Handily etching nickel foams into catalyst-substrate fusion self-stabilized electrodes toward industrial- level water electrolysis


 The key challenge of industrial water electrolysis is to design catalytic electrodes that can stabilize high current density with low power consumption (i.e., overpotential), while industrial harsh conditions make the balance between electrode activity and stability more difficult. Here we develop an efficient and durable electrode for water oxidation reaction (WOR), which yields a high current density of 10000 A m-2 at an overpotential of only 284 mV and shows robust stability even in 6 M KOH strong alkaline electrolyte with elevated temperature up to 80 °C. This electrode is fabricated from cheap nickel foam (NF) substrate through a simple one-step solution etching method, resulting in the growth of ultrafine phosphorus doped nickel-iron (oxy)hydroxide (P-NiFeOOH) nanoparticles embedded into abundant micropores on surface, featured as a self-stabilized catalyst-substrate fusion electrode. Such self-stabilizing effect fastens highly active P-NiFeOOH species on conductive NF substrate with significant contribution to catalyst fixation and charge transfer, realizing a win-win tactics for WOR activity and durability at high current densities in harsh environments. This work affords a cost-effective WOR electrode that can well work at large current densities, suggestive for rational design of catalyst electrodes toward industrial-scale water electrolysis.

Human's over-dependence on fossil fuels leads to serious energy crisis and environmental hazards. Renewable electricity generation from natural sources through solar panels, wind turbines, hydropower, etc., shows great promise for sustainable and green energy supplies for the world 1,2 . Further using surplus renewable electricity to produce fuels via electrochemical techniques will provide more room for power storage and utilization, attracting extensive interest in both academia and industry [3][4][5][6] . In this regard, electrochemical water splitting that promises for high-purity green hydrogen production offers an attractive prospect of eco-friendly hydrogen economy with zero carbon emitting [7][8][9][10] . However, water electrolysis generally exhibits sluggish kinetics especially for the energy-intensive water oxidation reaction (WOR) on the anode 11,12 , resulting in high electricity consumption inadequate for large-scale commercialization.
Despite great achievements, it remains an open question to apply these promising WOR catalysts in practical water electrolysis due to the criteria gap between lab-and industrial-scale performance [28][29][30] . Most of reported catalysts were shown to be active and stable only at low current densities (e.g., typical 10 mA cm -2 ) 31,32 , which might not necessarily translate into practical performance at industrial-level large current densities. Indeed, there are limited catalysts reported so far to meet the strict industrial standard of large current densities over 500 mA cm -2 operating at low overpotentials less than 300 mV 33 . Moreover, the catalyst electrode stability would encounter more serious trials at high current densities under industrially relevant harsh conditions (i.e., temperature between 70-80 °C and alkalinity in 25-30 wt.% KOH) 34,35 . Hence, paired success of high activity and long durability to deliver large current densities under industrial conditions is imperative to develop catalyst electrodes for practical application 36 . But, seeking such an activity-stability balance at high current densities becomes more challenging because of solid-liquid-gas interface exchange kinetics complicated in the context of intense mass/charge transfer and effervescence 37 .
To this end, self-supported electrodes in which active catalysts directly grown on conductive substrates have been considered to form their intimate contact beneficial for mechanical adhesion and charge/mass delivery without binders or additives 38,39 .
Several efficient self-supported electrodes have been noticeably emerged for largecurrent-density WOR in combination with some strategies, such as geometrical/structural design 40,41 , hetero assembly 42,43 and electronic modulation 44 .
Overall, most traditional approaches like electrodeposition and hydrothermal synthesis were generally adopted to guide the surface loading of active catalysts on support frameworks, whereas such hierarchical assembly might be still inadequate for robust solid-liquid-gas interface dynamics under industrial conditions because large-currentdensity WOR electrodes steady in strong alkali (≥500 mA cm -2 for over 100 h in ~6 M KOH) are rarely reported to date. Along this line, we suppose that an etching strategy on metal substrates with targeted catalyst layer to construct catalyst-substrate fusion electrodes might enable strong self-stabilizing effects on mechanical adhesion and charge transfer for industrial criterions.
In addition, species reported as WOR promoters at high current densities are few and are most limited to NiFe-based (oxy)hydroxides 40,43 , but their durability has yet to be further examined in industrially relevant harsh environments with strong alkali and high temperature. Hence, we preferentially target to NiFe (oxy)hydroxide configuration through the rational etching design over nickel foams (NF). Moreover, most synthetic methods for previously reported metal (oxy)hydroxides usually involve multi-step procedures even including post treatment or reconstruction 45,46 , which are inferior in respect to time-energy input, cost-effectiveness and scale-up, in comparison with etching process.
In this work, we develop a self-stabilized catalyst-substrate fusion electrode with industrially relevant WOR activity and durability through directly etching NF substrates, which in situ derives ultrafine phosphorus doped nickel-iron (oxy)hydroxide (P-NiFeOOH) nanoparticles embedded into abundant macropores etched on NF surface.
The P-NiFeOOH electrode achieves large current density 1000 mA cm -2 at a low overpotential of only 284 mV and exhibits robust durability at least 150 hours under industrial-level conditions. Such a self-stabilized electrode design strategic for industrial high current density features with several advantages as following. First, the surface etching method is extremely facile for targeted synthesis of active (oxy)hydroxides that differs from traditional hydrothermal or electrodeposition, more amenable to scaled-up production with economy. Second, the support-stabilizing catalyst construction avoids utilization of an expensive polymer binder (e.g., Nafion) that generally induces adverse effects of shielding active sites, deteriorating conductivity and insufficient stability especially at large currents in strong corrosive alkali. Third, the embedded fusion of catalyst-rooting at conductive substrate guarantees structural integrity with robust contact, favorable for electron transfer and catalyst fixation to overcome low conductivity and easy agglomeration commonly associated with metal (oxy)hydroxides. Fourth, the hierarchically porous structure offers large specific surface areas to maximize active-site exposure, electrolyte permeation and gas product release, which are especially useful under high current densities and violent gas evolution conditions. In addition, the targeted NiFeOOH is widely accepted as the most efficient WOR catalyst 47 , and here the trace P dopant into NiFeOOH species may improve electrical conductivity and optimize absorption energies for reaction intermediates, and the formed P-O bonds can contribute to resistance to corrosion 10,48 .

Results
Morphology and structural characterizations of P-NiFeOOH electrode. The support-stabilized P-NiFeOOH catalyst was prepared via chemically etching NF in mixture solution with specific amounts of Fe(NO3)3 and Na3PO4. The commercial NF presents a three-dimensional (3D) network with macropore structure and smooth surface under scanning electron microscopy (SEM) characterization ( Supplementary   Fig. 1). After etching reaction for 10 minutes, the treated NF maintains the overall 3D skeleton ( Supplementary Fig. 2), but the surface is etched with many pores several micrometers in size (Fig. 1A). The amplified SEM image shows a large number of nanoparticles (200 to 400 nm in diameter) embedded in these micropores (Fig. 1B). The coordinations, respectively. In the P-2p spectrum, two small peaks at 131.0 and 130.0 eV for spin-orbit P 2p1/2 and P 2p3/2 reveal a small amount of P conjugated by metals, and another noticeable peak at 133.1 eV as an indicator of P-O bonds suggests that P species mostly coordinates with O. The electronegative P dopant may improve the electronic conductivity and promote the deprotonation kinetics in WOR 10 . Moreover, surface P is easy to be oxidized to form P-O species, which is not only active for water oxidation but also resistance to oxide dissolution 49 Fig. 2A, it can be seen that P-NiFeOOH electrode exhibits an evident increase in WOR activity compared to pristine NF, and it is also much superior to IrO2 benchmark. For instance, small overpotentials of 221 and 251 mV are required for P-NiFeOOH to deliver current densities of 10 and 100 mA cm -2 , which are much lower than those for NF (375 and 553 mV) and IrO2 (310 and 408 mV) ( Supplementary Fig. 6). Especially, at an overpotential of 284 m V, P -NiFeOOH can achieve a large current density up to 1000 mA cm -2 , two orders of magnitude higher than that of IrO2 benchmark. Accordingly, P-NiFeOOH displays a smaller Tafel slope of 25.1 mV dec -1 than NF (114.2 mV dec -1 ) and IrO2 (97.2 mV dec -1 ), indicating faster catalytic WOR kinetics. These super parameters make P-NiFeOOH among the best for transition metal-based electrodes reported for alkaline WOR (Supplementary Table 1).
We further compare the Δ Δlog| | ⁄ ratio that is defined by the slope of overpotential versus logarithm current density for different electrodes with different current densities to appraise the overpotential required to raise the current intensity, which is an important descriptor of catalytic performance especially at large current densities and critically significative for practical availability. The ratio for IrO2 increases sharply as the current densities exceed 100 mA cm -2 , and reaches 455 mV dec -1 at 500 mA cm -2 ( Fig. 2C), whereas that for P-NiFeOOH remains small, only ~32 mV dec -1 at 1000 mA cm -2 , which indicates its remarkable WOR activity at large current densities.
We also tested the WOR activity of P-NiFeOOH electrodes obtained with different etching times. Longer etching treatment brings about higher WOR activity, and the NF etched with 10 minutes reaches an optimum ( Supplementary Fig. 7), beyond which the NF appears brittle upon excessive etching. Thus, 10-minute-etched electrode achieves a balance between the surface porosity and mechanical strength balance, and equips with desirable pore-rich surface serving for active species embedment and site exposure ( Supplementary Fig. 5). The double-layer capacitances ( dl C cyclic voltammetry (CV) method ( Supplementary Fig. 8). The dl C values increase with incremental etching times, and the 10-minute-etched electrode has a maximum of 12.6 mF cm -2 , which is over 3 times that of pristine NF (3.6 mF cm -2 ), offering a larger electrochemically active surface area (ECSA) with more active sites exposing to electrolyte. This suggests that the creation of abundant cavities embedded with active nanoparticles is very useful to maximize accessible active sites favorable for mass/charge transfer in catalysis.
The surface hydrophilicity and aerophobicity become very critical for the active-site utilization and mass transfer when catalytic electrode coping with high current densities.
The surface wettability of P-NiFeOOH electrode was examined through vacuolar contact manner tracked by a high-speed camera ( Supplementary Fig. 9) and pristine NF ( Supplementary Fig. 11), indicating its efficient charge-transport ability favorable to WOR dynamics.
Industrial adaptability of P-NiFeOOH electrode. We then tested the WOR performance of P-NiFeOOH electrode in industrially relevant strong alkali electrolytes (3 M and 6 M KOH). Fig. 2D shows that the polarization activity gradually enhances with increasing alkali concentration, and the performance of P-NiFeOOH remains excellent against strong alkali corrosion. Note that the overpotentials required at even industrial-scale high current densities from 500 up to 1000 mA cm -2 are essentially smaller than 300 mV (Fig. 2E). In particular, an ultralow overpotential of only 250 mV can deliver a large current density of 1000 mA cm -2 under strong mass transfer in 6 M KOH. Furthermore, the temperature rise in industrial electrolysis is inevitable, and so the electrolyte with heating was used to examine P-NiFeOOH electrode. As shown in  Fig. 12), which represent a record-high performance for current alkaline WOR (Supplementary Table 2). The chronopotentiometry (CP) tests demonstrates the steady WOR catalysis on P-NiFeOOH electrode at both 500 and 1000 mA cm -2 for 100 h in 1 M KOH (Fig. 2G).
The succedent CP test at 1000 mA cm -2 for 100 h in 6 M KOH with varying temperatures from 25 to 80 °C further verifies the electrode durability with industriallevel large current densities under strong alkali and high temperature conditions. After testing, the P-NiFeOOH electrode basically maintains its structural and component characteristics, as evidenced by SEM, TEM and EDS imaging (Supplementary Fig. 13).
Furthermore, the post-characterizations of XRD and XPS (Supplementary Fig. 14) show no detectable phase and valence changes. Of course, there are also little subtle changes noticed for active species, such as more flocculent morphology, clearer crystal domains and less P content. These micro-evolutions, though not reconstructing, may contribute to uncover atomic-scale catalytic centers and deserve further study in the future. The current work focuses on the applicability of P-NiFeOOH electrode to industrial harsh conditions, suggesting its potential practical application. Moreover, compared with other advanced electrodes reported recently, our P-NiFeOOH electrode exhibits both superior activity and stability that are among the best for alkaline WOR (Supplementary Table 2). These results confirm the exceptional WOR performance of P-NiFeOOH electrode especially suitable for industrial AWE, which mainly thanks to the targeted design with multiple advantages including the highly porous structure, as well as the robustly embedded contact and strong catalytic activity and corrosion resistance. Comparison of large-current overpotential and stability performance in 1 M and 6 M KOH for P-NiFeOOH and other WOR electrodes.
Practical application of water electrolyser. To demonstrate overall water electrolysis application, we assemble a two-electrode electrolyser using the present P-NiFeOOH as the anode coupled with the Pt foil as the benchmark cathode (Fig. 3A). Notably, as shown in Fig. 3B, the designed P-NiFeOOH ǁ Pt electrolyser displays remarkable activity for overall alkali water splitting in 1 M KOH, requiring small cell voltages of 1.653, 1.714, and 1.746 V to achieve current densities of 100, 500, and 1000 mA cm -2 , respectively, which are much lower than those for two benchmarks of IrO2 ǁ Pt and Ni mesh ǁ Pt. When in 6 M KOH, the cell voltages of P-NiFeOOH ǁ Pt electrolyser drop to 1.584, 1.652 and 1.692 V for delivering current densities of 100, 500, and 1000 mA cm -2 , respectively, even with only 1.743 V required to reach 2000 mA cm -2 ( Supplementary   Fig. 15). Gaseous produces were measured with an online gas chromatography for water electrolysis over P-NiFeOOH ǁ Pt electrolyzer operated at 100 mA cm -2 current density. As shown in the Supplementary Fig. 16, H2 and O2 evolving amounts match well with a stoichiometric ratio of 2:1 by overall water splitting, and the number of reacting electrons in catalysis is almost equal to that conducting in the circuit (Fig. 3C), indicating an almost 100% Faraday efficiency for overall water splitting.
Furthermore, the P-NiFeOOH ǁ Pt electrolyser proceeds long-term stable water electrolysis in the CP model with no noticeable performance decay for 150 h at large current densities of 500 and 1000 mA cm -2 in 6 M KOH electrolyte with different temperatures of 25 and 80 °C (Fig. 3D), and the polarization curve after the CP test is very close to the pristine level before stability test (Supplementary Fig. 17). The unit electricity consumption per cubic meter of H2 ( u ) and corresponding electricity-tohydrogenenergy conversion efficiency (ETH) were calculated to be 3.9 kW h and 90% ( Supplementary Fig. 18), respectively, whereas current commercial electrolysers commonly operate at an u of over 4.5 kW h m -3 with an STH less than 80% 54 . We summarize the cell voltages and durable times at industrial-level large current densities for reported water electrolysers consisting of transition-metal-based anodes with various cathodes for alkaline water splitting (Supplementary Table 3). The P-NiFeOOH electrode exhibits as a preferable alternative to costly IrO2/RuO2 anodes used in commercial hydrogen generators. In order to highlight the advantages of P-NiFeOOH in balancing activity and stability toward practical applications, we further compared the overpotentials and steady times at high current densities (5000 and 10000 A m -2 ) with other state-of-the-art WOR electrodes reported recently ( Fig. 3E and Supplementary Table 2). Evidently, the P-NiFeOOH electrode stands out from them and has evident superiorities both in high-current-density activity and long-term stability. Moreover, such win-win performance is creditable especially in 6 M KOH because large-current-density WOR electrodes steady in strong alkali are rarely reported to date. Combined with the simple and scalable preparation process ( Supplementary Fig. 19), the self-stabilized P-NiFeOOH electrode exhibits overall competitive advantages both from super performance and low capital cost toward practical applications.

Discussion
In summary, we have developed a non-noble metal based WOR electrode via facilely etching NF for industrially-relevant AWE with large current densities and harsh environments, by light of the support-stabilizing catalyst combination and porosityenabling catalysis acceleration. The produced electrode with in-situ-grown active P-NiFeOOH nanoparticles embedded in surface micropores exhibits an overpotential as low as only 284 mV to achieve high current density of 1000 mA cm -2 in 1 M KOH, which is among the best records for alkaline WOR, and also shows long durability for over 150 h at industrial-level large current densities in strong alkaline 6 M KOH electrolyte with elevated temperature up to 80 °C. The time-saving and low-cost production can be scaled up and make this electrode more cost-effective in commercial practice for massive water electrolysis to generate hydrogen. In particular, a new concept of "catalyst-substrate fusion electrode" based on targeted etching metal substrates was proposed to enable strong self-stabilizing effect on mechanical adhesion and charge/mass transfer at catalytic interfaces, which is instructive for designing highperformance electrodes competent for industrial-scale electrolysis.

Synthesis of P-
The electrochemically active surface areas (ECSA) were further evaluated as the ratio of dl to a constant specific capacitance ( S , 0.04 mF cm -2 ) by the following Formula 5: Real-time monitoring of gas evolution with a photocatalytic platform and gas chromatograph (Labsolar-IIIAG, Perfectlight Beijing; FULI 9790H) with a sealed electrolyser operating at 100 mA cm -2 current density from 0 to 60 minutes.
Calculation method. To evaluate the Faradaic efficiency ( F ), the reacting electron number ( re ) and conducting electron number ( ce ) were calculated and compared based on the following Formulas 6-8: where O is the mole amount of evolved O2, A is the avogadro number, is the electron charge, as well as , , and is the current density, effective electrode area, and the time, respectively.
The unit electricity consumption ( u , kW·h m -3 ) required to produce one standard cubic meter of H2 was calculated using the unit electric quantity ( u , A·h m -3 ) and cell voltage ( , V) at a certain current density of 500 mA cm -2 that is commonly operated in industrial alkali water electrolysis, according to the following Further, the electricity-to-hydrogen energy conversion efficiency (ETH) was estimated with a standard thermal neutral voltage ( = 1.48 V ) at which no waste heat is generated only for water splitting through the following Formula 11.

ETH =
1.48× u u ×1000 × 100% Data availability. All data supporting the results of this study are available from the corresponding authors upon reasonable request.