Polar Layered Intermetallic LaCo2P2 as a Water Oxidation Electrocatalyst.

We investigate LaCo2P2 as an electrocatalytic material for oxygen evolution reaction (OER) under alkaline and acidic conditions. This layered intermetallic material was prepared via Sn-flux high-temperature annealing. The electrocatalytic ink, prepared with the ball-milled LaCo2P2 catalyst at the mass loading of 0.25 mg/cm2, shows OER activity at pH = 14, reaching current densities of 10, 50, and 100 mA/cm2 under the overpotential of 400, 440, and 460 mV, respectively. Remarkably, the electrocatalytic performance remains constant for at least 4 days. Transmission electron microscopy reveals the formation of a catalytically active CoOx shell around the pre-catalyst LaCo2P2 core during the alkaline OER. The core serves as a robust support for the in situ-formed electrocatalytic system. Similar studies under pH = 0 reveal the rapid deterioration of LaCo2P2, with the formation of LaPO4 and amorphous cobalt oxide. This study shows the viability of layered intermetallics as stable OER electrocatalysts, although further developments are required to improve the electrocatalytic performance and increase the stability at lower pH values.

Introduction

The growing worldwide need for clean energy technologies, aiming to replace rapidly depleting and environmentally harmful fossil fuels, has been driving extensive research efforts into the viability of a hydrogen fuel economy. The centerpiece of these efforts is the decomposition of water into the constituent elements.[1] The overall water-splitting reaction can be divided into two half-reactions known as hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). In practice, however, these electrochemical processes suffer from a large overpotential, defined as the difference between the experimental and thermodynamic values of the electrochemical potential required to drive the water electrolysis reaction. The overpotential is caused by substantial kinetic barriers associated with the two-electron HER and especially with the four-electron OER mechanisms.[2,3] A broad range of electro- and photocatalysts has been investigated to address this problem.[4,5] The state-of-the-art electrocatalysts, such as RuO2 and IrO2, have afforded a substantial decrease in the OER overpotential,[6] but their use is not sustainable, given the scarcity and high cost of the platinum group metals.[7−9] In recent years, extensive research efforts have been devoted to the discovery of sustainable electrocatalysts based on 3d metal hydroxides, oxyhydroxides, and oxides,[4,10−14] chalcogenides,[15−17] phosphides,[18−23] and borides.[24,25]

The challenge in developing efficient electrocatalysts based on 3d metal compounds stems from the decomposition of such materials under conditions of electrocatalysis in acidic electrolytes. As a result, examples of such electrocatalysts that can operate under acidic OER conditions even for a few hours, without the loss of performance, are extremely rare. Among the materials mentioned above, transition-metal phosphides (TMPs) and transition-metal borides (TMBs) are well known to show high stability toward acids, in the absence of applied voltage, while OER activity in alkaline solutions has been demonstrated for several binaries, such as CoP, Ni2P, NiP2, FeB, Co2B, and Ni2B.[26−31] Hence, TMPs and TMBs offer appealing alternatives to the more extensively studied OER electrocatalysts based on 3d metal hydroxides, oxyhydroxides, and oxides.[32]

Recently, we have reported a promising performance by AlFe2B2 in alkaline OER electrocatalysis.[29] Our studies revealed that AlFe2B2 acts as a precatalyst by governing the formation of a thick and very stable shell of catalytically active Fe3O4 nanoparticles around the particles of AlFe2B2. Remarkably, this material far outperformed stand-alone Fe3O4 nanoparticles by showing a substantially higher long-term stability, faster reaction kinetics, and a lower overpotential in the electrocatalytic OER in a 1 M KOH electrolyte. Interestingly, AlFe2B2 also showed much better catalytic properties as compared to the Al-free FeB counterpart. We attributed this improved performance to the crystal structure of AlFe2B2, in which layers of Al atoms alternate with [Fe2B2] slabs (Figure a). The catalytically inactive Al layer provides additional structural stability and increased electrical conductivity within the structure, thus improving the electron-transfer kinetics and longevity of the catalytic system. At the onset of the OER, the surface layers of AlFe2B2 undergo reconstruction due to the partial etching of the Al layers by 1 M KOH and electro-oxidation of the [Fe2B2] slabs to the shell of Fe3O4 nanoparticles. Nevertheless, the remaining AlFe2B2 precatalyst core provides an excellent structural support and improved electron-transfer rate between the underlying electrode and the catalytically active oxide shell.

Figure 1

Side-by-side comparison of the crystal structures of AlFe2B2 (a) and LaCo2P2 (b).

Despite its excellent performance in the alkaline electrocatalytic OER, AlFe2B2 is not stable under acidic conditions.[33−35] In contrast, many ternary phosphides with related layered structures (Figure b) are known to exhibit high stability toward acids.[36] Building on this knowledge, we have decided to explore the use of such structures in both acidic and alkaline water oxidation. Herein, we report a study of OER electrocatalysis on LaCo2P2, which was previously investigated by us and a few other research groups as an interesting ferromagnetic material with the ordering temperature of 132 K.[37−40] We demonstrate that, despite its high general stability in acidic environment, this material quickly deteriorates once the voltage is applied to drive the OER at pH = 0. On the other hand, it acts as a stable OER precatalyst at pH = 14, showing only a slightly higher overpotential but similar electrochemical kinetics as compared to the reported performance of binary CoP. We also discuss the possible reasons for our observations and strategies to improve the stability and electrocatalytic performance of layered-structure TMPs.

Materials and Methods

Starting Materials

All manipulations during sample preparation were carried out in an Ar-filled dry box (content of O2 < 0.5 ppm). A lanthanum rod (≥99.9%), cobalt powder (99.9%), phosphorus powder (99.9%), tin powder (99.85%), and concentrated hydrochloric acid (36.5–38.0%) were obtained from VWR. The lanthanum rod was filed to powder immediately before the reaction. Cobalt powder was additionally purified by heating in a flow of H2 gas at 500 °C for 5 h. Nafion ionomer solution (5% in aliphatic alcohols and water) and platinum wire (99.9%) were acquired from Sigma-Aldrich, while the IrO2 (99.99%, #43396) reference material was purchased from Alfa Aesar. Ultrapure water (18.2 MΩ cm–2) was produced using a Milli-Q Advantage A10 system (Millipore). For electrolyte preparation, purified NaOH (98.5%) from Acros Organics and H2SO4 (95–98%) from Sigma-Aldrich were used.

Synthesis

LaCo2P2 was synthesized according to the previously reported Sn-flux method.[37] The starting materials were mixed in the La/Co/P/Sn = 1.6:2:2:20 ratio (the total mass = 2 g) and sealed in a fused silica tube of 10 mm inner diameter under vacuum (≈10–4 mbar). The mixture was annealed at 880 °C for 7 days and cooled to 600 °C at 10 °C/min, at which point the tube was quenched into ice water. The Sn-rich matrix was dissolved in dilute HCl (1:1 v/v) until the gas evolution ceased. The product was recovered by filtration, washed successively with water and ethanol, and dried.

Powder X-ray diffraction (PXRD) was carried out on a SmartLab diffractometer (Rigaku) equipped with a D/teX Ultra 250 1D detector and a Cu Kα radiation source (λ = 1.54187 Å). Each pattern was recorded in the 2θ range from 10 to 80° with a step of 0.03° and the total collection time of 1 h. The analysis of PXRD patterns was carried out with the SmartLab Studio II (Rigaku).[41]

Electrocatalyst Preparation

The phase-pure sample of LaCo2P2 was ball-milled at 1725 rpm for 1 h in an 8000 M high-energy mixer/mill (SPEX), using a stainless steel ball-milling set. The milling process was performed under Ar atmosphere to minimize surface oxidation. The PXRD analysis of the ball-milled sample revealed no new impurity phases, although the diffraction peaks broadened, in accord with the decreased particle size and increased strain. The specific surface areas of the ball-milled LaCo2P2 and reference electrocatalyst were evaluated by measuring N2 physisorption using an Autosorb iQ2 analyzer (Quantachrome). For that purpose, ≈100 mg of a sample was placed in the sample holder tube and degassed at 120 °C for 2 h. Subsequently, the sample holder tube was placed into a liquid N2 bath for the analysis. The specific surface area of the materials was determined by the Brunauer–Emmett–Teller (BET) method.

The ball-milled sample of LaCo2P2 was converted to an electrocatalyst ink by dispersing 5 mg of the material in 50 μL of Nafion ionomer solution and 1.0 mL of anhydrous ethanol (Honeywell). The mixture was homogenized for 30 min in a bath sonicator USC-TH (VWR) and for 1 min with an ultrasonic probe Vibra-Cell 75185 (Thermo Fisher Scientific). Catalytic anodes were prepared by depositing the as-derived homogeneous ink on Ni foam (Heze Jiaotong, 110 pores per in., 0.3 mm thick) for alkaline OER or Ti felt (Bekaert Fibre Technologies, 20 μm fiber diameter, 1 mm thick) for acidic OER. Before the ink deposition, both the Ni foam and the Ti felt were cleaned by sequential 30 min ultrasonication in acetone, ethanol, and Milli-Q water. Prior to ultrasonic cleaning, the Ti felt was additionally purified by heating it under Ar atmosphere to 400 °C at 3 °C/min and maintaining it at this temperature for 2 h. In each case, 640 μL of the ink was loaded in 20 μL batches on the surface of the Ni foam or the Ti felt current collectors, letting ethanol evaporate between the batches. The exposed surface area of the anode was fixed to be 1 cm2, and the total mass of the ball-milled LaCo2P2 catalyst or the reference IrO2 catalyst loaded on the anode was varied from 0.25 to 3 mg/cm2. Finally, the obtained anode was air-dried.

Electrocatalytic Measurements

Electrochemical studies were conducted at room temperature using an Autolab PGSTAT302N potentiostat (Metrohm). The OER performance of the electrocatalysts was evaluated under moderate Ar bubbling (≈1 bubble/s) while stirring at 150 rpm in a three-electrode system filled with a purified 1.0 M NaOH (alkaline OER) or 0.5 M H2SO4 (acidic OER) electrolyte. The catalytic anode, a calibrated saturated calomel electrode (SCE), and a Pt wire served as the working, reference, and counter electrodes, respectively. Unless stated otherwise, all potentials reported in this work were converted to a reversible hydrogen electrode (RHE) reference scale according to the following equation: ERHE = ESCE + 0.059pH + 0.241. An iR correction of 85% was applied in the polarization experiments to compensate for the voltage drop between the reference and working electrodes, which was evaluated by a single-point high-frequency impedance measurement. OER anodic polarization curves were recorded using cyclic voltammetry (CV) with a scan rate of 5 mV/s. In the case of electrocatalyst activation, the scan rate was augmented to 50 mV/s. The catalytic stability of the anodes was evaluated as a function of time by means of chronopotentiometry at a constant current density of 10 mA/cm2.

Electrochemical impedance spectroscopy (EIS) was carried out on stable electrocatalytic systems at an overpotential of 0.42 V in the frequency range from 105 to 0.01 Hz with a 10 mV sinusoidal perturbation. The EIS measurements and the interpretation of results were conducted in accordance with an aqueous electrochemical assembly, the so-called supported system.[42]

Chemical Analysis

Inductively coupled plasma-optical emission spectroscopy (ICP-OES) was carried out using an ICPE-9000 spectrometer (Shimadzu). Each sample was measured three times to ensure the reproducibility of results.

Transmission Electron Microscopy

Transmission electron microscopy (TEM), high-resolution TEM (HRTEM), high-angle annular dark-field scanning TEM (HAADF-STEM), selected area electron diffraction (SAED), and energy-dispersive X-ray spectroscopy in STEM mode (STEM–EDX) were performed using a JEM-ARM200F microscope (JEOL) equipped with a cold field-emission gun, a probe, an image aberration correction, a CENTURIO EDX detector, and a GIF Quantum filter. TEM samples were prepared by crushing a sample in an agate mortar in ethanol and depositing the obtained suspension on a copper carbon holey grid.

Results and Discussion

Bulk LaCo2P2 was prepared by annealing the constituent elements in Sn flux. After the Sn-rich matrix had been dissolved in dilute HCl, a phase-pure material was obtained, as evidenced by PXRD (Figure ). The material was ball-milled for 1 h to increase the surface area for improved catalytic performance. According to the BET method, the surface area of the ball-milled sample was 9.9 cm2/g. PXRD of the ball-milled material revealed that the sample remained phase-pure, but the diffraction peaks broadened due to the well-known combined effects of the decreased particle size and the strain introduced by ball-milling. The crystallite size was estimated as D = Kλ/[(ws – wi)·cos(θ)],[43] where K is a shape factor (set to 0.9), λ is the X-ray wavelength, θ is the diffraction angle of the observed peak, and ws and wi are the full widths at half-maximum for the sample and an instrumental standard (highly crystalline Si powder), respectively. Using this equation, the lower bound for the crystallite size was estimated at ≈25 nm. This value does not represent the actual average crystallite size because the Scherrer equation does not account for the strain induced by the ball-milling process, which will necessarily increase the ws value.

Figure 2

PXRD patterns for the bulk (red) and ball-milled (blue) samples of LaCo2P2. The calculated pattern (black) for LaCo2P2 is provided for comparison.

The TEM characterization prior to electrochemical testing revealed that the sample of ball-milled LaCo2P2, in general, showed good crystallinity and contained a mixture of nanosized (≈10 nm) and submicron particles. The STEM–EDX elemental mappings (Figure a) indicate homogeneous chemical distribution of all constituent elements (La, Co, and P), as well as a negligible amount of oxygen that can be attributed to minor surface oxidation. High-resolution HAADF-STEM (Figure b) and TEM (Figure c) images, along with the corresponding Fourier transform (FT) patterns, confirm the tetragonal structure of LaCo2P2 (space group I4/mmm, a = 3.8145 Å, c = 11.041 Å, ICSD-624010).

Figure 3

TEM analysis of LaCo2P2 particles after ball-milling: (a) low-magnification overview HAADF-STEM image and simultaneous STEM–EDX elemental mappings acquired at the L-line of La (red) and K-lines of P (purple), O (blue), and Co (green), and their mixture; (b) high-resolution HAADF-STEM image of a single LaCo2P2 particle viewed along the [111] zone axis (the corresponding FT pattern is shown in the inset); (c) bright-field HRTEM image of the LaCo2P2 sample, together with the insets showing the FT patterns taken from [100] (A) and [110] (B) oriented particles.

Electrochemical Testing

The electrochemical OER testing in a purified 1 M NaOH electrolyte revealed that LaCo2P2 requires initial activation for ca. 100 CV cycles to achieve steady-state conditions, after which the material exhibits constant OER performance. Experimentally, it was found that LaCo2P2 with a loading of 0.25 mg/cm2 was the most OER-active (Figure S1), reaching anodic current densities of 10, 50, and 100 mA/cm2 at overpotentials of ca. 400 mV (η10), 440 mV (η50), and 460 mV (η100), respectively (the blue curve in Figure a). A reference state-of-the-art IrO2 catalyst (Figure S2) showed a higher performance at an industrially relevant loading of 1 mg/cm2 (the red curve in Figure a), but at the same loading as that of LaCo2P2, that is, at 0.25 mg/cm2, the overpotentials achieved with IrO2 were quite similar (the yellow curve in Figure a). We would like to emphasize that at the same loading of 0.25 mg/cm2, the BET surface area of the IrO2 catalyst (25.5 cm2/g) was substantially larger than that of the LaCo2P2 material (9.92 cm2/g), while the cost of the IrO2 catalyst at such loading would be prohibitively high. Nevertheless, it might still be possible to achieve lower overpotentials with the LaCo2P2-based catalyst, if the particle size can be further reduced to increase the electrocatalytically active surface area. Such an effect could be achieved, for example, by a solvent-assisted ball-milling process.

Figure 4

(a) Alkaline OER anodic polarization curves for Ni foam (after 100 activation cycles), Ni foam-supported ball-milled LaCo2P2 at 0.25 mg/cm2 loading (after 100 activation cycles) and Ni foam-supported reference IrO2 catalyst at 0.25 and 1 mg/cm2 loadings. (b) Respective Tafel plots. (c) Comparison of Nyquist plots for Ni foam-supported LaCo2P2 and Ni foam-supported reference IrO2 at the applied overpotential η = 420 mV. The inset shows an equivalent electrical circuit model used to fit the Nyquist plots. (d) Chronopotentiometric stability tests under alkaline OER for Ni foam-supported LaCo2P2 and Ni foam-supported reference IrO2 at the same loading of 0.25 mg/cm2.

Given these promising results, the kinetic behavior of LaCo2P2 was investigated using a combination of Tafel and Nyquist plots. The Tafel plot relates the rate of the electrochemical reaction to the overpotential as η = b log(j/j0), where j is the current density, j0 is the exchange current density (i.e., the current density at zero overpotential), and b is the Tafel slope, which represents the decade change in the reaction rate as a function of applied voltage.[44] Analysis of the electrocatalytic data according to this equation led to b = 57 mV dec–1 for LaCo2P2 and b = 69 mV dec–1 for IrO2 at the same 0.25 mg/cm2 loading (Figure b). Subsequently, EIS data were collected at low frequencies (Figure c), and the obtained semicircles were fit with an equivalent circuit model (Figure c, inset) consisting of a resistor (Rs) in series with two parallel combinations of a resistor (R1, Rct) and a constant phase element (CPE1, CPE2).[45]Rs represents the Ohmic resistance from the electrolyte and all contacts. The time constant R1–CPE1 accounts for the interfacial resistance from the electron transport between the LaCo2P2 material and supporting Ni foam. Rct–CPE2 is the charge-transfer resistance (Rct) at the interface between the catalyst and the electrolyte, and smaller Rct values typically reflect faster charge-transfer kinetics. The derived parameters, listed in Table , reveal that the LaCo2P2 anode shows a significantly smaller Rct value (1.470 Ω) as compared to that of IrO2 (2.820 Ω), indicating that the LaCo2P2 catalyst offers favorable charge-transfer kinetics, competitive with that of IrO2.

Table 1

Impedance Parameters Calculated by Fitting the Nyquist Plots (Figure c)

circuit element	LaCo2P2	IrO2
equivalent series resistance (Rs)	1.460 Ω	0.985 Ω
electron-/charge-transport resistance (R1)	0.961 Ω	1.860 Ω
charge-transfer resistance (Rct)	1.470 Ω	2.820 Ω

Lastly, we performed chronopotentiometry to study the stability of LaCo2P2 and reference IrO2 with the same mass loading of 0.25 mg/cm2 under the alkaline OER conditions (pH = 14). Over 100 h, LaCo2P2 compares favorably to IrO2 and, more importantly, demonstrates an essentially constant performance for more than 4 days, driving the current density of 10 mA/cm2 with η ≈ 400 mV in the 1 M NaOH electrolyte solution (Figure d). Notably, the ICP-OES chemical analysis of the electrolyte after the stability testing showed the presence of only 0.077 ppm of Pt admixture, which, most likely, stems from leaching of the counter electrode.[46] Importantly, no traces of La, Co, or Ni were identified by the chemical analysis of the alkaline electrolyte, suggesting excellent stability of the LaCo2P2 catalyst, in particular, and the LaCo2P2/Ni foam anode assembly, in general.

After the studies of LaCo2P2 under alkaline OER, the material was tested under acidic OER (pH = 0) to determine its possible bifunctionality. The electrochemical station and the sample were prepared in a manner identical to that described for the alkaline OER, with the catalyst ink deposited on a Ti felt and 0.5 M H2SO4 used as the electrolyte. Under such conditions, the LaCo2P2 catalyst showed a much higher overpotential, η = 1.01 V, at 10 mA/cm2 (Figure ). Chronopotentiometric testing (Figure , inset) for driving current density of 10 mA/cm2 showed that LaCo2P2 is not stable, causing a gradual increase in the overpotential with time. Thus, this material is impractical as an OER electrocatalyst under acidic conditions.

Figure 5

Acidic OER anodic polarization curves and the continuous chronopotentiometric profiles (shown as an inset) for the Ti felt, as well as Ti felt-supported ball-milled LaCo2P2, and Ti felt-supported reference IrO2 materials, both at 3 mg/cm2 loading.

Post-electrochemical Testing

PXRD

To monitor changes to the LaCo2P2 phase under the OER conditions, the samples obtained after electrochemical testing in the alkaline and acidic electrolytes were harvested from the Ni foam and Ti felt, respectively, and subjected to PXRD analysis. The PXRD patterns obtained for the sample used in alkaline OER revealed that LaCo2P2 remains stable under the harsh alkaline conditions (Figure a). The large unresolved amorphous peaks that appear in the low-angle region are due to a Nafion ionomer admixture in the postcatalytic sample. The lower bound for the crystallite size, estimated from the Scherrer equation, decreased from 25 nm before the catalysis to 19 nm after the catalysis. This change is in agreement with the OER-induced in situ surface reconstruction that converts the outer layers of the LaCo2P2 particles to a shell of a catalytically active oxide–(oxy)hydroxide phase,[29,32] as suggested by the observations made in the TEM studies discussed below.

Figure 6

PXRD patterns of LaCo2P2 after 100 h of chronopotentiometric testing at 10 mA/cm2 in 1 M NaOH (a) and 0.5 M H2SO4 (b). Broad amorphous peaks present in both samples are due to the Nafion ionomer. The calculated patterns for LaCo2P2 and LaPO4 are shown as references.

In sharp contrast to the alkaline OER testing, the sample obtained after the acidic OER testing contained predominantly LaPO4 and minor impurities that could not be assigned due to the large amorphous peak observed in the low-angle region due to the Nafion ionomer (Figure b). As seen later from the EDX mapping results, the other impurity is Co-based due to the high content of Co found in this sample.

Electron Microscopy

The samples obtained after the alkaline and acidic OER testing were subjected to TEM imaging and STEM–EDX elemental mapping. In comparison to the sample prior to testing (Figure ), an extreme increase in the oxygen content was observed after 100 h of stability testing under alkaline conditions (Figure a). HAADF-STEM and SAED were employed to determine the nature of the oxygen-containing phase formed on the surface of the LaCo2P2 particles. These particles showed well-defined lattice planes (Figure b) with a number of defects, while the respective SAED (Figure b, inset) could be indexed with the lattice parameters corresponding to the tetragonal I4/mmm structure of LaCo2P2 (a = 3.814 Å, c = 11.041 Å, ICSD-624010). The HAADF-STEM image simulation based on this structure showed a good agreement with the experimental one (Figure b, inset). Interestingly, the edge of the LaCo2P2 particle is decorated by an amorphous layer of several nanometers thickness, and according to STEM–EDX elemental mapping, it can be attributed to the CoO phase. The high-resolution HAADF-STEM image (Figure b) clearly reveals a transition from the region of the CoO shell to the layered structure of the LaCo2P2 core, indicating the in situ surface reconstruction caused by alkaline OER electrocatalysis.

Figure 7

HAADF-STEM images together with simultaneously collected STEM–EDX elemental mappings of La, P, O, Co, and their mixture for LaCo2P2 particles after OER electrocatalysis in alkaline (a) and acidic (c) electrolytes. (b) High-resolution [201] HAADF-STEM image with the corresponding SAED (upper corner inset) and the magnified HAADF-STEM image with an overlaid simulated image (bottom corner inset) for LaCo2P2 particles after alkaline OER electrocatalysis. (d) Low-magnification TEM overview image for the LaPO4 needle-like particles formed after acidic OER electrocatalysis over LaCo2P2.

Under acidic OER conditions, the LaCo2P2 catalyst underwent a complete change in its morphology, as compared to the material before electrocatalysis. The particle shape changed from plate-like (Figure a) to needle-like (Figure c). This observation is in agreement with the formation of LaPO4 as the major phase observed by PXRD (Figure b). Indeed, LaPO4 was shown to form rodlike particles[47,48] that exhibit an increasing aspect ratio with decreasing pH.[49] The STEM–EDX elemental mapping of the postcatalytic sample showed an increase in the O content and a decrease in the La and P contents, while the content of Co remained relatively high (Figure c). These observations support the PXRD findings, which showed that LaCo2P2 decomposes during the acidic OER electrocatalysis to form LaPO4, and also suggest that the crystalline phosphate particles are surrounded by an amorphous cobalt oxide phase. This phase was unstable under the electron beam during TEM measurements even at low voltage (80 kV), which impeded the high-resolution imaging to identify the exact nature of this oxidic Co phase.

Concluding Remarks

Based on the results presented above, we rationalize that LaCo2P2 acts as pre-catalyst in the alkaline water oxidation. Under the applied voltage at pH = 14, its surface undergoes in situ reconstruction, most likely, according to the following equation

Although this equation is a much-simplified version of what actually occurs on the surface, it allows us to highlight two major points. First, during approximately the first 100 CV cycles of required activation, as established experimentally, the surface of the LaCo2P2 particles is converted to the oxidic CoO shell. The formation of the La(OH)3 phase is postulated according to the Pourbaix diagram for La at the specific applied voltage and pH = 14.[50] Second, the surface is reconstructed to the catalytically active amorphous CoO shell (or perhaps the CoOOH shell[51]), as evidenced by the electron microscopy analysis and the experimentally observed decrease in the overpotential over the first 100 cycles of the electrocatalysis. While CoO serves as an active OER electrocatalyst, the underlying LaCo2P2 phase provides a robust support, allowing the catalyst to maintain stability for at least 4 days of electrocatalysis. In contrast, under acidic conditions, LaCo2P2 rapidly decomposes to LaPO4 and amorphous cobalt oxide or (oxy)hydroxide, resulting in a rapid increase in the overpotential.

To determine the effectiveness of LaCo2P2 as an alkaline OER electrocatalyst, we compared its performance to that of other Co-based OER electrocatalysts studied under alkaline conditions (Table ). The overpotential at 10 mA/cm2 (η10 = 400 mV) observed for the CoO/LaCo2P2 electrocatalytic system in the present work is slightly higher than the values reported for other Co-based electrocatalysts, which typically show η10 above 300 mV. On the other hand, the Tafel slope of 57 mV, measured for CoO/LaCo2P2 in this work, is comparable to the values observed for the other Co-based systems at similar catalyst loading. Xing et al. added polythiophene to increase the electrical conductivity of the catalyst to achieve an overpotential of 338 mV at 10 mA/cm2 and a low Tafel slope of 52 mV/dec.[52] Nevertheless, the stability of such a system is substantially lower than that of LaCo2P2, as the performance decreased by 5% after 15 h, whereas our LaCo2P2 catalyst maintained constant performance for 100 h (Figure a, inset). Li et al. achieved good electrocatalytic results by applying an external magnetic field during the synthesis of the catalyst (Ni–S–CoFe2O4), which they attributed to the formation of a larger concentration of catalytically active sites and an effective electrochemical surface area on the active site surface.[53]

Table 2

Overpotential and Performance Parameters Reported for Cobalt Oxides and Phosphides Used in Alkaline OER Electrocatalysis

catalyst	electrolyte	catalyst loading (mg/cm2)	η10 (mV)	b (mV/dec)	references
LaCo2P2	1 MNaOH	0.25	400	57	this work
IrO2	1 M NaOH	1.0	340	73	this work
IrO2	1 M NaOH	0.25	380	69	this work
co-polythiophene	1 M KOH	1.4	338	52	(52)
Ni–S–CoFe2O4	1 M KOH	n/a	228	72	(53)
La0.9CoO3−δ	0.1 M KOH	0.24	380	83	(54)
Co3O4 (nanoparticles)	0.1 M KOH	n/a	310	53	(55)
CoP (film)	1 M KOH	2.5	345	47	(56)
CoP (nanoframes)	1 M KOH	0.27	323	50	(57)
CoP (N-doped carbon)	1 M KOH	0.27	354	60	(57)
CoP (graphitic carbon)	1 M KOH	n/a	345	56	(58)

Overall, the performance of LaCo2P2 in OER electrocatalysis is comparable to the previously reported performance by CoP, as well as to the performance of the IrO2 benchmark. The presence of the additional La layer does not have a substantial effect on the electrochemical activity, which is in contrast to our recent finding for AlFe2B2,[29] where the extra layer of Al appeared to improve the charge-transfer kinetics in comparison to the performance of binary iron borides. The lack of improved kinetics may be attributed to the higher polarity of the LaCo2P2 structure that contains alternating electropositive La layers and electronegative [Co2P2] layers. Another detrimental factor might be the presence of insoluble La(OH)3 in the amorphous surface layer. Lowering the polarity of the intermetallic structure and using an electropositive element that forms a more soluble hydroxide might alleviate these problems. In this vein, ACo2P2 structures that contain alkaline-earth metals (A) can serve as promising targets for future studies. These efforts are currently underway in our laboratories, and the results or such studies will be reported in due course.