Introduction

Artificial photosynthesis is a sustainable method to generate hydrogen, which provides effective strategies for solving environmental issues and energy storage [1]. Water electrolysis powered by solar energy is mainly fed with high-purity water [2]. However, considering the scarcity of freshwater resources, especially in some arid desert scenarios, it is necessary to find easily available raw materials for hydrogen production by artificial photosynthesis. Therefore, the direct electrolysis of low-grade or saline water, avoiding the high energy consumption of desalination, breaks the restrictions of freshwater on artificial photosynthesis [3]. Photoelectrochemical (PEC) seawater splitting combines two of the most widespread and abundant renewable resources, seawater and solar energy, which is a promising conversion technology for energy sustainability [4, 5].

However, PEC seawater splitting systems suffer from severe corrosion and low efficiency because of the presence of several inorganic salts, organic matter, and microorganisms, especially high-halogen ion content in seawater [6]. A feasible strategy for suppressing the complicated chloride electro-oxidation chemistry on the photoanode is using alkaline seawater, especially by adjusting the electrolyte pH to a value higher than 7.5 [7, 8]. However, when using alkaline seawater, the performance of most semiconductor materials deteriorates because of the corrosion of cocatalysts from complex seawater compositions and photocorrosion of semiconductors in the aqueous solutions [9]. Moreover, electrodes work in a high-alkaline electrolyte (30 wt.% KOH) in conventional industrial water electrolysis scenarios, and the poor stability of semiconductors in alkaline environments directly limits their industrial applications.

A viable method for improving the efficiency and stability of the PEC alkaline seawater splitting system is to introduce a dense protective layer or cocatalysts on the photoelectrodes [10,11,12,13,14]. Stainless steel (SS), known for its anticorrosion properties in chloride-containing conditions, is an ideal candidate used as a protective layer or cocatalyst [15, 16], isolating the semiconductor from harmful ions in seawater and efficiently catalyzing the hydrogen evolution reaction due to the presence of transition metals [17,18,19]. Additionally, SS is considered ideal materials as cocatalysts for hydrogen production because of their abundant iron, nickel, and chromium contents, as well as their low cost. However, the preparation of SS films on the semiconductor surfaces remains a great challenge as it may introduce interfacial defects and other damage to semiconductor substrates.

Another effective strategy for improving the energy utilization efficiency of the PEC seawater splitting is combining solar energy conversion with industrially proven electrolyzer configurations that meet the requirements for industrial hydrogen production. A PEC flow cell can reduce the mass transfer resistance between electrodes, such as the bubble accumulation problem. However, combining photoelectrodes with a PEC flow cell remain a great challenge because applying conventional photoelectrodes in the flow cells are difficult because of the parasitic light absorption by thicker cocatalysts due to the front-illumination nature of photoelectrodes.

To explore the potential of SS materials as cocatalysts in seawater splitting, this paper describes the design and fabrication of a back-illuminated silicon photoanode, on which 304 SS and 316 SS films were sputtered by a direct current (DC) magnetron as the cocatalyst layer. The photoanodes coated with SS were tested in natural seawater with 30 wt.% KOH, which inhibited harmful side reactions with a high O2 Faradaic efficiency of over 95%. Consequently, a back-illuminated PEC flow cell was constructed with the photoanodes coated with SS, reducing the mass transfer resistance from bubble accumulation. Under the two-electrode configuration, this PEC seawater splitting flow cell is stable for over 70 h.

Results and Discussion

The heterojunction Si photoanode consisted of an n-type Si substrate, and TiO2 prepared using atomic layer deposition (ALD). SS thin films were then deposited on the heterojunction silicon using a DC magnetron sputtering with AISI 316L and AISI 304 targets, and a Ni cocatalyst was prepared using the same method as the control sample. Due to the different sputtering yields of targets, the sputtering durations of SS were adjusted to obtain similar thicknesses [20]. The thicknesses of cocatalyst layers (including those of SS and Ni films) measured using a step profiler (Table S1) were approximately 200 nm. After an acid treatment, the contents of the SS films were measured using inductively coupled plasma-mass spectrometry (ICP-MS). The Fe content in the sputtered 316 SS film was slightly larger than that in the AISI 316L target, while the Ni content of sputtered 304 SS film is higher than that in the AISI 304 film target (Table S2). In addition, the contents of Mo in sputtered 316 SS and 304 SS films disappear, while AISI 316L target contains 2% Mo and AISI 304 does not include Mo element. The different compositions of the targets and sputtered SS films can be attributed to preferential sputtering caused by the distinct relative atomic masses and atom surface binding energies. A uniform distribution of Ni, Fe, and Cr atoms in the sputtered 316 SS and 304 SS films was observed through scanning electron microscope–energy-dispersive spectrometer (SEM–EDS), indicating the homogeneous feature of the sputtered SS films (Figs. 1a and S1). Furthermore, the SS films exhibit rough surfaces composed of abundant nanoscale particles, as evidenced by side-view SEM (Fig. 1b). The crystal structure of the sputtered 316 SS and 304 SS films was analyzed using X-ray diffraction (XRD). The characteristic peaks of Ni–Fe–Cr planes (220), (311), and (111) were detected in the sputtered 316 SS and 304 SS film (Fig. 1c). Meanwhile, there are obvious crystal structure differences between the sputtered SS films and austenitic steel targets (AISI 304/316L). This is because the cascade collision breaks the pristine structure of the metallic target, and growth of room temperature results in low crystallinity [21].

Fig. 1
figure 1

Characterizations of SS cocatalysts: a scanning electron microscopy (SEM) images and corresponding energy-dispersive spectroscopy mapping and b cross-sectional SEM images of 316 SS film. c X-ray diffraction patterns of 304 SS and 316 SS films. X-ray photoelectron spectroscopy results of d Ni, e Fe, and f Cr for 316 SS and 304 SS cocatalysts

Full size image

Photoanodes coated with sputtered 316 SS and 304 SS cocatalysts were activated through chronoamperometry (1.5 V vs. RHE, 1 mol/L KOH plus 0.5 mol/L NaCl), and X-ray photoelectron spectroscopy (XPS) was conducted to evaluate the chemical states and compositions of surface elements. The XPS of Ni illustrates two major spin–orbit peaks, located at 856 eV and 873 eV for Ni 2p3/2 and Ni 2p1/2, respectively, with two associate satellites, which are ascribed to Ni2+ oxidation state (Fig. 1d) [18]. The XPS signals of Fe 2p display two spin–orbit peaks at 710.6 eV (Fe 2p3/2) and 724.9 eV (Fe 2p1/2), indicating the presence of Fe3+ oxidation state (Fig. 1e) [22]. The peaks located at 576.1 eV and 586.5 eV in the Cr 2p spectrum are assigned to Cr 2p3/2 and Cr 2p1/2, respectively, which matches well with the characteristics of Cr3+ (Fig. 1f) [17]. The deconvolute peaks assigned to O–H and metal–O in terms of O 1 s spectra (Fig. S2) indicate the formation of NiFe (oxy)hydroxide. Additionally, the sputtered 316 SS and 304 SS cocatalysts exhibit similar surface compositions, as evidenced by XPS (Table S3). The surface element proportions of 316 SS and 304 SS films illustrate that more nickel species migrate to the surface, and the sum of the Ni and Fe atomic percentages is approximately 95% (Table S3), whereas the contents of Cr maintain at low condition, indicating that the SS cocatalysts are composed of chromium-incorporated NiFe (oxy)hydroxide upon activation [17, 23].

The photocurrent density (J) vs. potential (V) of heterojunction photoanodes coated with different cocatalysts, including 316 SS film, 304 SS film, and Ni film, were evaluated in chloride-containing alkaline electrolytes under simulated air mass (AM) 1.5 G sunlight illumination to investigate the effects of SS films on the PEC performance [24,25,26]. The XPS results of Ni, Fe and Cr indicate the formation of chromium-incorporated NiFe (oxy)hydroxide (Fig. 1d, e, and f). The sputtering durations of stainless steel and Ni targets were adjusted in order to achieve similar thicknesses (~ 200 nm, Table S1), thereby ensuring complete isolation between the semiconductor substrate and electrolyte with chloride ion [27]. The photoanodes loaded with 316 SS and 304 SS films illustrate the improved onset potential of approximately 0.88 V vs. RHE, while that of photoanodes coated with Ni is approximately 0.91 V vs. RHE (Fig. 2a). Moreover, the photocurrents of 316 SS and 304 SS samples increased more rapidly and reached the saturation photocurrents in a shorter period of time, compared with sample coated with Ni. The applied bias photon-to-current efficiency (ABPE) of 316 SS sample peaks is 3.45% at a bias of 1.06 V vs. RHE, while that for 304 SS sample approaches 3.40% at 1.07 V vs. RHE (Fig. 2b) and photoanode coated with Ni cocatalyst is 2.29% at 1.1 V vs. RHE [28]. Electrochemical impedance spectroscopy (EIS) of different photoanodes was measured to investigate their charge-transfer resistance between interfaces [29]. The Nyquist impedance plots of these photoanodes were evaluated by illuminating them at 1.23 V vs. RHE (Fig. 2c). Compared with the sample coated with Ni, the smaller diameters of the Nyquist impedance plots for the 316 SS and 304 SS films indicate that their carrier transfer resistances between the solid/electrolyte interfaces obviously decreased. Therefore, the photoanode coated with SS cocatalysts presents an approach for realizing increased oxygen evolution activity, which can be attributed to the chromium-incorporated NiFe (oxy)hydroxide [30].

Fig. 2
figure 2

a Current density–potential (J–V) curves, b ABPE, and c EIS of photoanodes coated with 316 SS film, 304 SS film, and Ni under AM 1.5 G illumination in chloride-containing alkaline electrolytes. d Stability tests of photoanodes with different cocatalysts at 1.5 V vs. RHE in chloride-containing alkaline electrolytes. The data in a–d were measured in 1 mol/L KOH plus 0.5 mol/L NaCl

Full size image

Photoanode stability in seawater is a great challenge, especially when photoanodes are exposed to complex chlorine chemistry. Their stability was assessed via chronoamperometry at a potential of 1.5 V vs. RHE in chloride-containing alkaline electrolytes under simulated AM 1.5 G illumination. The photoanodes coated with SS maintained stability for at least 50 h (Fig. 2d), while the photocurrent of the sample coated with Ni cocatalyst continuously decreased from the very beginning, indicating the superior anticorrosion properties of the SS films [31, 32]. As mentioned above, the Ni and Fe species in SS films are crucial for the catalytic activity of OER. Additionally, the function of Cr present in the 316 SS and 304 SS films for seawater splitting is non-negligible for this OER activity [33]. Chromic oxide was used as the Lewis acid layer to adjust the local reaction microenvironment of catalysts, avoiding the attack from chloride ions and locally generating hydroxide ions, which enhanced the direct seawater electrolysis performance of photoanodes [34]. The positive influence of chromic oxide is consistent with the improved stability performance of the photoanodes coated with SS. Therefore, the design rule of SS cocatalysts is that the activated SS films are expected to exhibit an appropriate Ni/Fe ratio (~ 2) for remarkable OER activity and chromic oxide to modify the microenvironment between seawater and cocatalysts.

Photoanodes coated with different SS films were evaluated in alkaline natural seawater collected from Bohai Bay. Compared with chloride-containing alkaline electrolytes, various dissolved ions in natural seawater result in competitive reactions in the PEC seawater splitting system. The precipitation of calcium and magnesium ions on the cathode is the first problem to be solved [35]. When the photoanode was operated in natural seawater without a buffer solution, precipitation occurred in a short period (Fig. S3), decreasing the performance of PEC. Thus, natural seawater solutions with different basicity were prepared [36] for the pre-precipitation of Ca(OH)2 and Mg(OH)2. Moreover, for conventional industrial water electrolysis applications, catalysts operate in high-alkaline solutions (30 wt.% KOH) to reduce the electrical resistance from the electrolyte. Therefore, the evaluation of photoelectrodes under such high-alkaline conditions remains a challenge in estimating their potential for industrial application [37].

To clarify the anticorrosion properties of SS cocatalysts for seawater splitting, the J–V curves and stabilities of these photoanodes were measured in two types of alkaline natural seawater (natural seawater with 1 mol/L KOH and 30 wt.% (7.64 mol/L) KOH). In natural seawater containing 30 wt.% KOH, the photoanode coated with 316 SS film exhibits an early onset potential and its current density increases at a sharper slope than that in natural seawater with 1 mol/L KOH (Fig. 3a). The overpotentials required to achieve current densities of 10 mA/cm2 for this photoanode in natural seawater with 30 wt.% KOH decreased by 110 mV compared with that in natural seawater with 1 mol/L KOH. Additionally, the ABPE of the photoanode in natural seawater with 30 wt.% KOH reached 5.54%, which was twice as much as that in 1 mol/L KOH plus natural seawater (Fig. 3b). The PEC performance of the photoanode coated with 304 SS film was also measured in these two different electrolytes, and it exhibited a similar trend, confirming the positive effect of the increased alkaline concentration for natural seawater splitting (Fig. S4).

Fig. 3
figure 3

a J–V curves, b the corresponding applied bias photon-to-current efficiency curves, and c stability tests of the photoanode coated with 316 SS film in alkaline natural seawater

Full size image

Although the photoanode coated with SS exhibits a remarkable enhancement in PEC seawater splitting, the semiconductor substrate is susceptible to deactivation in alkaline solutions, especially in alkaline natural seawater consisting of complex ions and high basicity [28, 38, 39]. Impressively, the current density of the photoanode coated with 316 SS film remained stable for over 50 h (Fig. 3c). This indicates that the SS cocatalysts increase the OER activity in natural seawater and can tolerate harsh environments with high basicity and corrosive ions. The surface morphology of the photoanodes after the stability test in alkaline natural seawater provided substantial information about the corrosion process, indicating that the nanotextured structure was unaffected during the prolonged stability test, whereas salt precipitation appeared on the surface due to the high salinity of electrolyte (Fig. S6).

The SS films coated on the photoanode ensure the complete isolation between the alkaline seawater and semiconductor with negligible structural change, enabling the photoanode to operate in harsh alkaline environments. Alkaline seawater inhibits drastic local pH changes on the photoanode surface, thereby preventing the cocatalyst from degradation and chlorine oxidation [40]. Additionally, alkaline seawater provides sufficient hydroxide ions, which obviously inhibits the adsorption behavior of halogen ions [6, 41, 42]. Thus, photoanodes coated with SS provide a promising strategy for alkaline seawater splitting, with abundant Ni and Fe species for water splitting and Cr species for anticorrosion against complex ions in seawater.

When the photoanodes coated with SS were tested in natural seawater with 30 wt.% KOH in an H-type PEC cell, the OER activity in alkaline seawater increased because of the higher concentration of OH−, resulting in the formation of a large number of bubbles. However, bubbles accumulated on the electrode will hinder the mass transfer between the photoanode and cathode (Fig. S7).

The PEC flow cell provides a promising method for solving bubble accumulation problems because it effectively shortens the distance between the photoanode and cathode to reduce the electrolyte resistance while accelerating the detachment of bubbles accumulated on the electrode surfaces. Moreover, the PEC flow cell configuration is highly compatible with the back-illuminated photoanode configuration in which the SS cocatalyst is coated on the opposite side of light illumination [43, 44] (Fig. 4). The flow cell is featured in the outside facing photoanode acting as the front side wall of the flow cell, which is directly illuminated, avoiding the optical losses due to quartz cell and electrolyte (Figs. 4 and S8).

Fig. 4
figure 4

a Schematic of the PEC flow cell. b Cross-section of the photoanode coated with SS coupled with flow cell

Full size image

The PEC performance of photoanodes coated with SS was evaluated in a three-electrode flow cell system. Compared with conventional H-type cells, the photoanode coated with 316 SS film also exhibited excellent onset potential (0.58 V vs. RHE) and higher saturated current density (37 mA/cm2) with 5.04% ABPE (Fig. 5a and 5b). The photoanode coated with 304 SS film also exhibited a remarkable OER activity, indicating the effective combination of flow cells and photoanodes (Fig. S9).

Fig. 5
figure 5

a J–V curves and b applied bias photon-to-current efficiency of photoanodes coated with 316 SS film in a three-electrode system. c J–V curves, d hydrogen production rate, and e stability tests with a two-electrode system in the flow cell. The data in a–e were measured using photoanodes coated with 316 SS film under 30 wt.% KOH + natural seawater

Full size image

To further verify the advancement of the PEC flow cell for seawater splitting, photoanodes in a two-electrode configuration with Pt foil as the cathode were also measured [44]. The cell voltage required for the seawater splitting system is only 1.75 V to achieve the saturated current density (Fig. 5c). Moreover, the PEC seawater splitting system coupled with the flow cell exhibited a record hydrogen production rate of approximately 600 μmol/(h·cm2) (Fig. 5d). Impressively, this PEC seawater splitting system remained stable for nearly 70 h (Fig. 5e), whereas the morphology and J–V curves before and after stability changed slightly (Fig. S10). The ion concentrations in electrolytes before and after stability tests were detected using ICP-MS to investigate the dissolution mechanisms. The changes of concentration of Ni and Cr are negligible, while the content of Fe in the electrolyte after stability increases, indicating the stable structure of the chromium-incorporated NiFe (oxy)hydroxide. The decreased performance of this PEC seawater splitting system resulted from salt precipitation because of the harsh environment with complex ions and high alkalinity, leading to a rough surface with slight corrosion (Fig. S10b). Thus, the photoanodes coated with SS in the PEC flow cell exhibited superior activity and stability, indicating an efficient design strategy for sputtered SS and flexible application environments.

Conclusions

This paper describes the design and fabrication of SS cocatalysts on photoanodes, demonstrating the efficient seawater splitting performances of photoanodes with improved OER and anticorrosion activities for halide ions in the PEC flow cell configuration. We selected SS films sputtered with easily available AISI 316 L and AISL 304 SS film targets because of their appropriate Ni/Fe ratios and chromic species content, modifying the microenvironment near cocatalysts and adapting successfully to the super alkaline environment of seawater with complex ions. Coupled with the PEC flow cell, the back-illuminated photoanodes coated with SS cocatalysts solve the bubble accumulation problem due to the higher OH− concentration in natural seawater with 1 mol/L KOH and 30 wt.% KOH, inhibiting the mass transfer resistance and blocking effect of adsorbed bubbles on reaction sites. Consequently, the photoanodes coated with 316 SS and 304 SS films exhibited remarkable PEC performance in natural seawater with 30 wt.% KOH, achieving more than 5% ABPE and stability for 50 h. Additionally, the photoanode coated with SS exhibited a higher saturated current density of 37 mA/cm2, and its current density remained stable during the 70 h test. Therefore, SS films in this study provide new insights into the fabrication of cocatalysts suitable for seawater splitting, and their combination with flow cells is promising for further industrial applications.