A Simple and Scalable Approach To Remarkably Boost the Overall Water Splitting Activity of Stainless Steel Electrocatalysts.

The stainless steel mesh (SSM) has received growing consideration as an electrocatalyst for efficient hydrogen and oxygen evolution reactions. Recently, the application of SSM as an oxygen evolution reaction (OER) electrocatalyst has been more promising, while its hydrogen evolution reaction (HER) catalytic activity is very low, which definitely affects its overall water splitting activity. Herein, a simple chemical bath deposition (CBD) method followed by phosphorization is employed to significantly boost the overall water splitting performance of SSM. The CBD method could allow the voids between the SSM fibers to be filled with Ni and P. Electrocatalytic studies show that the CBD-treated and phosphorized stainless steel (denoted SSM-Ni-P) exhibits an HER overpotential of 149 mV, while the phosphorization-free CBD-treated SSM (denoted as SSM-Ni) delivers an OER overpotential of 223 mV, both at a current density of 10 mA cm-2. An asymmetric alkaline electrolyzer assembled based on the SSM-Ni-P cathode (HER) and SSM-Ni anode (OER) achieved an onset and 10 mA cm-2 current densities at an overall potential of 1.62 V, granting more prospects for the application of inexpensive and highly active electrocatalysts for electrocatalytic water splitting reactions.

Introduction

Highly efficient electrolytic processes such as hydrogen and oxygen evolution reactions are in high demand for the production of clean energy such as hydrogen fuel.[1−7] Compared to the mature hydrogen evolution reaction (HER) in the acidic medium,[8−10] oxygen evolution reactions (OER) have intrinsic advantages in alkaline media (4OH– → 2H2O + O2 + 4e–) in terms of easy production of oxygen molecules evolving from the electrocatalysts[11−14] and straightforward assembly of the overall alkaline electrolytic device.[15,16] However, the sluggishness in the reaction kinetics[17,18] usually leads to the unsatisfactory catalytic activity of the electrocatalysts during the overall water splitting process.[19,20] The utilization of not only noble metal oxides such as IrO2 and RuO2[21,22] but also other transition metal compounds as promising OER catalysts[23,24] has shown undoubtedly enhanced performance in the assembly of the overall water splitting devices.[25,26] These electrocatalysts are mostly prepared on 2D planar[27] and 3D substrate structures[28−30] especially glassy carbon[31] and nickel foam/carbon cloth,[32−34] respectively, thereby forming substrate-assisted catalysts.[35,36] However, despite the successful fabrication of these catalysts on 2D and 3D structures, the tendency of the active materials peeling off from the substrates remains an unquestionable challenge,[33,37−39] which should be urgently addressed.

One of the promising approaches to address the tendency of the catalysts peeling off from the substrates is fabricating self-supporting electrocatalysts.[38] On the race to accomplishing the abovementioned approach, the stainless steel mesh (SSM) especially the 304-, 316-, and AISI 304-type has attracted great attention[38,40−42] not only due to its low cost but also its chemical stability in the alkaline environment.[43−45] SSM has shown impressive performance as an OER electrocatalyst in its pristine form[37,38] and significant performance enhancement upon different facile surface modification or exfoliation methods to improve its surface area.[46,47] Nevertheless, the HER performance of SSM is less attractive due to its limited active sites.[47] Impressive efforts have been explored to boost the HER performance of SSM,[41,47] with less advancement, which definitely affects the development of self-supporting SSM-based alkaline electrolyzers.[38,41,47]

Herein, we demonstrated that simple chemical bath deposition (CBD) followed by phosphorization strategy clearly enhanced the HER catalytic activity of SSM and its application as a cathode in the assembly of the alkaline electrolyzer successfully boosted its overall water splitting activity. Phosphorization is an effective strategy to enhance the HER properties of electrocatalysts due to its stability and tendency to promote hydrogen adsorption and water dissociation during HER.[38,48,49] However, fewer reports have been found to enhance the HER catalytic performance of SSM via phosphorization.[41] In addition, incorporation of Ni could effectively increase the active sites of electrocatalysts, which is beneficial for enhancing both the HER and OER performances of the electrocatalysts.[48,50] The facile and scalable CBD process of the SSM involves simple immersion of pristine SSM in boiled nickel chloride solution for only 120 s followed by phosphorization. As a result, Ni-rich CBD-treated SSM (SSM-Ni) exhibits an overpotential of 223 mV at a current density of 10 mA cm–2, while the Ni-rich phosphorus-doped SSM (SSM-Ni-P) reaches a current density of 10 mA cm–2 at an overpotential of 149 mV, resulting in an overall water splitting potential of 1.62 V. The enhanced water splitting activity is based on the enriched Ni active sites of SSM for the OER and the dual incorporation of Ni and P active sites for the HER. This work opens more opportunity for the optimization, cost-effective, and scalable fabrication of overall water splitting devices based on SSM.

Experimental Section

Synthesis of SSM-Ni

First, 304 stainless steel SSM (1000 mesh) (Foshan Guangmei Stainless Steel Co., Ltd.) with 2 × 3 cm was ultrasonically cleaned with acetone, ethanol, and distilled water and then dried in an oven at 60 °C. After that, the stainless steel was put into a boiling solution containing 0.5 M NaNO3 and 2 M NiCl2·6H2O for 120 s and dried naturally at room temperature.

Synthesis of SSM-Ni-P

For phosphorization, 500 mg of sodium hypophosphite (NaH2PO2·H2O) and SSM-Ni were put at two separate positions in a porcelain boat with NaH2PO2·H2O at the upstream side of the three-temperature zone tube furnace and the SSM-Ni at the second-temperature zone. The sample was heated at 400 °C for 60 min with a heating rate of 5 °C/min under a nitrogen atmosphere with a flow rate of 200 sccm. After natural cooling, the SSM-Ni-P sample was obtained.

Synthesis of SSM-P

SSM-P was prepared under the same conditions as SSM-Ni-P, but pristine SSM was used instead of SSM-Ni.

Materials Characterization

The morphology was characterized by a scanning electron microscope (FESEM, Quanta 400/INCA/HKL). An X-ray diffractometer (XRD, Rigaku SmartLab) was used to derive the crystalline structure of the as-prepared samples. X-ray photoelectron spectroscopy (FESEM, Quanta 400/INCA/HKL) was performed on an ESCALab250 to determine the chemical compositions and valence states in the samples.

Electrochemical Characterization

All the electrochemical tests were carried out on a CH Instruments electrochemical workstation (CH760E) with a three-electrode system in which the Pt wire was used as the counter electrode, Ag/AgCl in saturated KCl solution was used as the reference electrode, and the prepared sample was directly used as the self-supporting working electrode. During the test, the prepared sample was sealed with epoxy resin, leaving only an area of 1.0 cm2 for contact with the 1 M KOH solution and a small area at the other end for ohmic contact.

Oxygen Evolution Reaction Test

OER overpotential was calculated according to the following equation: overpotential (mV) = (measurement potential (vs SCE) + 0.197 + 0.059 × pH – 1.229 (vs REH)) × 1000. The polarization curve (LSV) was measured between 0.0 and 1.0 (vs SCE) at a scanning rate of 1 mV/s, and the Tafel graph was measured at the same scanning speed. The Tafel slope was derived from the equation η = b log j + a, where η, b, and j are the overpotential, Tafel slope, and current density, respectively. Electrochemical impedance spectroscopy (EIS) measurements were performed at a current density of 20 mA/cm2 with an ac amplitude applied of 5 mV and frequency range of 0.01–1000 kHz. The constant current stability of the working electrode was carried out for 25 h under the condition of intense stirring of the electrolyte.

Hydrogen Evolution Test

HER overpotential was calculated according to the following equation: overpotential (mV) = (measurement potential (vs SCE) + 0.197 + 0.059 × pH – 0 (vs REH)) × 1000. The polarization curve (LSV) was measured between −1.0 and −2.0 (vs SCE) at a scanning rate of 1 mV/s. Tafel, EIS, and stability tests were the same as those of the OER tests above.

Overall Water Splitting Test

SSM-Ni and SSM-Ni-P were used as anode and cathode materials, respectively, to measure the LSV curve in 1 M KOH solution at a scanning rate of 1 mV/s between 1.0 and 2.0 V. Also, the constant current stability analysis for 25 h was carried out in the process of intense agitation.

Results and Discussion

Morphological and Electronic Properties

SSM-Ni can be obtained by a simple CBD method, and SSM-Ni-P can be obtained by phosphorization of CBD-derived SSM-Ni as shown in Scheme . The CBD process involved simple immersion of clean SSM into boiling solution for just 120 s to obtain the SSM-Ni sample (Scheme a). The solution contains 2 M NiCl2·6H2O and 0.37 mmol of NaNO3. After drying, the pristine SSM and CBD-treated SSM (SSM-Ni) were subjected to morphological characterization. Figure a,b displays the scanning electron microscopy (SEM) images of pristine SSM and SSM-Ni, respectively. The surfaces of both samples appear very smooth. However, the voids in SSM is open (yellow dazed area in Figure a), while the voids in SSM-Ni are filled up (red dazed area in Figure b). Compared to the elemental mapping of SSM (Figure S1), the elemental mapping of SSM-Ni in Figure S2 shows that the filled-void area consists of the Ni element. After phosphorization of SSM-Ni using N2 gas at 400 °C for 60 min (Scheme b), the morphology of the newly formed SSM-Ni-P sample also shows that the voids are filled (blue dazed areas in Figure c). The mapping spectrum shows the presence of P confirming successful phosphorization (Figure d). Elemental mapping in Figure e–j clearly shows that the filled-void positions in the SSM-Ni-P sample are characterized with Ni and P. According to the results obtained from the EDS elemental mapping of SSM-Ni and SSM-Ni-P samples in Figure e–j and Figure S2, we believe that Ni exists both at the surface and voids between the SSM fibers. According to Table S1 and Figure d inset, the Fe atomic composition in the SSM sample reduces from 70.41 to 66.06 in SSM-Ni and 58.71 in SSM-Ni-P, like those of Cr, while the atomic composition of Ni increases from 7.45 in SSM to 8.71 in SSM-Ni and slightly reduces to 8.16 in SSM-Ni-P. It should be pointed out that previous reports have demonstrated the reduction of both Fe and Cr compositions could lead to an increase in their catalytic performances.[42,44] Hence, we proposed that the catalytic properties of SSM-Ni and SSM-Ni-P should be improved with respect to that of SSM.

Scheme 1

Synthetic Route of Both SSM-Ni and SSM-Ni-P Samples

Figure 1

Morphological characterization. SEM images of (a) SSM, (b) SSM-Ni, and (c) SSM-Ni-P samples. (d) Map sum spectrum of SSM-Ni-P sample. The inset is the atomic composition of SSM-Ni-P sample. (e–j) EDS elemental mapping of SSM-Ni-P sample.

Figure 3

Electrocatalytic properties. (a-I) HER and (a-II) OER LSV curves of SSM, SSM-Ni, and SSM-Ni-P. (b) HER and (c) OER Tafel plots of SSM, SSM-Ni, and SSM-Ni-P electrocatalysts.

X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses were used to study the phase and electronic structure characterization of the SSM, SSM-Ni, and SSM-Ni-P samples. Based on the XRD spectra in Figure S3, the three samples consist of the austenite-type stainless steel (PDF card #33-0397)[38] and the three samples depict no notable difference.

The result implies that the CBD and phosphorization processes have no specific effect on the phase properties of the samples. The XPS spectra of the three samples confirmed the presence of Fe, Cr, Ni, and O, while that of SSM-Ni-P is characterized with an additional P peak (Figure S4). The Fe 2p XPS spectra of SSM, SSM-Ni, and SSM-Ni-P are shown in Figure a. The peak around 707.0 eV is assigned to the Fe0 peak,[38] and the peak around 710.8 eV is assigned to the Fe peak in the FeCr alloy.[51,52] It can be observed that the Fe0 peak of SSM shifted to the higher binding energy, suggesting oxidation of the Fe0 and changes in the electronic states of SSM-Ni and SSM-Ni-P. In addition, the Cr 2p XPS spectra of the three samples are compared in Figure b. The peak around 573.2 eV corresponding to Cr0 disappears in SSM-Ni-P, suggesting the removal of elemental Cr in the phosphorized sample. Moreover, the peak around 576.2 eV corresponding to Cr in the FeCr alloy[51,52] in the SSM sample also shifted to the higher binding energy in both SSM-Ni and SSM-Ni-P, indicating oxidation or modification in the electronic structures of both SSM-Ni and SSM-Ni-P. According to Figure c, the Ni peak around 852.8 eV that is assigned to the zero oxidation state of Ni (i.e., Ni0) is negligible in SSM but prominent in those of SSM-Ni and SSM-Ni-P, confirming that both samples are Ni-rich. Furthermore, the peak around the binding energy of 856.1 eV in SSM-Ni suggests the presence of the Ni peak of Ni2+ from NiCl2,[38,52] while the intensities of these peaks reduced in SSM-Ni-P prescribing the binding of Ni with phosphorus. Moreover, XPS results further justify that Ni exists in both Ni0 and Ni2+ forms. It should be pointed out that the Ni2+ does not turn into Ni0 after phosphating, indicating the presence of Ni2+ from NiCl2 in both SSM-Ni and SSM-Ni-P samples. Moreover, the origin of Ni0 might be related to the strong order of electroactivity of Fe compared to Ni; that is, Fe can easily reduce Ni. Hence, due to the higher composition of Fe in SSM, more composition of Ni0 can be found observed according to the XPS results. Indeed, only SSM-Ni-P features the presence of P, while those of SSM and SSM-Ni are phosphorus-free according to the P 2p XPS spectra in Figure d. The XPS analysis confirms that, after CBD treatment and phosphorization, the surface and electronic properties of both SSM-Ni and SSM-Ni-P have been modified. Such modifications have also been proven to be of positive significance to improve OER and HER properties of electrocatalysts.[53]

Figure 2

Phase and electronic structure. (a) Fe 2p, (b) Cr 2p, (c) Ni 2p, and (d) P 2p XPS spectra of SSM, SSM-Ni, and SSM-Ni-P samples.

Electrocatalytic Properties

Both the HER and OER catalytic properties were tested in a three-electrode configuration system using our three as-prepared electrocatalysts as the working electrode, carbon rod counter electrode, saturated Ag/AgCl reference electrode, and 1.0 M KOH electrolyte solution. First, the HER linear sweep voltammetry (LSV) curves of the electrocatalysts are shown in Figure a-I.

Among the compared three catalysts, SSM-Ni-P delivers a 10 mA cm–2 current density at an overpotential of 149 mV, displaying the best HER performance compared to those of SSM-Ni and SSM at 346 and 402 mV, respectively. Insights on the kinetics of the electrocatalysts were studied using Tafel slopes and electrochemical impedance spectroscopy (EIS). Figure b shows the HER Tafel plots of SSM, SSM-Ni, and SSM-Ni-P electrocatalysts. The Tafel slope of SSM-Ni-P is 80 mV dec–1, which is also significantly smaller than those of SSM (136 mV dec–1) and SSM-Ni (145 mV dec–1). Moreover, the charge transfer resistance Rct (i.e., semicircle in the Nyquist plots) of SSM-Ni-P also remains the smallest among the compared samples, further confirming the enhanced kinetics of SSM-Ni-P over those of SSM-Ni and SSM (Figure S5a). The improved performance can be attributed to the synergistic contribution of both Ni and P, creating more active surface and sites for efficient evolution of hydrogen.

Thus, the active electrochemical surface areas (ECSAs) of the three tested electrocatalysts were further performed to justify the superiority in the performance of SSM-Ni-P as shown in Figure S6. Figure S6a–c shows the cyclic voltammetry curves in the voltage range of −0.71 to −0.73 V and different scan rates of 20, 50, 100, and 200 mV s–1. According to the double-layer capacitance (Cdl) value that were obtained from the different CV curves at different scan rates, the Cdl of SSM-Ni-P is 18.8 mF cm–2, while those of SSM-Ni and SSM are 40 and 1.7 mF cm–2, respectively (Figure a). Thus, with an electrode area of 0.5 cm2, the HER ECSAs of SSM, SSM-Ni, and SSM-Ni-P are 28, 35, and 313 cm–2ECSA, respectively, assuming that the specific capacitance of the catalysts is 60 μF cm–2.[54,55] The normalized LSV curves to ECSA further reveal that SSM-Ni-P displayed the lowest potential and Tafel slope compared to SSM-Ni and SSM, indicating intrinsic catalytic activity of the SSM-Ni-P electrocatalyst (Figure b). Due to the fact that we study the electrocatalytic performance of the electrocatalysts using the Hg/HgO reference electrode, we redetermined the double-layer capacitance values of the electrocatalysts. The SSM-Ni-P electrocatalyst still displayed a much higher double-layer capacitance value than SSM-Ni and SSM (Figure S7). Thus, based on similar morphological structures of SSM-Ni-P and SSM-Ni, we proposed that the higher double-layer capacitance value of SSM-Ni-P might be attributed to the phosphorization process, which increases the HER catalytic sites compared to that of SSM-Ni.

Figure 4

(a) Plots of the capacitive currents as a function of scan rate for the three electrocatalysts during HER and (b) the normalized LSV curves obtained from panel (a) to ECSA. (c) Plots of the capacitive currents as a function of scan rate for the three electrocatalysts during OER and (d) the normalized LSV curves obtained from panel (c) to ECSA.

The higher Cdl value of SSM-Ni-P over those of SSM-Ni and SSM indicates that SSM-Ni-P is characterized with the highest active sites for the HER process, further indicating the reasons for the superior performance of the SSM-Ni-P electrocatalyst. To have deep insight on the impact of phosphorization on the SSM-Ni-P catalyst, pristine SSM was directly annealed with the same condition as SSM-Ni-P (details in the Experimental Section) without CBD treatment and the performance was compared with that of SSM and denoted as SSM-P. Interestingly, the HER performance of SSM-P is not only superior to that of SSM (Figure S8a) but also superseded that of SSM-Ni (Figure S8b). Moreover, the Tafel slope, Rct, and Cdl values of SSM-P further confirmed the better HER of SSM-P over SSM and SSM-Ni (Figure S8c–f), justifying that the phosphorization process greatly contributed to the HER performance of SSM-Ni-P electrocatalysts.

During the OER, both SSM-Ni and SSM-Ni-P nearly show the same overpotential at 10 mA cm–2 current density, but SSM-Ni displays superior performance to SSM-Ni-P at higher current densities (Figure a-II), suggesting SSM-Ni as the optimized OER electrocatalyst. In addition, the Cdl value obtained from the CV curves in Figure S9 is 2.9 mF cm–2 for SSM-Ni, which is also higher than those of SSM-Ni-P (1.7 mF cm–2) and SSM (1.2 mF cm–2) (Figure c). By assuming that the specific capacitance of the electrocatalysts is 60 μF cm–2,[54,55] the OER ECSAs of SSM, SSM-Ni, and SSM-Ni-P are 20, 48, and 28 cm–2ECSA, respectively. After normalization of the OER LSV curves to ECSA, SSM-Ni still revealed the best catalytic performance compared to SSM-Ni-P and SSM, also confirming superior intrinsic catalytic activity of the SSM-Ni-P catalyst (Figure d), indicating that the main active sites for the superior OER performance are from CBD-treatment-induced Ni. Furthermore, the Tafel slope of SSM-Ni is the smallest (42 mV dec–1), while that of SSM (49 mV dec–1) is even smaller than that of SSM-Ni-P (43 mV dec–1) (Figure c), suggesting that the phosphorization process may have no effect on the OER kinetics. To have further information on the OER kinetics, the Nyquist plot recorded shows that the charge transfer resistances in both SSM-Ni-P and SSM are nearly the same (Figure S5b), indicating that the phosphorization process really has no effect on the OER performance. As we performed the HER, the performance of SSM-P was also compared with that of SSM. According to Figure S10a–d, both SSM and SSM-P catalysts display approximately the same features, finally justifying that the process of phosphorization has no effect in enhancing the OER performance of SSM. Thus, SSM-Ni was selected as the optimized OER electrocatalyst and utilized for further OER measurements.

The most important parameter of an electrocatalyst is its durability. Both SSM-Ni-P and SSM-Ni were subjected to chronoamperometry measurements at different current densities of 10, 20, 30, and then to 10 mA cm–2 as shown in Figure . Both electrocatalysts exhibit excellent stability up to 25 h. The HER electrocatalyst (SSM-Ni-P) displayed an overpotential retention of about 95% (Figure -i), while the OER electrocatalyst (SSM-Ni) achieved 100% overpotential retention (Figure -iii) after 25 h of durability testing. Both the HER and OER electrocatalysts still exhibit excellent kinetics after the stability test (Figure S11a,b). Moreover, both electrocatalysts also show excellent phase stability (Figure S11c,d).

Figure 5

HER and OER chronoamperometry stability measurements of SSM-Ni-P and SSM-Ni electrocatalysts, respectively, up to 25 h. (i) LSV curves before and after HER stability test and (ii) SEM image of SSM-Ni-P electrocatalyst after HER stability test. (iii) LSV curves before and after HER stability test and (iv) SEM image of SSM-Ni-P electrocatalyst after HER stability test.

According to the post-morphological characterization of the SSM-Ni-P electrocatalyst, the SEM image shows that its morphology could be retained (Figure -ii). Elemental mapping in Figure S12 confirmed the presence of the elements including their various distributions. The compositions of Fe, Ni, and P elements reduce, while those of Ni and O increase, suggesting the formation of nickel oxide and hydroxide species on the catalyst surface (Table S2), which could play a role in protecting the catalyst during HER electrolysis. For the post-OER analysis, the morphology of SSM-Ni could also be maintained (Figure -iv). Moreover, same as that of the HER catalyst, all the elements present in the elemental mapping of the SSM-Ni OER electrocatalyst were observed (SSM-Ni) (Figure S13), suggesting excellent stability of the OER catalyst. Additionally, the atomic compositions of Ni and O also increase as those of other elements decrease (Table S3) further suggesting that the oxides or hydroxides of nickel were formed, which could also serve as protection for the catalyst upon the OER electrolytic process. All these results confirm the excellent and impressive stability of SSM-Ni-P and SSM-Ni as efficient and stable HER and OER electrocatalysts, respectively.

Based on the fact that the testing stability performance of the electrocatalyst for a longer period of time using the Ag/AgCl reference electrode is not quite suitable, we also tested both the HER and OER performances of the three electrocatalysts using the Hg/HgO reference electrode. Interestingly, the HER performance of SSM-Ni-P in the Hg/HgO reference electrode is superior to that in the Ag/AgCl reference electrode (Figure S14). However, the OER performance of SSM-Ni in the Hg/HgO reference electrode is inferior to that in the Ag/AgCl reference electrode (Figure S15). It should be pointed out that the enhanced HER performance difference of SSM-Ni-P using the Ag/AgCl reference electrode compared to the Hg/HgO reference electrode is the same as the OER performance difference using the Hg/HgO reference electrode compared to the Ag/AgCl reference electrode. Hence, the overall water splitting activity of SSM-Ni-P//SSM-Ni should not be affected. The HER LSV curves using Hg/HgO after the stability test reduce, while that of OER almost maintains its initial LSV curve.

Overall Water Splitting Activity

Having discover the optimized HER and OER electrocatalysts, an alkaline electrolyzer or overall water splitting based on these electrocatalysts was designed. The device was denoted as SSM-Ni-P//SSM-Ni. Other alkaline electrolyzers based on other electrocatalysts were also designed for comparison such as SSM//SSM, SSM//SSM-Ni-P, SSM-Ni//SSM, SSM-Ni-P//SSM-Ni-P, SSM-Ni//SSM-Ni, and SSM-Ni-P//SSM-Ni, as shown in Figure a. Among the electrolyzers assembled as mentioned above, SSM-Ni-P//SSM-Ni displayed the best performance.

Figure 6

Overall water splitting activity. (a) LSV curves of different self-assembled alkaline electrolyzers based on the controlled samples. (b) LSV curves of SSM-Ni-P//SSM-Ni compared with SSM/Pt-C//SSM-IrO2. (c) Comparisons of our SSM-Ni-P//SSM-Ni electrolyzer with other recently reported SS-based alkaline electrolyzers.[33−35,40] (d) 25 h stability measurements of SSM-Ni-P//SSM-Ni electrolyzer. (e) LSV curves of SSM-Ni-P//SSM-Ni before and after stability test.

When compared with commercial Pt-C and IrO2 coated on SSM (denoted SSM/Pt-C//SSM/IrO2), our as-designed SSM-Ni-P//SSM-Ni delivered an overall potential of 1.62 V, which was comparable to that of SSM/Pt-C//SSM/IrO2 (Figure b), superior to previously reported stainless steel based overall water splitting devices such as SSFS//SSFS,[41] and also comparable to NASSM//NASSM[47] (see Table for details and Figure c). Finally, the SSM-Ni-P//SSM-Ni electrolyzer was subjected to 25 h of chronopotentiometric stability testing at different current densities displaying excellent stability (Figure d) with about 99% potential retention as shown in Figure e, potentially demonstrating the utilization of stainless steel as a promising electrocatalyst for the production of H2 and O2.

Table 1

Comparisons of SSM-Ni-P//SSM-Ni Different SS-Based Alkaline Electrolyzers

alkaline electrolyzers	HER (mV) (10 mA cm–2)	OER (mV) (10 mA cm–2)	overall water splitting (V) (10 mA cm–2)
SSM-Ni-P//SSM-Ni (this work)	149	223	1.62
SSM//SSM (this work)	402	297	1.97
SSM/Pt-C//SSM/IrO2 (this work)			1.57
NASSM//NASSM 47	146	225	1.61
SS-scrubber//SS-scrubber[40]	315	418	1.98
SSFS//SSFS[41]	136	262	1.64
NESSP//NESS[38]	230	278	1.74

Conclusions

In conclusion, the overall water splitting performance of the stainless steel mesh (SSM) was significantly enhanced by simple chemical bath deposition (CBD) followed by phosphorization. The optimized HER and OER electrocatalysts denoted as SSM-Ni-P and SSM-Ni could reach a current density of 10 mA cm–2 at lower overpotentials of 149 and 223 mV, respectively, which allow the assembly of an overall water splitting device with an overall potential of 1.62 V at 10 mA cm–2 current density. The CBD method introduced Ni as the active site, which not only filled up the voids of the SSM but also was evenly distributed on the catalyst surface for efficient OER. Phosphorization of the CBD-treated SSM induced more active sites for achieving efficient HER activity. This work did not only show great significance toward a simple strategy to improve the catalytic activities of steel-based electrocatalysts but also portray an easy scaling-up strategy for commercial utilization.