Laplace‐Pressure‐Stabilized Rutile Solid‐Solution Catalysts for Acidic Water Oxidation: Enabling DSA‐Inspired Compositions

ABSTRACT

An early phase diagram study in 1967 revealed surprisingly wide immiscible gaps in the rutile‐type IrO₂─TiO₂ and IrO₂─SnO₂ systems, even though the three oxides share the same crystal structure and nearly identical ionic radii. This finding indicates that the formation of IrO₂‐based solid solutions, especially with oxides of different structures, is thermodynamically challenging. Using more than 20 foreign metal (M) cations, we demonstrate that single‐phase rutile (Ir, M)O₂ and (Sn, M)O₂ solid solutions containing up to 30 at.% M can be obtained when the crystal size is below 10 nm, whereas larger, submicron‐scale crystals exhibit immiscible behavior. In particular, complete solid solutions are achieved across the entire composition range in the nanoscale IrO₂─SnO₂ system. This enhancement in miscibility under high Laplace pressure is not restricted to specific foreign cations but represents a general phenomenon. These results are exploited to synthesize rutile‐phase nanocrystals incorporating DSA‐inspired quaternary cations (Sn, Nb, Ru, and Ir), which show high corrosion resistance and catalytic activity under anodic potentials. Quantitative electrochemical analyses further show that the catalytic durability of these quaternary nanocrystals, despite the significantly reduced Ir content, is comparable to that of IrO₂ during the oxygen evolution reaction in acidic water electrolysis.

The miscibility of foreign elements in rutile‐type IrO₂ and SnO₂ is greatly enhanced under high Laplace pressure, where their crystal size is reduced to the nanoscale. This enhancement leads to the successful synthesis of DSA‐composition solid‐solution nanocatalysts with notable durability during the acidic OER in water electrolysis.

1. Introduction

IrO₂ is used as a benchmark catalyst for the oxygen evolution reaction (OER) in proton exchange membrane (PEM) electrolyzers, as Ir uniquely offers both high OER activity and long‐term stability under the strongly acidic conditions imposed by PEM operation. The scarcity and high cost of Ir, however, are significant obstacles to the gigawatt‐scale development of PEM electrolyzers for green hydrogen production [1]. Although many alternative catalyst chemistries have been proposed over the past decade [2, 3], both the catalytic activity and the durability of Ir‐based nanoscale catalysts are unrivalled. For example, 3M's nanostructured thin film (referred to as NSTF) Ir catalyst/electrode [4, 5] has demonstrated over 6000 h of operation at 10 A/cm², and the P2X catalyst developed under the Kopernikus project [6, 7] has also achieved over 35 000 h of operation at 2 A/cm² with low Ir loading. Despite ongoing efforts to replace Ir, it still plays a critical role in meeting the stringent performance and longevity requirements of PEM electrolyzers [8, 9, 10, 11, 12].

One of the most effective strategies for reducing reliance on platinum group metals in catalysts is the synthesis of single‐phase solid solutions, as extensively demonstrated in Pt‐based alloys [13, 14]. The formation of rutile‐type IrO₂ solid solutions incorporating a substantial amount of foreign element, however, is particularly challenging. An early phase diagram study [15] in 1967 and recent phase analyses [16] revealed that even the IrO₂─TiO₂ system shows significantly limited miscibility (see Figure 1a), despite that all three major criteria of the Hume‐Rothery rules are completely satisfied: both oxides crystallize in the rutile structure, the difference in ionic radii between Ir⁴⁺ (63 pm) and Ti⁴⁺ (61 pm) is less than 15%, and both cations possess the same tetravalence. Nearly immiscible behavior of IrO₂ with rutile SnO₂ (Sn⁴⁺: 69 pm) is also consistently observed [15], as illustrated in the phase diagram of Figure 1b, which strongly indicates the peculiar difficulty of IrO₂‐based solid solutions.

FIGURE 1.

Low solubility of foreign elements in large IrO₂ crystals. (a, b) The two phase diagrams demonstrate that less than 5% of Ti and Sn are soluble below 900°C in IrO₂. (c) The additional Bragg reflections, denoted by asterisks in the XRD patterns, verify the presence of secondary phases. (d) As indicated by white arrows, the formation of secondary phases rich in foreign elements is directly visualized in the EDS maps.

Mixed metal‐oxide composite electrodes known as ‘dimensionally stable anodes’ (DSAs) [17, 18, 19, 20], which consist of oxide mixtures of IrO₂ (and RuO₂) with highly stable TiO₂, SnO₂, ZrO₂, Ta₂O₅, or Nb₂O₅, are another representative example that results from the limited miscibility in rutile IrO₂. Consequently, major efforts to reduce the Ir content in catalysts while maintaining high activity have concentrated on fabricating IrO _x nanostructures and nanoclusters on stable supports [21, 22, 23].

In this study, contrary to the long‐standing view that IrO₂ is immiscible with foreign elements, we demonstrate the successful synthesis of rutile‐type IrO₂‐based solid solutions. To achieve this, we utilized the phase equilibria under high Laplace pressure, attained through nanoscale crystals. Although only a few percent of foreign metal (M) cations are typically soluble in (sub)micron‐scale IrO₂ crystals, we found that 30 at.% of M could be substituted into IrO₂ particles smaller than 10 nm. More importantly, this solubility enhancement was not limited to several specific foreign cations but was identified as a universal phenomenon across 20 different cations. We further demonstrated that this improvement in solubility via crystal size reduction also occurs in rutile‐type SnO₂ and enables complete solid solution formation over the entire composition range of IrO₂─SnO₂.

These findings establish a significant compositional design principle for high‐stability rutile‐structured nanoscale oxide crystals with substantially lowered Ir usage for the OER in acidic water electrolysis. Despite containing only one‐fifth the amount of Ir compared to pristine IrO₂, the resulting solid‐solution nanocrystals with DSA‐based compositions exhibit catalytic activity and durability comparable to those of IrO₂. For an objective and precise evaluation, we suggest quantitatively measuring the durability degradation rate during accelerated tests and comparing the degradation rates between our solid‐solution sample and the high‐stability rutile IrO₂ reference instead of merely presenting the preserved electrode‐assembly‐based performance.

2. Results and Discussion

2.1. Immiscible Nature of IrO₂ and SnO₂

Using powder samples with the initial composition of Ir_0.7M_0.3O₂ (30 at.% M), we first verified the immiscibility behavior of IrO₂. More than 20 different foreign M cations were utilized during the sample preparation. As annealing for crystallization was carried out at a relatively high temperature of 800–1000°C, the crystal size was fairly large (more than 100 nm). The X‐ray diffraction (XRD) analysis results in Figure 1c clearly show two‐phase separation into IrO₂ and counterpart M _x O _y in most cases, as denoted by asterisks in the diffraction patterns (Figure S1 for details). The energy dispersive X‐ray spectroscopy (EDS) maps acquired in scanning transmission electron microscopy (STEM) in Figure 1d directly visualize the presence of an M‐rich secondary phase in each case. Indeed, the EDS quantification shows that less than 5 at.% was detected within IrO₂ crystals in most of the samples (Figure S2 for the EDS spectra), confirming the low solubility in IrO₂. During the XRD and EDS analyses, only V, Cr, and Mo cations, in addition to Ru, among the 22 foreign cations were found to be soluble without forming any secondary phases (Figure S3 for the XRD patterns and EDS maps). We also observed that rutile‐type SnO₂ consistently exhibits low miscibility of 17 different foreign cations in samples with a starting stoichiometry of Sn_0.7M_0.3O₂. XRD and EDS results verifying the two‐phase mixture in each sample are provided in Figures S4–S6.

2.2. Solubility Enhancement by Crystal Size Reduction

One of the key findings in this work is that the solubility limit in IrO₂ and SnO₂ is remarkably enhanced when the crystal size diminishes at a nanoscale. To suppress particle coarsening and obtain nanocrystals that are less than 10‐nm in size, all the starting Ir_0.7M_0.3O₂ materials were annealed at a much lower temperature of 450–600°C (note that in the cases of Zr, Nb, Hf, and Ta, Ir_0.8M_0.2O₂ was the starting composition). In stark contrast to the phase separation observed in Figure 1c for large‐crystal samples, the series of XRD patterns of the nanocrystalline Ir_0.7M_0.3O₂ samples in Figure 2a clearly demonstrates the formation of a rutile‐type single phase without any extra Bragg reflections for the secondary phases in all 20 samples with foreign elements. Furthermore, as indicated by short vertical lines in each pattern, the positions of the major (110), (101), or (211) Bragg reflections shifted, revealing the variation of lattice parameters in the solid solutions (Figure S7 and Table S1 for the lattice parameters estimated from the XRD results).

FIGURE 2.

XRD patterns and EXAFS data of nanoscale (Ir, M)O₂ and (Sn, M)O₂ solid solutions. (a, c) No extra Bragg peaks associated with secondary phases are observed in both sets of solid solutions, indicating that a pure rutile phase is obtained in all samples. Small vertical markers are added at the three main reflections to emphasize the peak shifts caused by solid‐solution formation. (b, d) From the EXAFS analysis, the M─O and M─M radial distances in the samples agree well with the Ir─O and Ir─Ir distances in IrO₂ and the Sn─O and Sn─Sn distances in SnO₂, respectively.

For further clarification, we examined the interatomic radial distances and peak intensities obtained from an extended X‐ray absorption fine structure (EXAFS) during X‐ray absorption spectroscopy (XAS) analysis. Figure 2b shows that the M─O and M─M (M═Fe, Nb, and Hf in this example) radial distances and their peak intensities are consistent with those of Ir─O and Ir─Ir in the spectrum from pristine IrO₂. As seen in Figure S8 for all the samples analyzed by XAS, the EXAFS peaks of the foreign cations in the solid solutions differ from those of M‐rich secondary oxide phases and are analogous to the peaks of the rutile IrO₂. Consequently, both the XRD and EXAFS analyses provide consistent evidence regarding the formation of Ir_0.7M_0.3O₂ solid solutions.

Another important feature is that this solubility enhancement is not exclusively observed in IrO₂ but occurs in SnO₂ with many foreign cations as well. We verified the successful synthesis of nanoscale Sn_0.7M_0.3O₂ solid solutions with 15 distinct foreign cations via the same analytical methods. Figure 2c,d reveal the formation of a rutile phase without showing any additional Bragg reflections in each XRD pattern and the appearance of foreign‐cation EXAFS peaks aligning with the rutile framework (see Figures S9 and S10 and Table S2). Therefore, the reduction in crystal size toward attaining nanocrystals is an efficient approach that is applicable to many different elements to significantly enhance their miscibility in IrO₂ and SnO₂.

EDS chemical mapping in STEM with integrated silicon‐drift detectors [24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42] was also employed to directly visualize and quantitatively confirm the presence of foreign cations incorporated in the lattice. In particular, we analyzed individual single‐crystalline particles at high magnifications to avoid uncertain quantification due to particle overlapping. During the EDS analysis, Ir‐L _α, Sn‐L _α, and M‐K _α (or M‐L _α) spectra were used to quantitatively estimate M concentrations. As numerically denoted in the composition maps of Figure 3a,b for Ir_0.7M_0.3O₂ and Sn_0.7M_0.3O₂ nanocrystals, respectively, the target concentration of 30 (or 20) at.% was detected in each case. This clarifies that IrO₂ and SnO₂ solid solutions successfully formed. Additional sets of EDS maps and corresponding spectra are provided in Figures S11–S14 as consolidated evidence (also see Tables S3 and S4 for the average concentration of each sample).

FIGURE 3.

EDS maps verifying the formation of solid solutions. Composition mapping illustrates the incorporation of foreign elements into (a) Ir_0.7M_0.3O₂ and (b) Sn_0.7M_0.3O₂ nanocrystals. The quantitative substitution levels are also indicated from the EDS spectra. For Zr, Nb, Hf, and Ta within the IrO₂ lattice, the target concentration was adjusted to 20%.

2.3. Complete Solid‐Solution Formation Between IrO₂ and SnO₂

In agreement with the phase diagram of Figure 1b, the series of XRD patterns in Figure 4a consistently verifies mutually immiscible behavior and resulting two‐phase separation in the IrO₂─SnO₂ system annealed at 950°C. The enhanced miscibility observed in both IrO₂ and SnO₂ strongly implies that high miscibility can be attained in the nanocrystalline IrO₂─SnO₂ system. In stark contrast to the conventional phase equilibrium based on (sub)micron‐scale particles in Figure 1b, we indeed identified complete solid solution formation across the entire composition range between IrO₂ and SnO₂. A set of XRD data, obtained from nanocrystalline Ir_{1− x} Sn _x O₂ (0 ≤ x ≤ 1) samples that were significantly smaller than 10 nm, in Figure 4b clearly demonstrates that each pattern exclusively consists of the Bragg reflections corresponding to the rutile phase. Furthermore, the positions of the Bragg reflections gradually shift toward lower 2θ angles with increasing Sn content, denoted by x, which directly indicates that the lattice parameters continuously vary based on Vegard's law. The EDS composition analysis of individual nanocrystals in each stoichiometric composition shown in Figure 4b also strongly supports that complete solid solutions are formed in Ir_{1− x} Sn _x O₂ (Figure S15 for additional EDS results to verify the solid solution formation).

FIGURE 4.

Two distinct sets of XRD patterns for the IrO₂─SnO₂ system. (a) These XRD results, obtained from large crystals, show typical two‐phase separation into IrO₂ and SnO₂. (b) In contrast, continuous rutile single‐phase solid solutions are acquired when the crystal size is controlled to be less than 10 nm. The downward‐slanted vertical lines in the XRD patterns indicate systematic shifts of the Bragg‐peak positions that demonstrate compliance with Vegard's law. The EDS analyses also consistently support the formation of continuous solid solutions over the entire composition range.

2.4. Phase Equilibria Under High Laplace Pressures

As the conventional phase equilibria are typically considered under constant ambient pressure (1 atm), most binary phase diagrams are constructed in two dimensions as functions of composition and temperature. However, it should be noted that pressure, P, is no longer constant and becomes an important thermodynamic variable when high external pressures, far beyond 1 atm, are applied or when nanoscale particles under large Laplace pressure are considered. Indeed, the well‐known Young‐Laplace equation provides a reasonable estimate of the pressure exerted as the particle size decreases: ΔP = 2 γ/r, where γ is the surface energy of the nanoparticle and r is its radius (assuming a spherical shape). Based on previous literature [43, 44] on the surface energy of IrO₂ and SnO₂, their values range from 1.2 to 2.4 J/m². Therefore, if the median value of γ, 1.8 J/m², for spherical crystals is adopted, the Laplace pressure (ΔP) is estimated to reach as high as 1.8 GPa for nanocrystals with a radius of 2 nm. The key findings in this work thus reveal that the miscibility and resulting phase equilibria in IrO₂ and SnO₂ are considerably affected by pressure.

From the observations regarding the enhanced solubility in nanoscale crystals shown in Figures 2, 3, 4, it is anticipated that the solubility limit of foreign cations should proportionally increase with the Laplace pressure. In other words, as the crystals become smaller, a higher concentration of foreign cations in the lattice is expected. To examine this variation in solubility depending on the crystal size, we conducted further EDS compositional analyses for a sample annealed at 650°C with the starting stoichiometry of Ir_0.7Sn_0.3O₂ and a wide range of crystal sizes (see Figure S16 for a series of XRD patterns). To improve the reliability of the data, more than 60 crystals of different sizes were investigated. As denoted by the dashed red curve in Figure 5a, the Sn concentration rapidly decays from the initial stoichiometry to less than 5% Sn when the crystals were larger than ∼60 nm. The STEM images and EDS spectra in Figure 5b exemplify analysis results showing a much lower Sn content in larger IrO₂ crystals in the sample.

FIGURE 5.

Effect of crystal‐size‐dependent Laplace pressure on the phase equilibria. (a) The plot shows Sn concentrations obtained from the EDS spectra of individual crystals in the sample with added Sn that is annealed at 650°C. The red curve in the plot represents the fitted result from the dataset showing the size‐dependent variation of Sn concentration. (b) Two representative EDS results, for an 8‐nm nanocrystal and a 24‐nm large crystal, are provided to verify the size‐dependent solubility. (c) The conventional IrO₂─SnO₂ phase diagram at ambient pressure is shown. To account for the influence of Laplace pressure, the diagram should be extended into three dimensions. Above the critical pressure (ΔP _c), complete solid solution behavior is expected. (d) The regular solution model was employed to describe the evolution of phase equilibria in the IrO₂─SnO₂ system. At ΔP _c, the Gibbs free energy curve becomes fully concave, eliminating the presence of two minima (see Note S1 for details of the estimation based on the regular solution model).

Figure 5c schematically illustrates the Laplace‐pressure‐dependent phase equilibrium of the IrO₂─SnO₂ system, represented with three variables: composition, temperature, and pressure. The results showing the enhancement of the solubility of Sn in IrO₂ (Figure 5a), combined with the formation of complete solid solutions in nanocrystalline IrO₂─SnO₂ (Figure 4b), directly indicate that the miscibility is improved with increasing pressure. Complete solid solutions can be ultimately achieved above the critical Laplace pressure, ΔP _c (or equivalently, below the critical crystal size, d _c), as denoted by the blue line in the illustration in Figure 5c. We therefore used the classical regular solution model [45], together with the data in Figure 5a, to quantitatively estimate the crystal‐size‐dependent Gibbs free energy change of mixing, ΔG _mix. Based on this thermodynamic calculation, we found that d _c is approximately 5 nm, as shown in the ΔG _mix diagram in Figure 5d. Details regarding the application of the regular solution model and the estimation of the ΔG _mix curves as a function of crystal size are provided in Note S1.

2.5. Solid Solutions for Catalyzing Acidic Water Electrolysis

From the findings in this work, we suggest plausible solid‐solution compositions that can achieve both sufficiently high catalytic activity and durability, while minimizing the required Ir content for acidic OER catalysis. To select suitable elements among many composition candidates, we first measured the S‐number [46] regarding Ir and the elemental dissolution for the Ir_0.7M_0.3O₂ solid solutions with 20 different M cations. As denoted by asterisks in Figure 6a, the S‐number improved in the ten cases of M═Mn, Ni, Cu, Zn, Nb, Mo, Ru, In, Sn, and Ta. Meanwhile, the M dissolutions in the bar graph in Figure 6b reveal relatively high levels (more than 1 nmol) for the cases of Mg, Mn, Fe, Co, Ni, Cu, Zn, Ga, Mo, and In, as denoted by red triangles.

FIGURE 6.

Solid solutions and comparison of S‐number and elemental dissolution. (a,b) S‐number values and dissolution amounts of cations are provided for 20 different samples, respectively. As indicated by gray shadings, four elements, Nb, Ru, Sn, and Ta, are selected since they contribute to the enhancement of the S‐number without substantial dissolution.

It is noted that the S‐number is the ratio between the oxygen evolving catalytic capability and the Ir dissolution, and it thus represents the activity‐stability trade‐off [46, 47, 48] (see Figure S17 for the OER current densities of the solid‐solution samples). Consequently, the S‐number can be enhanced once substantially higher oxygen‐evolving catalysis is obtained, despite a similar or slightly larger amount of Ir dissolution. For long‐term durability, however, we believe that this way of acquiring a better S‐number is not the ideal approach. In this regard, we ruled out the rapidly dissolving Mg, Mn, Fe, Co, Ni, Cu, Zn, Ga, Mo, and In cations without further consideration. Therefore, as denoted by gray shading in Figure 6a, four M candidates, Nb, Ru, Sn, and Ta, were finally selected from the ten candidates to prepare rutile‐type single‐phase solid solutions with the aim of significantly reducing Ir concentration.

2.6. Synthesis of DSA‐Composition Solid Solutions

Interestingly, these four final M candidates correspond to the major compositions of the DSAs. While the DSAs are mixed‐oxide composites with multiple phases, the nanocrystals synthesized in this study consist of a rutile‐type single phase. It is thus noted that the M cations in single‐phase crystals have two different roles in the solid‐solution lattice. As can be verified from the E−pH Pourbaix diagrams (Figure S18) [49, 50], Nb, Sn, and Ta cations in the oxide framework are electrochemically stable in acidic conditions without substantial dissolution during the anodic OER. Consequently, they offer a robust structural basis in the rutile phase, although they are catalytically inactive, while Ir and Ru are major OER active sites.

As continuous solid solutions can form in nanoscale SnO₂─IrO₂, we utilized this (Sn, Ir)O₂ system as a basic compositional framework to preserve the rutile structure. The XRD results and EDS maps in Figures S19 and S20 and Figure 7a directly demonstrate the successful synthesis of ternary (Sn_0.6Nb_0.1Ir_0.3)O₂ and (Sn_0.6Ta_0.1Ir_0.3)O₂, and even quaternary (Sn_0.2Nb_0.2Ru_0.4Ir_0.2)O₂ nanocrystals, all of which are single‐phase and contain significantly reduced Ir content. In addition to these M candidates (Nb and Ta), other ternary nanocrystals consisting of Zr, Sb, and Hf were successfully synthesized (also see Figure S19 for additional EDS maps and Figure S21 for X‐ray photoelectron spectroscopy regarding quaternary (Sn_0.2Nb_0.2Ru_0.4Ir_0.2)O₂). Although we attempted to synthesize (Sn_0.2Ta_0.2Ru_0.4Ir_0.2)O₂, substantial separation of a Ta‐rich phase was unfortunately observed during the EDS analysis (see Figure S22). It is worthwhile to note that the standard Gibbs free energy changes (ΔG°) of oxide formation of Ir and the four M elements in the Ellingham diagram [51] considerably vary, showing the highest ΔG° for IrO₂ (−80 kJ/mol at 400°C), an intermediate value of SnO₂ (−450 kJ/mol), and the lowest ΔG° for Ta₂O₅ (−1750 kJ/mol) (Figure S23). The synthesis of quaternary nanocrystalline oxides thus appears to be challenging if the starting elements have significantly different oxide formation energies.

FIGURE 7.

DSA‐inspired quaternary (Sn_0.2Nb_0.2Ru_0.4Ir_0.2)O₂ solid solution and comparison of catalytic durability. (a) The XRD and EDS analyses show that single‐phase quaternary (Sn_0.2Nb_0.2Ru_0.4Ir_0.2)O₂ nanocrystals are successfully synthesized. (b) Accelerated electrochemical tests based on square‐type voltage profiles demonstrate comparable variations of the OER specific current and S‐number between the quaternary sample and the IrO₂ reference. (c) MEA‐based accelerated tests under square‐type current profiles also consistently support the notable durability of the quaternary nanocrystals despite their substantially low Ir content.

When we measured the OER activities and S‐numbers of these multi‐cation solid solutions, we found that adding more inactive elements, such as Sn, Ti, Zr, Sb, Hf, Nb, and Ta, resulted in considerably lower OER activity, as expected (see Figure S24). Therefore, high‐activity Ru should be utilized to ensure sufficient catalytic activity, which suggests that quaternary (Sn_0.2Nb_0.2Ru_0.4Ir_0.2)O₂ is a suitable candidate. This formulation features a balanced combination of activity and durability while containing remarkably low amounts of Ir (only 20%). For theoretical clarification, we further carried out density functional theory calculations to acquire the density of states (DOS) [27, 29, 32, 33, 52] of the five elements, Sn, Nb, Ru, Ir, and O, in a single phase and evaluate the near‐Fermi‐level states fractions. From the DOS results, Ru is expected to have higher activity than that of Ir, compensating for the reduced OER activity under a low Ir concentration. Details are provided in Note S2.

2.7. Quantitative Comparison of Catalyst Durability

One of the widely used methods to present the durability of newly suggested OER catalysts for PEM electrolyzers is based on a chronopotentiometric test using a membrane electrode assembly (MEA) at a certain level of current density (e.g., 1–2 A/cm²). Although this type of test has been frequently adopted to demonstrate the longevity performance of catalysts, it is very difficult to assess how much their intrinsic durability would differ from that of the IrO₂ reference. Therefore, to objectively evaluate the durability of this DSA‐inspired quaternary (Sn_0.2Nb_0.2Ru_0.4Ir_0.2)O₂, its activity degradation behavior and quantitative S‐number should be compared with those of pristine IrO₂ as a reference under identical test conditions. We thus used the same sample preparation method for both the quaternary solid solution and the IrO₂ reference and tested them under an identical overpotential range and electrochemical protocol. Furthermore, we applied periodic square‐wave potential/current cycles to accelerate the overall test procedure [53, 54, 55, 56].

Figure 7b shows the variations of both the specific OER current and the S‐number of (Sn_0.2Nb_0.2Ru_0.4Ir_0.2)O₂ and reference IrO₂ during more than 50 000 cycles of accelerated durability tests (ADTs) in the range of 1.23–1.68 V_RHE with a 10‐s hold at each end voltage in 0.1 m HClO₄ electrolyte. The dissolved elements in each electrolyte were quantitatively measured by inductively coupled plasma mass spectrometry (ICP‐MS) every 6000 cycles to acquire the S‐numbers with respect to Ir for reference IrO₂ and to Ir and Ru for (Sn_0.2Nb_0.2Ru_0.4Ir_0.2)O₂ (see Figure S25 for details). The electrolytes were completely replaced with fresh electrolytes every 6000 cycles as well. One of the key features readily observed in Figure 7b is that the OER current variations of both samples are similar, showing gradual decay at the initial stage followed by stabilization.

In particular, considering the low Ir content (20%) in (Sn_0.2Nb_0.2Ru_0.4Ir_0.2)O₂, this similar variation without distinctively rapid degradation in the solid solution is noteworthy. Although a substantial amount of Ru is included, the initial S‐number of the solid solution has a fairly high value on the order of 10⁵ (5.1 × 10⁵). The degradation rate of the S‐number, Δlog[S‐number] per cycle, of the solid solution (−6.1 × 10⁻⁶/cycle), also has the same order of magnitude with that of IrO₂ (−2.5 × 10⁻⁶/cycle). As a consequence, this solid solution is a suitable candidate that simultaneously satisfies the multiple criteria regarding activity, durability, and Ir content for future OER catalysts in PEM electrolyzers.

We performed post‐mortem STEM‐EDS analysis on the quaternary (Sn_0.2Nb_0.2Ru_0.4Ir_0.2)O₂ catalyst after 12 000 cycles. The elemental maps reveal completely homogeneous cation distribution across multiple ∼5 nm nanoparticles, with no signs of phase segregation, surface enrichment/depletion. Although gradual dissolution of the constituent cations occurs during cycling (Figure S25), quantitative EDS results confirm that the average compositions remain consistent with the initial target stoichiometry (see Figure S26), indicating that the single‐phase rutile solid solution structure is fully preserved even after prolonged acidic OER operation. In addition, the Tafel slopes remain essentially unchanged, from 94.6 mV/dec (before cycling) to 105.0 mV/dec and 109.0 mV/dec after 6000 and 12 000 cycles, respectively (see Figure S27). This negligible variation in slope, together with the preserved overpotential at a given current density, indicates that the reaction kinetics and underlying OER mechanism are largely maintained during extended cycling and that no severe degradation or phase transformation occurs under the applied conditions.

We examined the compatibility of our solid‐solution catalyst with proton‐exchange membranes by performing MEA‐based tests using Nafion 212. To accelerate the overall testing process and prevent negative voltage degradation, which is typically associated with reduced membrane resistance at high current densities, square‐wave current cycles were applied in the range of 0.1–1.0 A/cm² with a 20 s hold at each end current density. Figure 7c compares the overpotential degradation behavior of the solid‐solution catalyst and the reference during the MEA‐based ADTs. After stabilization, the overpotential degradation rates of our (Sn_0.2Nb_0.2Ru_0.4Ir_0.2)O₂ solid solution and the IrO₂ reference were measured to be +5.0 and +2.1 µV/cycle, respectively. This difference in overpotential degradation rates is in excellent agreement with the difference in the degradation rates of the S‐number of the two samples shown in Figure 7b. These results demonstrate that both ADT methods used in this work provide a reliable basis for comparing durability.

3. Implications

The findings of the present study offer several important implications from the perspectives of solid‐solution behavior, synthesis, and performance characterization. First, although our study specifically focuses on rutile‐type oxide materials, pressure appears to be a critical variable governing the overall phase equilibria, including miscibility, in many other oxide systems (see the schematic illustrations in Figure S28). Because applying gigapascal‐level high pressure during material synthesis is challenging, utilizing Laplace pressure by controlling nanocrystal size provides an efficient approach to achieve high‐pressure equilibrium conditions. Accordingly, the significance of oxide synthesis at a nanoscale deserves renewed attention.

Second, as summarized in Table S5, many inconsistent and ambiguous results have been reported regarding the incorporation of foreign elements in IrO₂, often without precise structural and compositional evidence. As demonstrated in Figures 4 and 5, which reveal a pronounced dependency of solubility on crystal size, we suggest that such inconsistencies may originate from differences in particle size. In particular, when the particle size distribution within a sample is relatively broad, as exemplified in Figure 5a, inhomogeneous doping levels among particles become unavoidable, which potentially can lead to misinterpretation of the catalytic durability.

Finally, although numerous studies over the past decade have proposed various oxide catalysts as potential replacements for pristine IrO₂, prior reports as well as the present study indicate that their intrinsic stability is unlikely to surpass that of rutile IrO₂. Consequently, MEA‐based longevity tests conducted without reference to IrO₂ at a constant current density may not be adequate to objectively consider the stability of newly proposed catalysts. In this respect, quantitative comparisons of durability with that of reference IrO₂ under identical test conditions, combined with accelerated degradation protocols, as shown in Figure 7b,c, are crucial for accurately assessing the intrinsic stability of new catalysts.

4. Conclusions

This study not only overturns the conventional understanding that rutile‐type IrO₂ is intrinsically immiscible with foreign elements but also demonstrates that this immiscibility can be efficiently overcome by the formation of nanoscale IrO₂ under high Laplace pressures. Through systematic structural and compositional analyses across more than 20 different cations, we reveal a universal size‐dependent enhancement of solubility in both IrO₂ and SnO₂ that enables the complete formation of solid solutions, such as Ir_{1− x} Sn _x O₂, across the entire composition range. Thermodynamic modeling based on the regular solution framework further consolidates the contention that this enhanced miscibility arises from distinct phase equilibria governed by Laplace pressure. Building on these fundamental insights, we developed DSA‐inspired multi‐component solid‐solution OER catalysts, most notably (Sn_0.2Nb_0.2Ru_0.4Ir_0.2)O₂, that simultaneously achieve high catalytic activity, long‐term durability, and a remarkable reduction in Ir utilization for acidic water electrolysis. Quantitative electrochemical analyses showed that the degradation rate of the S‐number and overpotential decay of this quaternary oxide are of the same order of magnitude as those of pristine IrO₂, despite the substantially lower Ir content.

Funding

This work was supported by the National Research Foundation of Korea (NRF) under grant nos. RS‐2024‐00435493, RS‐2024‐00347287, RS‐2026‐25468150, RS‐2026‐25607361, and RS‐2026‐25607648. K.H.P and S.‐Y.C. were also supported by Samsung Research Funding and Incubation Center of Samsung Electronics (project no. SRFC‐MA2401‐04) and the KAIST InnoCORE program.

Conflicts of Interest

The authors declare personal financial interests in a commercial entity to which patent filings related to certain subject matter in this publication have been submitted by the Korea Advanced Institute of Science and Technology.

Supporting information

Supporting File: adma73273‐sup‐0001‐SuppMat.pdf.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.