Climate change, rising energy needs, and the depletion of fossil fuels make it hard to power the world with sustainable energy sources like solar and wind. The creation of electrochemical machines that can use power from green sources to change electrical energy into chemical energy is one possible answer. Hydrogen can be made from water molecules by water electrolysers. Fuel cells can use hydrogen to make energy. The reduction of oxygen and the oxidation of hydrogen or metals work together in fuel cells and metal-air batteries to turn chemical energy into electrical energy. But these tools for changing energy aren’t very good because some of the processes they use are very slow.
To make these devices work better, electrocatalysis would need to change the speed of the chemical processes involved with the help of active and stable electrocatalysts. Electrocatalysis is a type of catalysis that changes the speed of an electrochemical process that is happening where an electrode and a liquid meet. Even though electrocatalysis is very important for green energy systems, we still don’t know enough about how it works on a basic level.
Recently, perovskite materials have become a new group of nonprecious metal catalysts that are both cheap and very good at speeding up reactions like the ORR, OER, and HER reactions. Perovskite materials are known for having a wide range of chemical, crystal, and electronic structures. Because they are so flexible, the electronic structure of perovskites can be tuned, which makes them perfect for studying the link between structure and function in electrocatalysis.
It has been seen that there has been an increase of study into making perovskite catalysts in oxygen electrocatalysis. But the part that perovskites play in hydrogen electrocatalysis isn’t studied as much, even though it’s been getting more and more attention lately. This chapter is mostly about perovskite materials used for ORR, OER, and HER processes that use electrocatalysis. The electrocatalytic processes are explained to give a clear picture of the reactions happening on perovskite catalysts. This is followed by a full talk on how to build perovskite materials in a way that makes them useful for electrocatalysis, with a focus on both stability and catalytic activity.
Mechanisms in Electrocatalysis on Perovskite Materials
To understand electrocatalysis on catalyst surfaces, you need to know how reactions work. There isn’t a single process that controls all electrocatalytic reactions because the electrodes interact with each other in many ways. Catalyst chemistries can change how the same reaction works in different ways. Noble metals, dimeric molecules, and perovskite oxides all have one-metal-site mechanisms suggested. The oxygen-evolving complex of photosystem II and electrodeposited oxides, on the other hand, have two-metal-site mechanisms. The usual ways that oxygen and hydrogen electrocatalyze reactions on perovskite surfaces are emphasised.
Oxygen Electrocatalysis
It is common to write the oxygen electrocatalysis (ORR) process in alkaline liquids as O2 + 2H2O + 4e–! 4OH–. But, new experiments and computer studies have shown that the ORR on perovskite oxides is a four-step process that only happens on one metal spot on the surface. Some of these steps are the shift of hydroxide, the formation of peroxide and oxide, and the renewal of hydroxide. Molecular oxygen or H2O from the solution and electrons combine with metal sites on the surface to make four types of oxygenated adsorbates: OO, OOH, O, and OH. Surface metal sites whose oxidation state changes between n + and n + 1 also undergo oxidation and reduction (redox).
The binding energies of the different reactive oxygen adsorbates in the ORR and OER processes are very closely linked because of scaling relationships. This is where the non-zero theoretical overpotential seen in many catalysts comes from. Because of scaling relationships, the binding energies of the different reactive oxygen adsorbates in the ORR are very closely linked to perovskite oxide surfaces that use the same one-metal-site process. This is where the possible overpotential that isn’t zero comes from in many catalysts.
When perovskite oxides are in alkaline media, on the other hand, the OER process happens in four steps: hydroxide deprotonation, peroxide formation, peroxide deprotonation, and hydroxide renewal. These work by hydroxyl ions (OH–) from the solution interacting with metal sites on the surface, which causes the metal to redox. The reactive intermediates made for the OER in base are the same as those made for the ORR, but they come in a different order: O, OOH, OO, and OH.
This difference in the intermediates that stick to perovskite oxide surfaces shows how different reaction processes are suggested for oxygen electrocatalysis. It also shows how complicated the oxygen chemicals are on catalyst surfaces. Time-resolved methods are still not well developed enough to make it easy to reach the reaction stages on the surfaces of perovskite oxide.
Hydrogen Electrocatalysis
The Volmer–Heyrovsky or Volmer–Tafel mechanism can be used for the hydrogen adsorption reaction (HER) in basic fluids. It is a two-step electron transfer process. The Volmer reaction happens in both ways. Molecular H2O binds to empty reactive sites, the adsorbed H2O is reduced electrochemically to adsorbed hydrogen atoms (H) and OH–, the OH– is released to clean the catalyst surface, and H is formed for H2 evolution.
A lot is known about how HER works on metal catalysts, but not as much is known about how it works on oxide surfaces, especially in alkaline conditions. In the 1990s, Goodenough et al. were the first to study how hydrogen changed on Sr1–xNbO3–d perovskite oxides when they were exposed to strong acids. They suggested a possible HER reaction route and said that the oxygen anion on the surface of perovskite oxides seems to be the place where the HER reacts.
However, it is still not clear whether the Heyrovsky reaction or the Tafel reaction leads to the second step. The two-electron HER has only one intermediate (H), so there is no scaling relation and the overpotential is perfect at zero on high-tech catalysts like Pt. The general rate of the HER reaction depends on how strongly the H intermediate binds, which is also known as the hydrogen adsorption free energy (DGH). The best HER catalyst should have a DGH number close to zero and a binding strength to hydrogen that is neither too weak nor too strong.
Rational Design of Perovskite Materials Toward
Efficient Electrocatalysis
The suggested reaction mechanisms help us understand how perovskite materials work electrochemically, but they can’t be used to find materials that have really cool catalytic effects. Because there are so many possible perovskite materials and they all have different physical features, it is important to create them in a way that makes the most of electrocatalysis. Most of the time, you can boost the activity of an electrocatalyst by either making each reaction site more active or adding more of them. “Activity descriptors,” which are links between properties and activities, have been created to help with the creation of better perovskite catalysts. These descriptors have been useful for predicting the discovery of new perovskite catalysts, with a focus on descriptions for the OER. But these words don’t fully describe all perovskite materials; they have some limits. We talk about specific design methods that aim to boost intrinsic or secondary activity, with a focus on changing factors in perovskite oxides to improve their catalytic performance. Another important thing for electrocatalysts that work well is that they are stable.
Activity Descriptors
The binding energy of oxygen intermediates is the main thing that controls how fast the OER reacts. Rossmeisl and others used DFT calculations to find the difference (DGO* – DGOH) in the adsorption free energy of O and OH* intermediates. They used this to describe the OER activity on perovskite oxide surfaces in a way that is uniform. However, it is hard to find active perovskite catalysts using this description because it is hard to tell the difference between reaction stages. It has been suggested that the redox potential of transition metal cations before the catalytic OER could be an easier way to describe the activity. However, this method is limited because it is hard to identify the redox transition in perovskite oxides. It has been easier to make perovskite materials work well as electrocatalysts by using activity parameters from molecular orbital theory and band theory.
Activity Descriptors from Molecular Orbital Theory
In the structure of perovskite oxide, the B-site transition metal is linked to the oxygen six times, making an octahedron. Along with the O 2p orbitals, the d orbitals of the transition metals combine to form the t2g orbital, which is made up of p-bonding and p-antibonding, and the eg orbital, which is made up of r-bonding and r-antibonding. Molecular orbitals that show the electronic states of metals and oxygen can be used to explain how electrocatalysis works in perovskite materials. Bockris and Otagawa were the first to use the d-electron number (both t2g and eg electrons) of transition metal cations to describe the OER activity seen on ABO3 perovskites (B = Ni, Co, Fe, Mn, Cr, V). It was discovered that the OER overpotential went down as the amount of transition metal d electrons went up. This was thought to be a major factor in setting the binding strength of the OH* adsorbate by filling the metal–OH antibonding orbitals.
Recent DFT studies showed that the binding energy of possible OER adsorbates is related to the number of transition metal d-electrons and the oxidation state of perovskite oxides on a scale. Instead, the filling of the eg orbital of surface transition metal cations by Shao-Horn and coworkers is a better way to describe the action based on molecular orbital theory. The eg orbital overlaps with a surface-anion adsorbate more than the t2g orbital does. This means that the occupation of the eg orbital can have a more direct effect on the binding of oxygen intermediates on surface transition metals and, in turn, on the OER activity.
We found a volcano connection between the OER activity (measured by the overpotential needed to provide a 50 lA cm^2 ox specific current density) and the eg filling of B-site ions. This was shown by carefully testing more than 10 perovskite oxides with different B-site elements in basic solutions. Too little or too much eg orbital occupation in perovskites can cause too strong or too weak of a contact with oxygen intermediates, both of which are bad for OER activity. At the very top of the volcano plot are perovskites whose eg filling is very close to unity. These materials are great for linking process intermediates and having high OER performance. In line with the Sabatier concept, this finding makes sense.
To make OER electrocatalysts that work well, the eg-filling description has been successfully used to screen oxides with structures other than perovskite, like spinel and rock salt structures. The eg-filling description has helped find a lot of useful perovskite ORR catalysts, including LaNi0.25Co0.75O3–d, LaNi1–xMgxO3, LaNi1–xFexO3, and La0.8Sr0.2Mn0.6Ni0.4O3.
The molecular orbital-based eg filling can be useful as a general way to choose perovskite ORR/OER catalysts, but it also has some problems. On the one hand, molecular orbital theory only sees the transition metal as the reacting site, and it’s still not clear how to figure out the eg electron number.
Activity Descriptors from Band Theory
The eg-filling description stresses how important the electronic structure of the catalyst is to the ORR/OER process. Band theory can help us understand the electronic structure of catalyst materials, like how metal or alloy surfaces work as catalysts and how perovskite oxides change over time in oxygen electrocatalysis. Researchers Grimaud et al. recently found a link between the O p-band centre and the OER activity of cobalt-based perovskites. They calculated this centre in relation to the Fermi level. The electronic structure of perovskite oxides is better understood by the O p-band, which is spread out more. The transition metal d-character is still shown by the hybridised density of states. To get better OER performance, move the O p-band centre closer to the Fermi level. This is because the O p-band centre of several double and single perovskite oxides scales linearly with OER performance. But, the calculated O p-band centre needs to be confirmed through experiments on metal and oxygen states, and it also needs to be checked to see if it works on all perovskite systems, not just ones with cobalt.
Design Strategies
It has been established that activity descriptions are not enough to guide catalyst design, which could slow down the progress in making active perovskite electrocatalysts. It is possible to make a perovskite catalyst more active in two ways: by increasing the activity of each reactive site intrinsically by controlling the electronic structures of the perovskite, such as its composition, oxygen vacancies, and crystal structure; or by increasing the number of reactive sites extrinsically by looking for new ways to make perovskite nanostructures or composites. This part talks about how to build a catalyst based on these parameters and shows how each parameter can be changed separately in perovskites to make electrocatalysis better. There is a link between these factors, and changes in makeup can have an effect on the crystal structure or the oxygen gap. Perovskite oxides can also have changes in their electronic structure when they are shrunk down. Other factors related to catalyst design, like strain and conductivity, can also affect how well perovskites work as electrocatalysts, but they are not included because they have not been studied as much.
Composition
Perovskite materials can change their chemical makeup easily. Adding or taking away elements from the A- or B-site can change the electronic structure of the B-site transition metal, which in turn changes the electrocatalytic activity. To describe a doped perovskite oxide, use the formula A1−xA′xB1−yB′yO3, where A′ and B′ are the dopants at the A- and B-sites, with x and y values for doping. Most of the time, A′ is a rare-earth or alkaline-earth metal for the A-site replacement. Alkaline-earth metals can be used instead of some lanthanides to change the catalytic function even more.
It was seen that adding A-sites to perovskite systems like La1−xCaxMnO3, La1−xSrxMnO3, and La1−xCaxCoO3 changed the ORR activity as well. In the OER (e.g., La1–xSrxCoO3 [29, 73, 81] and La1–xSrxFeO3 [82]) and HER (e.g., Prx(Ba0.5 Sr0.5)1–xCo0.8Fe0.2O3–d [42]), similar research has been done.
The B-site doping has a bigger impact on the catalytic activity of perovskite oxides than the A-site doping. This is because the B-site transition metal is thought to be the reacting site in normal ORR/OER processes. In an alkaline environment, Sunarso et al. looked at how different transition metals affected the ORR activity of LaBO3 (B = Cr, Mn, Fe, Co, and Ni). They discovered that the ORR’s diffusion-limited current density drops in this order: LaCoO3 > LaMnO3 > LaNiO3 > LaFeO3 > LaCrO3.
The catalytic behaviour can also be changed by adding more than one dopant to the B-site. The OER activity of a group of iron and tin co-doped BaCo0.9−xFexSn0.1O3−d (x = 0.2, 0.3, 0.4, labelled as BCFSn-721, BCFSn-631, and BCFSn-541, respectively) was studied by Xu et al. A bigger amount of Co in the B-site makes the OER activity go up. This shows that transition metal Co ions are naturally better at OER than Fe ions in a perovskite structure.
It can be trickier for composition to work when alien parts are added to both the A- and B-sites. In this case, the link between composition and action needs to be studied in a planned way, usually case-by-case, before the best composition can be found.
Oxygen Vacancy
The oxygen stoichiometry of perovskite oxides is not the usual value of 3. The effect of extra oxygen (oxygen stoichiometry of 3 + d) in perovskites is only studied in electrocatalysis. A lot of study has been done, though, on the role of oxygen void (oxygen stoichiometry of 3–d). There was a strong link between the oxygen vacancy concentration (d) and the OER activity of the La1−xSrxCoO3−d series, with SrCoO2.7 having the most vacancies and the greatest activity. They came up with a plan for an OER process called lattice oxygen-mediated (LOM) that considers the part played by surface oxygen holes and lattice oxygen species. DFT modelling added to the evidence that this LOM process was correct. Later, in situ 18O isotope labelling mass spectrometry was used to prove that lattice oxygen was involved in the OER.
Most perovskite oxides have an oxygen void that changes with any changes in makeup to keep the charge balance in the perovskite structure. There are two main ways to add oxygen vacancies to certain perovskites: directing the manufacturing steps, which may be important for adding oxygen vacancies; and treating an already existing perovskite after it has been made, usually by reducing the H2 or vacuum. For instance, oxygen-stoichiometric CaMnO3 was changed into oxygen-deficient CaMnO2.5 (d = 0.5) through a low-temperature reductive annealing process in 5% H2/Ar. The crystal structure stayed the same, though, as it was orthorhombic. It was found that adding oxygen flaws to CaMnO2.5 made its OER activity much better than in CaMnO3. Chen et al. made BaTiO3d perovskite with a lot of oxygen holes under a much harder reduction process in vacuum. This material worked better in both the OER and ORR.
However, adding more oxygen flaw sites does not always mean that the catalytic activity goes up. Changes in the oxygen void cause changes in the metal’s electronic states, such as its oxidation state and coordination, as well as the covalency between the metal and oxygen. Wang and others looked into the HER activity of different H2-reduction-treated NdBaMn2O6−d double perovskites that had different amounts of oxygen vacancies. They found that perovskites with a reasonable number of oxygen vacancies may have the best catalytic activity.
Crystal Structure
It is closely linked to the electrocatalytic activity of perovskite minerals, both in the bulk and on the surface. The bulk crystal structure can be different for the same theoretical makeup depending on how it was made. For example, heating a La0.7Sr0.3MnO3 starter at 650, 750, and 850 °C created tetragonal, cubic, and orthorhombic phase structures, in that order.
According to the electrochemical tests, the tetragonal La0.7Sr0.3MnO3 had the most ORR activity. Post-treatment can also change the bulk crystal structure. For example, when cooled from 800 °C to room temperature, solid LaNiO3−d perovskite can change into a cubic-phase LaNiO3−d. The perovskite mass structure can also be changed by reducing it in H2. For example, a cubic-phase Pr0.5Ba0.5MnO3−d perovskite can be changed into a tetragonal-phase PrBaMn2O5+d stacked perovskite by treating it with H2.
Amorphous perovskites also show good catalytic performance, according to new study. Trudel and his colleagues created a photochemical thin-film layering method to make a number of amorphous perovskite oxides, including BSCF, LaCoO3−d, and La0.7Ca0.3CoO3−d. These oxides all had good OER activity. Magnetron sputtering layering was used to make amorphous BSCF nanofilms that worked very well for OER.
The crystal structure on the surface of perovskite materials can also change how well they work as electrocatalysts. After a normal sol–gel process, BSCF perovskite crystallised after being heated to 950 °C for 5 hours. It had an amorphous oxide layer on the top that was about 20 nm thick. This amorphous layer grew incredibly thick to a thickness of 180–200 nm after being heated in an argon atmosphere. This stopped BSCF from working as an ORR catalyst. However, applying the same Ar treatment to pure Ba0.5Sr0.5Co0.2Fe0.8O3–d with a 20-nm-thick amorphous layer did not cause any more amorphization while increasing the amount of oxygen vacancies, which made the ORR activity much better.
These works show that perovskite structures can be changed in both their bulk and surface forms. This is possible by doing experiments outside of the material itself. In real-life electrochemical settings, perovskite structures can change on the spot. For example, the structure can change from BaNiO3 to BaNi0.83O2.5 during OER tests in alkaline media.
Nanostructure
Perovskites play an important role in electrocatalysis, which has mostly been studied in large materials made using standard sol–gel methods, solid-state reactions, and high-pressure synthesis. Because there are a lot of inactive atoms in the bulk, these bulk perovskites usually have big particles, a small surface area, and a shape that doesn’t have any features. This means that they can only do limited catalysis. Nanostructuring has been suggested as a technically possible way to solve this problem.
When a perovskite material is shrunk down to the nanoscale, its surface area and number of reacting sites are likely to grow. This extra surface area creates more places for reactions to happen, which usually leads to better catalytic activity on a mass level. The fact that the OER activity was higher on the ball-milled BSCF catalyst shows this to be true.
To make perovskite nanostructures, you can change the synthetic factors of normal preparation methods. On the other hand, Cho and his colleagues found a fascinating pattern of the perovskite phase and particle growth by changing the calibrated temperatures and lanthanum-dopant ratios during the sol–gel synthesis of Lax(Ba0.5Sr0.5)1−xCo0.8Fe0.2O3−d perovskites. A perovskite called La0.7(Ba0.5Sr0.5)0.3Co0.8Fe0.2O3−d was made at a lower temperature of 700 °C and with a La doping content of x = 0.7. The particles were as small as 50 nm, and the alkaline ORR/OER activity got a lot better.
Nanostructured perovskites have been made using a number of different techniques, such as the precipitation method, alkaline synthesis, the shaping approach, and electrospinning. Physical vapour deposition (PVD), chemical vapour deposition (CVD), and electrodeposition are some of the deposition-based methods that have been used to make nanosized perovskite materials.
It’s possible that nanostructured perovskites’ better catalytic performance isn’t just due to their larger surface area. It is possible to change the electronic state of perovskites by making the nanostructures smaller. By heating a sol–gel precursor at 600, 700, 800, and 1000 °C, Zhou et al. made LaCoO3 nanoparticles that were 60, 80, and 200 nm in size, as well as bulk LaCoO3 particles.
Composite
Perovskite/carbon alloys have gotten a lot of attention in study because they might make perovskite materials more active. It was discovered that carbon, which is an electrical base, plays a part in the electrocatalytic ORR pathway by speeding up the change of O2 to HO^2. It can also change the electronic structure of perovskites by reducing cobalt cations, like when it was mixed with BSCF and cobalt cations were lowered to a lower oxidation state, which improved ORR/OER action.
Perovskite/carbon composites can be made by physically putting them together. These composites have better ORR and OER activity. The ORR/OER dualfunctionality should get even better as the perovskites and carbons get closer to each other. This can be done with high-tech chemical production methods like electrospinning, the CVD process, and the electrolytic method. However, it is still hard to make perovskite/carbon compounds with a pure-phase perovskite component because reductive carbon materials and oxidative perovskite oxides don’t mix well.
For the smart creation of effective perovskite catalysts, it is important to know where the increased catalytic activity on perovskite/carbon compounds comes from. The higher activity seen in perovskite/carbon composites could have come from the ligand effect at the interface, the creation of interfacial heterostructures, or the spread effect. People think that carbons are more active towards the ORR and perovskites are more active towards the OER.
Even though it helps make electrocatalysis better, perovskite/carbon alloys may be affected by carbon rust, which is a common problem for carbon-based materials, especially in the case of the OER. One way to help with this is to use graphitised carbon or nitrogen-doped carbon, which is more resistant to rust. Perovskites have also been mixed with other conducting materials, like Ni foam, using high-tech ways for making them, like the alkaline method and electrodeposition.
Perovskites can be combined with metal nanoparticles as well as electrical materials to make new materials. Exsolution is a chemical process that is almost only used for perovskite oxides and is often used to make these kinds of mixed materials. If you heat treat perovskite in a reductive atmosphere, an active metal that is first inserted into the lattice can go through a reduction process and then come out on the top of the perovskite as metal nanoparticles. These metal nanoparticles can change the original perovskite electronic structure and add more catalytic sites. This makes the electrocatalysis work in a different way.
Other materials that can speed up reactions, like hydroxides and metal oxides, can also be mixed with perovskite oxides through a chemical synthesis or an infiltration process. The catalytic behaviour of these hybrid materials was better than that of the single perovskite component. To get the best action, you should be very careful when choosing both the perovskite oxide and the added functional material.
In the future, people should work on making perovskite/carbon compounds that are strongly linked so that they can work better as catalysts.
Stability Concerns
People often forget to think about how stable an electrocatalyst is in order to make it more catalytic. In particular, this is true for perovskite OER catalysts, which change shape during the OER process and often have A- or B-site cations leak into the solution. To make long-lasting perovskite electrocatalysts, it is important to understand these changes. One way to help with this is to learn more about how perovskite oxides and water ions combine in a modelling system that works a lot like the OER.
Perovskite oxides can go through big changes in their local chemical and electronic structures, which is different from how activity is described by physical qualities that are not present in the system. Using X-ray absorption near-edge structure (XANES) studies in real-life OER settings shows that a self-assembled layer of OER-active metal (oxy)hydroxides forms. This layer is thought to be the cause of the much better OER performance.
Active perovskite OER catalysts can have changes in their local chemical and electronic structures. This is very different from describing activity using physical qualities that are not present in the catalyst. Sometimes, perovskite oxide that was first thought to be “instable” can change through operando changes into a “stable” catalyst. In this case, the original perovskite can be seen as a “precursor” or “pre-catalyst” of a real active catalyst.
The amount of changes on perovskite surfaces is different for different types of materials and chemicals. This may be because perovskites have different amounts of oxygen vacancies. When the O p-band centre is in the best place, the trigger is very stable. However, not all perovskite materials can change into catalysts that work and last a long time. For instance, it was found that some nickel- and iron-based perovskite materials lost their ability to work after being tested for a long time in alkaline media for OER.
The business world is more interested in the security problem than the academic world. Researchers usually say that perovskite catalysts are “stable” based on how steady they are in terms of electrochemistry, without looking into the structure of the catalyst either outside or inside the reaction. When used in industry, the safety of the catalyst has to meet stricter standards. To close the gap between basic and applied research, more work needs to be done to figure out how perovskites change (for example, how they break down) during OER. This will also help in the creation of a better catalyst design principle that combines stability and catalytic activity.
In conclusion, it is important to know how stable perovskite electrocatalysts are in order to make electrocatalysts that work well and last a long time.
Perovskite Materials for Electrocatalysis-Related
Applications
The ORR/OER reactions are very important for metal-air batteries, and the OER/HER reactions are very important for the efficiency of water electrolysers. People have used perovskite materials in these kinds of devices for a long time, but their low catalytic activity and big particle sizes, low surface areas, and lack of structural traits may make them unsuitable for real-world use. Nanosized and nanostructured perovskites are better for practical use because they have more reaction sites, are easier to get to, and allow gas reactants and products to move through them more easily. For real-world products, though, the cost of an electrocatalyst and how stable it is are important things to think about. This part talks about how to use perovskite nanocatalysts made from elements that are common on Earth in metal-air batteries and water electrolysers.
Metal–Air Batteries
Metal-air batteries are energy-converting devices that might have a higher specific energy than Li-ion batteries that are sold in stores. They have a metal anode, a fluid, and an air cathode. Li and Zn are two anode materials that have been studied a lot. But the ORR/OER processes in these batteries move slowly, which affects how well they use energy generally. A two-in-one electrocatalyst that supports both the ORR and OER is needed to make a metal-air battery that works well and can be charged again and again.
In order for perovskite catalysts to be useful in metal-air batteries, porosity is a key factor. Zhang and his colleagues wrote about using a three-dimensionally ordered macroporous LaFeO3 perovskite (3DOM-LFO) as a cathode catalyst in a Li-air battery that doesn’t contain water. This porous nanostructure makes it easier for O2 to move through the electrolyte during discharge and charge processes. It also provides a lot more surface area for ORR/OER reactions, which leads to better performance and cycle stability of the rechargeable Li-air battery compared to the nanoparticulate LaFeO3 catalyst. One-dimensional (1D) porous nanomaterials have a lot of holes and a high surface-to-volume ratio. This makes it easier for O2 gas to move through them when they are used as cathode catalysts for Li-air batteries.
But the oxygen electrocatalysis in Li-air batteries that aren’t in water is very different from that in water-based solutions. This makes it hard to use modern perovskite catalysts that work in alkaline water in Li-air batteries that aren’t in water. To make better perovskite catalysts for Li-air batteries that don’t use water, more organised research is needed. Zn-air batteries, on the other hand, usually use a watery alkaline solution because the zinc anode is stable and the battery design is cheap to make. So, the ORR/OER on perovskite materials in alkaline media that has been studied in the past is thought to be more useful for use in Zn-air batteries.
In the past few years, different perovskite nanoparticles have been created for use in Zn–air batteries. Perovskite/carbon mixtures are getting more study attention because they carry electricity much better than sole perovskites. For instance, a mixture of LaNiO3 nanorods and reduced graphene oxide that was made using a chemical method had better conductivity than the LaNiO3 nanorod alone. This led to higher ORR/OER activity in alkaline solutions and better cycle performance in rechargeable Zn–air batteries.
According to Prabu et al., perovskite LaTi0.65Fe0.35O3−d nanoparticles were found on the surface of and inside nitrogen-doped carbon nanorods. These nanorods were made by electrospinning them and then heating them up in an Ar atmosphere. This led to more catalytic sites, better electrical conductivity, and highly accelerated rates towards both the ORR and OER, which made the Zn–air battery work much better.
These composites are made on a small scale, though, using hydrothermal or electrospinning methods that are done on a bench. This may make it hard for them to be widely used. CVD has recently become a flexible way to make large amounts of nanohybrids of perovskites and carbons, especially carbon nanotubes (CNTs). Transition metal-containing perovskites are used as supports for the growth of CNTs quickly.
Chen and his colleagues wrote about using CVD to make a core-corona structured bifunctional catalyst (CCBC) and using it as a cathode catalyst in a refillable Zn–air battery. The CCBC-2 catalyst had great discharge and charge performance, about the same as standard catalysts in the ORR (Pt/C) and OER (Ir/LaNiO3).
But it’s not as clear what causes the triggers to be so active and stable physically. As a reducing agent, carbon may break down the crystalline structure of perovskite oxides. This means that more study needs to be done on how to make perovskites that are still whole but have conductive carbons mixed in with them.
To sum up, understanding the connection between structure, shape, and function in real-world situations is very important for creating useful electrocatalysts for batteries.
Water Electrolyzers
Water electrolysers are important parts of the “indirect” sun to hydrogen production process because they make it possible to make high-purity hydrogen. The equation for water electrolysis is H2O! = H2 + 1/2O2. This means that water is broken down into hydrogen and oxygen. At room temperature and pressure, this process needs 286 kJ mol−1, which is equal to a possible minimum cell voltage of 1.23 V. But the OER at the anode and the HER at the cathode, which are the two half-reactions of water breaking, move slowly and can waste a lot of energy.
To make water electrolysis technology use less energy, it would be nice to create perovskite materials that are options and contain transition metals that are common on Earth and don’t cost much. These materials would still have a high catalytic activity. Strong alkaline electrolytes are used in industrial electrolysers to keep them from corroding in acids and to keep costs down. Perovskite oxides have been shown to speed up the HER reaction in basic solutions. This has led to a lot of research on using perovskites as two-in-one catalysts for both the HER and OER reactions, as well as for splitting water in general.
One example is the electrospinning creation of perovskite SrNb0.1Co0.7Fe0.2O3–d nanorods (SNCF-NR) and their use as HER/OER catalysts in an alkaline cell that splits water. HER and OER activity were much higher in the porous 1D nanostructure of SNCF-NR compared to the bulk-sized SNCF. When placed on a Ni foam base, the SNCF-NR//SNCF-NR pair provided a voltage of 1.68 V and a geometric current density of 10 mA cm−2. This is a good result for solar-to-hydrogen production.
It is possible to improve the electrolysis of alkaline water by connecting perovskite materials with nanocarbons. One example is electropunct La0.5(Ba0.4Sr0.4Ca0.2)0.5Co0.8Fe0.2O3–d perovskite nanorods to reduced graphene oxide nanosheets. It is possible to get even better performance by adding reacting sites besides perovskites, like Co/CoOx nanoparticles.
However, the perovskites that are currently on the market only have modest catalytic activity towards the HER. This means that the alkaline water electrolysis will always have an overpotential of about 0.2 V. One way to get around this problem is to use the cutting edge Pt metal instead of the perovskite-based cathode. Magnetron sputtering was used by Chen et al. to put amorphous BSCF nanofilms on Ni foam surfaces to make very active OER catalysts. For these electrodes to work so well, they had a catalyst mass loading that was almost 20 times smaller than that of SNCF-NR.
Researchers have tested perovskite catalysts in a basic room-temperature electrolysis cell in the lab, but they haven’t looked at how the water electrolysers are set up. Fabbri et al. recently described a more sensible method. They used an alkaline membrane water electrolysis cell to compare how well BSCF-FS worked as an anode OER catalyst to the most advanced IrO2. We put together membrane electrode assemblies (MEAs) with Pt as the cathode and BSCF-FS or industrial IrO2 as the anode. The catalyst loading was 3 mg cm−2. For example, when the temperature was 50 °C and the current density was 200 mA cm∹, the BSCFFS-based MEA had a lower and more stable cell voltage. These were conditions that are more common in industrial settings. This test provides stronger proof that perovskite BSCF-FS nanocatalyst can be used instead of IrO2 as a cheap option for effective water electrolysis.
An important step forward has been made in using perovskite materials as electrocatalysts for water electrolysers, but more work needs to be done, especially with the HER. The fact that perovskites can be used for both HER and OER may seem like a huge task, since the reactions that happen on their surfaces are very different. Perovskite HER enzymes, on the other hand, are still in their early stages of research. It might be easier to find highly active perovskite HER catalysts if we have a better understanding of how reactions work on perovskite surfaces and better ways to build catalysts.
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