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Johnson Matthey Technol. Rev., 2022, 66, (4), 393
doi: 10.1595/205651322X16529612227119

Development of New Mixed-Metal Ruthenium and Iridium Oxides as Electrocatalysts for Oxygen Evolution: Part I

Survey of crystal structures and synthesis methods

  • Jasmine A. Clayton, Richard I. Walton*
  • Department of Chemistry, University of Warwick, Gibbet Hill Road, Coventry, CV4 7AL, UK
  • *Email: r.i.walton@warwick.ac.uk

PEER REVIEWED

Received 19th January 2022; Revised 4th May 2022; Accepted 19th May 2022; Online 19th May 2022

Article Synopsis

We review recent research into oxides of platinum group metals (pgms), in particular those of ruthenium and iridium, for use as electrocatalysts for the oxygen evolution reaction (OER). These are used in membrane electrode assemblies (MEAs) in devices such as electrolysers, for water splitting to generate hydrogen as fuel, and in fuel cells where they provide a buffer against carbon corrosion. In these situations, proton exchange membrane (PEM) layers are used, and highly acid-resilient electrocatalyst materials are required. The range of structure types investigated includes perovskites, pyrochlores and hexagonal perovskite-like phases, where the pgm is partnered by base metals in complex chemical compositions. The role of chemical synthesis in the discovery of new oxide compositions is emphasised, particularly to yield powders for processing into MEAs. Part I introduces the electrocatalytic splitting of water to oxygen and hydrogen and provides a survey of ruthenium and iridium oxide structures for oxygen evolution reaction catalysis.

1. Introduction

A key process in developing future devices for various energy-related applications is the electrocatalytic splitting of water to oxygen and hydrogen, and this is highly relevant for the sustainable and clean production of electricity or fuels in the move away from carbon-focused technologies. In an electrolyser, the anodic OER is a major limiting step in improving efficiency since it inherently possesses unfavourable thermodynamics, being a four-electron transfer reaction (1, 2). This leads to high overpotentials to drive the reaction, giving unfavourable kinetics, and can lead to energy losses within any electrochemical device. The activation barrier of the OER is much larger than that of the hydrogen evolution reaction (HER) at the cathode, and thus much attention is focused on discovery of electrocatalysts to overcome this barrier in order to optimise device performance. The two relevant half-equations in acid conditions can be represented in Equation (i) and Equation (ii):

HER at cathode:

(i)

OER at anode:

(ii)

OER in aqueous acid electrolytes is desirable since it provides high current densities and high voltage efficiency, and the fast kinetics of the partnering HER are beneficial (3, 4). Furthermore, contamination by aerial carbon dioxide and precipitation of carbonates is avoided, which is not the case for alkaline electrolytes. The benefits of acid electrolysis are exploited in PEM devices that make use of proton-conducting polymer electrolytes, that are typically fluorocarbon or hydrocarbon polymers with sulfonic-acid side chains to provide transport of protons (5), and these polymers have been optimised over many years to give a well-developed technology (6). The PEM permits high gas purity and a compact system design in which gas crossover is low, and these are further significant advantages over alkaline devices (7). As well as obvious uses in water splitting to produce gases for fuels, an OER electrocatalyst is also of benefit in PEM fuel cells where it can be used to limit the carbon corrosion reaction that may occur under conditions of fuel starvation or with other excursions to high electrode potentials, instead evolving oxygen (8–10). This application thus mitigates against fuel cell degradation and prolongs their life. Electrocatalytic oxygen evolution is also closely associated with the charging process in metal–air batteries (11), and so understanding and optimising the OER is of key importance in this respect. With the rapidly growing demand, and indeed expectation, for efficient energy conversion and storage devices it is evident that electrocatalysis of the OER reaction is a crucial bottleneck to overcome.

While it is clear that acid-resilient electrocatalysts are in high demand to meet these applications, it is unfortunate that most metal oxide materials that might be candidate electrocatalysts are unstable at low pH and readily dissolve. While many oxides of many metals have been studied for OER electrocatalysis under alkaline conditions (12), presently the most promising materials for the acid conditions of PEM devices are oxides of pgms, in particular those of ruthenium and iridium (13–15). The binary dioxides RuO2 and IrO2 and solid-solutions thereof can be considered benchmark materials in this respect (16–20). These materials share the rutile-type structure, as would be found for the thermodynamically stable form of TiO2, consisting of octahedrally coordinated tetravalent cations that share corners and edges to yield a three-dimensionally extended structure, Figure 1.

Fig. 1.

The rutile structure as found for IrO2 and RuO2 shown as: (a) ball-and-stick view of tetragonal unit cell; and (b) a polyhedral representations showing corner- and edge-shared connectivity. Green spheres are iridium and red are oxygen

A tremendous amount of work has been focused on IrO2-based materials, prepared by various synthesis routes, as both films and powders (21, 22). Nanocrystalline forms have been developed, which may permit surface reactivity to be tuned, as well as allowing processing for device fabrication (22–24). Many iridium oxide materials studied for electrocatalytic OER are poorly crystalline, or indeed amorphous to X-rays, and often the active phase is proposed to be a hydrous iridium oxide (25, 26). Examination of local atomic order of these poorly crystalline materials has identified structural motifs that are favourable for high electrocatalytic activity and that minimise dissolution of the solid under operating conditions; interestingly the rutile-like structure may be more prone to collapse than more open IrOx structures (27). The phase IrO3 has also been isolated via an ion-exchange process from β-Li2IrO3, via a protonated intermediate (28). Insertion of lithium into IrO2 forms an amorphous phase with increased activity over crystalline IrO2, with the structural flexibility of the amorphous structure proposed to enhance turnover of OER (29).

Partial substitution of iridium by non-pgms in IrO2 has been extensively studied, with the aim of not only diluting the pgm content but also tuning activity and stability. For example, isovalent replacement of Ir4+ by Ti4+ (30, 31) or by Sn4+ (32, 33) has yielded materials with improved stability over pure IrO2, although it may be noted that some of these materials actually consist of small particles of IrO2 supported on the second oxide (31). In other cases, a substituent cation may adjust the oxidation state of iridium to modify electrocatalytic properties: for example, inclusion of manganese was found to give a higher concentration of surface Ir3+ (34). Similar chemistry has been developed for RuO2, which although suffers from lower stability than IrO2 in acid electrolytes (35), partial elemental substitution by a variety of elements has proved possible, including various aliovalent first-row transition-metal cations (manganese, cobalt, nickel or copper) that introduce defects, or modify the average oxidation state of ruthenium (36–43). Addition of these as dopants at the surface of RuO2 can have specific benefits: for example, adding nickel or cobalt enhances activity, which was rationalised computationally as from activation of a proton donor–acceptor functionality on conventionally inactive bridge surface sites (44). Combinations of two substituent elements introduce further tunability to these systems, such as strontium‐ruthenium‐iridium oxides (45), or cerium-ruthenium-iridium oxides (46). Such complex solid solutions must be carefully structurally characterised, since local segregation of the metals may occur, to give preferential surface enrichment of one (17, 47), although if carefully engineered this can lead to enhanced properties (48, 49).

Although precious-metal-free materials may be desirable for economic reasons (50), and indeed some manganese oxides have recently been suggested as alternatives that have been claimed to operate over a range of pH (51, 52), iridium and ruthenium oxides remain the most active and robust materials for OER under acidic conditions. One strategy to overcome the use of expensive and limited pgms is to use complex ternary oxides with the precious metal diluted by the presence of a non-precious metal. Goodenough and coworkers made the first investigations of such materials, with a study in 1990 of the electrocatalytic properties of pyrochlores Pb2(Ir2–x Pbx )O7–y and Pb2(Ru2–x Pbx )O7–y as a function of pH (53). This was followed by work in 1995 by ten Kortenaar et al. who studied a range of ternary iridium oxides for their activity towards OER (54), and although they found no correlation between crystal structure and activity, they discovered some materials with promising properties including those with pyrochlore and fluorite structures (see Section 2). With the resurgence of interest in electrocatalysts for OER, recent work in the past five years has focused on discovery of new acid-stable and highly active ruthenium and iridium oxides with an emphasis on thrifting of precious metals (55). This includes multi-element oxides so that the pgm is not only diluted, but also present in various crystal structures that offer differing connectivity of the metal centres, thereby offering the possibility of developing structure-property relationships to optimise electrocatalytic behaviour. While a number of recent reviews have examined various aspects of the development and implementation of new oxides of ruthenium and iridates specifically for acid-resilient electrocatalysts (2–4, 7, 56–60), it is the purpose of this two-part article to survey the various crystalline structures recently discovered for iridium and ruthenium oxides, to consider their synthesis and to summarise some of the mechanistic findings made, with emphasis of the degradation pathways of the new oxide electrocatalysts that have emerged. This will include some of our own work on the use of solution-based synthesis methods for formation of mixed-metal oxide materials.

2. Survey of Ruthenium and Iridium Oxide Structures for Oxygen Evolution Reaction Catalysis

The crystal chemistry of ruthenium and iridium oxides is associated with a range of possible oxidation states of the pgm, each of which may have various coordination preferences, in turn leading to distinctive structural chemistry (61, 62). For ruthenium, in Group 7, oxidation states in oxides can range from as low as +2, reported in the phase SrFe0.5Ru0.5O2, formed by topochemical reduction of perovskite SrFe0.5Ru0.5O3 using CaH2 (63), up to +8 as seen in RuO4, although most commonly the oxide chemistry is dominated by the +4 and +5 oxidation states for ruthenium. In the case of iridium in Group 8, a smaller range of oxidation states is seen: from +3 to +6 (although it may be noted that molecular IrO4, containing the +8 oxidation state was isolated in noble gas matrices at extremely low temperatures (64), and even the +9 oxidation state detected in the species [IrO4]+ in the gas phase (65)). As was mentioned above, the rutile structure-type is found for the metal dioxides of both ruthenium and iridium, and the +4 oxidation state appears most stable for a binary oxide of each of the two elements. Other binary oxides of ruthenium and iridium are less well-characterised, with evidence for Ru2O3 and Ir2O3 as distinct crystalline phases not compelling.

Instead, a variety of structures of ternary and higher oxides of ruthenium and iridium are found, where electropositive partner cations, such as alkali, alkaline earth or rare earth metals, promote increased orbital overlap between oxygen and the electronegative pgms by destabilisation of the O-p states, which may provide a means of stabilising higher oxidation states of the pgm, as discussed by Kurzman et al . (66). In these materials, a variety of local structural arrangements may also be possible, such as the connectivity of pgm coordination polyhedra, which in turn may provide novel electrocatalytic properties. As well as diluting the amount of pgm, the substitutional chemistry possible in mixed-metal oxides to form solid‐solutions or an isomorphous series of materials, provides a means of tuning structural chemistry, such as local atomic environment (bond distances and local symmetry), and average pgm oxidation state (which will adjust the number of d electrons for conductivity, and the metal-oxide bond strength). Table I summarises and compares the various crystal structure types that we will consider, and these will be described in more detail in the following paragraphs, with examples of chemical compositions provided.

Table I

Crystal Structures of Ruthenate and Iridates Studied as Oxygen Evolution Reaction Electrocatalysts

Structure type Ideal chemical composition A-site coordination environment B-site coordination environment (ruthenium or iridium) Ruthenium/iridium oxidation state Examples
Rutile BO2 – Corner- and edge-shared octahedra +4 RuO2
IrO2
Pyrochlore A2B2O6O’ 8-coordinate Corner-shared octahedra +4 or +5 Bi2Ir2O7
Ln2Ir2O7 (Ln = La, Nd, Pr) (Na,Ca)2Ir2O6 · H2O
Perovskite ABO3 12-coordinate Corner-shared octahedra +3, +4 or +5 SrIr0.8Zn0.2O3
LaRuO3
Ruddlesden-Popper An +1Bn O3n +1 12-coordinate or 9-coordinate Corner-shared octahedra +4 or +5 Sr2IrO4
Sr3Ir2O7
Hexagonal perovskite ABO3 Distorted 12-coordinate Face-shared and corner-shared octahedra +4 6H-SrIrO3
9R-BaIrO3
α-Li2IrO3 A2BO3 6-coordinate, octahedral Edge-shared octahedra +4 Li2IrO3
Hollandite Ax BO2 Distorted 8-coordinate Edge-shared and corner-shared octahedra +4 or +5 K0.25IrO2

Oxide materials with the A2B2O6O’ pyrochlore structure stand out as being of use for many heterogeneous catalysis applications, as well as being of interest for their electronic and magnetic properties (67). The pyrochlore structure may be viewed as an oxygen deficient fluorite A2B2O8–1 where half the cations (B) have coordination number 6 and the remainder (A) maintain their coordination number of 8. Depending on the positions of the oxide ions, the B sites may have regular octahedral geometry accompanied by irregular 6+2 coordination for the A site, or rather the B site may be distorted octahedral and the A-site a cubic 8-coordinate site. For the former case, the structure may be viewed as constructed from corner-shared BO6 octahedra that create a network that incorporates the A site cations along with additional oxide ions, Figure 2.

Fig. 2.

The cubic unit cell of the A2B2O7 pyrochlore structure showing the corner-shared connectivity of the B-site octahedra (green, ruthenium or iridium in ruthenates and iridates), with A-site cations as blue spheres and additional oxide ions as the larger red spheres

There are a variety of possible combinations of A and B metals in the pyrochlore structure, and oxygen non-stoichiometry is possible by partial occupation of the O’ site, or replacement of oxide by hydroxide or water leading to inherently defective materials with redox properties for the B-site metal and pathways for oxide-ion migration in the solid-state. Partnering iridium on the B-site with a range of A-site metals offers the possibility of tuning crystal chemistry, electronic structure and, potentially, surface chemistry to optimise electrocatalysis properties (68). We have previously described the electrocatalytic activity of the pyrochlore Bi2Ir2O7, prepared via a facile hydrothermal route (69), and the mixed ruthenate‐iridate phases (Na0.33Ce0.67)2(Ir1–x Rux )2O7 (70). This led to the discovery of phases (Na,Ca)2−x (Ir2−y My )O6·n H2O (M = antimony, zirconium, ruthenium, rhodium) that offer favourable OER activities when fabricated into MEAs, with certain compositions minimising the unfavourable carbon corrosion reaction (71, 72).

Sun et al. compared the OER activity of the pyrochlores Bi2Ir2O7 and Pb2Ir2O6.5 with IrO2 and proposed a correlation of OER activity with the local atomic distortion of iridium environment (73). Lebedev et al. prepared Y2Ir2O7, Bi2Ir2O7 and Pb2Ir2O7, including mixed A-site variants of these end members, and produced electrocatalysts with OER activities approaching that of IrO2 nanoparticles in acid conditions (74). Shang et al. studied R2Ir2O7 (R = holmium, terbium, gadolinium, neodymium and praseodymium) and proposed that Pr2Ir2O7 was most active due to enhanced covalency in Ir–O bonds and a higher conductivity (75). Abbot et al. compared the iridate pyrochlores with ruthenate analogues with A-site neodymium, gadolinium or ytterbium and found all to be more stable than IrO2, with the ruthenates and Yb2Ir2O7 being more active (76). The pyrochlores are also active towards OER in alkali conditions: Parrondo et al. studied A2B2O7–y (A = lead or bismuth, and B = ruthenium, iridium or osmium) for OER and correlated activity with composition (77).

Various B-site ruthenate pyrochlores have been prepared as OER electrocatalysts, including with the trivalent A-site cations yttrium, neodymium, gadolinium, bismuth (78–80), implying an oxidation of +4 for ruthenium. A-site substitution has also been investigated for these materials with materials Y2–x Mx Ru2O7−δ formed for M = zinc (78), M = copper, cobalt, nickel, iron (81), M = barium (82), M = strontium (83), and M = magnesium (84). With these divalent substituents the charge may be compensated by oxide-ion deficiencies, as indicated by the chemical formula, but there is also the possibility of partial oxidation of ruthenium to the +5 oxidation state, or higher, as seen in the phase Ca1.5Ru2O7 (85). Mixed ruthenium-iridium pyrochlores have also been studied, and a synergistic effect of combining the two cations was investigated in the materials A2 B2O7 with A = neodymium, gadolinium or ytterbium and B = Ir1–x Rux with x = 0.2, 0.4, 0.6, 0.8 (86).

The ABO3 perovskite structure is common in solid-state chemistry, and consists of a corner-shared network of BO6 octahedra in which the 12-coordinate A-site cations are found, Figure 3(a) (87). The versatility of the perovskite structure arises from the degree of tilting of the octahedra to give a range of possible B-O-B angles, lowering the coordination of the A-site to allow accommodation of smaller cations, Figure 3(b). For ruthenate and iridate perovskites, with their preference for octahedral coordination, ruthenium and iridium are expected to be found on the B-site, although in fact the perovskite phases that have been prepared for electrocatalysis are more complex solid solutions or substitutional series. SrIrO3 as a perovskite phase was prepared as an epitaxial layer on SrTiO3 as substrate using pulsed laser deposition by Seitz et al. (88). Pseudocubic SrIrO3 thin films were also grown on (La,Sr)(Al,Ta)O3 substrates by molecular-beam epitaxy (89). Orthorhombic SrIr0.8Zn0.2O3 (90) and the cubic phase SrTi0.67Ir0.33O3 (91) are examples of bulk materials with the classical perovskite structure, and the former contains mixed-valent iridium, formally in the +4.5 oxidation state. The SrTiO3-SrIrO3 solid solution has been produced in the form of nanotubes (92). The orthorhombic ruthenate perovskites LaRuO3 (93), SrRuO3, Sr0.95Na0.05RuO3 and S0.90Na0.10RuO3 (94) have been studied and the oxidation state of ruthenium depends on the choice of A-site cation(s).

Fig. 3.

Polyhedral views of ABO3 perovskite structures: (a) the cubic parent structure; (b) orthorhombic distorted perovskite from octahedral tilting; (c) an ordered double perovskite structure A2BB’O6. The green octahedra would be occupied by ruthenium or iridium in ruthenates and iridates and the A-site cations are shown as blue spheres. The red spheres are oxide ions and the purple octahedra in (b) contain the second B-site cation (see text)

With certain combinations of metal cations, double perovskites can be produced where the B-site cations form an ordered superstructure, Figure 3(c). Examples of iridates and ruthenates include the sets of materials Ba2MIrO6 M = yttrium, lanthanum, cerium, praseodymium, neodymium and terbium (95), A2BIrO6 (A = praseodymium, neodymium or yttrium; B = barium or strontium) (96), Sr2MIrO6 (M= iron, cobalt) (97), La2LiMO6 (M = iridium or ruthenium) (98), A2BIrO6, (A = barium and strontium; B =lanthanides, yttrium and tin) (99), and Sr2MIrO6 (M = nickel, cobalt, scandium and iron) (100). Less commonly, the A-site cations may be ordered, and the phase CaCu3Ir4O12 provides an example, studied for OER electrocatalysis under alkaline conditions (101). The analogous ruthenate, CaCu3Ru4O12, on the other hand, has shown high activity and stability for OER under acid conditions (102).

Related to the perovskite structure are the Ruddlesden-Popper phases, Figure 4 (103). These have general chemical formula An +1Bn X3n +1 where A and B are appropriate cations (B will be iridium or ruthenium in this discussion) and X an anion, oxide in this case. The Ruddlesden–Popper structure may be viewed as a layered variant of the perovskite structure, where sheets of corner-shared B-centred octahedra are interleaved with rock-salt (NaCl type) layers of A-centred octahedra and so may be written (ABX3)n ·AX. The n = ∞ composition corresponds to the conventional perovskite structure. For n = 1, the iridates Sr2IrO4 (95, 104) Ca2IrO4 (105) and Sr2Fe0.5Ir0.5O4 (97) have been reported, with the first two containing Ir4+ and the last, Ir5+. Ion-exchange of Sr2IrO4 using aqueous perchloric acid leads to the protonated n = 1 Ruddlesden-Popper phase H3.6IrO4·3.7H2O (106). The n = 2 Ruddlesden-Popper Sr3Ir2O7 has also been studied for its OER activity (107).

Fig. 4.

Ruddlesden–Popper structures n (ABX3)n ·AX for: (a) n = 1, as found for Sr2IrO4; and (b) n = 2, as found for Sr3Ir2O7. The green octahedra would be occupied by ruthenium or iridium in ruthenates and iridates and the A-site cations are shown as blue spheres

Another set of so-called perovskite phases have the ABO3 chemical composition but different connectivity of the octahedral building units. An example is SrIrO3, whose structure is an example of a hexagonal perovskite, Figure 5(a), in this case of the 6H type, where alternating face-sharing IrO6 octahedral dimers corner-share with isolated IrO6 octahedra. The structure is actually monoclinic, being a distorted version of the 6H-BaTiO3 structure, where iridium–iridium metal-metal bonding is present in the face-shared IrO6 octahedra. It is only by B-site substitution that the classical perovskite structure is produced for SrIrO3 under ambient pressure, such as with Ti4+, or Zn2+, as mentioned above (108). 6H-SrIrO3 has been synthesised as a polycrystalline powder and studied by a number of authors (89, 104, 109–111). Recently, SrIrO3 was modified with Ruddlesden-Popper Sr2IrO4 to give a composite two-phase sample (112).

Fig. 5.

Examples of iridates with hexagonal perovskite structures: (a) 6H-SrTiO3 (drawn using the idealised 6H-BaTiO3 structure); (b) 9R-BaIrO3; (c) the triple perovskite Ba3IrTi2O9. In all cases the green polyhedra represent octahedrally coordinated iridium with red oxide, and the blue spheres are the alkali-metal cations. The blue polyhedra in (c) are titanium-centred octahedra

BaIrO3 has likewise been studied, and this phase exists in the 9R polymorph when prepared at ambient pressure, Figure 5(b), also of monoclinic crystal system, but with trimers of face-shared octahedra (89). Amorphous IrOx on 9R-BaIrO3 was found to be a particularly active electrocatalyst (113). The triple perovskites Ba3M’M’’2O9 (M’ = titanium, indium or zinc; M’’ = IrIr, IrTi, IrRu or RuRu) have structures consisting of face-shared M’’2O9 dimers and isolated MO6 octahedra, Figure 5(c) (114). Ba4MIr3O12 with M = praseodymium, bismuth, niobium adopt 12L-perovskite structures with isolated trinuclear Ir3O12 units with MO6 octahedra alternately linked in a corner-sharing fashion, thus generating the 12-layer structure (115). La3.5Ru4O13 and La2RuO5 are further examples of materials with perovskite-derived structures that have been prepared for investigation as OER electrocatalysts (93).

Various other ruthenate and iridate complex oxides have been studied, although none as systematically as the rutiles, pyrochlores and perovskites described so far. This includes materials with fluorite-related structures, Ln3IrO7 with Ln = neodymium, europium (54) or praseodymium (95), which contain Ir5+. More open structures have also been considered, including those with tunnel-like structures, such as potassium iridate hollandites (116, 117), Figure 6(a). In the case of layered α-Li2IrO3, Figure 6(b), ion-exchange with potassium in alkali conditions yields a birnessite-type structure (118). The spinel NiCo2–x O4 has been substituted with iridium (119). Some other iridate compositions with crystalline structures, such as Sr4IrO6 with isolated iridium-centred octahedra (104), have also been studied. Finally, composite materials have been considered, where a two-phase mixture of oxides is purposely produced, examples being iridium-tungsten oxide (120), and a mixed-phase BaIrO2.937/La3IrO7 material consisting of hexagonal perovskite and fluorite-structured materials (121).

Fig. 6.

Iridates with open structures: (a) view of layered α-Li2IrO3 parallel to layers with blue octahedra containing lithium, green octahedra containing iridium and blue spheres representing lithium interlayer; (b) view of layered α-Li2IrO3 perpendicular to layers, showing iridium oxide honeycomb structure; and (c) the hollandite tunnel structure with blue spheres representing the occluded ions, such as potassium

3. Synthesis Methods for Oxides of Ruthenium and Iridium

As with many ternary and higher mixed-metal oxides, the simplest approach to synthesis is from the individual, binary oxides, via a solid-state reaction. This requires intimate grinding or milling of the solid precursors to ensure homogeneity, heating to high temperature, often in excess of 1000°C, followed by cycles of regrinding and reheating until phase-pure crystalline material is produced. Examples of materials produced by such methods include the iridate perovskites La2LiIrO6 (98), Ba2MIrO6 (M = yttrium, lanthanum, cerium, praseodymium, neodymium and terbium) (95), A2BIrO6 (A = praseodymium, neodymium or yttrium; B = barium or strontium) (96), Ba3MM2O9 (M= titanium, indium or zinc; M2 = IrIr, IrTi, IrRu or RuRu) (114), and the hexagonal perovskites AIrO3 (A = strontium or barium) (89) and 12L-Ba4MIr3O12 (M = praseodymium, bismuth, niobium) (115). Such reactions may be facilitated by using carbonate precursors, where the loss of gaseous CO2 provides an additional entropic driving force for the reaction. Disadvantages of such a synthesis approach are that volatility of any of the component binary oxides may result in loss of material so that metal ratios are difficult to control, and that crystallite growth is unpredictable and difficult to control so that typically large crystallites with low surface area are produced. Furthermore, only the most thermodynamically stable phases are accessed under such conditions limiting the range of possible compositions that might be available. Mechanical activation may be used to ensure greater homogeneity of precursors and lower the temperature of synthesis, such as in the case of SrIrO3 (110). In other cases, high pressure must be employed to drive the reaction and an example is provided by the case of Sr3Ir2O7 (107).

Low temperature routes to solid state materials have been developed to allow control of homogeneity and formation of materials with small crystallite size and high surface area and examples are the cases of coprecipitation and sol-gel methods, where a disordered precursor is formed from a solution and then subsequently heat treated under moderate temperature to yield the desired crystalline phase. This has been less studied than solid-state synthesis for iridates, but the cases of 6H-SrIrO3 (109), Sr2MIrO6 (M = nickel, cobalt, scandium and iron) (100) and SrTi0.67Ir0.33O3 (91) illustrate this approach. Hydrothermal synthesis employs solution chemistry in combination with heat treatment in a sealed vessel to generate a moderate pressure and under these conditions the formation of many multinary oxides has proved possible including a number of ruthenium and iridium oxides (122). The method has been particularly useful for the preparation of pyrochlore phases, as we have shown in our own work, leading to the discovery of a variety of new compositions for both iridium and ruthenium materials, and mixed ruthenate-iridates (70, 71, 85, 123, 124), as well as being applied for the convenient preparation of others that would usually be formed at high temperature (69, 73, 125). The solids are typically produced as fine powders with crystallites of only a few tens of nanometres in dimension meaning that they can be easily dispersed with a polymer binder, for example, and employed in an electrochemical device. In some cases molten salts can be used for the formation of mixed oxide materials and the Adams fusion method has been used for the crystallisation of some iridium oxides using molten NaNO3 as a reaction medium. Here, temperatures of around 500°C are used and this has allowed the formation of materials such as pyrochlores (B,Pb,Y)2Ir2O6–x (74). A spray-freeze, freeze-drying technique has also been applied to prepare mixed ruthenium-iridium pyrochlores A2(Ru,Ir)2O7 with A = ytterbium, gadolinium or neodymium with particle size in the submicron regime (86). While not every synthesis approach may be applicable to every composition it is apparent that there exists a wide choice of preparative chemistry suited for the formation of oxides of iridium and ruthenium. For the formation of layers and thin films, more complex experimental design is required. Seitz et al. produced films of SrIrO3 by epitaxial growth on SrTiO3 using pulsed laser deposition from a SrIrO3 polycrystalline target (88), while pseudocubic SrIrO3 thin films were grown on (La,Sr)(Al,Ta)O3 substrates by molecular‐beam epitaxy (89). The majority of iridate and ruthenate materials have been formed, however, as polycrystalline powders and layer deposition has not yet been reported for many of the complex compositions recently studied.

Part II (126) will cover mechanistic details and acid stability of pgm oxides and the conclusions and outlook.

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Acknowledgements

We thank our colleagues at Johnson Matthey, particularly the group of Jonathan Sharman, for their continued collaboration in work in this field. Jasmine Clayton thanks Johnson Matthey for part funding of a PhD studentship with the EPSRC Doctoral Training Partnership award EP/R513374/1. Richard Walton thanks the Royal Society for provision of an Industry Fellowship with Johnson Matthey during which some of ideas in this article were developed.

The Authors

Jasmine Clayton studied Chemistry at the University of Warwick, UK, graduating with MChem (Hons) in 2019. As a Masters student she undertook an international placement at the University of Modena and Reggio Emilia, Italy, under the supervision of Alfonso Zambon. Her final year research project on gallium substituted AlPO zeotypes was under the supervision of Professor Richard Walton. She is presently undertaking research for a PhD in collaboration with Johnson Matthey on acid-resilient OER catalysts and catalyst supports.

Richard Walton is Professor of Chemistry at the University of Warwick, and director of Warwick’s X-ray facility. His research group investigate the synthesis and structures of inorganic materials, particularly from solution-based crystallisation, such as hydrothermal chemistry. This includes open-framework zeotypes and MOFs, as well as condensed oxide structures, that may have practical applications in areas related to energy and heterogeneous catalysis. He has collaborated with various scientists at Johnson Matthey for the past 20 years, and was Royal Society Industry Fellow from 2015–2019 at Johnson Matthey.

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