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. 2023 Mar 27;3(4):299–309. doi: 10.1021/acsmaterialsau.2c00080

A Little Nickel Goes a Long Way: Ni Incorporation into Rh2P for Stable Bifunctional Electrocatalytic Water Splitting in Acidic Media

Tharanga N Batugedara 1, Stephanie L Brock 1,*
PMCID: PMC10347692  PMID: 38090124

Abstract

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In acidic media, many transition-metal phosphides are reported to be stable catalysts for the hydrogen evolution reaction (HER) but typically exhibit poor stability toward the corresponding oxygen evolution reaction (OER). A notable exception appears to be Rh2P/C nanoparticles, reported to be active and stable toward both the HER and OER. Previously, we investigated base-metal-substituted Rh2P, specifically Co2–xRhxP and Ni2–xRhxP, for HER and OER as a means to reduce the noble-metal content and tune the reactivity for these disparate reactions. In alkaline media, the Rh-rich phases were found to be most active for the HER, while base-metal-rich phases were found to be the most active for the OER. However, Co2–xRhxP was not stable in acidic media due to the dissolution of Co. In this study, the activity and stability of our previously synthesized Ni2–xRhxP nanoparticle catalysts (x = 0, 0.25, 0.50, 1.75) toward the HER and OER in acidic electrolyte are probed. For the HER, the Ni0.25Rh1.75P phase was found to have comparable geometric activity (overpotential at 10 mA/cmgeo2) and stability to Rh2P. In contrast, for OER, all of the tested Ni2–xRhxP phases had similar overpotential values at 10 mA/cmgeo2, but these were >2x the initial value for Rh2P. However, the activity of Rh2P fades rapidly, as does Ni2P and Ni-rich Ni2–xRhxP phases, whereas Ni0.25Rh1.75P shows only modest declines. Overall water splitting (OWS) conducted using Ni0.25Rh1.75P as a catalyst relative to the state-of-the-art (RuO2||20% Pt/C) revealed comparable stabilities, with the Ni0.25Rh1.75P system demanding an additional 200 mV to achieve 10 mA/cmgeo2. In contrast, a Rh2P||Rh2P OWS cell had a similar initial overpotential to RuO2||20% Pt/C, but is unstable, completely deactivating over 140 min. Thus, Rh2P is not a stable anode for the OER in acidic media, but can be stabilized, albeit with a loss of activity, by incorporation of nominally modest amounts of Ni.

Keywords: phosphide, nanoparticle, hydrogen evolution reaction, oxygen evolution reaction, overall water splitting

Introduction

Electrochemical water splitting is a prospective method to produce green hydrogen fuel.1 However, the large overpotential arising from the kinetically slow four-electron transferring anodic oxidation reaction creates a significant barrier to practical water splitting.2 In addition, establishing water splitting systems with different cathodic and anodic materials is challenging because optimization of each half-reaction may require different electrolyte pH, which increases the complexity and the cost of the electrolyzer.2,3 In this regard, catalysts active for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are considered to be critical for the realization of simple, low-cost electrolyzer systems.4

The development of inexpensive and efficient water electrolyzers for acidic media is a particular challenge because of the instability of the anodic catalysts, which perform the OER, generating electrons for hydrogen production at the cathode via the HER, and a lack of clear understanding of the OER mechanism and the nature of the catalytic sites under acidic conditions.3−10 However, acidic water splitting is worth pursuing for several reasons: (1) due to the high mobility of H+ ions relative to OH– ions, acidic media has a higher ionic conductivity than base, hence enabling a high current density; (2) acidic media dissolves CO2, thereby enabling long electrolyzer lifetimes relative to alkaline electrolyzers, where metal carbonates precipitate and poison active catalytic sites.5,7,8,11−14

Recently, transition-metal phosphides have been documented to be effective electrocatalysts for HER and precatalysts for OER, opening up exciting new avenues of investigation.4,6,11,15−33 However, there are few examples that pair HER and OER studies in acidic media; typically, HER studies are performed in acid (although full pH spectrum studies are not unusual) and OER in base. An activity survey of supported base-metal phosphides revealed good activity of Ni2P and Co2P in acid for HER and OER, but poor anode stability.28 In contrast, work done by Duan et al. on noble-metal phosphide Rh2P nanocube catalysts reported higher activity and stability relative to Pt for HER and OER in acidic media, at least over short time periods (33 and 17 min, respectively).22 We are investigating phosphide solid solutions that combine noble- and base-metal components to explore possible synergies that may enable high activity and stability at lower noble-metal concentrations. Our studies on Co2–xRhxP (x = 1.75 for HER and x = 0.25 for OER) and Ni2–xRhxP (x = 0.25 for OER) in basic media showed higher electrocatalytic activities compared to their monometallic end members, with Ni0.25Rh1.75P (x = 1.75) exhibiting a higher overpotential for HER relative to Rh2P when compared on geometrical surface area, but a lower overpotential when normalized against the electrochemical surface area (ECSA).7,8,13 In acidic media, we found that adding 37.5% Co into the Rh2P cubic antifluorite structure to form Co0.75Rh1.25P retained the crystal structure and high (initial) HER activity of Rh2P.14 However, Co dissolution under HER conditions led to a loss in activity of ca 50% over 30 min before leveling off, whereas under positive potentials required for OER, the polarization curves were irreversible and broad, indicating the anode is being consumed.14,34

In this study, we report the performance evaluation of our previously synthesized Ni2–xRhxP (x = 0.25, 0.5, and 1.75) nanomaterial catalyst8 toward HER, OER, and overall water splitting (OWS) in acidic media, and in comparison to Rh2P and Ni2P end members. In contrast to Co2–xRhxP, we are able to identify (relatively) stable catalysts for both HER and OER, with the most stable bimetallic phosphide being Ni0.25Rh1.75P for both processes. Notably, while Ni0.25Rh1.75P is comparable to Rh2P in terms of activity and stability for HER, we find the relative behavior toward OER to be distinct: Rh2P has an initial activity >2x that of Ni0.25Rh1.75P but rapidly deactivates, whereas Ni0.25Rh1.75P is surprisingly stable, with modest declines in current density over time, comparable to RuO2. We attribute the unusual stability of Ni0.25Rh1.75P under OER conditions in acid to synergy between surface NiOx catalytic sites that “protect” the reduced Rh underlayers, and the subsequent moderation by Rh of the electronic structure of the Ni catalytic sites to enable turnover.

Experimental Section

Reagents

Nickel acetylacetonate (Ni(acac)2, Alfa Aesar, 95%), hydrated rhodium chloride (RhCl3·nH2O, Sigma-Aldrich, 38–40% Rh), tri-n-octyl phosphine (TOP, STREM Chemicals, 97%), 1-octadecene (Sigma-Aldrich, 90%), oleylamine (TCI America, >50%), n-octyl ether (TCI America, 95%), chloroform (Fisher Scientific), hexanes (technical grade, Fisher Scientific), isopropyl alcohol (Sigma-Aldrich, 99.9%), ethanol (200 proof) (Decon Laboratories), Nafion (5%, LQ-1105, Ion Power), and sulfuric acid (H2SO4, Fisher Scientific, 93–98%) were used as received.

Synthesis of Ni2–xRhxP Nanoparticles

All reactions were carried out under an argon atmosphere using standard Schlenk line techniques. To synthesize Ni2–xRhxP our previously developed synthesis method was utilized.8 Briefly, (0.42–x) mmol Ni to x mmol Rh was combined with 15.0 mL of oleylamine and 5.0 mL of 1-octadecene in a 200 mL Schlenk flask with an attached condenser. Then, the mixture was degassed at 120 °C for 30 min. Then argon was purged for 20 min at the same temperature. The temperature was then increased up to 260 °C and maintained for 3 h. After 3 h, 8.0 mL of TOP was quickly injected into the mixture. Next, the temperature was set to 350 °C and was maintained for another 3 h. After naturally cooling the mixture to room temperature, 20 mL of ethanol was added to the solution and centrifuged to isolate the precipitate. The precipitate was dispersed in 5 mL of chloroform, sonicated for 5 min, and reprecipitated, adding ethanol, followed by centrifugation. This sonication and precipitation process was carried out two times.

Synthesis of Ni2P Nanoparticles

Ni2P nanoparticles were synthesized using a method developed by Muthuswamy et al. with minor variations.35 Ni(acac)2 (0.59 g), 6.0 mL of oleylamine, 15.0 mL of n-octyl ether, and 2.0 mL of TOP were added to a 200 mL Schlenk flask. The mixture was degassed at 120 °C for 30 min, and then argon was purged for 20 min at the same temperature. The temperature was then increased up to 230 °C and maintained for 90 min. After 90 min, 3.0 mL of TOP was quickly injected into the mixture. Next, the temperature was set to 350 °C and was maintained for another 3 h. After naturally cooling the mixture to room temperature, the sample was purified three times using chloroform and ethanol, as described in the procedure for the synthesis of Ni2–xRhxP.

Synthesis of Rh2P Nanoparticles

The Rh2P nanoparticle synthesis procedure was adopted initially from a method developed by the Schaak group and then modified by Mutinda et al.13,36 RhCl3·nH2O precursor (0.1 g), oleylamine (15.0 mL), and 1-octadecene (5.0 mL) were added to a 200 mL Schlenk flask. The mixture was degassed at 120 °C for 30 min, and then argon was purged for 20 min at the same temperature. The temperature was then increased up to 230 °C and maintained for 90 min. After 90 min, 6.0 mL of TOP was quickly injected into the mixture. Next, the temperature was set to 350 °C and was maintained for another 3 h. After naturally cooling the mixture to room temperature, the sample was purified three times using chloroform and ethanol, according to the procedure described for the synthesis of Ni2–xRhxP.

Ink Preparation

50 mg of purified nanoparticles was mixed with 25 mg of Ketjen-300J carbon (C) in hexane and sonicated for 30 min to form nanoparticles/C composite. The formed composite was then washed with hexane and reprecipitated by centrifugation. The precipitate was then dried under vacuum, followed by annealing in a furnace under 5% H2/Ar at 400 °C for 1 h. An ink was prepared by mixing 15 mg of annealed composite, 2 mL of ethanol, 1 mL of isopropanol, and 1 mL of nano pure water, followed by sonicating for 20 min. Then, 1 mL of a 5 wt % Nafion solution was added and the mixture was again sonicated for 20 min.

Scanning Transmission Electron Microscopy with Energy-Dispersive Spectroscopy (STEM-EDS) Analysis

EDS elemental mapping data were collected using a Thermo Fisher Talos F200X G2 scanning transmission electron microscope, operating at 200 kV and equipped with a high angle annular dark-field (HAADF) and EDAX detectors. For STEM-EDS analysis, nanoparticles/C ink were dispersed in chloroform and a drop of solution was deposited on a formvar carbon-coated 200 mesh Cu grid and allowed to dry completely in air. Analysis of postcatalysis samples involved sonicating the carbon cloth support in chloroform to form a dispersion, followed by deposition as described above. Three samples of the same targeted composition were averaged to calculate the composition from STEM-EDS analysis.

X-ray Photoelectron Spectroscopy (XPS) Analysis

A NEXSA, Thermo Fisher Scientific instrument was used with a monochromatic Al Kα (1486.7 eV) X-ray source operating at 6 mA and 12 kV. The work function was calibrated to give binding energy of 83.98 eV for Au 4f7/2. The spot size is 400 μm. The surface charges were neutralized using a focused low-energy electron beam. Spectrometer resolution details: Ag 3d5/2 core line full width at half-maximum (FWHM) at 10 eV pass energy = 0.5 eV; instrumental resolution = 0.38 eV. XPS was conducted for selected precatalyst and postcatalyst samples supported on C cloth. Samples were sputtered at 2 keV with Ar+ for 90 s before collecting the spectra. The scan details are shown in Table S1. Thermo Avantage v5.9922 software was used to analyze the collected high-resolution spectra, and binding energies were calibrated against the C 1s peak at 282.8 eV.

Electrochemical Measurements

Cyclic voltammograms (CVs) were collected using an EC Epsilon potentiostat equipped with a rotating disk electrode (RDE) using a standard three-electrode setup. An Ag/AgCl electrode was used as the reference electrode, and the potential was checked against a master Ag/AgCl reference electrode periodically to ensure there is no potential drift under strongly acidic conditions. A carbon electrode was used as the counter electrode for collecting HER polarization curves, while a Pt wire counter electrode was used to collect OER polarization curves. A glassy carbon electrode with a surface area of 0.07 cm2 was modified by drop-casting 10 μL of nanoparticles/C ink onto the surface to get a catalytic loading of 0.285 mg/cm2. The electrode was then dried under an infrared heat lamp for 2–3 min. HER and OER polarization curves were obtained in 0.5 M H2SO4 electrolyte at a scan rate of 10 mV/s with the RDE operating at 1600 rpm. IR compensation was performed prior to collecting polarization curves by applying IR-COMP using the epsilon software. The potential was measured in reference to the Ag/AgCl electrode and converted to the reversible hydrogen electrode (RHE) scale using eq 1.

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Electrochemical surface area (ECSA) analysis employed the same RDE setup. After activating the catalytic material by applying positive potential for OER and negative potential for HER, CVs were collected at various scan rates (50, 100, 150, and 200 mV/s) under nonfaradaic conditions (Figures S1a and S2a). The double-layer capacitive current difference (eq 2) was taken from the middle of the potential window and was plotted against the scan rate (υ) (Figures S1b and S2b).37 The linear behavior confirms the ideal capacitor behavior, and according to the relationship in eq 3, Cdl was determined by calculating the slope.37 The slope was then divided by the specific capacitance value (Cs = 0.035 mF/cm2 for acidic medium) to obtain the ECSA (eq 4, Tables S2 and S3).38,39

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Evaluation of the stability of the current density at fixed potential as a function of time was performed in an H-type cell using similar reference electrodes, counter electrodes, and electrolytes as described for CV measurements. The working electrode was prepared by drop-casting 140 μL of selected catalyst ink on a carbon cloth substrate with a geometric surface area of 1 cm2 and drying under an IR lamp (0.280 mg/cm2). Before ink drop-casting, the substrate material was cleaned and preheated at 450 °C for 2 h in air in a furnace to remove any residual organic material. The fabricated carbon cloth electrode with nanoparticles/C was then used to carry out the stability tests by applying a constant potential on the activated working electrode. In all cases, a blank experiment was carried out with the working electrode made with ink without nanoparticles. The blank was subtracted to produce the reported data.

Overall water splitting activity evaluation was carried out in a two-electrode system, where Ni0.25Rh1.75P/C composites were utilized as the anode and cathode. The electrodes were prepared as described for the long-term stability tests on carbon cloth. CV data were recorded in a single flask with anode and cathode located approximately 1 cm apart. Subsequently, a long-term stability test was carried out at a constant potential of 2.0 V vs RHE for 180 min.

Results

Electrocatalytic Studies

Our previous work showed that the crystal structure of Ni2–xRhxP nanoparticles varies with composition, where Rh-rich and Ni-rich end members adopt the cubic antifluorite- and hexagonal Fe2P-type structures, respectively (Figure 1), and phase segregation/impurity formation occurs for x = 0.75–1.5 compositions.8 Accordingly, three different compositions of Ni2–xRhxP (x = 0.25, 0.5, and 1.75) nanoparticles, and their end members Ni2P and Rh2P, all determined to be phase-pure within the limitations of powder X-ray diffraction (see Figure S3) were prepared by the solution-phase arrested precipitation method according to our established protocol.8 The isolated quasi-spherical particles (Figure S4) were used to make nanoparticle inks, Ni2–xRhxP/C. For comparison studies, commercial catalysts (RuO2 and 20% Pt/C) were used to prepare inks with the same catalytic material concentration (RuO2 and Pt: 2 mg/mL). For RuO2/C ink preparation, a similar protocol as for Ni2–xRhxP/C was followed using RuO2 as nanoparticles, while for 20% Pt/C ink, no additional C was added as the Pt was already supported by C in the commercial catalyst.

Figure 1.

Figure 1

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(a) Ni2P structure with Ni (silver balls) in tetrahedral sites (Ni(1), coordination outlined in orange), square pyramidal sites (Ni(2), coordination outlined in green), and adlayer P on the (0001) facet. Other compositions that adopt this structure type are indicated in parentheses. (b) Rh2P structure with Rh atoms (gold balls) in tetrahedral sites. The other composition that adopts this structure type is shown in parentheses. Phosphorus atoms are shown in lavender.

Hydrogen Evolution Reaction in Acid

Figure 2 shows HER activity and current density stability studies normalized to geometric surface area in acidic media for the different compositions that were examined. Polarization curves clearly reveal that the HER performance of Ni2–xRhxP nanoparticles in acidic media is composition-dependent (Figure 2a), with the Rh-rich cubic antifluorite Ni0.25Rh1.75P phase being the most active and comparable to Rh2P (ca 85 mV@10 mA/cmgeo2) (Figure 2b). In contrast, Ni-rich samples adopting the hexagonal Fe2P structure have overpotentials that are above 150 mV. Because geometrical surface area can overestimate activity for complex surfaces, we also collected ECSA data and plotted ECSA-normalized polarization curves. As shown in Figure S5, the ECSA-normalized plots exhibit the same trend as the geometric surface area-normalized plots, supporting our conclusions.

Figure 2.

Figure 2

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(a) HER polarization curves for Ni2–xRhxP nanoparticles compared to those for Rh2P and Ni2P end members and 20% Pt/C. (b) Overpotential values at 10 mA/cmgeo2 (geometrical surface area). (c) Corresponding Tafel slopes. (d) Cathodic current density vs time (carried out on C-cloth substrate with a surface area of 1 cm2, mass loading 0.285 mg/cm2) at an applied potential of 0.08 V vs RHE for Ni0.25Rh1.75P nanoparticles compared to Rh2P and 20% Pt/C in 0.5 M H2SO4.

Examination of Figure 2a also reveals that at higher current densities (>30 mA/cmgeo2), both Rh2P/C and Ni0.25Rh1.75P/C catalytic materials outperform 20% Pt/C in terms of activity (Figure 2a and Table S4). As shown in Figure 2c, the Tafel slopes of Rh2P (35 mV/dec) and 20% Pt/C (36 mV/dec) are close to the value for a rate-limiting chemical step preceded by the Volmer–Tafel mechanism (∼30 mV/dec),40 whereas the slopes for Ni2–xRhxP phases are considerably higher, ranging from 59 to 204 mV/dec.41 The trend in Tafel slopes is generally in agreement with the observed HER activity, with the most active group of samples (Rh-rich) having the smallest slope while the least active samples (Rh-poor) have the highest slope. The time dependence of the current density at the fixed potential for Rh2P, Ni0.25Rh1.75P, and 20% Pt/C is shown in Figure 2d. After modest declines in the first few minutes, the current density for all of the samples remains stable over the 10 h run.

Oxygen Evolution Reaction in Acid

OER data were collected on the same samples (Figure 3a). When comparing potential values required to obtain 10 mA/cmgeo2 current density, Rh2P activity is shown to be comparable to RuO2 with an overpotential of 0.403 V, and the performance of the Ni-containing phases is comparatively poor (ca. 1 V overpotential, Figure 3b). The same trend is observed in the ECSA-normalized data (Figure S6). However, it is clear that to achieve higher current densities, the applied voltage for Rh2P must be increased, whereas this is not the case for RuO2 and the Ni2–xRhxP phases. Indeed, at 50 mA/cmgeo2, the voltage required by Rh2P is not that much less than for the Ni2–xRhxP phases (Table S4). This is also evident in the Tafel slopes (Figure 3c), where Rh2P exhibits the largest slope (ca. 450 mV/decade), RuO2 the smallest (ca 120 mV/decade), and the Ni2–xRhxP samples fall in between (190–280 mV/decade).

Figure 3.

Figure 3

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(a) OER polarization curves for Ni2–xRhxP nanoparticles compared to those for Rh2P and Ni2P endpoints and RuO2. (b) Overpotentials at 10 mA/cmgeo2. (c) Tafel slopes. (d) Anodic current density as a function of time (carried out on a C-cloth substrate with a surface area of 1 cm2, mass loading 0.285 mg/cm2) at 2.0 V vs RHE applied potential for Ni2–xRhxP, Rh2P, Ni2P, and RuO2 in 0.5 M H2SO4.

In addition, although the initial activity of Rh2P far outperforms any of the Ni2–xRhxP samples, the current density of Rh2P rapidly decreases over time (Figure 3d). A similar fall-off in current density as a function of time is observed for the Ni-rich compositions, including Ni2P, but the current density for Ni0.25Rh1.75P is demonstrated to be quite stable, similar to RuO2, although Ni0.25Rh1.75P is considerably less active (lower current density at fixed potential). According to the original report on Rh2P OER, Rh2P current density drops by 18% over 16.7 min (1000 s), which is in line with what we observe (Rh2P current density drops by 76% after 1 h).22 However, with Ni addition, the current density decrease was much smaller (21 vs 76%) for the Ni0.25Rh1.75P sample, while the commercial catalyst only showed a 9% decrease in current density over a 1 h period. The time-dependent current density studies were repeated twice and the same trend was observed.

Overall Water Splitting in Acid

The Ni0.25Rh1.75P/C composite was evaluated as a catalyst for overall water splitting. For comparison, overall water splitting electrolyzers were also prepared using Rh2P/C (Rh2P/C||Rh2P/C) and commercial catalysts (RuO2/C||20% Pt/C). The overall water electrolyzers were prepared in a two-electrode configuration (see Figure S7). The electrolytic activities of the three constructed overall cells were evaluated by collecting cyclic voltammograms (Figure 4a). For the Ni0.25Rh1.75P/C composite, the overall cell potential to deliver 10 mA/cmgeo2 was 1.76 V (without IR correction). This value is ∼200 mV higher than the voltage required by Rh2P/C (1.59 V to achieve 10 mA/cmgeo2) and the commercial catalyst (1.55 V to achieve 10 mA/cmgeo2).

Figure 4.

Figure 4

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Overall water splitting for the cells Ni0.25Rh1.75P/C||Ni0.25Rh1.75P/C, Rh2P/C||Rh2P/C and RuO2/C||20% Pt/C: (a) activity and (b) current density vs time.

The time-dependent current densities of the overall cells were also evaluated at a constant potential of 2.0 V for 180 min (Figure 4b). For the commercial catalysts, the observed current density was much higher than our catalysts, but in both cases, the current density was found to decrease over time (decrease is 26% for the commercial catalyst and 47% for Ni0.25Rh1.75P). However, the Rh2P/C catalyst rapidly lost its high initial activity, with the current density dropping to zero after 140 min.

Structural Elucidation: STEM-EDS Mapping and XPS

STEM-EDS and XPS analyses were performed on Ni0.25Rh1.75P before and after long-term stability tests to better understand the structural, compositional, and morphological changes in the catalyst as a consequence of use. The STEM-EDS images are shown in Figure 5, and the EDS compositions are compiled in Table 1. Data acquired on precatalyst inks reveals some aggregation has occurred during the process of supporting the discrete particles on carbon and performing the reductive annealing step to pyrolyze the ligand shell; however, individual particles are clearly discernable, and the sizes do not appear to be impacted by the processing. According to STEM-EDS mapping images, Ni, Rh, and P were uniformly distributed in the structure of the nanoparticles before catalysis and oxygen is present throughout, attributed to surface oxidation upon exposure to air. After 10 h of HER, the elemental components in the Ni0.25Rh1.75P sample were still uniformly distributed with no apparent changes. However, after OER catalytic testing, the O channel is more intense and clearly co-localized on the nanoparticles reflecting in situ oxide/hydroxide formation, which is commonly found in OER activity studies of transition-metal phosphide materials in alkaline media.20,23,28 Notably, the average EDS compositions collected from different areas of the TEM grid sample before and after catalytic testing were found to have no significant difference for both targeted compositions: Ni0.25Rh1.75P and Rh2P (Table 1).

Figure 5.

Figure 5

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EDS mapping analysis of Ni0.25Rh1.75P/C before catalysis, after 10 h HER activity, and after 1 h OER activity (scale bar is 10 nm).

Table 1. EDS Elemental Compositions before and after Catalytic Testinga.

targeted composition before catalysis after 10 h HER after 1 h OER
Ni0.25Rh1.75P1.00 Ni0.28(4)Rh1.72(4)P1.03(16) Ni0.24(2)Rh1.76(2)P0.98(10) Ni0.29(2)Rh1.71(2)P1.01(21)
Rh2.00P1.00 Rh2.00(10)P1.01(6) Rh2.00(9)P1.04(9) Rh2.00(8)P0.97(18)
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a

P was normalized assuming the metals sum to 2.0. Values in parentheses reflect the standard deviation in the composition.

Detailed XPS analyses were carried out before and after stability tests with both Ni0.25Rh1.75P/C and Rh2P/C materials to assess how the surface changes during the catalytic process (transformation of the precatalyst to the catalyst). Figure 6 summarizes the XPS spectra of Ni 2p, Rh 3d, and P 2p before and after the 10 h HER and 1 h OER stability tests of Ni0.25Rh1.75P, and the corresponding data for Rh2P are shown in Figure 7. According to the quantification analysis of Ni0.25Rh1.75P before catalysis (see Table 2), the sample surface was found to be quite rich in Ni and deficient in P, with a composition of Ni1.3Rh0.7P0.2.8 The same 10:1 metal:P composition is found for Rh2P (Rh2P0.2). Thus, the surface composition has little correlation to the bulk composition measured by EDS analyses. In terms of speciation, the Ni 2p3/2 peaks at 853.8, 856.5, and 858.7 eV are attributed to Ni–P, Ni2+, and Ni3+, respectively.8,42 The corresponding Rh 3d5/2 spectrum has peaks at 307.6 and 309.4 eV attributed to phosphorus-bound Rh0 (Rh–P) and Rh3+. Finally, the P 2p1/2 spectrum has peaks at 129.5 and 133.3 eV for reduced phosphorus (M–P) and phosphate (PO43–), respectively. Compared to Ni0.25Rh1.75P, Rh2P is also found to have a larger proportion of reduced P and Rh (Rh–P).

Figure 6.

Figure 6

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XPS spectra of N0.25Rh1.75P/C before catalysis and after 10 h HER and 1 h OER stability testing.

Figure 7.

Figure 7

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XPS spectra of Rh2P/C before catalysis and after 10 h HER and 1 h OER stability testing.

Table 2. XPS Elemental Ratios and Oxidation State Ratios before and after Catalytic Stability Tests with N0.25Rh1.75P/C and Rh2P/C.

  Ni0.25Rh1.75P (ideal Rh/Ni/P = 58:9:33)
 Rh2P (ideal Rh/P 67:33)
  Rh/Ni/P Rh Ni P Rh/P Rh P
    (Rh–P):Rh3+ (Ni–P):Ni2+/Ni3+ (P-M):PO43–   (Rh–P):Rh3+ (P-M):PO43–
before 31:59:10 63:37 18:38:44 16:84 92:8 80:20 48:52
after HER 25:72:3 59:41 17:30:53 19:81 95:5 75:25 56:44
after OER 30:68:2 65:35 9:45:46 27:73 94:6 62:38 42:58
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As shown in Table 2, after HER catalysis in acidic media, there is a notable change in the surface Rh:Ni ratio in Ni0.25Rh1.75P/C from 0.54:1 to 0.35:1—that is, the surface is even more Ni-rich after HER catalysis. A similar, but less extreme change is noted post OER, where the Rh:Ni ratio is 0.44:1. At the same time, the atomic percentage of surface P decreases from 10 to 2–3% (Table 2). This decrease in P is greater than is observed for Rh2P, where the initial atomic percentage is 8% decreasing to 4–5% post catalysis. In terms of Rh speciation, significant changes are observed between Rh2P/C and Ni0.25Rh1.75P/C. Comparing Rh–P to Rh3+, the ratio is 4:1 in Rh2P but only 1.7:1 in Ni0.25Rh1.75P/C—there is more oxidized Rh in the presence of Ni. However, the Rh–P to Rh3+ ratio in Ni0.25Rh1.75P/C does not change significantly after HER (1.4:1) or OER (1.9:1), whereas significant conversion of Rh–P to Rh3+ occurs in Rh2P after HER (ratio changes from 4:1 to 3:1) and OER (ratio changes from 4:1 to 1.6:1). With respect to Ni speciation, there is little change in Ni–P concentration post HER, although the distribution of Ni2+ and Ni3+ changes to favor the latter. After OER, Ni–P is depleted by 50%, replaced by a near-equal balance of Ni2+ and Ni3+.

Discussion

Many transition-metal phosphide materials are reported to be stable catalysts for HER under acidic condition.15,17−22,24−28,31 HER mechanisms for metal phosphides in acidic media tend to presume the active catalyst to be the metal phosphide, with reduced P sites acting to bind protons. However, more detailed studies have revealed that phosphides are not immune to corrosion processes that may result in catalytic restructuring (possibly improving activity) or dissolution (loss of activity due to solubilization of active sites).28,38 In the case of base-metal phosphides, both Co2P and Ni2P are reported to dissolve under active HER conditions, although the rate of dissolution appears much greater for Co2P relative to Ni2P.25,28,38 In our previous work with Co2–xRhxP, we observed an immediate activity loss of 50% (within the first 30 min of stability testing) followed by a leveling off for the remainder of the test (1 h or 9h).14 The activity loss correlates to Co dissolution, reducing surface Co from 45 to <10% on the surface after a 1 h stability test, with a composition that remains unchanged for a comparable 9h stability test. This corresponds to a change in the surface Rh/Co ratio from 0.75 to 8.8.14 In contrast, Ni2–xRhxP (x = 1.75, 2.00) does not show appreciable loss in activity over the extent of the test. In the case of Ni0.25Rh1.75P, we actually find the already Ni-rich surface becomes even more Ni-rich, at the expense of Rh, the opposite of the case for Co. It is surprising that the activities for Rh2P and Ni0.25Rh1.75P are so similar because their surface compositions are far more different than expected based on the nominal concentration, with Ni0.25Rh1.75P having a 2:1 ratio of Ni/Rh on the surface. The similarity in activity and stability suggests the most important factor may be the cubic antifluorite structure, which is adopted only at the Rh-rich end of the composition space (Figure 1b). Unfortunately, to our knowledge, there are no published studies comparing Rh2P and Ni2P as a function of structure type (i.e., Rh2P and Ni2P in hexagonal vs cubic) that would enable the roles of composition to be separated from structure. Indeed, since Ni2P does not adopt the cubic structure, nor Rh2P hexagonal, any studies would necessarily be computational. However, our prior computational studies focused on antifluorite Rh2–xCoxP suggested that (a) the active sites are P-terminated, not metal terminated, {100}, and (b) the computed free energy of hydrogen adsorption changes only modestly with metal compositional variation (introduction of Co).14 That is, the underlying metal composition per se is not expected to significantly impact HER activity in the antifluorite structure type, which is also what we see experimentally (Figure 2b). The other compositions that yielded pure phosphides appeared at the base-metal-rich end (x = 0.25, 0.5), adopting the Fe2P hexagonal structure type (Figure 1a) and were found to be considerably less active, though more active than Ni2P. Computational studies on Ni2P have identified the most active surface as the (0001), and specifically, the adlayer P sites (Figure 1a), but have also shown that the surface restructures outside the potential window −0.21 ≥ U ≥ −0.36 V vs NHE, where low-activity Ni3 hollow sites dominate.43 Such restructuring may be responsible for decreased activities, and the window for restructuring may be composition-dependent, leading to more variation in activity at the base-metal end of the composition space.

In contrast to the case for HER, transition-metal phosphides are not catalysts for OER, but rather precatalysts that convert to oxide or oxyhydroxide phases at the positive potentials required to oxidize water.16,23,28 While OER catalysis employing metal phosphide precatalysts has not been extensively studied in acidic media, a comparative analysis of base-metal phosphides revealed that Co2P and Ni2P were more active than IrO2, but they degraded rapidly during cycling.28 Considerable effort has been devoted to identifying why poor stability is observed in some high-activity catalysts for OER.3 It has been proposed that with most noble-metal oxides, when catalysts undergo an intermediate species-sharing mechanism, the overoxidation of the active material and lattice oxygen participation in the mechanism can induce instability.3 The intermediate species-sharing mechanism was first reported by Stucki et al. for Ru and RuO2 electrodes. In the mechanism, RuO4 serves as the reaction intermediate and the OER takes place by a cyclic transition between Ru8+ and Ru6+ (RuO2(OH)2).44 A similar result was found for iridium-based electrodes;45 Stucki et al. suggested that for IrO2, IrO3 serves as the intermediate species for the OER reaction and Ir dissolution happens due to IrO4 formation from IrO3. However, recent studies carried out with the help of scanning flow cells and inline electrochemical mass spectrometry revealed that both Ir3+ species and IrO3 are formed as intermediates, and both are possible dissolution species for Ir during the OER.46 Thus, the low stability of phosphides might be due to the intermediate species-sharing mechanism forming metal oxides and/or metal ions and dissolution of those species resulting in low stability.

Rh2P, as discussed earlier, is reported to be stable toward OER in acid, at least over a 17 min window.22 In the present study for the case of OER, in situ metal oxide/hydroxide formation was observed with both Ni0.25Rh1.75P and Rh2P, as expected. Because of the intermediate species-sharing mechanism, metal oxides are not stable under acidic conditions (even noble-metal oxides, as discussed above), and a decrease in the OER activity over time was expected with both materials. Intriguingly, the noble-metal phosphide Rh2P, with comparable initial activity to RuO2, was found to be far less stable than Ni0.25Rh1.75P to OER (Figure 3). The OWS activity and current density stability data show the rapid deactivation of the Rh2P/C||Rh2P/C electrolyzer, whereas Ni0.25Rh1.75P/C||Ni0.25Rh1.75P/C shows a much slower deactivation, comparable to RuO2/C||20%Pt/C, although considerably less active (Figure 4). Since XPS data show that the Ni0.25Rh1.75P surface is quite Ni-rich (2:1 Ni/Rh), we propose that the active catalytic sites are Ni oxyhydroxy moieties. This analysis is consistent with composition-dependent polarization data revealing that all Ni-containing samples have the same overpotential at 10 mA/cmgeo2, and more than twice the initial overpotential for Rh2P. In this analysis, the stability of the Ni catalyst towards OER may be associated with the underlying Rh. That is, the more favorable nickel oxidation may assist in preserving reduced Rh, with the reduced Rh in part facilitating the turnover of the Ni catalyst, thus leading to sustained activity. That this is not found in the corresponding Co phases, which deactivate after one cycle, may be due to the faster dissolution of CoOx phases in acid relative to NiOx phases. Notably, the kinetics of dissolution for Co2O3 in sulfuric acid are demonstrated to be faster than the corresponding Ni2O3 and Fe2O3 phases.47

Conclusions

In this work, we have shown that in acidic media, Rh2P-like activity and stability can be obtained for Ni0.25Rh1.75P as a water reduction electrocatalyst. In contrast, Ni0.25Rh1.75P is far less active than Rh2P for OER, and yet it is the combination base-metal/noble-metal phosphide that is the most stable. Overall water splitting can be achieved at driving potentials of 1.76 V with the Ni0.25Rh1.75P/C catalyst, about 200 mV higher than RuO2/C||20% Pt/C, but with comparable stability. Although the exact OER mechanism of our catalyst is yet to be explored, we surmise that Rh catalytic sites in Rh2P are subject to rapid deactivation, as are Ni sites in Ni2P, but the combination of Ni + Rh, which yields Ni-rich surfaces (even at nominally low Ni loadings) with protected Rh underlayers, results in Ni catalytic sites that can “turn over”, due to electronic effects of the Rh.

Acknowledgments

This work was supported by NSF-DMR 1904775. Some data were acquired on UPS/XPS, Talos F200X G2 S/TEM, and PXRD facilities in the Lumigen Instrument Center at Wayne State, which are partially supported by NSF (grant numbers: 1849578, 2018587, and 1427926, respectively).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsmaterialsau.2c00080.

  • ECSA data, PXRD patterns, TEM images, and ECSA-normalized HER and OER data for Ni2–xRhxP nanoparticles; picture of the cell for overall water splitting; corresponding tables of XPS scan details; and ECSA data acquired at negative (for HER) and positive (for OER) potentials and overpotentials for HER and OER at 50 mA/cmgeo2 (PDF)

This work was supported by the National Science Foundation under grant numbers 1904775, 1849578, 2018587, and 1427926.

The authors declare no competing financial interest.

Supplementary Material

mg2c00080_si_001.pdf (2.5MB, pdf)

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