributed to its low bubble adhesive force (fig. S11). Although the issue of bubble coverage can be greatly avoided, numerous released H₂ bubbles still exist near the electrode interface, i.e., bubble dispersion, which also negatively influences the mass transfer in HER (Fig. 3B). On the SAL/flat Pt electrode, the gas cushion at the SAL stripes can provide an avenue to efficiently enhance the diffusion of dissolved H₂ but only partially address the issue of bubble coverage because of the large adhesive force of flat Pt to the generated H₂ bubbles (Fig. 3C). Nanostructured Pt with superaerophobicity is adapted in the SAL/SAB cooperative electrode (marked as SAL/SAB Pt; Fig. 3D). Benefiting from the low adhesive force of nanostructured Pt, the generated H₂ bubbles are efficiently transferred through the SAL stripes in 2 ms, and, consequently, only few H₂ bubbles could be observed on the SAB Pt region, indicating that the SAL/SAB Pt electrode can achieve more efficient bubble transfer than the SAL/flat Pt electrode.

Fig. 3. H₂ bubble behaviors and electrochemical tests on the flat Pt electrode, the SAB Pt electrode, the SAL/flat Pt electrode, and the SAL/SAB Pt electrode.

(A to D) Optical images and schematics of H₂ bubble behaviors on these four electrodes. Serious H₂ bubble adhesion can be observed on the flat Pt electrode (A), while H₂ bubbles can timely detach from the nanostructured SAB Pt electrode (B). Introducing SAL stripes can facilitate H₂ bubble transfer on the SAL/flat Pt electrode (C). SAL stripes, coupled with superaerophobicity of nanostructured Pt, further promote bubble transfer on the SAL/SAB Pt electrode, which can achieve ultrafast bubble transfer in 2 ms (D). (E and F) HER polarization curves (E) and current densities of these four electrodes at −50, −100, −300, and −500 mV versus RHE (F). The SAL/SAB Pt electrode exhibits the best HER performance. (G) Tafel plots of these four electrodes.

We carefully characterized the HER performances of the abovementioned electrodes in 0.5 M H₂SO₄. HER polarization curves in Fig. 3E and current densities at various overpotentials in Fig. 3F show that the SAL/SAB Pt electrode has the best HER performance. At an overpotential of −300 mV, the current densities of the flat Pt electrode, the SAB Pt electrode, the SAL/flat Pt electrode, and the SAL/SAB Pt electrode are ~−47.7, ~−149.5, ~−231.3, and ~−471 mA cm⁻², respectively. In addition, similar Tafel slopes of 26.3, 29.4, 30.2, and 30.1 mV dec⁻¹ are achieved on the SAL/SAB Pt electrode, the SAL/flat Pt electrode, the SAB Pt electrode, and the flat Pt electrode (Fig. 3G), suggesting the Volmer Tafel mechanism reported earlier for HER on Pt (45).

The Ti-based SAL/SAB cooperative electrode with an excellent HER performance

To further advance our concept toward practical applications, we selected titanium (Ti) as the substrate and deposited Pt clusters at electrode regions between SAL stripes to achieve the Ti-based SAL/SAB cooperative electrode (marked as the Ti-based SAL/SAB Pt; Fig. 4A). Deposited Pt catalysts with pine cluster morphology are composed of single crystals (Fig. 4B and fig. S12), of which bubble contact angle is ~161.6°, exhibiting its SAB property (fig. S13). The generated H₂ bubbles can be timely transferred through the SAL stripes in 2 ms, showing its elegant capability of bubble transfer (Fig. 4C). Because of efficient bubble transfer and enhanced hydrogen diffusion, the Ti-based SAL/SAB electrode exhibits distinguished HER performance in 0.5 M H₂SO₄, which only requires an extremely low overpotential of −19 mV to reach the current density of −10 mA cm⁻², compared to −49 mV for the Ti-based SAB Pt electrode (Fig. 4D). The current densities of the Ti-based SAL/SAB Pt electrode at various overpotentials are also much higher than that of the Ti-based SAB Pt electrode, showing excellent HER performance (Fig. 4E). The Tafel tests demonstrate that the Ti-based SAL/SAB Pt has much faster HER kinetics than the Ti-based SAB Pt electrode. For example, the Tafel slope of the Ti-based SAL/SAB Pt electrode is 25.9 mV dec⁻¹, while for the Ti-based SAB Pt, the Tafel slope is 36.8 mV dec⁻¹ (Fig. 4F). The Ti-based SAL/SAB electrode also shows great stability at −500 mA cm⁻² with little attenuation of ~29 mV for at least 30 hours (fig. S14).

Fig. 4. The Ti-based SAL/SAB cooperative electrode with excellent HER performance.

(A) Schematic of the Ti-based SAL/SAB electrode. The SAL stripes and SAB Pt are introduced on the Ti substrate. (B) SEM image of the electrodeposited Pt cluster on the Ti substrate. (C) Bubble transfer on the Ti-based SAL/SAB Pt electrode; H₂ bubble can be timely transferred in 2 ms. (D and E) HER polarization curves and current densities at various overpotentials. The Ti-based SAL/SAB Pt electrode exhibits better HER performance than the Ti-based SAB Pt. (F) Tafel plots of the Ti-based SAL/SAB electrode and the Ti-based SAB Pt electrode in 0.5 M H₂SO₄. (G) Comparison of overpotential at 10 mA cm⁻² (red columns, left y axis) and Tafel slope (blue columns, right y axis) for various efficient HER catalysts in 0.5 M H₂SO₄ solution. Values were plotted from references (table S1) (46–55). (H) HER polarization curves of the Ti-based SAL/SAB electrode in solutions with various H₂SO₄ concentration, which exhibits the best HER performance in 4 M H₂SO₄. (I) Comparison of the current densities of the Ti-based SAL/SAB Pt electrode with various 2D materials, single atoms, or Pt-based HER catalysts under different potentials. Values were plotted from references (table S2) (40, 42, 48–50, 59–64). (J) Stability test of the Ti-based SAL/SAB Pt electrode in 4 M H₂SO₄ at −500 mA cm⁻², which exhibits a stable HER performance over 10 hours with ~31 mV potential change.

By comparing with the recently reported electrocatalysts, the Ti-based SAL/SAB cooperative electrode presents competitive merits for HER (Fig. 4G and table S1) (46–55). Of note, to avoid divergence caused by oversimplified and overlooked quantitative and experimental issues in using foam-type electrodes, including surface area matters, capillary action, and foam type (56), only two-dimensional electrodes, such as glassy carbon, metal plate, and metal foil–based electrodes were selected for comparison. All work electrodes here were insulated at the back side, and the geometry area of the electrode region was used to normalize current to achieve current density. Toward practical applications, large current density at low overpotential is urgently required. According to Le Chatelier’s principle, improving reactant (H⁺ in HER) concentration is beneficial for accelerating the HER process. Consequently, HER performance of the Ti-based SAL/SAB Pt electrode was further explored in the electrolyte with various H₂SO₄ concentrations. As shown in Fig. 4H and fig. S15, the Ti-based SAL/SAB Pt electrode exhibits the best HER performance in 4 M H₂SO₄ electrolyte and can achieve −1867 mA cm⁻² at an overpotential of −500 mV, which is attributed to the highest conductivity and relatively high H⁺ concentration (57, 58). This value is almost the best in recently reported HER catalysts (Fig. 4J and table S2) (40, 42, 48–50, 59–64). The stability of the Ti-based SAL/SAB Pt electrode in 4 M H₂SO₄ was assessed by a long-term chronopotentiometry test with a current density of −500 mA cm⁻², which shows stable HER performance over 10 hours with only ~31-mV potential change (Fig. 4J). The bubble contact angles of SAL stripe and SAB Pt during the long-term test only have a minor change, which indicates stable wettability of the Ti-based SAL/SAB Pt (fig. S16). These electrochemical results illustrate that the fabricated SAL/SAB electrode has great potential to be used for industrial H₂ production.

DISCUSSION

We have developed a SAL/SAB cooperative electrode through integrating the SAL stripes with the SAB electrocatalysts, which can greatly enhance H₂ bubble transfer and promote dissolved H₂ diffusion. Enhanced H₂ bubble transfer can effectively address the issues of bubble coverage on electrode and bubble dispersion in electrolyte, which releases more active electrocatalytic sites and enables sufficient contact of electrode with electrolyte. Promoted dissolved H₂ diffusion is beneficial for lowering the H₂ concentration overpotential. Consequently, intensified HER performance was achieved by the SAL/SAB cooperative electrode.

Fundamentally, the improvement of the SAL/SAB cooperative electrode on HER benefited from fast bubble transfer and efficient H₂ diffusion. Although fast bubble transfer is observed on the designed electrode, more detailed microfluidic physics–focused investigations are further required to comprehensively understand the intrinsic mechanism of bubble transfer. For an in-depth understanding of the influence of SAL stripes on the dissolved H₂ diffusion, in situ characterizations of dissolved H₂ concentration at the electrode interface still need to be explored. Besides, the stability of gas cushion on SAL stripes is of vital importance on both bubble transfer and dissolved H₂ diffusion, which should be also improved toward practical applications. On the basis of this work, we can envision that the SAL/SAB cooperative strategy is an efficient method to improve HER efficiency and thus provide a previously unreported electrode design for other gas evolution reactions.

MATERIALS AND METHODS

Fabrication of the SAL/flat Pt electrode

We prepared the SAL/flat Pt electrodes in three major steps as shown in fig. S17. First, we fabricated the stripe-shaped polyethylene terephthalate (PET) mask on a flat Pt substrate. The front facet of the flat Pt plate (1.5 cm by 1.5 cm by 0.1 mm) was covered by a layer of PET film, while its back facet was treated by a polymer adhesive tape to be insulated. Then, the PET film was cut into a stripe-shaped mask by a CO₂ laser device. Second, we modified the Pt plate with the stripe-shaped PET mask to be SAL. The Pt plate with the PET mask was immersed into polydimethylsiloxane (PDMS) solution consisting of 1.0 g of PDMS [Dow corning SYLGARD184, with 10 weight % (wt %) curing agent] in 10 ml of n-hexane (Beijing Chemical Works) for 30 s. After the evaporation of n-hexane and after being cured at 80°C for 2 hours, the treated Pt plate with the stripe-shaped PET mask was decorated with the SAL SiO₂ particles (particle size, ~14 nm, Aerosil R202, Evonik Degussa Co.). Third, removing the PET mask out of the flat Pt substrate can be capable of achieving the SAL/flat Pt electrode. Of note, all work electrodes here were treated to be insulated at their back sides.

Fabrication of the SAL/SAB Pt electrode

We fabricated the SAL/SAB Pt electrode by electrodepositing pine-shaped Pt on the SAL/flat Pt surface with a deposition potential of −0.264 V versus Ag/AgCl. H₂PtCl₆ solution (3 mM, Aladdin) with KNO₃ (100 mM, Aladdin) was used as the electrolyte in the electrodeposition process (40). The working electrode was the SAL/flat Pt substrate, while the Pt plate was used as the counter electrode.

Fabrication of the Ti-based SAL/SAB Pt electrode

First, we polished the Ti substrate (1.5 cm by 1.5 cm by 0.5 mm) with sandpaper and sonicated it twice in ethanol to remove silicon particles. Next, the polished Ti substrate was etched in 10 wt % oxalic acid (Beijing Chemical Works) at ~90°C for 1 hour and then rinsed by ample deionized water. The SAL/flat Ti substrate was prepared by following the fabrication process of the SAL/flat Pt electrode. Then, the pine-shaped Pt was electrodeposited on the SAL/flat Ti substrate with a deposition potential of −0.264 V versus Ag/AgCl. H₂PtCl₆ (3 mM) solution with KNO₃ (100 mM) was used as electrolyte in the electrodeposition process. The working electrode was the SAL/flat Ti substrate, while the Pt plate was used as the counter electrode (fig. S18). Pt mass loading is ~4.5 mg cm⁻² on the Ti-based SAL/SAB Pt electrode.

Electrochemical measurements

We carried out all electrochemical measurements at room temperature in a traditional three-electrode system cell using a Pt and an Ag/AgCl electrode as the counter electrode and the reference electrode, respectively. These electrochemical experiments were conducted in a static state (without rotation) via an electrochemical workstation (CHI 660E, Chenghua, Shanghai). Note that the SAL/SAB cooperative electrode was partially immersed in electrolyte to make sure that the SAL stripes connect with atmosphere. Before the test measurements, hydrogen bubbles were blown into the electrolyte to eliminate the dissolved oxygen and maintain a fixed Nernst potential for the H⁺/H₂ redox couple. The working electrode was cycled at least five times in test electrolyte to achieve a stable cyclic voltammogram before our data were recorded. HER polarization curves with a scan rate of 10 mV s⁻¹ were conducted in H₂SO₄ solution and without iR compensation. Tafel slopes were derived from HER polarization curves obtained at 1 mV s⁻¹. The Ag/AgCl electrode was calibrated with respect to reversible hydrogen electrode (RHE) by using Pt plate as standard electrode (for example, Ag/AgCl 0.226 V versus RHE in 0.5 M H₂SO₄; see details in the “RHE calibration” section). Potentials without any special statement in this work were normalized versus RHE. For example, in the 0.5 M H₂SO₄ solution, E_RHE = E_Ag/AgCl + 0.226 V, where the E_Ag/AgCl is the applied potential (versus Ag/AgCl).

RHE calibration

We used the Ag/AgCl electrode as the reference electrode in all electrochemical measurements. The potentials at various H₂SO₄ concentrations were calibrated with respect to RHE. The calibration was performed in high-purity hydrogen–saturated electrolyte with a Pt plate as the working electrode. Cyclic voltammetry (CV) was performed at a scan rate of 1 mV s⁻¹, and the average of the two potentials at which the current crossed zero was taken to be the thermodynamic potential for the hydrogen electrode reactions (for example, calibration CV in 0.5 M H₂SO₄ is presented in fig. S19).

Finite element simulation

We performed finite element simulations by using the COMSOL Multiphysics (Comsol, Inc.) software. To simplify the physical model, the gaseous hydrogen bubble was not considered. H⁺ at the Pt electrode interface was reduced to H₂ at an overpotential of −350 mV. Then, the generated hydrogen molecules diffused into the bulk electrolyte and finally reached steady state (65, 66). Temperature and pressure at simulation were set to be 293.15 K and 1 atm, respectively. Because the gas cushion on the SAL stripes connects with atmosphere and electrochemical tests were measured in H₂-purged electrolyte, the dissolved hydrogen concentration at the SAL/electrolyte interface was set to 0.8 mM [saturation concentration of H₂ in 0.5 M H₂SO₄ at 1 atm and room temperature (66)]. The simulation geometry, mesh, and boundary conditions are provided in fig. S20.

Characterization

We obtained SEM images through a scanning electron microscope (SU8010, Hitachi, Japan). Bubble behavior on the electrode was recorded by a high-speed camera (i-speed 3, Olympus, Japan). Bubble contact angles were measured by using a video-based contact angle measuring device (OCA 20, DataPhysics, Germany). LCSM images were obtained through laser confocal microscopy (OLS-4500, Olympus, Japan). The adhesive forces between gas bubble and various surfaces, including the flat Pt, SAL coating, and pine-shaped Pt, were evaluated by a high-sensitivity microelectromechanical balance system (DCAT 11, DataPhysics, Germany). An optical microscope lens and a charge-coupled device camera system were used to take photographs during adhesive force measurement.

Supplementary Materials

This PDF file includes:

Figs. S1 to S20

Tables S1 to S3

Notes S1 and S2

Other Supplementary Material for this : manuscript includes the following:

Movie S1