A Self‐Reconstructed Bifunctional Electrocatalyst of Pseudo‐Amorphous Nickel Carbide @ Iron Oxide Network for Seawater Splitting

Abstract Here, a sol‐gel method is used to prepare a Prussian blue analogue (NiFe‐PBA) precursor with a 2D network, which is further annealed to an Fe3O4/NiCx composite (NiFe‐PBA‐gel‐cal), inheriting the ultrahigh specific surface area of the parent structure. When the composite is used as both anode and cathode catalyst for overall water splitting, it requires low voltages of 1.57 and 1.66 V to provide a current density of 100 mA cm−2 in alkaline freshwater and simulated seawater, respectively, exhibiting no obvious attenuation over a 50 h test. Operando Raman spectroscopy and X‐ray photoelectron spectroscopy indicate that NiOOH2–x active species containing high‐valence Ni3+/Ni4+ are in situ generated from NiCx during the water oxidation. Density functional theory calculations combined with ligand field theory reveal that the role of high valence states of Ni is to trigger the production of localized O 2p electron holes, acting as electrophilic centers for the activation of redox reactions for oxygen evolution reaction. After hydrogen evolution reaction, a series of ex situ and in situ investigations indicate the reduction from Fe3+ to Fe2+ and the evolution of Ni(OH)2 are the origin of the high activity.


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Instruments Autosorb AS-6B. The samples were degassed in N 2 at 80 o C for 8 h before the measurements. The specific surface area was determined by the multi-point Brunauer-Emmett-Teller (BET) method and the pore-size distribution was calculated based on the Barrett-Joyner-Halenda (BJH) method. XPS analysis was conducted on a PHI-5000 VersaProbe X-ray photoelectron spectrometer using an Al Kα X-ray source. Operando Raman spectra was obtained with an inVia Renishaw confocal Raman microscope operated with an incident laser beam at 532 nm focused through a 50x objective (Leica). The laser intensity was set to < 1 mW and Raman spectra were collected in static mode, with an exposure time of few seconds every 5 minutes to minimize the sample heating. To monitor the evolution of catalyst samples during OER process, each Raman spectrum was collected after a constant potential was applied to the catalyst electrode for 5 min. Each Raman spectrum was obtained using an integration time of few seconds with accumulating 5 times. The laser shutter remained closed between spectrum collections. The gas product was extracted and analyzed using gas chromatography-mass spectrometry (GC-MS, 7890A and 5975C, Agilent). Static contact angles were measured with a contact angle meter, OCA20.

Electrochemical measurements
The electrochemical tests of the catalysts were performed using a Metrohm Autolab electrochemical workstation PGStat-12 (Utrecht, the Netherlands) connected to a three-electrode cell. A glassy carbon electrode (GCE) of 3 mm diameter served as the substrate for the working electrode. A carbon rod and a saturated Ag/AgCl/Clwere employed as the counter electrode and reference electrode, respectively. 5 mg of NiFe-PBA-gel-cal was dispersed in 4.5 mL of a water/isopropanol solution (1:3) containing 500 μL Nafion (5%). The resulting solution was sonicated for 0.5-1 h. When the solution was well dispersed, 4 μL of the above solution was dropped onto the clean GCE for electrochemical studies.
An O 2 -purged aqueous solution of 1 M KOH (alkaline freshwater) and 1 M KOH and 0.5 M NaCl (alkaline simulated seawater) were the electrolytes for OER and HER experiments. Cyclic voltammetry (CV) curves were recorded at a sweep rate of 100 mV s −1 for multiple cycles. Linear sweep voltammetry (LSV) was carried out at a scan rate of 5 mV s −1 for polarization curves. LSV was performed several times until the signals were stabilized. The Tafel and EIS plot measurements were performed under the same conditions as for OER evaluation. CV curves with 4 different scan rates (10-60 mV s -1 ) were measured over a potential range in which redox processes were absent to calculate the electrochemical double-layer capacitance: C dl = I c /ν, where C dl , I c , and ν are the double-layer capacitance (F cm -2 ) of the electroactive materials, charging current (mA cm -2 ), and scan rate (mV s -1 ). All results reported in this work were converted to the RHE scale according to the Nernst equation without any iR-correction, Turnover frequency (TOF) was calculated using the equation below, where j is the current density, A is the geometric area of electrode, F is the Faraday constant (96485 C mol -1 ), and n is the moles of the corresponding metal atom (mol) within the catalyst loading.
A water-splitting device with a two-electrode configuration was assembled. Both the cathode and the anode electrodes were made by depositing NiFe-PBA-gel-cal onto Ni foam (2 × 1 cm 2 ) and then drying in air. To obtain a total catalyst loading of approximately 1 mg cm -2 , the deposition process was repeated several times. Then, the Ni foams loaded with catalyst were fixed as electrodes on both sides of an "H" tube with each component containing electrolyte, separated by a Nafion membrane.

Computational details
Density functional theory (DFT) calculation was performed using the generalized gradient approximation (GGA) Perdew-Burke-Ernzerhof (PBE) functional, and the projected augmented plane-wave method implemented in the Vienna ab initio simulation program (VASP) software code. A trigonal primitive unit cell containing two-unit formula of Ni(OH) 2 was used, and the plane-wave basis-set cutoff was set to 400 eV. The structure is of P-3m1 space group, the same as our experimental XRD obtained data. The unit cell is D3d symmetry. The subsequent deprotonation modifications were carried out on the primitive unit cell. To obtain the projected density of state (PDOS), all structures were subjected to full relaxation until reaching a force threshold of 0.01 eV Å -1 . The Brillouin zone was sampled in a Monkhorst-Pack 3 × 3 × 2 k-points mesh for structural relaxation and 6 × 6 × 4 for static calculation. The Hubbard U correction was used to compensate the electron delocalization originated from GGA-PBE functional. The Ueff 5 was chosen to be 4 eV enforcing on Ni 3d atomic orbitals.
The Gibbs free energy of the adsorbed intermediate can be calculated as: where E ads is the adsorption energy of intermediate, ∆E ZPE is the zero-point energy difference between the adsorption state and gas state, T is the temperature (300 K), ∆S is the entropy various between the adsorption and gas phase.
6 Figure S1. SEM images of FeFe-PBA-gel.   FeFe-PBA-gel-cal for electrochemical OER in both alkaline freshwater and alkaline simulated seawater. Figure S4. Time-series mass spectra of the produced gas during OER in alkaline simulated seawater.           Figure S13. The OER performance of commercial IrO 2 and the HER performance of commercial Pt/C (10% wt. Pt).