Cobalt–Iron–Phosphate Hydrogen Evolution Reaction Electrocatalyst for Solar-Driven Alkaline Seawater Electrolyzer

Seawater splitting represents an inexpensive and attractive route for producing hydrogen, which does not require a desalination process. Highly active and durable electrocatalysts are required to sustain seawater splitting. Herein we report the phosphidation-based synthesis of a cobalt–iron–phosphate ((Co,Fe)PO4) electrocatalyst for hydrogen evolution reaction (HER) toward alkaline seawater splitting. (Co,Fe)PO4 demonstrates high HER activity and durability in alkaline natural seawater (1 M KOH + seawater), delivering a current density of 10 mA/cm2 at an overpotential of 137 mV. Furthermore, the measured potential of the electrocatalyst ((Co,Fe)PO4) at a constant current density of −100 mA/cm2 remains very stable without noticeable degradation for 72 h during the continuous operation in alkaline natural seawater, demonstrating its suitability for seawater applications. Furthermore, an alkaline seawater electrolyzer employing the non-precious-metal catalysts demonstrates better performance (1.625 V at 10 mA/cm2) than one employing precious metal ones (1.653 V at 10 mA/cm2). The non-precious-metal-based alkaline seawater electrolyzer exhibits a high solar-to-hydrogen (STH) efficiency (12.8%) in a commercial silicon solar cell.


Introduction
Hydrogen is a next-generation energy source that can solve environmental pollution and the energy-depletion crisis [1,2]. Among the various methods for producing hydrogen, electrochemical water splitting represents an ecofriendly, sustainable, and efficient route. To electrochemically produce hydrogen, enormous efforts have been devoted to the development of highly active electrocatalysts for water splitting in acidic or alkaline electrolytes containing high-purity fresh water. However, with the increasing demand for high-purity fresh water owing to the development of water splitting through electrolysis, the possibility of challenges, such as water distribution, must be considered [3,4]. Thus, the electrolysis of seawater is a promising alternative for mitigating the challenges accompanying the supply of high-purity freshwater. Seawater is the most abundant source of water resources on Earth; it can be employed as an inexpensive electrolyte for electrochemical water splitting [5]. However, despite these advantages, the side reactions caused by the chlorine ions (Cl − ) in seawater prevent seawater electrolysis [6][7][8][9]. Recently, it has been reported that the selectivity of the oxygen evolution reaction (OER) can be improved by changing the thermodynamic potential of the chlorine evolution reaction (ClER) via the adjustment of the pH of the seawater; thus, many ongoing studies have focused on developing catalysts for OER [10][11][12]. However, since the hydrogen evolution reaction (HER) is 2.2. Synthesis of (Co,Fe) 3

O 4 and (Co,Fe)PO 4
The prepared (Co,Fe)OOH was converted into a (Co,Fe) 3 O 4 sample via calcination for 2 h in the air at 500 • C and a heating rate of 5 • C/min employing a tube furnace. The (Co,Fe) 3 O 4 sample, which was named (Co,Fe) 3 O 4 , was obtained after cooling to room temperature.
The (Co,Fe)PO 4 sample was synthesized via a phosphidation process. Briefly, (Co,Fe) 3 O 4 and 2.0 g of sodium hypophosphite (NaH 2 PO 4 , Sigma-Aldrich Inc., St. Louis, MO, USA) were placed in two separate ceramic boats in a tube furnace. Next, NaH 2 PO 4 and (Co,Fe) 3 O 4 were placed at the upstream and downstream sides of the Ar gas flow, respectively. Subsequently, the tube furnace was heated for 2 h to 500 • C in Ar atmosphere at a heating rate of 5 • C/min and air cooled to room temperature. The (Co,Fe)PO 4 sample, which was obtained via the phosphidation of (Co,Fe) 3 O 4 for 2 h, was named (Co,Fe)PO 4.

Characterization of Physical Properties
X-ray diffraction (XRD) patterns were recorded on an X-ray diffractometer (Ultima IV, Rigaku, Tokyo, Japan) employing a Cu-Kα radiation source over the 2θ range of 10 • -90 • at a scan rate of 2 • /min. The surface morphologies of the samples were examined by field-emission scanning electron microscopy (FE-SEM, MIRA 3, TESCAN, Brno, Czechia). FE-transmission electron microscopy (FE-TEM), high-resolution TEM (HR-TEM), selected area electron diffraction (SAED), and elemental distribution spectroscopy (EDS) were performed on a TALOS F200X (Thermo Fisher Scientific, Waltham, USA). Further, the chemical states were investigated by X-ray photoelectron spectroscopy (XPS, K-Alpha + XPS System, Thermo Fisher Scientific, Waltham, USA).

Electrochemical Characterization
The electrochemical properties of the electrocatalysts were investigated using a potentiostat (VersaSTAT 4, AMETEK, Oak Ridge, USA) in a three-electrode cell at room temperature. The synthesized (Co,Fe)OOH, (Co,Fe) 3 O 4 , and (Co,Fe)PO 4 electrocatalysts were employed as the working electrode with dimensions of 1 cm × 1 cm. Hg/HgO (1 M KOH) and a graphite rod were employed as the reference and counter electrodes for the HER, respectively. The polarization curves for the HER activity were recorded via linear sweep voltammetry (LSV) at a scan rate of 1 mV/s in N 2 -purged 1 M KOH, 1 M KOH + 0.5 M NaCl, and 1 M KOH + seawater as the electrolyte. Real seawater was collected from the sea of Haeundae (Busan, Korea). The recorded potentials were converted into reversible hydrogen electrode (RHE) according to Nernst's equation (E RHE = E Hg/HgO + 0.0591 × pH + 0.098). All the electrochemical tests were performed with 90% iR compensation, and the Tafel slopes were measured from the corresponding polarization curves. Electrochemical impedance spectroscopy (EIS) was performed at an overpotential of −0.25 V RHE for HER in the frequency range from 100 kHz to 0.01 Hz with an amplitude of 10 mV. The double-layer capacitance (C dl ) was estimated in the 1 M KOH solution via cyclic voltammetry (CV) at different scan rates (10,20,40,80, and 160 mV/s) in the non-faradaic region. The durability tests for HER were performed at a constant current density of −100 mA/cm 2 for 72 h. The Faradaic efficiency (FE) was determined via the water displacement method. The volume of the generated H 2 was measured by collecting the amount of H 2 gas at a constant current density of 50 mA/cm 2 . To prepare the Pt/C noble metal electrocatalysts for comparison, an ink solution was fabricated by mixing commercial Pt/C powder (20 mg), 5 wt.% Nafion solution (100 µL), and ethanol (900 µL). Thereafter, the ink solution was coated onto the surface of an iron foam (1 cm × 1 cm) after ultrasonic dispersion for 15 min. The loading mass of Pt/C was~3 mg/cm 2 . Figure 1 shows the schematic for synthesizing the (Co,Fe)PO 4 electrocatalysts. Firstly, (Co,Fe)OOH was directly synthesized on the iron foam via surface corrosion in a CoCl 2 aqueous solution at room temperature. The prepared (Co,Fe)OOH was converted into (Co,Fe) 3 O 4 through calcination, after which the nanoneedle shape of (Co,Fe)PO 4 was synthesized through phosphidation. To determine the crystalline structures of the synthesized (Co,Fe)OOH, (Co,Fe)3O4, and (Co,Fe)PO4 electrocatalysts, the XRD patterns were obtained ( Figure S1). The diffraction peaks of the (Co,Fe)OOH sample appeared at 2θ = 27.0°, 36.4°, 46.9°, 60.8°, and 79.6°, and were indexed to the (021), (130), (150), (132), and (202) planes, respectively, of iron oxyhydroxide (FeOOH, JCPDS # 01-073-2326). The diffraction peaks of the (Co,Fe)3O4 sample that appeared at 2θ = 18.3°, 30.1°, 35.5°, 37.1°, 43.1°, 57.0°, and 62.6° were indexed to the (111), (220), (311), (222), (400), (511), and (440) planes, respectively, of iron oxide (Fe3O4, JCPDS # 01-075-0033). However, for the (Co,Fe)PO4 sample, both the iron phosphate and iron oxide phases were discerned, corresponding to FePO4 (JCPDS # 00-029-0715) and Fe3O4 (JCPDS # 01-075-0033). This result indicated the hybrid structure of (Co,Fe)3O4 and (Co,Fe)PO4. The peaks of Fe3O4 exhibited almost identical diffraction peaks with the (Co,Fe)3O4 sample, and the peaks from the (100) and (102) planes of FePO4 were observed at 20.3° and 25.8°, respectively. Generally, chemical transformation reactions, such as sulfurization, phosphidation, and selenization, also occurred at the surface. [45][46][47] Therefore, after phosphidation, the outer region of (Co,Fe)3O4 was converted into (Co,Fe)PO4, and the inner region of (Co,Fe)3O4 did not participate in the chemical transformation reaction. [48,49].

Results and Discussion
The surface morphologies of (Co,Fe)OOH, (Co,Fe)3O4, and (Co,Fe)PO4 were observed via the FE-SEM images. (Co,Fe)OOH exhibited thin nanosheets ( Figure S2), and (Co,Fe)3O4, which was obtained by calcining (Co,Fe)OOH, exhibited a nanoneedle morphology ( Figure S3). Interestingly, (Co,Fe)PO4 and (Co,Fe)3O4 exhibited almost the same surface morphologies even after phosphidation (Figure 2a,b). Particularly, the shape of the nanoneedles could extensively increase the concentrations of the reactants in the active sites and enhance local electric fields that promote the intrinsic catalytic activity. [50] Therefore, the shape of (Co,Fe)PO4 is suitable for electrochemical water splitting.  [45][46][47]. Therefore, after phosphidation, the outer region of (Co,Fe) 3 O 4 was converted into (Co,Fe)PO 4 , and the inner region of (Co,Fe) 3 O 4 did not participate in the chemical transformation reaction [48,49].
The surface morphologies of (Co,Fe)OOH, (Co,Fe) 3 O 4 , and (Co,Fe)PO 4 were observed via the FE-SEM images. (Co,Fe)OOH exhibited thin nanosheets ( Figure S2), and (Co,Fe) 3 O 4 , which was obtained by calcining (Co,Fe)OOH, exhibited a nanoneedle morphology ( Figure S3). Interestingly, (Co,Fe)PO 4 and (Co,Fe) 3 O 4 exhibited almost the same surface morphologies even after phosphidation (Figure 2a,b). Particularly, the shape of the nanoneedles could extensively increase the concentrations of the reactants in the active sites and enhance local electric fields that promote the intrinsic catalytic activity [50]. Therefore, the shape of (Co,Fe)PO 4 is suitable for electrochemical water splitting.
The TEM images were obtained to confirm the surface morphology and phase information. (Co,Fe)OOH exhibited a nanosheet morphology ( Figure S4a). After calcination, Nanomaterials 2021, 11, 2989 5 of 13 (Co,Fe) 3 O 4 exhibiting a nanoneedle shape was obtained ( Figure S5a) owing to the escape of the water molecules in (Co,Fe)OOH during calcination. Interestingly, the nanoneedle shape was maintained well after phosphidation ( Figure 2c). Furthermore, the phase information was obtained from the SAED patterns. The ring patterns of (Co,Fe)OOH were indexed to the planes of the (021), (130), (150), and (202) reflections of FeOOH (inset of Figure S4a). Additionally, the ring patterns of (Co,Fe) 3 O 4 were indexed to the planes of the (111), (220), (311), and (222) reflections of Fe 3 O 4 (inset of Figure S5a). Further, the elemental distributions of (Co,Fe)OOH and (Co,Fe) 3 O 4 were uniform ( Figures S4b and S5b). The lattice fringes and ring pattern of (Co,Fe)PO 4 exhibited both Fe 3 O 4 and FePO 4 patterns, which are consistent with the XRD results (Figure 2d-g). The EDS mapping of (Co,Fe)PO 4 confirmed that each element was uniformly distributed therein ( Figure 2h). The EDX spectrum in the collected area is shown in Figure S7. Interestingly, the high-magnification TEM-EDS mapping images revealed that elemental P was mainly distributed in the outer region and that elemental Co and Fe were mainly distributed in the inner region. Additionally, elemental O was uniformly distributed in the inner and outer regions ( Figure S6). These results indicated that the chemical transformation reaction proceeded on the surface. The TEM images were obtained to confirm the surface morphology and phase information. (Co,Fe)OOH exhibited a nanosheet morphology ( Figure S4a). After calcination, (Co,Fe)3O4 exhibiting a nanoneedle shape was obtained ( Figure S5a) owing to the escape of the water molecules in (Co,Fe)OOH during calcination. Interestingly, the nanoneedle shape was maintained well after phosphidation (Figure 2c). Furthermore, the phase information was obtained from the SAED patterns. The ring patterns of (Co,Fe)OOH were indexed to the planes of the (021), (130), (150), and (202) reflections of FeOOH (inset of Figure S4a). Additionally, the ring patterns of (Co,Fe)3O4 were indexed to the planes of the (111), (220), (311), and (222) reflections of Fe3O4 (inset of Figure S5a). Further, the elemental distributions of (Co,Fe)OOH and (Co,Fe)3O4 were uniform (Figures S4b and S5b). The lattice fringes and ring pattern of (Co,Fe)PO4 exhibited both Fe3O4 and FePO4 patterns, which are consistent with the XRD results (Figure 2d-g). The EDS mapping of (Co,Fe)PO4 confirmed that each element was uniformly distributed therein (Figure 2h). The EDX spectrum in the collected area is shown in Figure S7. Interestingly, the high-magnification TEM-EDS mapping images revealed that elemental P was mainly distributed in the outer region and that elemental Co and Fe were mainly distributed in the inner region. Additionally, elemental O was uniformly distributed in the inner and outer regions ( Figure S6). These results indicated that the chemical transformation reaction proceeded on the surface.  XPS analysis was performed to investigate the surface chemical states of (Co,Fe) 3 O 4 and (Co,Fe)PO 4 ( Figure 3). Figure 3a shows the full XPS survey spectra of (Co,Fe) 3 O 4 and (Co,Fe)PO 4 , which clearly confirmed the existence of Co, Fe, P and O. Figure 3b-e shows the HR-XPS profiles of Co, Fe, P and O. Notably, the binding energies of Co 2p and Fe 2p shifted along a higher direction after phosphidation. Additionally, the binding energy of O 1s shifted along a higher direction. These observations indicated that electrons were transferred from (Co,Fe) 3 O 4 to (Co,Fe)PO 4 in the hybrid (Co,Fe) 3 O 4 and (Co,Fe)PO 4 structures [51]. This changed local charge-density distribution was expected to reduce the energy barrier of HER, thus facilitating the adsorption and desorption processes between the reactant and resultant molecules [52][53][54][55].
XPS analysis was performed to investigate the surface chemical states of (Co,Fe)3O4 and (Co,Fe)PO4 ( Figure 3). Figure 3a shows the full XPS survey spectra of (Co,Fe)3O4 and (Co,Fe)PO4, which clearly confirmed the existence of Co, Fe, P and O. Figure 3b-e shows the HR-XPS profiles of Co, Fe, P and O. Notably, the binding energies of Co 2p and Fe 2p shifted along a higher direction after phosphidation. Additionally, the binding energy of O 1s shifted along a higher direction. These observations indicated that electrons were transferred from (Co,Fe)3O4 to (Co,Fe)PO4 in the hybrid (Co,Fe)3O4 and (Co,Fe)PO4 structures. [51] This changed local charge-density distribution was expected to reduce the energy barrier of HER, thus facilitating the adsorption and desorption processes between the reactant and resultant molecules. [52][53][54][55]. LSV was performed to measure the HER activity in a 1 M KOH solution (Figure 4a). For comparison, Pt/C, which is a benchmark precious metal electrocatalyst for HER, was tested; it exhibited a low overpotential of 48 mV at −10 mA/cm 2 . Moreover, (Co,Fe)OOH and (Co,Fe)3O4 exhibited overpotentials of 215 and 191 mV at −10 mA/cm 2 , respectively. Interestingly, (Co,Fe)PO4, obtained through phosphidation, exhibited a significantly reduced overpotential (122 mV at −10 mA/cm 2 ). Although the overpotential of (Co,Fe)PO4 was relatively higher compared with that of Pt/C, it still outperformed Pt/C at a high current density. This result is because the nanoneedle shape increased the concentration of the reactant in the active site and concurrently enhanced the local electric field. [50] The Tafel plots were calculated to elucidate the electrocatalytic kinetics. Figure 4b shows the Tafel slopes that were derived from the HER polarization curves. (Co,Fe)PO4 displayed a lower Tafel slope (−71 mV/dec) compared with those of (Co,Fe)3O4 (−77 mV/dec), (Co,Fe)OOH (−85 mV/dec), and the bare iron foam (−111 mV/dec). These results indicate that (Co,Fe)PO4 exhibited faster reaction kinetics for HER. Generally, HER proceeds via two different reaction routes: the Volmer-Heyrovsky and Volmer-Tafel mechanisms. [56,57] Considering the Tafel slope of (Co,Fe)PO4, it was inferred that (Co,Fe)PO4 followed the Volmer-Heyrovsky mechanism. [58] The electrochemically active surface area (ECSA) was estimated employing Cdl that was derived via CV in the non-Faradaic region ( Figures  S8 and S9). (Co,Fe)PO4 and (Co,Fe)OOH exhibited the highest and the smallest Cdl values, respectively, indicating that (Co,Fe)PO4 exhibited the largest ECSA. Since ECSA was directly proportional to the number of active sites, as well as the efficiency of the mass and charge transports of catalysts, the largest ECSA of (Co,Fe)PO4 indicated that it exhibited the most active sites, as well as the most effective mass-and charge-transport capabilities, LSV was performed to measure the HER activity in a 1 M KOH solution (Figure 4a). For comparison, Pt/C, which is a benchmark precious metal electrocatalyst for HER, was tested; it exhibited a low overpotential of 48 mV at −10 mA/cm 2 . Moreover, (Co,Fe)OOH and (Co,Fe) 3 O 4 exhibited overpotentials of 215 and 191 mV at −10 mA/cm 2 , respectively. Interestingly, (Co,Fe)PO 4 , obtained through phosphidation, exhibited a significantly reduced overpotential (122 mV at −10 mA/cm 2 ). Although the overpotential of (Co,Fe)PO 4 was relatively higher compared with that of Pt/C, it still outperformed Pt/C at a high current density. This result is because the nanoneedle shape increased the concentration of the reactant in the active site and concurrently enhanced the local electric field [50]. The Tafel plots were calculated to elucidate the electrocatalytic kinetics. Figure 4b shows the Tafel slopes that were derived from the HER polarization curves. (Co,Fe)PO 4 displayed a lower Tafel slope (−71 mV/dec) compared with those of (Co,Fe) 3 O 4 (−77 mV/dec), (Co,Fe)OOH (−85 mV/dec), and the bare iron foam (−111 mV/dec). These results indicate that (Co,Fe)PO 4 exhibited faster reaction kinetics for HER. Generally, HER proceeds via two different reaction routes: the Volmer-Heyrovsky and Volmer-Tafel mechanisms [56,57]. Considering the Tafel slope of (Co,Fe)PO 4 , it was inferred that (Co,Fe)PO 4 followed the Volmer-Heyrovsky mechanism [58]. The electrochemically active surface area (ECSA) was estimated employing C dl that was derived via CV in the non-Faradaic region ( Figures S8 and S9). (Co,Fe)PO 4 and (Co,Fe)OOH exhibited the highest and the smallest C dl values, respectively, indicating that (Co,Fe)PO 4 exhibited the largest ECSA. Since ECSA was directly proportional to the number of active sites, as well as the efficiency of the mass and charge transports of catalysts, the largest ECSA of (Co,Fe)PO 4 indicated that it exhibited the most active sites, as well as the most effective mass-and charge-transport capabilities, which imparted it with the best HER activity [59]. EIS was performed to confirm the charge-transfer resistances of (Co,Fe)OOH, (Co,Fe) 3 O 4 , and (Co,Fe)PO 4 . Figure 4c shows the Nyquist plots, which were fitted into an inserted equivalent-circuit model, where R s is the solution resistance and R ct is the charge-transfer resistance [60]. (Co,Fe)PO 4 exhib-ited the smallest semicircular diameter (R ct = 0.60 Ω), indicating the lowest R ct compared with those of (Co,Fe) 3 O 4 (R ct = 1.44 Ω) and (Co,Fe)OOH (R ct = 1.89 Ω). To confirm the catalytic activity for HER in alkaline seawater, the LSV graphs were measured for different electrolytes (Figure 4d): alkaline solution (1 M KOH), artificial alkaline seawater (1 M KOH + 0.5 M NaCl), and real alkaline seawater (1 M KOH + seawater). The overpotentials of (Co,Fe)PO 4 were 134 and 137 mV at a current density of 10 mA/cm 2 in the 1 M KOH + 0.5 M NaCl and 1 M KOH + seawater electrolytes, respectively. The HER activities of (Co,Fe)PO 4 in 1 M KOH + 0.5 M NaCl and 1 M KOH + seawater were slightly lower than that of 1 M KOH. In the seawater environment, including real alkaline seawater, the HER activity was reduced owing to the blocking of Mg(OH) 2 or Ca(OH) 2 by the active site via precipitation [61]. Furthermore, impurities, such as bacteria, in the seawater interfered with the electrochemical reaction [6]. Compared with Pt/C, (Co,Fe)PO 4 exhibited better HER activity in real alkaline seawater, as well as the 1 M KOH solution, at a high current density (Figure 4e). FE was measured by collecting the generated H 2 gas via the water displacement method at a constant current density of −50 mA/cm 2 (Figure 4f). The FEs of (Co,Fe)PO 4 in 1 M KOH, 1 M KOH + NaCl, and 1 M KOH + seawater were still >98.6%, 96.5%, and 96.3% after 60 min, indicating that most of the electrons that participated in the reaction were consumed during HER. In addition to the catalytic activity, durability is also an essential factor for evaluating the performance of catalysts in practical applications [62,63]. The long-term stability of (Co,Fe)PO 4 for HER was tested by measuring the potentials in different electrolytes for over 72 h at a constant current density of −100 mA/cm 2 (Figure 4g-i). The measured potential indicated high stability during the continuous operation in all electrolytes (no noticeable deterioration was observed), demonstrating its excellent HER durability. Nanomaterials 2021, 11, x 7 of 13 which imparted it with the best HER activity. [59] EIS was performed to confirm the charge-transfer resistances of (Co,Fe)OOH, (Co,Fe)3O4, and (Co,Fe)PO4. Figure 4c shows the Nyquist plots, which were fitted into an inserted equivalent-circuit model, where Rs is the solution resistance and Rct is the charge-transfer resistance. [60] (Co,Fe)PO4 exhibited the smallest semicircular diameter (Rct = 0.60 Ω), indicating the lowest Rct compared with those of (Co,Fe)3O4 (Rct = 1.44 Ω) and (Co,Fe)OOH (Rct = 1.89 Ω). To confirm the catalytic activity for HER in alkaline seawater, the LSV graphs were measured for different electrolytes ( Figure 4d): alkaline solution (1 M KOH), artificial alkaline seawater (1 M KOH + 0.5 M NaCl), and real alkaline seawater (1 M KOH + seawater). The overpotentials of (Co,Fe)PO4 were 134 and 137 mV at a current density of 10 mA/cm 2 in the 1 M KOH + 0.5 M NaCl and 1 M KOH + seawater electrolytes, respectively. The HER activities of (Co,Fe)PO4 in 1 M KOH + 0.5 M NaCl and 1 M KOH + seawater were slightly lower than that of 1 M KOH. In the seawater environment, including real alkaline seawater, the HER activity was reduced owing to the blocking of Mg(OH)2 or Ca(OH)2 by the active site via precipitation. [61] Furthermore, impurities, such as bacteria, in the seawater interfered with the electrochemical reaction. [6] Compared with Pt/C, (Co,Fe)PO4 exhibited better HER activity in real alkaline seawater, as well as the 1 M KOH solution, at a high current density (Figure 4e). FE was measured by collecting the generated H2 gas via the water displacement method at a constant current density of −50 mA/cm 2 (Figure 4f). The FEs of (Co,Fe)PO4 in 1 M KOH, 1 M KOH + NaCl, and 1 M KOH + seawater were still >98.6%, 96.5%, and 96.3% after 60 min, indicating that most of the electrons that participated in the reaction were consumed during HER. In addition to the catalytic activity, durability is also an essential factor for evaluating the performance of catalysts in practical applications. [62,63] The long-term stability of (Co,Fe)PO4 for HER was tested by measuring the potentials in different electrolytes for over 72 h at a constant current density of −100 mA/cm 2 (Figure 4g-i). The measured potential indicated high stability during the continuous operation in all electrolytes (no noticeable deterioration was observed), demonstrating its excellent HER durability.  Regarding the full-cell applications, a two-electrode alkaline water electrolyzer, which was assembled with (Co,Fe)PO 4 and NiFeOOH as the cathode and anode, respectively, was set up for overall seawater splitting employing alkaline natural seawater (1 M KOH + seawater) (Figure 5a). NiFeOOH, which is known as the best OER catalyst, was prepared, following a reported method, [64] and the polarization curve of the OER activity is shown in Figure S10. To avoid interference with the oxidation current, a cell voltage of 10 mA/cm 2 was measured via reverse-swept CV [65]. Interestingly, Figure 5b shows that the NiFeOOH//(Co,Fe)PO 4 electrolyzer exhibited excellent activity in this two-electrode system for overall seawater splitting in 1 M KOH + seawater. This electrolyzer required low voltages of 1.625 (η = 395 mV at 10 mA/cm 2 ), 1.749 (η = 519 mV at 50 mA/cm 2 ), and 1.801 V (η = 571 mV at 100 mA/cm 2 ) in 1M KOH + seawater, demonstrating better overall water-splitting performance compared with the IrO 2 //Pt/C precious metal electrolyzer in both 1 M KOH ( Figure S11) and 1 M KOH + seawater (Figure 5b). The performance of the NiFeOOH//(Co,Fe)PO 4 electrolyzer in 1 M KOH + seawater was comparable with or outperformed the recently reported electrolyzer that was based on the transition metal electrolyzer (Figure 5d). The FE of the NiFeOOH//(Co,Fe)PO 4 alkaline water electrolyzer was calculated by collecting the generated O 2 and H 2 gases for 60 min from each electrode at a constant current density of 50 mA/cm 2 in 1 M KOH + seawater (Figure 5c). The measured FE demonstrated high energy conversion rates of 97.4% and 99.0% for HER and OER in 1M KOH + seawater, respectively. Moreover, the NiFeOOH//(Co,Fe)PO 4 electrolyzer in 1 M KOH + seawater exhibited excellent durability. To confirm the long-term stability of the (Co,Fe)PO 4 electrolyzer, the measured voltage at a constant current density of +100 mA/cm 2 remained very stable without any noticeable deterioration for 50 h in the 1 M KOH + seawater electrolytes (Figure 5e). The durability test, which was conducted in the 1 M KOH electrolyte at a constant current density (+100 mA/cm 2 ) for 50 h, further confirmed the high stability ( Figure S12). These results demonstrate that the NiFeOOH//(Co,Fe)PO 4 alkaline water electrolyzer exhibited a high potential for application as a high-efficiency and durable seawater electrolyzer in natural seawater environments. In order to confirm the change in the morphology and phase, the SEM image and XRD patterns were presented in Figure S13. The surface morphology after durability test was well maintained. In addition, the XRD pattern showed an almost identical pattern to that of (Co,Fe)PO 4 before the durability test. These results indicate that the morphology and crystal structure of (Co,Fe)PO 4 were still maintained after the durability test. Furthermore, driving the electrolysis with natural solar power without artificial current is an ecofriendly and attractive method for conserving the cost of hydrogen production. Thus, the NiFeOOH//(Co,Fe)PO4 seawater electrolyzer was combined with a commercial silicon solar cell to set up a photo-assisted water-splitting system (Figure 5f), after which the overall seawater splitting performance was evaluated in the 1 M KOH + seawater electrolyte under natural sunlight. Figure 5g shows the J-V curve of a commercial silicon solar cell, and the solar-to-hydrogen (STH) efficiency was calculated from the intersection of the power curve of the solar cell and the polarization curve of the electrolyzer,[8] yielding an STH of 12.8%. When this photo-assisted seawater splitting device was driven under natural sunlight, the continuous release of H2 and O2 bubbles from both electrodes was clearly observed, confirming the successful production of H2 (inset of Figure 5e). Therefore, the photo-assisted seawater splitting system developed in this study could be applied to cost-effective hydrogen production in the seawater-splitting industry.

Conclusions
In summary, a non-precious-metal catalyst, (Co,Fe)PO4, was developed as an HER electrocatalyst for alkaline seawater electrolysis. (Co,Fe)PO4 demonstrated impressive HER activity with a low overpotential of 134 mV at −10 mA/cm 2 in 1 M KOH + seawater, as well as excellent durability. The nanoneedle shape of (Co,Fe)PO4 enhanced the local electric field, and its electronic structure, which was modified via phosphidation, enhanced the HER activity. The assembled seawater electrolyzer employing the non- Furthermore, driving the electrolysis with natural solar power without artificial current is an ecofriendly and attractive method for conserving the cost of hydrogen production. Thus, the NiFeOOH//(Co,Fe)PO 4 seawater electrolyzer was combined with a commercial silicon solar cell to set up a photo-assisted water-splitting system (Figure 5f), after which the overall seawater splitting performance was evaluated in the 1 M KOH + seawater electrolyte under natural sunlight. Figure 5g shows the J-V curve of a commercial silicon solar cell, and the solar-to-hydrogen (STH) efficiency was calculated from the intersection of the power curve of the solar cell and the polarization curve of the electrolyzer [8], yielding an STH of 12.8%. When this photo-assisted seawater splitting device was driven under natural sunlight, the continuous release of H 2 and O 2 bubbles from both electrodes was clearly observed, confirming the successful production of H 2 (inset of Figure 5e). Therefore, the photo-assisted seawater splitting system developed in this study could be applied to cost-effective hydrogen production in the seawater-splitting industry.

Conclusions
In summary, a non-precious-metal catalyst, (Co,Fe)PO 4 , was developed as an HER electrocatalyst for alkaline seawater electrolysis. (Co,Fe)PO 4 demonstrated impressive HER activity with a low overpotential of 134 mV at −10 mA/cm 2 in 1 M KOH + seawater, as well as excellent durability. The nanoneedle shape of (Co,Fe)PO 4 enhanced the local electric field, and its electronic structure, which was modified via phosphidation, enhanced the HER activity. The assembled seawater electrolyzer employing the non-precious-metal catalysts delivered excellent performance (1.625 V in 1 M KOH + seawater), which surpassed those of precious-metal-based electrolyzers. Further, the combination of the non-precious-metalbased electrolyzer with a commercial silicon solar cell successfully generated H 2 gas under natural sunlight in alkaline natural seawater. This study demonstrates that non-preciousmetal-based electrolyzers can outperform precious-metal-based ones, indicating that costeffective hydrogen production without artificial current is feasible with commercial silicon solar cells.

Conflicts of Interest:
The authors declare no conflict of interest.