Perovskite Oxides as Electrocatalysts for Hydrogen Evolution Reaction.

Hydrogen generation through electrocatalytic splitting of water, i.e., hydrogen evolution reaction (HER), is an attractive method of converting the electricity generated from renewable sources into chemical energy stored in hydrogen molecules. A wide variety of materials have been studied in an effort to develop efficient and cost-effective electrocatalysts that can replace the traditional platinum/carbon catalyst. One family of functional materials that holds promise for this application is perovskite oxides. This mini-review discusses some of the progress made in the development of HER electrocatalysts based on perovskite oxides in the past decade. Given the diverse range of possible compositions of perovskite oxides, various studies have focused on compositional modifications to develop single-phase catalysts, whereas others have investigated heterostructures and composites that take advantage of synergistic interactions of different compounds with perovskite oxides. The recent advances indicate that this family of materials have great potential for utilization in HER electrocatalysis.

Introduction

The importance of hydrogen as a sustainable and environmentally friendly fuel, which can replace the traditional fossil fuels, is evident. Hydrogen evolution reaction (HER), as part of the water-splitting process, is at the center of the hydrogen generation research. The electrocatalytic HER is also a method of energy conversion and storage, where the electricity generated from solar and wind can be converted into chemical energy. The commonly accepted mechanism for the HER involves two steps. The process starts with the Volmer reaction, followed by either Heyrovsky or Tafel reactions, as shown below, where M represents the catalyst:

Volmer reaction in acidic conditions: H3O+ + M + e– ⇌ M–H* + H2O

Volmer reaction in alkaline conditions: H2O + M + e– ⇌ M–H* + OH–

Heyrovsky reaction in acidic conditions: M–H* + H3O+ + e– ⇌ M + H2 + H2O

Heyrovsky reaction in alkaline conditions: M–H* + H2O + e– ⇌ M + H2 + OH–

Tafel reaction in both acidic and alkaline conditions: 2M–H* ⇌ 2M + H2

The first step (Volmer) involves the interaction of a proton (in acidic) or water molecule (in basic) with active sites of the catalyst, leading to hydrogen adsorption and the M–H* intermediate. Therefore, this step in basic conditions involves the breaking of a bond in the water molecule. The next step will be the formation of the H2 molecule and can proceed through one of two pathways (Heyrovsky or Tafel). The Heyrovsky reaction involves the interaction of the adsorbed hydrogen with a proton from the solution (in acidic) or a water molecule (in basic) to form a H2 molecule. Therefore, in basic conditions, the Heyrovsky step also involves the breaking of a bond in the water molecule. An alternative process is the Tafel reaction, where two adsorbed hydrogens join together to form an H2 molecule.

The sluggish kinetics of this multistep process results in an overpotential. By convention, the overpotential required to reach a current response of 10 mA cm–2, η10, is used as a metric for comparison of the overpotential of different catalysts. This current density is related to solar fuel synthesis.[1,2] The most common HER catalyst is a composite of platinum and carbon. However, due to the high cost of platinum, there is a need for the design of inexpensive, earth-abundant, and efficient electrocatalysts. Various materials, such as carbides, borides, chalcogenides, and oxides, have been examined for this purpose with varying degrees of success. In this mini-review, we discuss some of the efforts in the past decade, in utilizing perovskite oxides as electrocatalysts for the HER. Perovskite oxides (Figure ) have the general formula, ABO3, where A is usually an alkaline-earth or rare-earth metal with coordination number of 12 and B is a 6-coordinated transition metal, although some main group metals can also be placed on the B-site. Many perovskite oxides contain more than one type of 12-coordinated (e.g., A and A′) and/or 6-coordinated metals (e.g., B and B′), leading to formulas such as A1–A′B1–B′O3. Also, ordered structures are possible, where the size or charge mismatch prevents A and A′ (or B and B′) from sharing the same crystallographic site. The most common of these ordered structures are double perovskites, such as AA′BB′O6, where there are two crystallographically distinct 12-coordinated and/or 6-coordinated sites. These features lead to a significant compositional diversity among the perovskite oxide family, resulting in a wide range of functional properties. The modification of the composition and structure of perovskite oxides has led to the development of a number of active electrocatalysts for the HER. While the performance of perovskite oxides still does not reach that of platinum, the recent advances have shown the great potential of this versatile family of materials for the development of active HER catalysts.

Figure 1

(a) Crystal structure of a typical perovskite oxide, ABO3. The oxygen atoms are represented by gray spheres, whereas A and B metals are shown in turquoise and purple. (b) Octahedral coordination of the B-site metal and (c) 12-coordinated A-site metal.

Methods of Improving HER Activity of Perovskite Oxides

Effect of A-Site Cation Type

The effect of A-site cation on HER activity has been discussed by a number of researchers.[3−7] In some cases, the substitution of the A-site cation has led to the enhancement of the electrical conductivity, Goldschmidt tolerance factor, A-site order, etc. There have been efforts to correlate these changes with the HER performance. However, currently, there are not enough studies to confirm the universality of the effect of these parameters. In other words, while a certain parameter might apply to one series of perovskite oxides, it may not be applicable to all materials from this family.

The transition metals, located on the B-site, are often considered the active sites in the HER. However, the A-site cations can have an indirect effect on the HER. For example, in the series LnBaCo2O5+δ (Ln = Gd–La),[3] with various sizes of the A-site cation, LaBaCo2O5+δ, which has the largest A-site cation, showed the best HER performance. This observation has been attributed to the change in bond angles, leading to the enhancement of the electrical conductivity.[3] With increasing the size of A-site cation, the O–Co–O bond angle increases and gets close to 180°, which enhances the overlap of the electron cloud.[3] This effect, combined with an increase in hole carrier concentration, leads to an improvement in electron transport and an increase in electrical conductivity, which correlates with a decrease in efficiency loss during the HER.[3,8−12] The overpotentials of this series in 1 M KOH range from η10 = 441 mV for EuBaCo2O5+δ to η10 = 156 mV for LaBaCo2O5+δ.[3] Other studies have also proposed that the enhanced overlap and hybridization between the orbitals of the B-site and oxygen atom, arising from larger A-site cations, can shift the position of the eg band center toward the Fermi level, which makes the eg electrons more easily transportable to the antibonding orbitals of the adsorbed H species.[8]

Some researchers have also correlated the HER activity to the change in the structural tolerance factor as a result of A-site substitution and have proposed that the near ideal cubic perovskite oxides with Goldschmidt tolerance factor close to unity are favorable for catalyzing the alkaline HER.[3,12] Goldschmidt tolerance factor, t, is a measure of the stability of perovskite oxides and is described by r = (rA + ro)/√2(rB + ro), where rA, rB, and ro represent the ionic radii of the A-site cation, the B-site cation, and the oxide ion, respectively.[3,12] A tolerance factor close to unity corresponds to a stable cubic perovskite oxide. In the LnBaCo2O5+δ (Ln = Gd–La) series,[3] the tolerance factor of the best catalyst, LaBaCo2O5+δ, is close to unity, 1.009, whereas the rest of the series has smaller tolerance factors.

The effect of an optimum A-site ionic radius is observed for other perovskite-type oxides, as well.[4] For example, CaSrFeMnO6−δ shows a HER overpotential of η10 = 310 mV in 0.5 M H2SO4,[11] compared to that of Sr2FeMnO6−δ, η10 ≈ 480 mV[11] (the symbol ≈ is used when values were approximated from graphs or were given as approximate values, indicated by ∼, in the original publication). The change in the ionic radius of the A-site cation can sometimes result in a change in the arrangement of oxygen vacancies, which will be discussed later. This was recently observed for La1/3Ca2/3FeO3–1/3, which has an ordered structure with a significantly better HER activity, compared to La1/3Sr2/3FeO3–1/3, which has a disordered structure.[5]

There have also been attempts to correlate the HER activity to the inductive effect of the A-site ionic electronegativity,[6] suggesting that the lower overpotential of (Gd0.5La0.5)BaCo2O5.5+δ, η10 ≈ 210 mV in 1 M KOH, is due to the optimum ionic electronegativity of A-site cations, compared to those of the analogues containing various combinations of La3+, Pr3+, Sm3+, Gd3+, and Ho3+.[6] However, others have asserted that the A-site ionic electronegativity is not an appropriate descriptor for the HER activity of cobalt-based perovskites.[7] They have instead suggested that the cobalt valence state is the main parameter in the HER activity, and that structural order can also contribute to the HER performance. For example, they have proposed that the A-site ordering in the double perovskite PrBaCo2O6 helps to improve the HER activity due to a periodic charge distribution, which helps to activate the surface states more effectively compared to the random arrangement of A-site cations in La0.5Ca0.5CoO3.[7]

Effect of B-Site Cation Type

The active sites in the HER are often the transition metals, which are located on the B-site. The use of precious metals in the perovskite composition has the expected effect of enhancing the electrocatalytic performance. However, in some cases, a synergy between a precious metal and a nonprecious metal can lead to an even higher activity. For example, partial substitution of Ti by Ru in SrTiO3 results in SrTi0.7Ru0.3O3−δ,[13] where the presence of synergistic active centers (Figure ) together with high electrical conductivity has been proposed to contribute to a superior HER activity with η10 = 46 mV, compared to that of SrRuO3, η10 ≈ 85 mV, in the same condition (1 M KOH). The relation between electrical conductivity and the HER performance has been pointed out by several authors.[9,14−16]

Figure 2

Schematic representation of a single-phase system with intrinsically atomic-scale synergistic active centers, such as SrTi0.7Ru0.3O3−δ, obtained from partial substitution of Ti with Ru in SrTiO3. Reproduced with permission from ref (13) (open access). Copyright 2020 The Authors; https://creativecommons.org/licenses/by/4.0/.

Other types of B-site substitutions can also lead to the enhancement of the HER activity. In a recent study, the gradual variation of the Mn content relative to Fe and Co in CaSrFe1–Co1–Mn2O6−δ led to an optimum composition with x = 0.25, which showed overpotentials of η10 ≈ 350 mV in 0.1 M HClO4 and η10 = 310 mV in 1 M KOH.[17] Another work showed that partial niobium substitution in SrCoO3 (η10 = 447 mV) results in SrCo0.9Nb0.1O3−δ (η10 = 427 mV), which has an improved HER overpotential in 1 M KOH.[18] Further substitution using nickel results in Sr0.95Nb0.1Co0.9–NiO3−δ (0.1 ≤ x ≤ 0.4), where x = 0.2 gives the lowest overpotential in the series, η10 = 299 mV in 1 M KOH,[18] although the underlying reasons behind this observation are not clear.

In another study, the molybdenum-doped compound, SrCo0.7Fe0.25Mo0.05O3-d, showed an overpotential of η10 = 323 mV in 1 M KOH compared to that of SrCo0.70Fe0.30O3-d with η10 = 378 mV.[19] The Mo-doped compound shows a slightly expanded lattice, a distorted octahedral coordination, and greater structural stability.[19]

Effect of B-Site Cation Oxidation State

Another impact of A-site doping can be a change in the valence state of the B-site cation. In the LnBaCo2O5+δ (Ln = Gd–La) series mentioned above,[3] the change of the A-site metal from Gd to La leads to an increase in the valence state of the B-site metal, cobalt, from 2.92 to 3.49. The increase in Co4+ concentration is proposed to facilitate the small polaron hopping through the Co4+–O–Co3+ pathway, leading to an enhancement of the HER performance from GdBaCo2O5+δ to LaBaCo2O5+δ, with latter having the lowest overpotential. On the other hand, the higher valence Co4+ may help this cation to serve as the active site for the initial adsorption of water molecules due to an enhanced electrostatic attraction.[3] It is proposed that the molecular water prefers to interact with more positive cations, which can lead to better HER activity in alkaline medium for B-site cations with a higher valence.[3,8,12,20,21] Another example is the series Pr(Ba0.5Sr0.5)1–Co0.8Fe0.2O3−δ (PrBSCF, 0 ≤ x ≤ 1),[22] where the HER activity as a function of Pr content increases from x = 0 to x = 0.5 but after that decreases, as a structural transformation from cubic to orthorhombic occurs. The Pr doping on the A-site partially oxidizes the cobalt species which acts as an active center for the HER. The overpotential in 1 M KOH is improved from η10 = 342 mV for BSCF to η10 = 237 mV for Pr0.5BSCF.[22]

The higher valence of cobalt has also been used to justify the enhanced HER activity of CaCoO3 compared to that in La0.5Ca0.5CoO3 and LaCoO3.[7] In this case, the effect of the valence state has been explained in terms of the enhanced charge-transfer kinetics due to strong covalency between the transition metal and adsorbed intermediates.[7]

Effect of Oxygen Vacancies

Oxygen vacancies in perovskite oxides could act as active sites for water adsorption and dissociation, a vital step in the Volmer reaction of HER in alkaline conditions.[10,15,20,23] One study[11] shows the concentration of oxygen vacancies in isostructural Sr2FeMnO6−δ and CaSrFeMnO6−δ, which were mentioned in the discussion of the A-site effect to be δ ≈ 0.22 and 0.57, respectively. CaSrFeMnO6−δ, which has a greater concentration of oxygen vacancies, shows an enhanced HER activity with η10 = 310 mV in 0.5 M H2SO4, compared to η10 ≈ 480 mV for Sr2FeMnO6−δ.[11]

However, oxygen vacancies can also have a detrimental effect on the HER activity.[3,8] As the concentration of oxygen vacancies increases, the B-site cation valence decreases, which affects the HER performance negatively.[3] In one study,[7] PrBaCo2O6−δ (PBCO6−δ) was synthesized with different amounts of oxygen vacancies, namely, PBCO6, PBCO5.9, PBCO5.8, PBCO5.7, PBCO5.5, and PBCO5.15. The HER performance showed a volcano shape correlation with the oxygen vacancy content, where PBCO5.8 had the best activity with an overpotential of η10 = 240 mV in 1 M KOH. The volcano plot is explained in terms of changes to the Co valence as a function of oxygen vacancies. It appears that PBCO5.8 has an optimum degree of vacancies, beyond which the negative effect of the Co valence state dominates the activity.[7]

However, a different study on this system showed that PrBaCo2O5.52 had better HER performance, η10 = 245 mV in 0.1 M KOH, compared to that of compositions with lower concentrations of oxygen deficiency, which showed overpotentials ranging from 291 to 429 mV in 0.1 M KOH.[24] In that study, four Pr0.5Ba0.5CoO3−δ systems were synthesized, where compositions with δ = 0.058 and 0.038 had a cubic simple perovskite structure, whereas a double perovskite structure was observed for the δ = 0.238 (PrBaCo2O5.52, orthorhombic) and δ = 0.121 (PrBaCo2O5.76, tetragonal) phases. The enhanced HER activity was correlated with the greater oxygen vacancy concentration and also the higher O p-band center.[24]

In another work,[25] La0.67Sr0.33MnO3 was synthesized under different oxygen pressures. Lower oxygen pressure yielded a greater Mn3+/Mn4+ ratio. With increasing oxygen pressure, the amount of Mn3+ and oxygen vacancies decreased, but the best HER overpotential was found for 75% Mn3+ content.[25] A similar effect was observed for NdBaMn2O5.46, which was found to contain an optimum amount of oxygen deficiency for HER electrocatalysis, resulting in η10 = 290 mV in 1 M KOH, compared to the related perovskites with lower and higher degrees of oxygen deficiency, namely, NdBaMn2O5.20 (η10 ≈ 410 mV) and NdBaMn2O5.65 (η10 ≈ 510 mV).[26] The better performance of NdBaMn2O5.46 was attributed to its distorted structure, optimized oxygen p-band center, and near-unity eg filling of manganese, given that its valence state varies depending on the degree of oxygen deficiency.[26]

In some perovskite oxides, oxygen vacancies can have an ordered arrangement, which sometimes leads to an enhanced HER activity. In a recent study,[5] La1/3Ca2/3FeO3–1/3, which contains an ordered arrangement of oxygen vacancies (Figure ), showed a HER performance significantly better than that of La1/3Sr2/3FeO3–1/3, where the vacancy distribution is random. Also, both compounds showed a HER performance greater than that for LaFeO3, which does not have oxygen vacancies.[5]

Figure 3

(a,b) Perovskite structures with random and ordered distribution of oxygen vacancies for La1/3Sr2/3FeO3–1/3 and La1/3Ca2/3FeO3–1/3, respectively. White squares are a schematic representation of the random distribution of oxygen vacancies. (c) Polarization curves showing the HER activity in 1 M KOH. Reprinted from ref (5). Copyright 2021 American Chemical Society.

The oxygen vacancies can sometimes be created by cation-deficiency. For example, a small degree of barium deficiency in BaCo0.4Fe0.4Zr0.1Y0.1O3−δ leads to an increase in the concentration of oxygen vacancies. The compound containing 0.95 barium per formula unit shows a HER overpotential of η10 = 360 mV in 1 M KOH, compared to that of the parent compound, η10 ≈ 390 mV.[1] This enhancement has been attributed to the formation of low-coordinated active Fe/Co sites due to the oxygen deficiency, which can facilitate the adsorption of reaction intermediates.[1]

Anion Doping

Anion doping on the oxygen sites has been used in some cases to enhance the HER performance. For example, nitride-doped SrFe1.5Mo0.5O6−δ[20] has an overpotential of η10 = 251 mV in 1 M KOH compared to η10 = 400 mV for the undoped material. This improvement is attributed to the enhanced electrical conductivity due to a smaller band gap, arising from lower electronegativity of N compared to that of O, as well as the creation of oxygen vacancies and higher oxidation state of iron due to the partial substitution of O2– with N3–.[20] Another example is the phosphide-doped BSCF perovskite, Ba0.5Sr0.5(Co0.8Fe0.2)1–PO3−δ,[21] which shows an overpotential of η10 ≈ 415 mV for x = 0.05, compared to that of the parent BSCF oxide, η10 ≈ 465 mV, under the same conditions (0.1 M KOH).[21] Arguments concerning the increase in the oxidation state of transition metals and the creation of oxygen vacancies are made for phosphide doping,[21] similar to the nitride substitution discussed above.[20] However, other authors have shown that partial substitution of an oxide ion with an anion with lower valence can also lead to an enhancement of the HER performance. For example, fluoride doping into the oxygen site of La0.5Ba0.25Sr0.25CoO3−δ (LBSC) results in the formation of La0.5Ba0.25Sr0.25CoO2.9−δF0.1 (LBSCF). The overpotential for LBSCF is η10 ≈ 180 mV, compared to η10 ≈ 240 mV for LBSC in 1 M KOH.[27] This enhancement was explained in relation to the free energy of hydrogen adsorption (ΔGH*) during the electrocatalytic process. The ΔGH* was calculated to be less negative (closer to zero) for LBSCF compared to LBSC, indicating a more optimized ΔGH*, which allowed a faster proton/electron-transfer step for LBSCF.[27]

Nanostructuring

Some studies have shown that nanostructured perovskite oxides can show high HER performance. For example, SrNb0.1Co0.7Fe0.2O3−δ nanorods have higher electrochemically active surface area, greater surface oxygen vacancies, and faster charge transport, all of which can contribute to the enhanced HER activity of nanorods, which show overpotential of η10 = 262 mV in 0.1 M KOH, compared to that of the bulk SrNb0.1Co0.7Fe0.2O3−δ, η10 = 361 mV.[28]

In another study, hollow nanorods of PrBa0.5Sr0.5Co2O5+δ (PBSC) were reported to show η10 = 360 mV in 0.1 M KOH.[29] Also, a combination of nanostructuring and composite formation was used to obtain hollow nanorods of PrBa0.5Sr0.5Co2O5+δ (PBSC) covered with ultrathin amorphous FeOOH nanoflakes (PBSC@FeOOH), which led to an overpotential of η10 = 280 mV in 0.1 M KOH.[29] However, the HER activity of the bulk perovskite phase was not reported to allow a direct comparison.[29]

CV Activation

HER catalysts can often be activated by conducting cyclic voltammetry for around 100 cycles.[30,31] This activation process has been explained in terms of the enhanced wetting property of the catalyst surface or surface reconstruction.[31] For example, the SrIrO3 surface becomes rich in metallic Ir due to Sr2+ leaching, which increases the available active sites.[30] The enhanced HER activity is attributed to the presence of metal on the surface, improved electrical conductivity, and increased active surface area. The overpotential of activated SrIrO3 was found to be η10 = 139 mV in 0.1 M KOH, compared to η10 = 391 mV for fresh SrIrO3.[30]

Multicomponent HER Electrocatalysts Based on Perovskite Oxides

Some researchers have studied HER electrocatalysts that do not consist of a pure perovskite oxide. Instead, they contain more than one phase, comprising metals, metal sulfides/oxides, or other phases mixed with or located on the surface of a perovskite oxide.[31−42] These additional phases contribute synergistically for enhanced HER performance. Multicomponent HER catalysts based on perovskite oxides are often obtained from either surface decoration or heterostructure and composite formation.

Surface Decoration

Metals can be introduced on the surface by exsolution of B-site cation through reduction and subsequent treatment with other reagents. For example, one study involved the synthesis of Pr0.4Sr0.6Co0.2Fe0.7Nb0.1O3-δ (PSCFN), followed by reduction using hydrogen gas to convert it into a layered Ruddlesden–Popper perovskite (Pr0.4Sr0.6)3(Fe0.85Nb0.15)2O7 (CF-PSFN) with uniformly dispersed Fe-Co alloy nanoparticles on the surface.[32] This CF-PSFN was further treated with dicyandiamide to coat the alloy particles, leading to an oxide surface, decorated with nitrogen-doped carbon-coated Fe–Co alloy nanoparticles, NC-CF-PSFN.[32] This catalyst shows a good HER performance with overpotentials of η10 = 124 and 186 mV in acidic (0.5 M H2SO4) and basic (1 M KOH) electrolytes, respectively. At the same time, CF-PSFN without a carbon coating has poor performance and cannot reach the current density of 10 mA cm–2 in the experimental potential window.[32] Another example of metal exsolution is the formation of Co–Ni bimetallic nanoparticles on the surface of Sr0.95Nb0.1Co0.7Ni0.2O3−δ, which shows an overpotential of η10 = 208 mV in 1 M KOH, compared to η10 = 299 mV for the pristine perovskite oxide.[18] This improvement has been attributed to the role of exsolution in exposing more active sites and synergistic activity of the perovskite host lattice and the exsolved particles.[18]

Similarly, the Ni–Fe alloy is reported to be exsolved from perovskite oxide SrTi0.1Fe0.85Ni0.05O3−δ in the presence of H2 and CH4.[43] Here, CH4 is used as a source of carbon, and its decomposition into carbon nanotubes is catalyzed by the exsolved Ni–Fe particles. This perovskite oxide, featuring the Ni–Fe alloy and carbon nanotubes on the surface, shows overpotential of η10 = 340 mV in 0.1 M KOH, whereas the HER using the pure perovskite does not generate enough current to reach 10 mA cm–2 in the experimental potential window.[43]

In another study, nickel exsolution and subsequent phosphidization led to the formation of Ni2P nanoparticles on the surface of La0.8Sr0.2Cr0.69Ni0.31O3−δ.[44] As a result, an enhanced HER overpotential of η10 = 339 mV in 0.1 M KOH was achieved, compared with that of the pristine perovskite, η10 = 447 mV.[44]

In another study, the surface of the perovskite oxide K0.469La0.531TiO3 was modified by treatment with RuCl3, which resulted in the surface doping of Ru due to ion exchange, as well as the formation of Ti-doped RuO2 nanoparticles on the surface.[38] This catalyst showed a very low overpotential of η10 = 20 mV in 1 M KOH, whereas the perovskite oxide alone has a poor HER performance, where the current response does not reach 10 mA cm–2 in the experimental potential window.[38]

Heterostructure and Composite Formation

Efforts toward improving the HER activity have also included the formation of composites. An example of a multiphase composite catalyst is a RuO2/SrRuO3 (RSRO) heterostructure.[31] While this catalyst contains a precious metal instead of earth abundant metals, the enhancement of the HER overpotential is interesting to note. The overpotential values for RuO2, SrRuO3, and RSRO in 1 M KOH are η10 = 73, 68, and 7 mV, respectively. The superior performance of RSRO is attributed to the increased number of active site and synergistic effect of the two components of the heterostructure.[31] Density functional theory (DFT) calculations showed the energy barrier of water decomposition was lowest for RSRO surface compared with the individual components, RuO2 and SrRuO3. The H adsorption energy was found to be higher on the SrRuO3 component of the heterostructure, while the OH adsorption energy was higher for RuO2, which suggested the preferential adsorption of H and OH on SrRuO3 and RuO2 surfaces, respectively. The calculated electron density difference suggested that at the heterojunction interface, electrons flowed from Ru atoms of RuO2 to the O atoms of SrRuO3, which resulted in more negative O sites in SrRuO3 for H adsorption and more electrophilic Ru sites in RuO2 for OH– adsorption.[31]

Similar synergistic improvement of HER activity is found in several other reports.[38−40] An example is the decoration of the double perovskite PrBa0.94Co2O5+δ particles with in situ grown simple perovskite nanorods to obtain a double/simple perovskite heterostructure, which shows an overpotential of η10 = 186 mV in 1 M KOH, compared to those obtained for the simple perovskite, η10 ≈ 240 mV, and double perovskite, η10 ≈ 240 mV.[39] The enhanced performance of the heterostructure is attributed to the presence of more active sites, fast release of the generated hydrogen from the surface, better charge transfer, and synergistic effect on water adsorption and dissociation, as well as OH* desorption during the HER process.[39] DFT calculations showed H2O dissociation was energetically favored by simple perovskite. On the other hand, the Gibbs free energy for OH* desorption was lower for the double perovskite, helping to free up more active sites.[39]

Some studies have explored physical mixing of a perovskite oxide with another compound.[45−48] For example, a composite that forms as a result of ball-milling a mixture of the perovskite LaNiO3−δ and MoS2 (Figure ) showed enhanced HER performance over the individual components LaNiO3−δ and MoS2.[45] Another example is a mixture of the perovskite La0.5Sr0.5CoO3−δ with MoSe2 and carbon black. Upon ball-milling, the composite catalyst was able to achieve overpotential of η10 ≈ 240 mV in 1 M KOH compared to the higher overpotentials, η10 ≈ 425 and 650 mV for La0.5Sr0.5CoO3−δ and MoSe2, respectively.[46] Interestingly, a phase transition of MoSe2 from less conductive semiconducting 2H phase to the more conductive 1T metallic phase was observed upon formation of the composite, possibly due to the transfer of electrons from cobalt to the more electronegative molybdenum. It was proposed that the enhanced HER activity was due to this electron donation, making the cobalt site more electrophilic, upshifting the d-band, and facilitating the adsorption of the reaction intermediates.[46]

Figure 4

Schematic representation of the preparation of a composite catalyst containing perovskite LaNiO3−δ and MoS2. Reprinted from ref (45). Copyright 2018 American Chemical Society.

Another composite catalyst, obtained from ball-milling of a mixture of the perovskite Ba0.5Sr0.5Co0.8Fe0.2O3−δ, Co–Ni alloy nanoparticles and nitrogen-doped carbon, shows an overpotential of η10 = 183 mV in 1 M KOH compared to that of the pristine perovskite, η10 = 376 mV.[49] Cobalt/nickel metal particles were chosen due to their reported low H adsorption energy.[12,29,49,50] A control experiment using an ultrasonicated mixture of the above components led to a poor HER activity with overpotential of η10 ≈ 360 mV. This was attributed to the deficit of interfaces in this mixture, as opposed to the ball-milled composite, which had enhanced interfaces, facilitating the charge transfer.[49] The advantage of the interface between the Co–Ni alloy and the perovskite was described in terms of the ability of the perovskite oxide to bind OH– ions preferentially, while H was adsorbed onto the Co–Ni nanoparticles.[49]

The addition of reduced graphene oxide has also been shown to improve the HER activity. An example is a catalyst consisting of La0.5(Ba0.4Sr0.4Ca0.2)0.5Co0.8Fe0.2O3−δ perovskite nanorods attached to reduced graphene oxide nanosheets, which showed an enhanced overpotential of η10 = 144 mV in 1 M KOH compared to η10 ≈ 192 mV for the catalyst without the reduced graphene oxide.[12] Also, a significant enhancement of the HER activity is observed for a nanostructure composite, comprising LaFeO3 and reduced graphene oxide, showing overpotential of η10 = 130 mV in 0.5 M H2SO4, compared to η10 = 490 mV for LaFeO3.[51]

Concluding Remarks

Perovskite oxides feature a wide range of possible compositions and structures and readily lend themselves to various modifications, making them an attractive family of materials for applications in electrocatalysis. They have indeed been used as electrocatalysts for a variety of chemical processes, including the HER. Compositional changes have been a primary method in efforts toward the development of highly active HER electrocatalysts based on perovskite oxides. In addition, surface modifications by incorporation of other types of materials on the perovskite surface have led to enhancements in the HER activity. Furthermore, the formation of multiphase catalysts, where perovskite oxides are combined with other materials to take advantage of synergistic effects, have shown promising results. However, the methods of enhancing the HER activity have been tested for a limited number of perovskite oxides, and the broad application of those methods to a wide range of perovskites need to be studied. Nevertheless, the progress, especially in the past few years, in the utilization of perovskite oxides for the HER has been significant and indicates the great potential of this important family of functional materials as possible replacements for the costly precious metal catalysts of HER.