ABSTRACT
The kinetic process of a slow oxygen evolution reaction (OER) always constrains the efficiency of overall water electrolysis for H2 production. In particular, nonprecious metal electrodes for the OER have difficulty simultaneously possessing excellent electrocatalytic activity and stability in pH-universal media. In this work, urea is first used as a pore-forming agent and active C/N source to fabricate a nanoporous NiFeCoCN medium-entropy alloy (MEA) by high-temperature sintering based on the nanoscale Kirkendall effect. The NiFeCoCN MEA achieves an overpotential of 432 mV at a current density of 10 mA cm-2 and a lower Tafel slope of 52.4 mV dec-1 compared to the IrO2/Ti electrode (58.6 mV dec-1) in a 0.5 M H2SO4 solution. In a 1 M KOH solution, the NiFeCoCN MEA obtains an overpotential of 177 mV for 10 mA cm-2 and a Tafel slope of 36.1 mV dec-1, which is better than IrO2/Ni foam. This work proves a novel strategy to design and prepare nanoporous MEA materials with desirable C/N species, which provides promising prospects for the industrial production of H2 energy.
KEYWORDS: Oxygen evolution reaction; Medium-entropy alloy; Urea; Electrocatalytic activity; Stability
INTRODUCTION
Hydrogen energy is an important component of the future energy system, and it is an important carrier to achieve green and low-carbon development [1, 2]. The oxygen evolution reaction (OER) as an anode reaction during water decomposition is a four-electron transfer process, which requires a higher overpotential than the hydrogen evolution reaction (HER) in a two-electron transfer process [3, 4]. This phenomenon severely limits the efficiency of electrolyzed water to hydrogen. In industrial applications, the IrO2 electrocatalyst has good electrocatalytic activity and stability, especially in acidic solution [5, 6], but the scarcity of iridium and extremely high cost make it difficult to apply on a large scale in the industry of H2 production [7, 8]. Since the anode in the proton exchange membrane (PEM) water electrolytic cell is performed in a strongly acidic environment, the anodic electrocatalyst is usually defined as IrO2 [9, 10]. Moreover, the current electrocatalysts for the alkalious OER must further decrease the overpotential and increase the Faraday efficiency. Therefore, research on nonprecious metal-based electrocatalysts with high activity performance and long-term stability for pH-universal OER is significant [11-14].
High-entropy alloys (HEAs) can be used as OER electrocatalysts because of the synergistic effect among different elements in the solid solution. However, HEAs requires all atoms to exist in similar proportions to ensure an entropy of 1.5 R or higher, which has fundamental limitations in optimizing the composition of electrocatalysts [15-18]. Medium-entropy alloys (MEAs) are composed of three or four main elements, and their entropy is smaller than that of HEAs but higher than that of traditional alloys. Compared with HEAs, MEAs are easier to control the composition of the alloy to maximize catalytic activity while keeping the long-term stability, and due to the lattice distortion and synergy effects, MEAs also has excellent electrocatalytic activity [15, 19]. Meanwhile, the single phase and low free enthalpy of MEAs contribute to excellent corrosion resistance [20]. However, the application of MEAs in the field of OER electrocatalysis has rarely been reported. Han et al. prepared three-dimensional porous medium-entropy alloy aerogels (MEAAs) of Ni50Co15Fe30Cu5by using a self-propagating sintering method, which simultaneously achieved excellent methanol oxygen reaction (MOR) and HER electrocatalytic properties [21].
Compounds of Ni, Fe and Co have higher catalytic activity than Ir in OER, which is due to they are the first-row transition metals in the periodic table, have lower crystal field stabilization energy and smaller d orbitals than Ir [22-24]. The bulk Ni in the elemental Ni-based oxide for the OER is considered the active centre of the OER, and oxygen is released through the NiOO- precursor. The incorporation of Fe in electrocatalysts usually enables the OER to occur at lower overpotentials because new highly active sites are generated on the surface [25, 26]. The incorporation of Co in electrocatalysts can improve the conductivity of the surface (oxy)hydroxide layer based on the synergistic effect of multiple metal species and further promoting proton migration and charge transportation on the catalyst surface [27, 28]. Recently, studies have shown that the electrocatalyst with C and N elements produces higher OER activity than the pristine one by adjusting the binding energies of the adsorbed/desorbed intermediate (*O, *OH and *OOH) on the active sites [29-32]. For example, our previous work proves that C/N/S doping in Ni-based alloy electrodes improved the OER activity via the nitrocarburizing and sulfonitrocarburizing technology [33].
In this work, urea (CH4N2O) with theoretical carbon and nitrogen contents of 20 at.% and 46 at.%, respectively [34, 35], is employed as a pore-forming agent and active C/N source to fabricate a nanoporous NiFeCoCN medium-entropy alloy (MEA) based on the nanoscale Kirkendall effect via high-temperature sintering The NiFeCoCN MEA exhibits excellent electrocatalytic activity and long-term stability in pH-universal media.
EXPERIMENTAL SECTION
Preparation of bulk NiFeCoCN MEA
Iron (12.7 μm, > 99.5%), cobalt (13.7 μm, > 99.5%) and nickel (17.2 μm, > 99.7%) powders and urea (molecular biology grade, ≥ 99.5%) were purchased from Search Bureau Chemicals (Shanghai, China). First, Ni, Fe and Co powders were weighed with an atomic percent of 56 at.%, 33 at.%, 11 at.%, then, uniformly ground urea powders were added in different mass percentages (0-30 wt.%). For comparison, pure Ni, pure Fe, pure Co, NiFe, NiCo, FeCo, NiFeCo, NiFeCoCN, NiFeCoCN-1 and NiFeCoCN-2 were fabricated, and all as-prepared samples were described in Table S1. According to the Linear Sweep Voltammetry (LSV) data of all as-prepared samples (see Figure S1), the optimal composition of NiFeCoCN was identified for the preparation of nanoporous compound electrodes. A planetary ball mill (Nanjing University Instrument Factory XQM-2A) was used to mix all metal and urea at a rotational speed of 260 r/min for 6 hours, and ball grinding beads were not placed in the ball grinding tank to avoid element pollution. The mixed powder was pressed into a cylindrical compact of φ12 mm × 6 mm under 580 MPa. The tube furnace (Hefei Kejing Materials Technology Corp.,OTF-1200X) was employed to sinter the cylindrical compact at a high temperature of 900 °C for 1 hour. Argon gas was poured into evacuate the air in the tube for 20 minutes before sintering. The entire sintering process was performed in a high-purity argon atmosphere.
Materials characterizations
The phase analysis of the as-prepared samples was performed using XRD (Bruker D8 ADVANCE). The surface morphology, microstructure and element distribution of the electrode were determined by FESEM (FEI Nova Nano SEM450). Transmission electron microscopy (TEM) attached with an energy dispersive X-ray spectrometer (EDS), high-resolution TEM (HRTEM, FEI Talos F200X) equipped with selected area electron diffraction (SAED) were used to verify the crystal structure and element distribution. XPS (KRATOS ANALYTICAL LTD Axis Ultra DLD) was used to further determine the change in element valence state before and after the electrochemical ageing of the electrode. To determine the specific surface area and average pore size, BET (Micromeritics ASAP 2460) was tested by using N2 as the adsorbed gas. An infrared C-S analyzer (LECO CS-844) and O-N-H joint analyzer (LECO ONH-836) were used to quantify the contents of C and N in the sample, respectively.
Electrochemical measurements
Using a standard three-electrode system at an electrochemical workstation (CHI-660C, Shanghai CHI Instruments, Inc.) to finished all electrochemical tests in this work and electrolyte is 0.5 M H2SO4 and 1.0 M KOH. The as-prepared samples were connected with copper wires and subsequently sealed to become working electrodes (1 cm-2). A platinum sheet and a graphite rod were used as auxiliary electrodes in the acidic solution and alkalinous solution, respectively. Two calibrated saturated calomel electrodes (SCEs) were used as reference electrodes under different solutions of 1 M KOH [33] and 0.5 M H2SO4 [36], respectively. The polarization curve of the OER was determined by linear sweep voltammetry (LSV) with a scan rate of 1 mV s-1. Tafel slopes withiRs correction were obtained by the LSV curves, where Rs is the solution resistance obtained by EIS. Electrochemical impedance spectroscopy (EIS) was measured at 0.01 Hz-100 kHz with an amplitude of 5 mV. The ECSA was obtained by CV measurements of the Cdl capacitance at non-Faradaic potentials and different scan rates of 10-100 mV s-1 in a 1 M KOH solution, 100 to 1000 mV s-1 in a 0.5 M H2SO4solution. Electrode stability was measured at a constant step current density of 10, 50, 100 and 200 mA cm-2. Long-term stability for 28 hours was achieved using a two-electrode cell at a current density of 100 mA cm-2 in 0.5 M H2SO4 solution. Faraday efficiency formula placed in supplementary materials.
DFT calculations
The data of density functional theory (DFT) are calculated by Vienna ab initio simulation package (VASP). The generalized gradient approximation (GGA) is used to calculate the exchange correlation energy in the Perdew-Burke-Ernzerhof (PBE) functional framework, and the projector augmented wave (PAW) method is used to describe the electron-ion interaction. The energy cutoff for the plane wave expansions is set to 600 eV, while the energy and force for the convergence threshold are set to 10-5 eV and 0.03 eV Å-1, respectively. A vacuum space of 10 Å is increased to avoid the interaction between periodic images generated during the calculation of the flat plate model. In addition, the dipolar correction was adopted for slab models with the symmetrization switching off. The brillouin zone was sampled with 2 × 2 × 1 k-points.
Both EDS and XPS data (see Table S2 and Table S3) show that Ni in the as-prepared electrodes will dissolve in 0.5 M H2SO4 solution, so the γ-Fe2O3 after OER ageing is selected as the main model. Initially, constructing a (110) slab model of γ-Fe2O3 consisting of four layers to facilitate comparison. To conform with XPS data, randomly substituted some Fe atoms in the structure with Co and Ni atoms while maintaining the appropriate ratios of Fe, Co, and Ni atoms. Subsequently, employing the Ovito method in the Atomsk software to amorphize the first layer of the slab, resulting in an amorphous model.
RESULTS AND DISCUSSION
Figure 1a shows the electrode fabrication process of nanoporous NiFeCoCN MEAs with different urea contents. Interconnected nanopores and nanogaps are formed based on the nanoscale Kirkendall effect due to the different diffusion rate of atoms/vacancies between the diffusion couples (e.g., Ni/Co, Ni/Fe and Fe/Co) during the high-temperature sintering process [36, 37]. Urea is first isomerized into isocyanic acid (HNCO) during the heating process via a boiling reaction at 380 °C-400 °C. When the temperature increases to 900 °C, urea volatilizes to form pores, and part of the decomposed active C and N form solid solutions with Ni, Fe and Co [38-40]. Figure 1b shows the X-ray diffraction (XRD) curves of NiFeCoCN. The three peaks of NiFeCoCN at 44.3°, 51.5° and 75.8° correspond to the (111), (200), and (220) facets of the γ-Ni solid solution of the face-centred cubic (FCC) structure, respectively. This consequence is consistent with the MEA composed of NiFeCo elements analysed by XRD in other papers [41, 42]. The diffraction peaks of NiFeCo, NiFeCoCN, NiFeCoCN-1 and NiFeCoCN-2 were basically unchanged (see Figure S2), which indicates the urea content had little effect on the MEA structure. However, compared with the standard PDF card (PDF#04-0850) of pure Ni, the diffraction peaks of the NiFeCoCN exhibit an obvious negative shift, which should be ascribed to the incorporation of Fe, Co, C and N in the γ-Ni solid solution. Hence, nanoporous NiFeCoCN with a single γ-Ni solid solution structure is confirmed in combination with the latter TEM analysis in Figure 1e.
Figures 1c and d show the FESEM images of NiFeCoCN. As shown in the figures, NiFeCoCN has an interconnected porous structure with a nanosheet morphology of approximately 10-20 nm (see Figure S3). Figure S4 and Table S4 show that NiFeCoCN has a BET surface area of 0.63 m2 g-1 with an average pore diameter of 10.21 nm. This result indicates that urea plays a critical role in the formation of nanoporous structures of NiFeCoCN and favourably forms abundant active sites for OER. As shown in Figure 1e, the 0.211 nm and 0.182 nm interplanar spacings of NiFeCoCN correspond to the (111) and (200) facets of the γ-Ni solid solution, respectively. The selective diffraction pattern (SAED) also shows the (111), (200), and (220) facets of the γ-Ni solid solution, which is consistent with the XRD analysis. Figure 1f is the HRTEM taken from Figure 1e and clearly shows that the crystal lattice has many defects. Figure 1g is obtained by performing an inverse fast fourier transformation (IFFT) on Figure 1f. As shown in the figure, the dislocations can also demonstrate lattice distortion occured in NiFeCoCN. The high-angle annular dark-field scanning-transmission electron microscopy (HAADF-STEM) and energy dispersive X-ray (EDX) elemental mappings in Figures 1h-n show that Fe, Co, Ni and C, N, O atoms are uniformly dispersed in the nanoporous NiFeCoCN bulk. The atomic percentages of C and N in NiFeCoCN are 1.1 at.% and 0.25 at.%, respectively (see Table S5). These results prove the incorporation of C/N in the NiFeCoCN bulk and show that urea can be used as a C/N source. Based on the above analysis, NiFeCoCN is indeed a MEA with the nonmetallic incorporation of C/N.