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Article

Chromium Substitution Within Ruthenium Oxide Aerogels Enables High Activity Oxygen Evolution Electrocatalysts for Water Splitting

by
Jesus Adame-Solorio
1,2,
Samuel W. Kimmel
1,2,
Kathleen O. Bailey
2,† and
Christopher P. Rhodes
1,2,*
1
Material Science, Engineering, and Commercialization Program, Texas State University, San Marcos, TX 78666, USA
2
Department of Chemistry and Biochemistry, Texas State University, San Marcos, TX 78666, USA
*
Author to whom correspondence should be addressed.
†
Current address: Department of Chemistry, Texas A&M University, College Station, TX 77843, USA.
Crystals 2025, 15(2), 116; https://doi.org/10.3390/cryst15020116
Submission received: 19 December 2024 / Revised: 13 January 2025 / Accepted: 15 January 2025 / Published: 23 January 2025
(This article belongs to the Special Issue Advanced Materials for Applications in Water Splitting)
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Abstract

:
Acidic oxygen evolution reaction (OER) electrocatalysts that provide high activity, lower costs, and long-term stability are needed for the wide-scale adoption of proton-exchange membrane (PEM) water electrolyzers for generating hydrogen through electrochemical water splitting. We report the effects of chromium substitution and temperature treatments on the structure, OER activity, and electrochemical stability of ruthenium oxide (RuO2) aerogel OER electrocatalysts. RuO2 and Cr-substituted RuO2 aerogels (Ru0.6Cr0.4O2) were synthesized using sol–gel chemistry and then thermally treated at different temperatures. Introducing chromium into the synthesis increased the surface area (7–11 times higher) and pore volume (5–6 times higher) relative to RuO2 aerogels. X-ray diffraction analysis is consistent with s that Cr was substituted into the rutile RuO2 structure. X-ray photoelectron spectroscopy showed that trivalent Cr substitution altered the surface electronic structure and ratio of surface hydroxides. The specific capacitance values of Cr-substituted RuO2 aerogels were consistent with charge storage within a hydrous surface. Cr-substituted RuO2 aerogels exhibited 26 times the OER mass activity and 3.5 times the OER specific activity of RuO2 aerogels. Electrochemical stability tests show that Cr-substituted RuO2 aerogels exhibit similar stability to commercial RuO2. Understanding how metal substituents can be used to alter OER activity and stability furthers our ability to obtain highly active, durable, and lower-cost OER electrocatalysts for PEM electrolyzers.

1. Introduction

The ability to generate hydrogen through proton-exchange membrane (PEM) water electrolysis powered by renewable energy sources provides a sustainable pathway to “green hydrogen” that can be used for numerous industrial, transportation, and energy-storage applications [1,2]. The overall electrochemical water splitting reaction (2H2O → O2 + 2H2) in acid occurs via combination of the half-cell reaction at the cathode, the hydrogen evolution reaction (HER: 2H+ + 2e−  → H2), and the half-cell reaction at the anode, the oxygen evolution reaction (OER: 2H2O → O2 + 4H+ + 4e−). While the two-electron HER exhibits fast kinetics, the four-electron OER exhibits high activation barriers and slow kinetics, and OER catalysts must also remain active and stable over an extended time under the harsh conditions of high oxidative potentials and highly corrosive acidic environments [3,4].
Platinum-group metals (Ir, Ru, and Pt) and their oxides (IrOx, RuOx, and PtOx) have been studied as OER electrocatalysts and exhibit high OER activities [5,6,7,8,9,10]. Metallic Ir and Ir oxide catalysts are currently considered to provide the best balance of activity and stability for OER, but the scarcity and high price of iridium (USD 4450/oz) [11] limit the widespread adoption of PEM water electrolyzers. Metallic Ru and RuO2 have shown higher OER activities than Ir and IrO2 [12], respectively, and are potential lower-cost alternatives to Ir-based catalysts; however, the instability of Ru-based catalysts remains a major challenge [4]. Based on their higher activity and lower cost, developing more stable Ru-based catalysts can aid in achieving the U.S. Department of Energy’s goal of developing water electrolyzers that produce one kilogram of hydrogen per dollar in one decade [13].
RuO2 is more stable than metallic Ru [14], and efforts have been explored to improve the activity and stability of RuO2 OER catalysts. Different metal substituents within RuO2 (e.g., Ce [15], Zn [16], Sn [17], Co [18], and Ni [19] combined with RuO2) have been explored to understand the influence of the second element on the active site and influence the OER activity and stability. Our group has explored how Ti substituted into RuO2 [20] and Zr, Nb, and Ta substituted into RuO2 [21] affect its structure, OER activity, electrochemical stability, and dissolution. Our prior work showed that Ti substituted into RuO2 can lower the Ru dissolution rate [20]; however, Ti, Zr, Nb, and Ta substituted into RuO2 result in lowered OER activity [20,21].
Within rutile-type metal oxides, the OER activity is correlated with the M-O binding energy [22,23], and the M-O binding energy can be tuned to provide a balance between adsorption strengths of OER intermediates (adsorbed *O, *OH, *OOH) and improve catalytic activity [23,24]. Calculations of the binding energy of adsorbed oxygen (*O) on RuO2 have revealed that RuO2 binds oxygen slightly too weakly for the optimal balance between O* and *OH binding energies for the OER reaction [22]. Prior studies have reported that substitution of Cr into rutile RuO2 enhanced OER activity by changing the adsorption strength of OER intermediates [25] and that the Cr-O-Ru interaction can regulate the adsorption and desorption rates of oxygen intermediates of the OER in acid media [26]. A previous study of a chromium–ruthenium oxide solid solution prepared from a metal–organic framework template reported that incorporation of Cr into RuO2 improves the OER performance by tuning the electronic structure of RuO2 [27]. Other studies have also reported that the substitution of Cr within RuO2 influences the Ru-O binding energy and the rate formation of OER intermediates, increasing the OER activity [28,29]. The oxidation state of the substituted metal is also an important factor, since the metal substituent’s different electronegativity relative to Ru will influence the Ru-O binding energy. Prior studies have reported that Cr adopts 3+ and 4+ oxidation states within Cr-substituted RuO2 materials [26,27,28,29].
In addition to the effects of metal substituents on the active site, other factors, including particle size and morphology [30,31], crystallinity [32,33], oxidation treatments [34,35], synthesis methods [36,37], thermal treatments [38], and defects [39,40] affect the structure and OER performance of RuO2-based catalysts. Thermal treatments can significantly affect the structure and OER performance of catalysts [41]. Prior work revealed that increasing the temperature during thermal treatment in air can reduce the density of cracks and surface defects, increasing OER stability [42]. However, as temperature increases, the general trend is for the surface area and porosity of the materials to decrease along with the OER activity [42,43].
In addition to the importance of the catalytically active site, developing catalysts with high surface area and porosity is important because high surface area increases the number of active sites of catalysts for the OER [44,45,46], and mesoporosity facilitates faster mass transport within the catalyst and the removal of O2 gas from the surface of the catalyst, which enables accessible active sites at the catalyst surface [47,48,49]. Aerogels are of significant interest for obtaining improved OER catalysts due to the interconnected solid-pore structure of aerogels that provides characteristically high surface area and mesoporosity. Previous studies of Ru oxide aerogels have reported materials with high Brunauer–Emmett–Teller (BET) surface areas of up to 370 m2 g−1 [50,51]. Cr oxide aerogels have also been synthesized with high BET surface areas (up to 520 m2 g−1) [52]. Here, we report the sol–gel synthesis of Ru-Cr oxide aerogels and an evaluation of how Cr substitution and thermal treatments affect the morphology, atomic and electronic structure, and oxygen evolution reaction activity and stability, which has not been previously studied.

2. Materials and Methods

Chemicals. Ruthenium (III) chloride hydrate (42 wt % Ru), chromium (III) chloride hexahydrate (98%), and isopropyl alcohol (99%) were purchased from Alfa Aesar. Propylene oxide (>99%) and Nafion perfluorinated solution (5 wt %, item # 274704) was purchased from Sigma Aldrich. Anhydrous ethyl alcohol (99.5%, item # 2701G) was purchased from Decron Laboratories. Ultrapure water (≥18 MΩ-cm) was obtained from an ELGA PureLab Classic water purification system. Double-distilled perchloric acid (70%) was obtained from Veritas GFS Chemicals.
Material Synthesis. Ruthenium oxide and ruthenium–chromium oxide aerogels were synthesized via a sol–gel chemistry route adapted from previous studies [50,51,52]. To obtain ruthenium oxide gels, 240 mg (1 mmol) of RuCl3∙xH2O was placed in a 12 mL plastic vial containing 1.98 mL (34 mmol) anhydrous ethanol and 0.54 mL (30 mmol) ultrapure water. Then, the solution was magnetically stirred for 10 min, and 1.26 mL (18 mmol) of propylene oxide was added to the solution to obtain a gel, which formed in 5 h. The gel time was defined as the time that it took for magnetic stirring to stop due to gel formation, as previously reported [50]. Ru-Cr oxide gels were obtained by mixing 240 mg (1 mmol) of RuCl3∙xH2O and 170 mg (0.66 mmol) of CrCl3∙6H2O (nominal Ru0.60 Cr0.40 ratio) in a 12 mL plastic vial containing 2.63 mL (45 mmol) anhydrous ethanol and 0.54 mL (30 mmol) ultrapure water. Then, the solution was magnetically stirred for 10 min, and 1.26 mL (18 mmol) of propylene oxide was added to the solution to obtain a gel, which formed within one hour for the mixed Ru-Cr oxide gels. For comparison, we also synthesized chromium oxide gels (without Ru), which had a gel time of 20 min, a value consistent with a previous study that reported a 20 min gel time for chromium oxide gels synthesized under similar reaction conditions [52]. The Ru-Cr oxide gels had an intermediate gel time (1 h) compared with the gel time of ruthenium oxide gels (5 h) and chromium oxide gels (20 min). After the Ru-Cr oxide and Ru oxide wet gels formed, the plastic vials were covered with parafilm and left to age at room temperature for 6 days. After this aging step, the wet gels were transferred to 50 mL glass vials, and 30 mL of ethanol was added. The gels were washed over a three-day period, which involved replacing the ethanol every 12 h. The ethanol-washed gels underwent solvent exchange with liquid carbon dioxide (CO2) and were dried under supercritical CO2 conditions using a Leica EM-CPD300 supercritical dryer to obtain aerogels. The materials after supercritical drying (before additional thermal treatment) are notated as Ru-AG-AP for the as-prepared Ru oxide aerogel and RuCr-AG-AP for the as-prepared Ru-Cr oxide aerogel.
For thermal treatment, separate batches of 100 mg of the as-prepared Ru-AG-AP and RuCr-AG-AP aerogels were subjected to a thermal treatment in air at 500 or 600 °C for 5 h in a muffle furnace (Thermo Scientific Thermolyne, Waltham, MA, USA) using a 10 °C min−1 ramp rate. After a 5 h dwell time at 500 or 600 °C, the samples were left inside the muffle furnace until the material reached room temperature. The materials after thermal treatment are notated as Ru-AG-500 (Ru oxide aerogel treated at 500 °C), Ru-AG-600 (Ru oxide aerogel treated at 600 °C), RuCr-AG-500 (Ru-Cr oxide aerogel treated at 500 °C), and RuCr-AG-600 (Ru-Cr oxide aerogel treated at 600 °C). Commercial ruthenium oxide (>98%, item # 238058, lot # MKCL4231) obtained from Sigma-Aldrich (St. Louis, MO, USA) was used for comparison and is notated as RuO2-comm.
Structural Characterization. The as-prepared and thermally treated aerogel materials were ground using an agate mortar and pestle to obtain powders for structural characterization. X-ray diffraction (XRD) measurements of the powder aerogel samples were obtained with a Bruker AXS D8 Advance Powder X-ray diffractometer using Cu Kα (1.5406 Å). Diffraction patterns from 2θ° = 20 to 80 were analyzed with a step size of 0.01 2θ°. Nitrogen physisorption isotherms were obtained using a Micromeritics ASAP 2020 surface area and porosimetry analyzer. Before the surface area and porosity measurements, the samples were degassed under vacuum for 16 h at 120 °C. Pore volumes and mean pore diameters were calculated from the adsorption isotherm using the Barrett−Joyner−Halenda method. The morphology and elemental distribution of the materials were determined using an FEI Helios Nanolab 400 DualBeam scanning electron microscope (SEM) equipped with an energy-dispersive X-ray spectroscopy (EDS) detector. X-ray photoelectron spectra were obtained using a Thermo Fisher Scientific Nexsa X-ray photoelectron spectrometer with a monochromatic Al Kα X-ray source (1486.6 eV) and using a 400-micron diameter spot size. Survey and high-resolution spectra were collected using Constant Analyzer Energy (CAE) mode with pass energies of 200 and 10 eV, respectively. Adventitious carbon, C1s, was used for internal calibration (binding energy of 284.8 eV). AVANTAGE v5.91 software (Thermo Fisher Scientific) was used for data analysis. A Shirley-type background subtraction and a pseudo-Voigt function with Gaussian (70%)–Lorentzian (30%) for each component were used for the XPS peak fitting.
Electrochemical Characterization. Electrochemical measurements were conducted with an Autolab PGSTAT128N biopotentiostat (Metrohm USA, Inc., Riverview, FL, USA) coupled to a rotating disk electrode (Pine Instruments, Durham, NC, USA) placed into a three-electrode cell arrangement using 85 mL of 0.1 M perchloric acid (HClO4) electrolyte maintained at a constant temperature of 298 K. A Pt coil and a Pt wire (reversible hydrogen electrode, RHE) were used as counter and reference electrodes, respectively. A gold (Au) electrode (0.196 cm2) was used as the working electrode. A binder solution was prepared by combining 250 µL of Nafion perfluorinated solution with 75 mL of ultrapure water and 24.75 mL of isopropyl alcohol. Then, the binder solution was combined with 5 mg of the aerogel material or commercial RuO2 in a 20 mL glass scintillation vial to make a catalyst ink. The volume of the binder solution was adjusted to achieve a catalyst concentration of 0.55 mg mL−1. The inks were sonicated (Fisher Scientific Ultrasonic Bath) for 30 min in an ice bath to make a homogeneous ink. The inks were placed in a water bath at room temperature, and then 20 µL of the freshly prepared catalyst ink was placed on the surface of a Au electrode, which was put upside down in an electroderotator (Pine Instruments, Durham, NC, USA). The Au electrode with the cast ink was rotated at 800 rpm and allowed to dry for 30 min. Freshly prepared catalyst inks were used before each electrochemical evaluation. Before the first cyclic voltammetry (CV) measurement, the working electrode was immersed in the electrochemical cell under potential control at 0.1 VRHE to avoid any undesired catalyst modification. In addition, before all CV measurements, the electrolyte was deaerated for 15 min by bubbling argon gas through the solution. An initial CV procedure, 30 scans from 0.05 VRHE to 1.0 VRHE at 50 mV s−1, was performed to obtain stable voltammograms. Prior to the next evaluation steps, the working electrode rotation was set at 1600 rpm. A CV step consisting of 5 scans from 1.2 VRHE to 1.6 VRHE w at 50 mV s−1 was then carried out. Linear sweep voltammetry (LSV) was recorded between 1.3 and 1.6 VRHE using a scan rate of 20 mV s−1 under 1600 rpm rotation. A chronoamperometry (CA) test was then performed using a potential step of 0.01 V for 5 s from 1.36 to 1.55 VRHE at 1600 rpm. The currents used for the oxygen evolution reaction activity calculations and Tafel plots were obtained from the CA tests. Next, rotation was stopped, and a CV procedure (30 scans) from 0.05 to 1.00 VRHE at 50 mV s−1 was recorded to evaluate the catalyst’s surface after exposure to OER potentials. Following the CV step and prior to the durability test, 85 mL of fresh 0.1 M HClO4 solution was added to the electrochemical cell, and the electrolyte was deaerated for 15 min. To evaluate the OER stability of the catalysts, an accelerated durability test (ADT) was performed. During the ADT test, a constant potential of 1.6 VRHE was applied for 13.5 h at 1600 rpm. Following the ADT test, 50 mL of the electrolyte was collected to evaluate the metal dissolution into the electrolyte and was replaced with fresh HClO4 solution. The fresh HClO4 solution was deaerated for 30 min. Analysis of the concentrations of Ru and Cr within the electrolyte during the ADT step was performed by Galbraith Laboratories, Inc., using a Perkin Elmer Nexion 2000 inductively coupled plasma mass spectrometer (ICP-MS). A final CV test (30 scans) from 0.05 to 1.0 VRHE at 50 mV s−1 was performed to evaluate the catalyst’s surface condition after the ADT. The rotation was set at 1600 rpm, and potential was cycled (5 scans from 1.2 to 1.6 VRHE at 50 mV s−1). A final LSV test (20 mV s−1) from 1.3 to 1.6 VRHE at 1600 rpm and a chronoamperometry (potential step of 0.01 V for 5 s) procedure from 1.36 to 1.55 V at 1600 rpm were taken to evaluate the catalyst’s performance after the ADT. The amount of dissolved Ru during the ADT phase was considered when calculating the final mass activity for all the catalysts. Specific capacitance values were calculated from the CV test at low potentials (0.05 to 1.0 VRHE) using the integrated charge from 0.3 to 0.8 V of the voltammetric sweep, the scan rate, and the catalyst mass [53].

3. Results and Discussion

3.1. Analysis of Morphology and Elemental Composition

The morphologies of the as-prepared and thermally treated Ru oxide and Ru-Cr oxide aerogels were characterized by SEM (Figure 1). The as-prepared aerogels (Ru-AG-AP and RuCr-AG-AP) show the presence of micron-scale aggregates of nanoscale particles (Figure 1A,D). The thermally treated Ru oxide aerogels (Ru-AG-500, Ru-AG-600) and Ru-Cr oxide aerogels (RuCr-AG-500, RuCr-AG-600) have a nanostructured morphology composed of aggregates of interconnected nanoscale particles (Figure 1B,C,E,F), and the distinct, nanoscale particles for the thermally treated samples may result from the formation of crystalline domains, as evidenced by the X-ray diffraction analysis presented in the following section.
The elemental distribution and composition of the Ru oxide and Ru-Cr oxide aerogels were determined using energy-dispersive X-ray spectroscopy (EDS), and the EDS spectra are presented in Figures S1 and S2. EDS mapping analysis of the Ru oxide aerogels showed a uniform distribution of Ru and O for Ru-AG-AP, Ru-AG-500, and Ru-AG-600 (Figure 2A–O) and a uniform distribution of Ru, Cr, and O for RuCr-AG-AP, RuCr-AG-500, RuCr-AG-600 (Figure 2G–U) at the scale of the images, which supports that the sol–gel synthesis method resulted in well-integrated Cr within the structure rather than a separate phase.
EDS spectra were analyzed to determine the relative atomic ratios of Ru and Cr in the materials, which were compared with the nominal atomic ratios of the synthetic precursors. The as-prepared Ru-Cr oxide aerogel, RuCr-AG-AP, and thermally treated Ru-Cr oxide aerogels, RuCr-AG-500 and RuCr-AG-600, showed experimental Ru/Cr ratios that were very similar relative to their nominal atomic ratios (Table 1). The slightly lower experimental Ru ratios and higher Cr ratios relative to the synthesis ratios are within the reasonable experimental error of the EDS analysis, and for the thermally treated materials, some Ru may be lost in the vapor phase during the thermal treatment process, as previously reported by our group [20] and others [51]. EDS analysis showed a negligible amount of Cl ( < 2 atomic %) from the Ru precursor. Since carbon corrodes at OER potentials in acid media [54] and can affect the catalyst’s OER performance, we analyzed the weight % of carbon using EDS. The as-prepared materials Ru-AG-AP and RuCr-AG-AP had 17 ± 6 wt % and 14 ± 2 wt % carbon, respectively, which was expected due to the alkoxide precursors used for the sol–gel synthesis. After thermal treatment at 600 °C, the amount of carbon decreased to 6 ± 1 wt % for Ru-AG-600 and 5 ± 1 wt % for RuCr-AG-600.
We used nitrogen physisorption to determine the Brunauer–Emmett–Teller (BET) surface area, pore diameter, and pore volume of the Ru oxide and Ru-Cr oxide aerogels, and we also analyzed commercial RuO2 for comparison. The as-prepared Ru oxide aerogel, Ru-AG-AP, showed a high BET surface area of 234 ± 7 m2 g−1. After thermal treatment to 500 °C or 600 °C, the surface area significantly decreased to similar values of 7 ± 1 m2 g−1 for Ru-AG-500 and 7 ± 4 m2 g−1 for Ru-AG-600. The Ru-Cr oxide gels showed a similar decrease in surface area as temperature increased, with surface area decreasing from 285 ± 3 m2 g−1 for RuCr-AG-AP to 77 ± 10 m2 g−1 for RuCr-AG-500 and 51 ± 1 m2 g−1 for RuCr-AG-600. For both Ru oxide and Ru-Cr oxide materials, the substantial decrease in surface area with temperature treatment is consistent with the transition from an amorphous to a crystalline Ru oxide phase [50] and changes occurring due to the sintering process [55]. The comparison of the surface areas of Ru oxide and Ru-Cr oxide aerogels showed that the introduction of Cr resulted in an increase in surface area, which was particularly evident for the thermally treated samples. The aerogel with Cr heated to 600 °C, RuCr-AG-600, had a surface area of 51 ± 1 m2 g−1, which is approximately seven times higher than the surface area of the non-Cr-containing aerogel heated to 600 °C, Ru-AG-600 (7 ± 4 m2 g−1). In addition, the thermally treated aerogels with Cr, RuCr-AG-500 and RuCr-AG-600, both showed increased surface areas of 77 ± 10 m2 g−1 and 51 ± 1 m2 g−1, respectively, compared to commercial RuO2 (7 ± 1 m2 g−1). For pore volume, the trends for the effects of temperature and the introduction of Cr were similar to those observed for surface area. Thermal treatment lowers the pore volume, and the introduction of Cr results in higher pore volume (Table 1). Comparing aerogels heated to 600 °C, the pore volume of the thermally treated aerogel with Cr (RuCr-AG-600) was approximately six times higher than the pore volume of the thermally treated aerogel without Cr (Ru-AG-600). All aerogels showed average pore diameters in the nanometer range.In addition to the analysis of surface area, nitrogen physisorption measurements were used to determine the distribution of pores within the Ru oxide aerogels (Figure 3A) and Ru-Cr oxide aerogels (Figure 3B). The as-prepared Ru-AG-AP and RuCr-AG-AP aerogels exhibited a wide distribution of pore sizes, with a pore structure composed primarily of mesopores (≥2–50 nm) and macropores ( > 50 nm). The as-prepared Ru oxide, Ru-AG-AP, showed the presence of mesopores, with pore sizes centered at ~50 nm; however, after thermal treatment, the relative ratio of these pores significantly decreased (Figure 3A). In contrast, for the aerogels with Cr, after thermal treatment, the materials maintained the presence of mesopores (pore dimeters centered at ~50 nm) as well as macropores. Pores in the mesopore region could be associated with the pore structure of the original wet gel, which was maintained during the supercritical drying process. It is possible that larger particles in the Ru-AG-AP and RuCr-AG-AP samples (Figure 1) stuck together during thermal treatment, creating pores in the macropore range [56].
The Ru-Cr oxide aerogels showed larger pore volumes and a wide distribution of pore sizes, including mesopores and macropores, compared to Ru oxide aerogels and commercial RuO2, which is beneficial for OER. Prior work reported that porous materials improved OER activity by enabling faster mass transfer within the catalyst’s structure [57]. The presence of a significant degree of mesopores within the Ru-Cr oxide aerogels is desirable for OER because microporosity can hinder mass transport and mesoporous structures are preferred for catalysis [47]. The porosity and morphology are particularly important for OER, as supported by a prior study reporting that the porosity and morphology of RuO2 catalysts at the meso- and macroscale impact the OER activity by controlling the formation and removal of O2 gas bubbles [48].

3.2. X-Ray Diffraction Characterization

X-ray diffraction (XRD) was used to investigate the effects of Cr substitution and thermal treatment on RuO2 aerogels, which were compared to commercial RuO2 and indexed against RuO2 (rutile; PDF: 01-071-4825), Cr2O3 (trigonal; PDF: 38-1479), and metallic Ru (hexagonal; PDF 06-0663), as presented in Figure 4. The full-range XRD patterns are provided as supporting information, Figure S3. The as-prepared RuO2 aerogel and Cr-substituted RuO2 aerogels were amorphous, without any discernable diffraction peaks. After thermal treatment at 500 °C and 600 °C, the XRD patterns of the RuO2 and Cr-substituted RuO2 aerogels showed new diffraction peaks indicating a transition from amorphous structures (Ru-AG-AP and RuCr-AG-AP) to structures exhibiting periodic ordering. The most intense diffraction peaks were identified as the (110), (101), and (200) planes of rutile RuO2 [38]. Ru-AG-500 exhibited a peak at 2ϴ° = 44°, which was indexed as the (101) plane for metallic Ru. The metallic Ru phase present within Ru-AG-500 may have resulted from the reaction with products of partial oxidation of the organic matrix, as previously reported (details provided in SI) [51]. Based on the XRD pattern, Ru-AG-500 was not further analyzed with XPS or electrochemical testing, as the mixed phase containing a metallic Ru phase and a rutile RuO2 phase would complicate the analysis because the metallic phase would contribute differently than the oxide phase. When a temperature of 600 °C was used, no metallic Ru peaks were observed. No diffraction patterns of Cr2O3 were observed in RuCr-AG-500 or RuCr-AG-600. The XRD patterns of the thermally treated Cr-substituted RuO2 aerogels are consistent with Cr substitution within the rutile lattice.
The diffraction patterns were fit to determine the effect of the Cr substitution and thermal treatment on crystallographic lattice parameters relative to RuO2 aerogels and RuO2-comm. Without Cr substitution, the lattice parameters and unit cell volumes of Ru-AG-500 were similar to those of RuO2-comm; Ru-AG-500 had a smaller crystallite domain size (Table 2). Compared to Ru-AG-500, Ru-AG-600 showed an increased unit cell volume and increased crystalline domain sizes. Previous work has indicated that at higher calcination temperatures, larger crystallite sizes grow as crystallites are sintered together [58,59]. After thermal treatment, the Cr-substituted samples (RuCr-AG-500 and RuCr-AG-600) both had smaller c-lattice parameters, smaller unit cell volumes, and smaller domain sizes than their unsubstituted analogs (Ru-AG-500 and Ru-AG-600), as presented in Table 2 and Figure S4. The compression of the unit cell volume for the Cr-substituted aerogels is consistent with Cr substitution within the rutile RuO2 lattice considering the reduced ionic radius of Cr3+ (61.5 pm) compared to the ionic radius of Ru4+ (62.0 pm) [60].

3.3. X-Ray Photoelectron Spectroscopy Characterization

X-ray photoelectron spectroscopy (XPS) was used to evaluate the surface electronic structure of the Ru oxide and Ru-Cr oxide aerogels and RuO2-comm. XPS spectra of the Ru 3d region of RuO2-comm, Ru-AG-600, RuCr-AG-500, and RuCr-AG-600 are shown in Figure 5A, and binding energies and relative areas from peak fitting analysis are presented in Table 3. The Ru 3d region consists of the Ru 3d5/2 and Ru 3d3/2 peaks and their corresponding satellite peaks as well as a low-relative-intensity C1s peak [61]. The binding energies of the Ru 3d3/2 (280.8 eV) and Ru 3d5/2 (284.8 eV) of Ru-AG-600 and RuO2-comm are similar to the binding energies reported for RuO2 [62,63]; the binding energies of the Ru 3d peaks in all the materials are consistent with the presence of Ru4+ [61]. The Ru 3d5/2 peak of RuCr-AG-500 (281.0 eV) has a higher binding energy than those of RuO2-comm (280.8 eV) and Ru-AG-600 (280.8 eV). According to a previous study, a shift of the peaks in the Ru 3d region to higher binding energies suggests a lower electron density at the surface Ru atoms [27]. The binding energy of the Ru 3d5/2 peak of RuCr-AG-600 (280.9 eV) was within experimental error (relative error in binding energies: ±0.1 eV) of the other materials.
The XPS spectra of the aerogels and RuO2-comm in the O 1s region are shown in Figure 5B, and the O 1s band was fitted using three peaks: Oa (lattice oxygen, O-Ru, and O-Cr), Ob (Ru-O sat, Ru-OH, and Cr-OH), and Oc (C-O); binding energies are presented in Table 3 [61,64]. The increased relative intensity of the Ob peak in RuCr-AG-500 and RuCr-AG-600 relative to samples without Cr (RuO2-comm and Ru-AG-600) indicates that materials with Cr have a higher concentration of hydroxyl groups (Cr-OH and/or Ru-OH) at the surface [61]. The Oa peaks of the materials with Cr (RuCr-AG-500 and RuCr-AG-600) have contributions from Ru-O [61] and Cr-O [65]. The lower binding energies of the Oa peaks of the materials with Cr (RuCr-AG-500 and RuCr-AG-600) compared to those of RuO2-comm and Ru-AG-600 may result from different electronic environments of oxygen bonded to Ru and/or Cr within the Cr-substituted aerogels, which have a disordered, hydrous surface region, as supported by our voltammetric data and analysis presented below. The thermal treatment temperature also influences the relative surface composition: the Ru-Cr oxide aerogel treated at 600 °C (RuCr-AG-600) has a lower relative peak area for Ob than the Ru-Cr oxide aerogel treated at 500 °C (RuCr-AG-500), which indicates that higher-temperature treatment lowers the relative concentration of surface hydroxyl groups.
The XPS spectra of the Cr 2p region of the Ru-Cr oxide aerogels (Figure 5C) showed that Cr was present at the surface of the catalyst materials. The Cr 2p region revealed multiplet splitting and was deconvoluted (Table 3) into five peaks (Cra-e) associated with Cr-O and one peak (Crf) corresponding to Cr-OH, based on prior fitting analysis of Cr 2p3/2 peaks of surface Cr oxides and hydroxides [64,66,67], and the Cr 2p3/2 binding energies were consistent with Cr3+-O and Cr3+-OH surface species. The Cr-O and Cr-OH peaks contributed to the Oa and Ob peak intensities (Figure 5B and Table 3), and the higher relative intensity of the Ob peak for the materials with Cr (RuCr-AG-500 and RuCr-AG-600) was consistent with the presence of Cr-OH surface groups.

3.4. Cyclic Voltammetric Characterization

Cyclic voltammetry scans of Ru-AG-600, RuCr-AG-500, RuCr-AG-600, and RuO2-comm were obtained to characterize the electrochemistry of the catalyst materials within the 0.05–1.00 VRHE potential region (Figure 6) before the oxygen evolution activity measurements. The commercial RuO2 material (RuO2-comm) exhibited oxidation and reduction peaks around 0.6 and 0.5 VRHE, respectively, which are attributed to the Ru (III)/Ru (IV) redox couple [35,59,68]. The Ru oxide aerogel, Ru-AG-600, showed similar oxidation and reduction peaks compared to RuO2-comm, but also broader voltammetric features. The Ru-Cr oxide aerogels, RuCr-AG-500, and RuCr-AG-600, exhibited broad voltametric features, without discernable peaks, consistent with hydrous RuO2 (designated as RuOxHy or RuO2·xH2O) rather than anhydrous RuO2. The presence of voltammetric features of hydrous RuO2 within Ru-Cr oxide aerogels that also exhibit crystalline rutile RuO2 structures from X-ray diffraction (Figure 4) indicates the presence of a surface hydrous Ru-Cr oxide region, which is supported by our XPS data (Figure 5). The asymmetry of the CV voltammograms of RuCr-AG-500 and RuCr-AG-600 compared to RuO2-comm may result from a higher resistivity resulting from Cr within the rutile RuO2 structure.
The integrated charge from the cyclic voltammetry scans was used to determine the specific capacitance values of the aerogel materials and commercial RuO2. (Table 4). The Ru-Cr oxide aerogels exhibited higher specific capacitances than Ru oxide aerogels and RuO2-comm. The specific capacitances of Ru oxide materials have been reported to depend on their degree of hydration [69], and the higher currents observed in the cyclic voltammograms of RuCr-AG-500 and RuCr-AG-600 compared to Ru-AG-600 and RuO2-comm are consistent with their higher specific capacitance (F g−1) [35,50,53]. The specific capacitance values of all the Cr-substituted materials were between values reported for anhydrous RuO2 and hydrous RuO2 (RuO2·0.03H2O) [53]. Pseudocapacitive charge storage occurs within hydrous RuO2 and involves charge storage below the top surface layer [53]. The increased specific capacitance values of the Ru-Cr oxide aerogels compared to anhydrous RuO2 suggests that the materials have a disordered, hydrous surface region [70] rather than a compact anhydrous surface, and that charge storge occurs below the top surface in the Cr-substituted aerogels.

3.5. Electrochemical Oxygen Evolution Reaction Activity and Mechanism

The OER electrocatalytic activity was evaluated by chronoamperometry measurements, and the Ru-Cr oxide aerogels showed significantly higher geometric area-normalized currents than the Ru oxide aerogels and RuO2-comm (Figure 7A). Mass activities were determined by normalizing the current at 1.51 VRHE to the catalyst mass (Figure 7B) and Ru mass (Figure 7C). The Ru-normalized mass activity values of RuCr-AG-500 (242 ± 48 mA mg−1 Ru) and RuCr-AG-600 (206 ± 9 mA mg−1 Ru) were higher than those of Ru-AG-600 (8 ± 1 mA mg−1 Ru) and RuO2-comm (15 ± 2 mA mg−1 Ru). The comparison of aerogels thermally treated at 600 °C showed that Cr-substituted RuO2 aerogels (RuCr-AG-600) exhibited 26 times higher OER mass activity than RuO2 aerogels (Ru-AG-600). For further comparison of the catalyst activities, we determined the specific activity by normalizing the current to the BET surface area (Figure 7D). Ru-Cr oxide aerogels showed higher specific activities than Ru oxide aerogels and RuO2-comm. The OER specific activity showed a clear increase in Cr-substituted samples. The specific activity of RuCr-AG-600 (0.39 ± 0.01 mA cm−2) was ~3.5 times higher than that of the Ru oxide aerogel without Cr, Ru-AG-600 (0.11 ± 0.01 mA cm−2), and approximately twice that of RuO2-comm (0.21 ± 0.03 mA cm−2).
The enhanced specific activities of the Ru-Cr aerogels, RuCr-AG-500 and RuCr-AG-600, compared to Ru-AG-600 and RuO2-comm are attributed to the combination of the higher concentration of surface hydroxides within the hydrous surface layer and the influence of Cr on the atomic and electronic structure of the active site. Our XPS analysis showed that both Cr-substituted aerogels had a higher relative surface concentration of hydroxides than RuO2-comm and Ru-500-AG. Electrocatalysts with surface Ru hydroxides have been shown to be more active than those with surface anhydrous RuO2 [38]. From our XRD analysis, Cr substitution results in a smaller unit cell volume, which affects M-O bond distances and bond strengths. XPS analysis showed that the Cr-substituted aerogel, RuCr-500-AG, had a higher Ru3d5/2 binding energy, which is consistent with a more positive charge of Ru with Cr incorporation [27]. A higher positive charge of Ru would likely result in stronger Ru-O binding energy [25], which would provide a better balance between O* and *OH binding energies for the OER reaction [22] and therefore result in higher OER activity. A previous study of a Ru-Cr oxide catalyst reported that the lower electron density at Ru sites (more positive charge) was due to the electron-withdrawing effect of Cr and resulted in higher OER activity [27].
To understand the effect of Cr substitution into rutile RuO2 compared to metal substitution, we compared the specific activity of the Ru-Cr oxide aerogel (RuCr-AG-600) with the specific activities of Ti-, Zr-, Nb-, and Ta-substituted RuO2 from our prior studies [20,21]. Cr-substituted RuO2 exhibited a higher specific activity than Ti-, Zr-, Nb-, and Ta-substituted RuO2 (Figure 8). Cr was the only metal substituent from this comparison that enhanced the OER specific activity relative to RuO2-comm. Chromium is an interesting substituent, as it has a number of possible oxidation states, with Cr3+ and Cr6+ being the most pronounced [71]. From our XPS analysis described above, the substituted Cr existed in a +3 oxidation state in the synthesized Ru-Cr oxide aerogels; however, under the high applied potentials of the oxygen evolution reaction, we consider that is it possible that the Cr substituent could adopt a higher oxidation state (Cr6+) than these other metal substituents (Ti4+, Zr4+, Nb5+, and Ta5+). Previous calculations of Cr-substituted RuO2 considered the presence of Cr6+ to explain higher OER activity [27], and prior studies of chromium-doped cobalt–iron hydroxide alkaline OER catalysts reported the presence of Cr6+ after electrochemical testing [72,73]. However, the presence of Cr6+ under OER reaction conditions needs to be experimentally confirmed. The specific effects of Cr substitution within RuO2 on OER activity need to be further studied, as we also note that numerous factors including the metal substitution ratio, synthesis method, thermal treatments, surface structure, electronic conductivity, and pore structure can influence the OER activity.
To further understand the effect of Cr substitution within RuO2 aerogels, we prepared Tafel plots and determined the Tafel slopes (Figure 9) to probe how Cr substitution may influence the OER reaction mechanism. The linear behavior of the CA data in the potential range of 1.45–1.50 V indicates that the reaction is predominantly kinetically controlled within this potential region. The Tafel slope value for RuO2-comm (32 ± 6 mV dec−1) is in line with previous reports for RuO2 [27,74,75,76,77,78]. The Tafel slopes of the Ru oxide aerogel (Ru-AG-600) and Ru-Cr oxide aerogels (RuCr-AG-500 and RuCr-AG-600) were within error (~46 to 56 mV dec−1) of each other but higher than that of RuO2-comm (32 mV dec−1). Previous theoretical analyses reported that a Tafel slope of 40 mV dec−1 is obtained when the second electron transfer step is the rate-determining step [77,79]. The electrochemical oxide path [77] and the density functional theory-predicted peroxide path [23] are possible reaction pathways for RuO2-comm from the Tafel analysis. The Ru oxide aerogel and Ru-Cr oxide aerogels have higher Tafel slopes than RuO2-comm, which suggests the aerogels may have a different OER mechanism. The higher Tafel slopes and higher activities of Ru-Cr oxide aerogels are counter to the general trend that higher Tafel slopes result in lower activity [80]. However, factors such as the surface arrangement of active sites [78], adsorbate concentration and organization of adsorbates [81], mass transport, and ohmic resistance [82] have been reported to influence Tafel slopes. In addition, variables such as synthesis conditions [27,76], morphology [75], and catalyst layer differences [74] can also influence Tafel slopes, and further analysis is needed to understand how Cr may influence the reaction pathway with the aerogel catalysts.

3.6. Evaluation of Oxygen Evolution Electrochemical Stability and Dissolution

Catalyst electrochemical stability or durability is a key parameter for the development of viable OER electrocatalysts for PEM water electrolyzers [83]. The electrochemical stability of the catalysts was evaluated using an accelerated durability test (ADT), where the potential was held at 1.6 VRHE for 13.5 h based on previous reports [84,85,86]. The polarization curves from the CA test before (initial) and after the ADT evaluation (final) of RuO2-comm, Ru-AG-600, RuCr-AG-500, and RuCr-AG-600 are shown in Figure 10A. The comparisons of the initial and final OER mass activities and percentage mass activity retained over the accelerated durability test are presented in Figure 10B. RuO2-comm showed a reduction in OER mass activity from 15 to 3 mA mg−1Ru, representing 19 ± 13% OER activity retained after the ADT. The Ru oxide aerogel, Ru-AG-600, showed a higher OER mass activity retention of 72 ± 1% compared to RuO2-comm. The Ru-Cr oxide aerogels, RuCr-AG-500 and RuCr-AG-600, showed a reduction in mass activity from 242 and 206 mA mg−1 Ru to 62 and 50 mA mg−1 Ru, respectively, with ~24% of OER activity retained after the ADT, which is within error of the retained OER activity of RuO2-comm. The reduction in mass activity is associated with a number of factors including catalyst dissolution, surface area changes during the electrochemical process, specific activity changes due to modification of the active site, changes to the morphology and pore structure, and changes to mass transport within the catalyst layer [20,83,84,87]. Changes to the catalyst surface and/or surface area are evident from the comparison of initial CVs to CVs after ADT (Figure S5).
To determine the amounts of Ru and Cr dissolved over the ADT step, we analyzed the electrolyte using inductively coupled plasma mass spectrometry (ICP-MS), and the data are shown in Table 5. Ru-AG-600 showed the least amount of Ru dissolved over the durability test, which is in agreement with the high percentage of activity retained (Figure 10B). The Ru-Cr oxide aerogels, RuCr-AG-500 and RuCr-AG-600, had higher amounts of dissolved Ru than Ru-AG-600 and RuO2-comm, and Cr also dissolved from the catalyst during the ADT step. A prior study of well-defined RuO2 surfaces showed an initially increased Ru corrosion rate and a decreased Ru corrosion rate upon a second stability test, suggesting that the surface is reorganized following dissolution and reaches a steadier state [30]. There may be a higher initial rate of metal dissolution within the Ru-Cr oxide aerogels, which is then lowered upon further durability tests; however, additional testing is needed to determine this. Interestingly, the higher thermal treatment temperature used for the synthesis of RuCr-AG-600 lowered the dissolution of Ru and Cr relative to RuCr-AG-500. Previous studies have shown that thermal treatments may reduce the density of surface defects and cracks of OER catalysts, enhancing their stability in acid conditions [42,43].
Based on our data, metal dissolution occurred in all samples and contributed to the degradation of the OER activity over the durability test. The higher metal dissolution from Ru-Cr oxide aerogels may be related to the increased relative concentrations of surface hydroxides in RuCr-AG-500 and RuCr-AG-600 relative to Ru-AG-600 and RuO2-comm, which was evident from XPS results, discussed above. A previous report showed that electrocatalysts with surface Ru hydroxides were less stable than those with surface anhydrous RuO2 [38].

4. Conclusions

We developed chromium-substituted ruthenium oxide aerogels and evaluated how the Cr substitution within RuO2 and thermal treatments in air affected the atomic and electronic structure and influenced OER activity and electrochemical stability. Ruthenium oxide aerogels and chromium-substituted ruthenium oxide aerogels were synthesized using sol–gel chemistry and supercritical drying and then heated at 500 °C or 600 °C. The introduction of Cr resulted in aerogels with higher surface area and porosity, with thermally treated samples with Cr showing approximately seven times more surface area and approximately six times more pore volume than Ru oxide aerogels. X-ray diffraction analysis supported that Cr was substituted into the rutile RuO2 lattice, and Cr substitution resulted in a smaller unit cell volume, in agreement with the smaller ionic radius of Cr3+ relative to Ru4+. X-ray photoelectron spectroscopy showed that within the Cr-substituted aerogel heated to 500 °C, the incorporation of Cr shifted the Ru 3d3/2 peak to a higher binding energy, lowering the electron density at surface Ru sites. The surface region of the Cr-substituted ruthenium oxide aerogels had Cr3+-O and Cr3+-OH species, and Ru-Cr oxide aerogels had a higher relative concentration of surface hydroxides than Ru oxide aerogels. Although Cr-substituted RuO2 aerogels exhibited highly crystalline domains according to X-ray diffraction, their specific capacitance values indicated that these aerogels had a disordered, hydrous surface region rather than a compact anhydrous surface and that charge was stored below the topmost surface.
Chromium substitution within RuO2 aerogels affected the oxygen evolution activity. From the electrochemical tests, Cr substitution increased the OER mass and specific activity. The comparison of aerogels thermally treated at 600 °C shows that Cr-substituted RuO2 aerogels (RuCr-AG-600) exhibited 26 times the OER mass activity and 3.5 times the specific OER activity of RuO2 aerogels (Ru-AG-600). In addition, the Cr-substituted aerogel, RuCr-AG-600, showed approximately twice the OER specific activity of commercial RuO2. Compared with other metal substituents within rutile RuO2, Cr substitution showed the ability to increase specific OER activity, which may result from the combination of the influence of the higher concentration of surface hydroxides and better M-O binding energy. From accelerated durability testing, Cr substitution resulted in similar electrochemical stability compared to a commercial RuO2 material. However, the Ru-Cr oxide aerogels had higher amounts of Ru dissolution and exhibited Cr dissolution, and the higher metal dissolution may be related to the higher relative concentration of surface hydroxides. Using a higher thermal treatment temperature for the Ru-Cr oxide aerogels resulted in lowering the dissolution of Ru and Cr over the durability test. The ability to incorporate metal substituents into high-surface-area aerogels and the understanding of how metal substituents alter the atomic and electronic structure, OER activity, and stability furthers the development of OER catalysts that can provide high activity, high stability, and lower cost and enable improved water electrolyzers that generate green hydrogen.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/cryst15020116/s1, Figure S1: X-ray diffraction patterns in the 2θ = 20–75° range; explanation of formation of metallic Ru; Figure S2: Comparison of lattice parameters, unit cell volumes, and crystalline domain sizes from X-ray diffraction analysis; Figure S3: Cyclic voltammetry before and after accelerated durability testing; Figure S4: Comparison of lattice parameters, unit cell volumes, and crystalline domain sizes from X-ray diffraction analysis; Figure S5: Cyclic voltammetry before and after accelerated durability testing.

Author Contributions

Conceptualization, C.P.R.; methodology, C.P.R., J.A.-S. and S.W.K.; investigation, J.A.-S., S.W.K. and K.O.B.; writing—original draft preparation; J.A.-S. and S.W.K.; writing—review and editing, C.P.R., J.A.-S., S.W.K. and K.O.B.; supervision, C.P.R.; funding acquisition, C.P.R. All authors have read and agreed to the published version of the manuscript.

Funding

The authors acknowledge support given for this research by the Office of Naval Research (Award no. N00014-22-1-2144) for the material synthesis, structural and electrochemical characterization, and dissolution analysis, and they acknowledge support from the National Science Foundation (Award No. 2122041) Partnerships for Research and Education in Materials, Center for Intelligent Materials Assembly, for the XPS analysis.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are available upon request from the authors.

Conflicts of Interest

The authors declare no conflicts of interest.

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Crystals 15 00116 g001
Figure 1. Scanning electron microscopy (SEM) images of (A) Ru-AG-AP (as-prepared Ru oxide aerogel), (B) Ru-AG-500 (Ru oxide aerogel treated at 500 °C), (C) Ru-AG-600 (Ru oxide aerogel treated at 600 °C), (D) RuCr-AG-AP (as-prepared Ru-Cr oxide aerogel), (E) RuCr-AG-500 (Ru-Cr oxide aerogel treated at 500 °C), and (F) RuCr-AG-600 (Ru-Cr oxide aerogel treated at 600 °C).
Figure 1. Scanning electron microscopy (SEM) images of (A) Ru-AG-AP (as-prepared Ru oxide aerogel), (B) Ru-AG-500 (Ru oxide aerogel treated at 500 °C), (C) Ru-AG-600 (Ru oxide aerogel treated at 600 °C), (D) RuCr-AG-AP (as-prepared Ru-Cr oxide aerogel), (E) RuCr-AG-500 (Ru-Cr oxide aerogel treated at 500 °C), and (F) RuCr-AG-600 (Ru-Cr oxide aerogel treated at 600 °C).
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Crystals 15 00116 g002
Figure 2. Scanning electron microscopy (SEM) images and energy-dispersive X-ray spectroscopy (EDS) mapping of (A–C) Ru-AG-AP, (D–F) Ru-AG-500, (G–I) Ru-AG-600, (J–M) RuCr-AG-AP, (N–Q) RuCr-AG-500, and (R–U) RuCr-AG-600. Colors for EDS mapping for Ru (purple), Cr (brown) and O (green) were used for visualization of the different elements).
Figure 2. Scanning electron microscopy (SEM) images and energy-dispersive X-ray spectroscopy (EDS) mapping of (A–C) Ru-AG-AP, (D–F) Ru-AG-500, (G–I) Ru-AG-600, (J–M) RuCr-AG-AP, (N–Q) RuCr-AG-500, and (R–U) RuCr-AG-600. Colors for EDS mapping for Ru (purple), Cr (brown) and O (green) were used for visualization of the different elements).
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Crystals 15 00116 g003
Figure 3. Incremental pore volume vs. average pore width: (A) Ru-AG-AP (as-prepared Ru oxide aerogel), Ru-AG-500 (Ru oxide aerogel treated at 500 °C), and Ru-AG-600 (Ru oxide aerogel treated at 600 °C; (B) RuCr-AG-AP (as-prepared Ru-Cr oxide aerogel), RuCr-AG-500 (Ru-Cr oxide aerogel treated at 500 °C), and RuCr-AG-600 (Ru-Cr oxide aerogel treated at 600 °C).
Figure 3. Incremental pore volume vs. average pore width: (A) Ru-AG-AP (as-prepared Ru oxide aerogel), Ru-AG-500 (Ru oxide aerogel treated at 500 °C), and Ru-AG-600 (Ru oxide aerogel treated at 600 °C; (B) RuCr-AG-AP (as-prepared Ru-Cr oxide aerogel), RuCr-AG-500 (Ru-Cr oxide aerogel treated at 500 °C), and RuCr-AG-600 (Ru-Cr oxide aerogel treated at 600 °C).
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Crystals 15 00116 g004
Figure 4. X-ray diffraction patterns in the 2θ = 25–45° range for (A) Ru oxide aerogels and reference patterns: RuO2-comm (commercial RuO2), Ru-AG-AP (as-prepared Ru oxide aerogel), Ru-AG-500 (Ru oxide aerogel treated at 500 °C), and Ru-AG-600 (Ru oxide aerogel treated at 600 °C); (B) Ru-Cr oxide aerogels: RuO2-comm, RuCr-AG-AP (as-prepared Ru-Cr oxide aerogel), RuCr-AG-500 (Ru-Cr oxide aerogel treated at 500 °C), and RuCr-AG-600 (Ru-Cr oxide aerogel treated at 600 °C). Miller indices for rutile RuO2 are included. Reference diffraction patterns for reference compounds: RuO2 (rutile; PDF: 01-071-4825), Cr2O3 (trigonal; PDF: 38-1479), and metallic Ru (hexagonal; PDF 06-0663).
Figure 4. X-ray diffraction patterns in the 2θ = 25–45° range for (A) Ru oxide aerogels and reference patterns: RuO2-comm (commercial RuO2), Ru-AG-AP (as-prepared Ru oxide aerogel), Ru-AG-500 (Ru oxide aerogel treated at 500 °C), and Ru-AG-600 (Ru oxide aerogel treated at 600 °C); (B) Ru-Cr oxide aerogels: RuO2-comm, RuCr-AG-AP (as-prepared Ru-Cr oxide aerogel), RuCr-AG-500 (Ru-Cr oxide aerogel treated at 500 °C), and RuCr-AG-600 (Ru-Cr oxide aerogel treated at 600 °C). Miller indices for rutile RuO2 are included. Reference diffraction patterns for reference compounds: RuO2 (rutile; PDF: 01-071-4825), Cr2O3 (trigonal; PDF: 38-1479), and metallic Ru (hexagonal; PDF 06-0663).
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Crystals 15 00116 g005
Figure 5. X-ray photoelectron spectroscopy (XPS) spectra of Ru-AG-600 (Ru oxide aerogel treated at 600 °C), RuCr-AG-500 (Ru-Cr oxide aerogel treated at 500 °C), RuCr-AG-600 (Ru-Cr oxide aerogel treated at 600 °C), and RuO2-comm (commercial RuO2) in the (A) Ru 3d/C1s region and (B) O1s region; (C) XPS spectra of RuCr-AG-500 and RuCr-AG-600 in the Cr 2p region.
Figure 5. X-ray photoelectron spectroscopy (XPS) spectra of Ru-AG-600 (Ru oxide aerogel treated at 600 °C), RuCr-AG-500 (Ru-Cr oxide aerogel treated at 500 °C), RuCr-AG-600 (Ru-Cr oxide aerogel treated at 600 °C), and RuO2-comm (commercial RuO2) in the (A) Ru 3d/C1s region and (B) O1s region; (C) XPS spectra of RuCr-AG-500 and RuCr-AG-600 in the Cr 2p region.
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Crystals 15 00116 g006
Figure 6. Cyclic voltammograms of RuO2-comm, Ru-AG-600, RuCr-AG-500, and RuCr-AG-600 after initial conditioning step; 30th cycle shown.
Figure 6. Cyclic voltammograms of RuO2-comm, Ru-AG-600, RuCr-AG-500, and RuCr-AG-600 after initial conditioning step; 30th cycle shown.
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Crystals 15 00116 g007
Figure 7. Evaluation of oxygen evolution reaction (OER) activities of RuO2-comm (commercial RuO2), Ru-AG-600 (Ru oxide aerogel treated at 600 °C), RuCr-AG-500 (Ru-Cr oxide aerogel treated at 500 °C), and RuCr-AG-600 (Ru-Cr oxide aerogel treated at 600 °C): (A) initial chronoamperometry in OER region, (B) catalyst-based mass activity, (C) Ru-based mass activity, and (D) specific activity determined using BET surface area.
Figure 7. Evaluation of oxygen evolution reaction (OER) activities of RuO2-comm (commercial RuO2), Ru-AG-600 (Ru oxide aerogel treated at 600 °C), RuCr-AG-500 (Ru-Cr oxide aerogel treated at 500 °C), and RuCr-AG-600 (Ru-Cr oxide aerogel treated at 600 °C): (A) initial chronoamperometry in OER region, (B) catalyst-based mass activity, (C) Ru-based mass activity, and (D) specific activity determined using BET surface area.
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Crystals 15 00116 g008
Figure 8. Comparison of oxygen evolution reaction (OER) specific activities of RuO2-comm and RuCr-AG-600 in this study with specific activities of Ti-substituted RuO2 (notated with *, adapted from [20] and reproduced with permission, copyright 2002, American Chemical Society) and Zr-, Nb-, and Ta-substituted RuO2 (notated with **, adapted from [21] and reproduced with permission, copyright 2004, American Chemical Society). All specific activities were obtained from current at 1.51 V. The specific activity of RuO2-comm and RuCr-AG-600 were calculated based on the current at 1.51 VRHE from chronoamperometry and normalized to the BET surface area. Specific activities of Ti-, Zr-, Nb-, and Ta-substituted RuO2 were determined based on current at 1.51 VRHE from linear sweep voltammetry and normalized to the electrochemical surface area.
Figure 8. Comparison of oxygen evolution reaction (OER) specific activities of RuO2-comm and RuCr-AG-600 in this study with specific activities of Ti-substituted RuO2 (notated with *, adapted from [20] and reproduced with permission, copyright 2002, American Chemical Society) and Zr-, Nb-, and Ta-substituted RuO2 (notated with **, adapted from [21] and reproduced with permission, copyright 2004, American Chemical Society). All specific activities were obtained from current at 1.51 V. The specific activity of RuO2-comm and RuCr-AG-600 were calculated based on the current at 1.51 VRHE from chronoamperometry and normalized to the BET surface area. Specific activities of Ti-, Zr-, Nb-, and Ta-substituted RuO2 were determined based on current at 1.51 VRHE from linear sweep voltammetry and normalized to the electrochemical surface area.
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Crystals 15 00116 g009
Figure 9. Tafel plots in the 1.45–1.50 VRHE potential region and Tafel slopes of RuO2-comm (commercial RuO2), Ru-AG-600 (Ru oxide aerogel treated at 600 °C), RuCr-AG-500 (Ru-Cr oxide aerogel treated at 500 °C), and RuCr-AG-600 (Ru-Cr oxide aerogel treated at 600 °C).
Figure 9. Tafel plots in the 1.45–1.50 VRHE potential region and Tafel slopes of RuO2-comm (commercial RuO2), Ru-AG-600 (Ru oxide aerogel treated at 600 °C), RuCr-AG-500 (Ru-Cr oxide aerogel treated at 500 °C), and RuCr-AG-600 (Ru-Cr oxide aerogel treated at 600 °C).
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Crystals 15 00116 g010
Figure 10. Evaluation of oxygen evolution electrochemical stability of RuO2-comm, Ru-AG-600, RuCr-AG-500, and RuCr-AG-600. (A) Initial and final (after accelerated durability test) chronoamperometry in OER region. The inset graph shows an expanded view of the 1.45–1.55 V potential region. (B) Initial and final OER mass activity and percentage mass activity retained (% final/initial).
Figure 10. Evaluation of oxygen evolution electrochemical stability of RuO2-comm, Ru-AG-600, RuCr-AG-500, and RuCr-AG-600. (A) Initial and final (after accelerated durability test) chronoamperometry in OER region. The inset graph shows an expanded view of the 1.45–1.55 V potential region. (B) Initial and final OER mass activity and percentage mass activity retained (% final/initial).
Crystals 15 00116 g010
Table 1. Relative Ru:Cr atomic ratios from synthesis and energy-dispersive X-ray spectroscopy (EDS) and Brunauer–Emmett–Teller (BET) surface area, pore diameter, and pore volume obtained from nitrogen physisorption measurements of RuO2-comm (commercial RuO2), Ru-AG-AP (as-prepared Ru oxide aerogel), Ru-AG-500 (Ru oxide aerogel treated at 500 °C), Ru-AG-600 (Ru oxide aerogel treated at 600 °C), RuCr-AG-AP (as-prepared Ru-Cr oxide aerogel), RuCr-AG-500 (Ru-Cr oxide aerogel treated at 500 °C), and RuCr-AG-600 (Ru-Cr oxide aerogel treated at 600 °C).
Table 1. Relative Ru:Cr atomic ratios from synthesis and energy-dispersive X-ray spectroscopy (EDS) and Brunauer–Emmett–Teller (BET) surface area, pore diameter, and pore volume obtained from nitrogen physisorption measurements of RuO2-comm (commercial RuO2), Ru-AG-AP (as-prepared Ru oxide aerogel), Ru-AG-500 (Ru oxide aerogel treated at 500 °C), Ru-AG-600 (Ru oxide aerogel treated at 600 °C), RuCr-AG-AP (as-prepared Ru-Cr oxide aerogel), RuCr-AG-500 (Ru-Cr oxide aerogel treated at 500 °C), and RuCr-AG-600 (Ru-Cr oxide aerogel treated at 600 °C).
MaterialRu:Cr Atomic Ratio (Synthesis)Ru:Cr
Atomic Ratio (EDS)		BET Surface Area,
(m2 g−1)		Pore Diameter (nm)Pore Volume
(cm3 g−1)
RuO2-comm−−7 ± 124 ± 10.04 ±0.01
Ru-AG-AP−−234 ± 718 ± 11.00 ± 0.02
Ru-AG-500−−7 ± 166 ± 200.10 ± 0.03
Ru-AG-600−−7 ± 446 ± 10.08 ± 0.05
RuCr-AG-APRu0.60Cr0.40Ru0.56Cr0.44285 ± 323 ± 62.0 ± 0.6
RuCr-AG-500Ru0.60Cr0.40Ru0.54Cr0.4677 ± 1029 ± 50.51 ± 0.03
RuCr-AG-600Ru0.60Cr0.40Ru0.57Cr0.4351 ± 136 ± 60.46 ± 0.07
Table 2. Crystallographic parameters obtained from X-ray diffraction analysis: d-spacing, lattice parameters, unit cell volume, and crystalline domain size of Ru-AG-500, Ru-AG-600, RuCr-AG-500, RuCr-AG-600, and RuO2-comm; crystalline domain size obtained from rutile (110) peak.
Table 2. Crystallographic parameters obtained from X-ray diffraction analysis: d-spacing, lattice parameters, unit cell volume, and crystalline domain size of Ru-AG-500, Ru-AG-600, RuCr-AG-500, RuCr-AG-600, and RuO2-comm; crystalline domain size obtained from rutile (110) peak.
Materiald-Spacing
(Å)		Lattice Parameters (Å)Unit Cell Volume
(Å3)		Crystalline Domain Size
(nm)
(110)(101)a = bc (Å)
RuO2-comm3.172.554.493.1062.519.1 ± 0.4
Ru-AG-5003.192.564.493.1062.613.4 ± 0.7
Ru-AG-6003.202.564.513.1163.316.3 ± 4.2
RuCr-AG-5003.142.494.463.0560.73.5 ± 1.4
RuCr-AG-6003.182.514.503.0561.77.4 ± 0.2
Table 3. Peaks, binding energy (B.E.) values, and relative area percentages from peak fitting analysis of X-ray photoelectron spectra (XPS) of RuO2-comm, Ru-AG-600, RuCr-AG-500, and RuCr-AG-600 and assignments of Ru [61,62,63], O [61,64,65], and Cr [64,66,67] peaks from prior studies. “B.E.” notates binding energy; * represents species present in samples containing Cr only. The relative error in binding energies is estimated as ±0.1 eV.
Table 3. Peaks, binding energy (B.E.) values, and relative area percentages from peak fitting analysis of X-ray photoelectron spectra (XPS) of RuO2-comm, Ru-AG-600, RuCr-AG-500, and RuCr-AG-600 and assignments of Ru [61,62,63], O [61,64,65], and Cr [64,66,67] peaks from prior studies. “B.E.” notates binding energy; * represents species present in samples containing Cr only. The relative error in binding energies is estimated as ±0.1 eV.
RegionPeak LabelAssignmentRuO2-CommRu-AG-600RuCr-AG-500RuCr-AG-600
B.E.
(eV)		Area
%		B.E.
(eV)		Area
%		B.E.
(eV)		Area
%		B.E.
(eV)		Area
%
Ru 3d/
C 1s		Ru 3d5/2Ru 3d5/2280.842280.841281.041280.941
Ru 3d5/2 satRu 3d5/2 sat282.820282.820282.920282.820
Ru 3d3/2Ru 3d3/2285.123285.123285.321285.222
Ru 3d3/2 satRu3d3/2 sat286.612286.714286.816286.715
C 1sC 1s, C-C284.82284.82284.82284.82
O 1sOaO 1s, Ru-O, Cr-O*529.367529.360529.351529.262
ObO 1s, Ru-O sat, Ru-OH, *Cr-OH530.620530.728530.737530.525
OcO 1s, C-O532.113532.512532.512532.112
Cr 2pCraCr 2p3/2, Cr3+ oxide, peak 1−−−−574.230.0575.628
CrbCr 2p3/2, Cr3+ oxide, peak 2−−−−574.817.0574.715
CrcCr 2p3/2, Cr (III) hydroxide−−−−576.724.0576.524
CrdCr 2p3/2, Cr3+ oxide, peak 3−−−−577.713.6577.614
CreCr 2p3/2, Cr3+ oxide, peak 4−−−−578.610.4578.511
CrfCr 2p3/2, Cr3+ oxide, peak 5−−−−579.05.0579.18
Table 4. Specific capacitance values of RuO2, RuO2-comm, Ru-AG-600, RuCr-AG-500, and RuCr-AG-600. * notates values for anhydrous RuO2 (Alfa) and RuO2·0.03H2O obtained from a prior study [53] that utilized a scan rate of 2 mV s−1 and a potential range of 0.2 to 0.07 VRHE.
Table 4. Specific capacitance values of RuO2, RuO2-comm, Ru-AG-600, RuCr-AG-500, and RuCr-AG-600. * notates values for anhydrous RuO2 (Alfa) and RuO2·0.03H2O obtained from a prior study [53] that utilized a scan rate of 2 mV s−1 and a potential range of 0.2 to 0.07 VRHE.
MaterialSpecific Capacitance (F/g)
RuO2-comm (Sigma Aldrich)1.5 ± 0.2
Ru-AG-6001.4 ± 0.6
RuCr-AG-50013 ± 1
RuCr-AG-6007 ± 2
Anhydrous RuO2 (Alfa) *0.75
RuO2·0.03H2O *29
Table 5. Inductively coupled plasma mass spectrometry (ICP-MS) data of dissolved Ru and Cr within the electrolyte of RuO2-comm, Ru-AG-600, RuCr-AG-500, and RuCr-AG-600 after the accelerated durability test.
Table 5. Inductively coupled plasma mass spectrometry (ICP-MS) data of dissolved Ru and Cr within the electrolyte of RuO2-comm, Ru-AG-600, RuCr-AG-500, and RuCr-AG-600 after the accelerated durability test.
MaterialRu Dissolution
(ppb)		Cr Dissolution
(ppb)
Ru-AG-6000.15 ± 0.01−
RuCr-AG-6002.0 ± 0.82.2 ± 0.1
RuCr-AG-5007.6 ± 0.74.1 ± 0.3
RuO2-comm0.35 ± 0.08−
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Adame-Solorio, J.; Kimmel, S.W.; Bailey, K.O.; Rhodes, C.P. Chromium Substitution Within Ruthenium Oxide Aerogels Enables High Activity Oxygen Evolution Electrocatalysts for Water Splitting. Crystals 2025, 15, 116. https://doi.org/10.3390/cryst15020116

AMA Style

Adame-Solorio J, Kimmel SW, Bailey KO, Rhodes CP. Chromium Substitution Within Ruthenium Oxide Aerogels Enables High Activity Oxygen Evolution Electrocatalysts for Water Splitting. Crystals. 2025; 15(2):116. https://doi.org/10.3390/cryst15020116

Chicago/Turabian Style

Adame-Solorio, Jesus, Samuel W. Kimmel, Kathleen O. Bailey, and Christopher P. Rhodes. 2025. "Chromium Substitution Within Ruthenium Oxide Aerogels Enables High Activity Oxygen Evolution Electrocatalysts for Water Splitting" Crystals 15, no. 2: 116. https://doi.org/10.3390/cryst15020116

APA Style

Adame-Solorio, J., Kimmel, S. W., Bailey, K. O., & Rhodes, C. P. (2025). Chromium Substitution Within Ruthenium Oxide Aerogels Enables High Activity Oxygen Evolution Electrocatalysts for Water Splitting. Crystals, 15(2), 116. https://doi.org/10.3390/cryst15020116

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