Source: https://pubs.rsc.org/en/content/articlehtml/2018/ee/c8ee02495b
Timestamp: 2019-04-24 08:19:42+00:00

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The oxygen evolution and reduction reactions are two extremely important reactions in terms of energy applications. Currently, the Oxygen Evolution Reaction (OER) hinders the efficient running of electrolyzer devices which convert water into molecular H2. This H2 can subsequently be used in a H2/O2 fuel cell for the renewable generation of electricity with only H2O as a by-product. However, this fuel cell process is not economy feasible due to the sluggish kinetics of the Oxygen Reduction Reaction (ORR) at the device cathode, even with expensive state-of-the-art electrocatalytic materials. As of late, the amount of interest in the OER and ORR, from research laboratories from all over the globe, has risen rapidly in order to find cheap and efficient catalysts to replace the expensive platinum based catalysts currently used in the two aforementioned energy conversion/generation technologies. Layered transition metal oxides, based on the cheap transition metal oxides Mn, Co, Ni and Fe have been reported as viable catalysts for the OER and ORR. Layered structures have an added advantage over non-layered materials as the surface area can be increase by means of exfoliation, with potential for tailoring electrocatalytic activity. It has been shown that the fabrication process and post-synthetic treatments, e.g. anion exchange or exfoliation, of these materials can alter the catalytic activity of these materials. Here we summarise various fabrication methods and modifications utilised in literature to tailor the performance of layered transition metal and hydroxide based catalysts for the ORR and OER toward that of the state-of-the-art materials for these technologies.
Michelle Browne is a ChemJets research fellow in the group of Prof. Martin Pumera at the University of Chemistry and Technology Prague in the Czech Republic. Prior to this, she was a research fellow on an UK Catalysis Hub Project based in Queens University Belfast in the United Kingdom. In 2017, Michelle received her PhD from Trinity College Dublin, Ireland, in electrochemistry and material science under the supervision of Prof. Mike Lyons and Prof. Paula Colavita.
Zdeněk Sofer is an Associate Professor at the University of Chemistry and Technology Prague since 2013. He received his PhD also at University of Chemistry and Technology Prague, Czech Republic, in 2008. During his PhD he spent one year in Forschungszentrum Julich (Peter Grünberg Institute, Germany) and also one postdoctoral stay at University Duisburg-Essen, Germany. Research interests of Prof. Sofer concerning on nanomaterials graphene based materials and other 2D materials, its chemical modifications and electrochemistry. He is an associated editor of FlatChem journal. He has published over 310 articles, which received over 6000 citations (h-index of 39).
Martin Pumera is currently the Director of the Center for Advanced Functional Nanorobots at the University of Chemistry and Technology, Prague, Czech Republic (since 2017). Before joining UCT, he was a tenured faculty member at Nanyang Technological University, Singapore (since 2010). He received his PhD at Charles University, Czech Republic, in 2001. After two postdoctoral stays (in the United States and Spain), he joined the National Institute for Materials Science, Japan, in 2006 for a tenure-track arrangement and stayed there until Spring 2008, when he accepted a tenured position at NIMS. In 2010, he joined NTU Martin has broad interests in nanomaterials and microsystems, in the specific areas of electrochemistry and synthetic chemistry of 2D nanomaterials, micro and nanomachines, and 3D printing.
Research into finding economical and sustainable energy alternatives to the world's ever dwindling fossil fuel reserves has increased significantly in the last decade. One renewable energy generation/distribution route called the hydrogen economy concept is receiving significant attention from research groups. The hydrogen economy concept is one idea which utilises H2 gas as the main energy source for the efficient running of buildings, homes and vehicles. Unfortunately, due too many inefficiencies associated with the energy conversion device (electrolyser), needed to make the H2, and the fuel cell device, utilised to convert the H2 into electricity, this idea remains a concept. Many research laboratories all over the world are trying to fabricate cheap and active catalysts to improve the activity while lowering the cost of the materials needed in these two devices. Finding a cheap and active catalyst which rivals that of the state-of-the-art materials for these devices would make the hydrogen economy concept closer to a reality. Layered materials, compared to their bulk counterpart, have shown improved activity as catalysts for electrolyser and fuel cell technologies. Herein, the effect of the fabrication and post-fabrication methods on the catalytic activity used to make these layered materials for electrolysers and fuels are discussed.
Fig. 1 Schematic of a simple hydrogen economy concept.
The overall cell potential needed to drive water electrolysis is dependent on the thermodynamic potential of water electrolysis, the overpotentials associated with the reaction on the anode/cathode and the cells ohmic drop.
Fig. 2 Schematic of (a) water splitting and (b) fuel cell device.
Regardless of the class of the layered material, the 2D form of these layered TMOs often possess enhanced electrochemical properties compared to their bulk counterpart.70 The fabrication or synthesis of these layered catalysts spans a multitude of various routes including solvothermal,71 hydrothermal,72 microwave-assisted,73 thermal decomposition,74 low temperature synthesis,75 and precipitation methods.70 Then, other treatment processes including anion exchange and exfoliating processes, including chemical, mechanical and physical, are undertaken on these layered materials to enhance its electronic and chemical properties.76 A table of common layered TMO based materials, the synthetic route to produce the layered materials along with their exfoliated counterpart or anion exchange procedure, if applicable, found in literature that are utilised as catalysts in electrochemical energy applications are illustrated in Table 1.
Exfoliation: in IPA using sonic bath and centrifugation.
Sulfurisation process: MnO2 mixed with sulphur powder in a alumina crucible and calcined in a tube furnace for 12 hours under N2 flow. The temp was then raised to 250 °C and heated for an hour under N2 to evaporate the excess S.
Anion exchange: ethanol assisted anion exchange using sodium nitrate (NO3− ions).
Anion exchange: ethanol assisted anion exchange using sodium chloride (Cl− ions).
Exfoliation: Ni/Fe oxide dispersed in formamide and stirred under N2 atmosphere for 2 days. The solution was then centrifuged to remove un-exfoliated material.
Fig. 3 The structural model of molybdenum(VI) oxide (A), vanadium(V) oxide (B) and hydrated tungsten(VI) oxide (C). Yellow lines show elemental cells, blue balls are oxygen atoms, green balls are metals atoms and red ball are hydrogen atoms.
The other group of binary layered oxides with a layered structure are based on transition metal oxide which were synthesised using suitable planar templates, for example TiO2.86,87 However, these materials have amorphous or non-layered crystallographic structure and exhibit highly anisotropic shape (platelet shape with single or few layers atoms thickness). The synthesis is typically based on self-assembling methods or by topochemical reactions (e.g. conversion of layered TiS2 into TiO2 sheets).86,87 The flexibility and variability of these methods allows for the synthesis of the most basic binary oxides.
The mixed transition metal is formed with an alkali metal adopting layered structure. The most typical examples are LixCoO2 or KxMnO2 where the structure is based on hexagonally coordinated metal ions with oxygen and alkali metal atom layers. Due to the possibilities in the variation of alkali metal content these materials are highly popular for battery applications.88 The LixCoO2 structure is shown on Fig. 4(A). However, these materials don’t adopt true van der Waals layered structure therefore they can be exfoliated by chemical methods based on substitution/removal of alkali ions intercalated between the metal-oxide layers.89 The family of this type of layered oxides is very rich and from the ternary based systems KxMnO2, also called δ-MnO2 oxide or birnessite according its natural counterpart, is well known, Fig. 4(B). The other layered oxides in this family of materials can be based on iron, chromium, titanium and other transition metals.90 Transition metals form also several complex layered oxidic phases including titanates, tantalates, niobates, tungstates and its various mixed counterparts. Similarly, to birnessite type oxides, their exfoliation is based on the removal or substitution of alkali metals within layered structure. Their substitution with bulky ions, such as tetrabutylammonium ions, treatment with acids (hydrated protonic form) and subsequent reaction with tetrabutylammonium hydroxide significantly increase interlayer spacing and allows for their mechanical exfoliation to take place. These complex oxides exhibit several interesting physical properties including thermoelectric effects, ferroelectric and multiferroic properties, but to date their exploration as electrocatalysis for the OER and ORR has been very limited.
Fig. 4 The structural models of layered oxides containing alkali metal and transition metal. The structure of LiCoO2 (A) and (B) mineral birnessite (K0.48Mn1.97O5.18) based on layers of alkali metals and oxygen octahedrally coordinated transition metal layer.
Fig. 5 (A) The structures derived from layered oxo-hydroxide MO(OH); (B) the structure of double layer hydroxide (hydrotalcite with composition Al2Mg4(OH)12(CO3)·3H2O). (C) The structural model of layered hydroxide with brucite structural type of general formula M(OH)2; metal atoms are green, oxygen atoms blue, hydrogen atoms red and carbon atom black. Grey atoms in LDH.
Based on the layered hydroxide structure, there also exist a broad group of layered oxide-hydroxide of general formula MO(OH) which are generally formed by topochemical oxidation of layered hydroxides (e.g. lepidocrocite γ-FeO(OH)). The structure is shown on Fig. 5(A). The oxo-hydroxides are significantly more stable in comparison with metal hydroxides and can withstand relatively broad pH range, especially in alkaline environment.
An important group based on the brucite structure for the OER and ORR are the layered double hydroxides (LDH). The LDH structure consists of M2+ ions substituted with M3+ ions and in-between layers charge compensation anions, e.g. hydroxides, carbonates etc., are present. The structure of these materials are extremely varied; multiple possibilities for the anion and cation sites exist as well as the anion introduction into the interlayer space between hydroxide structures. The LDH structure can be observed in Fig. 5(B). The presence of multivalence ions results in a positive charge for the hydroxide layers compensated by lightly bonded anions. The general formula of LDH can be written as M1−x2+Mx3+(OH)2·Ax/nn−·mH2O where M2+ = Mg2+, Fe2+, Co2+, Ni2+, Zn2+ and others and M3+ = Al3+, Fe3+, Cr3+ and others. The synthesis is typically based on reaction of metallic ions at elevated pH (formed e.g. by thermal decomposition of urea under hydrothermal conditions). The presence of organic long chain anions like dodecylsulfate or lactate can form LDH with controlled and extremely high interlayer spacing. Such LDH with large interlayer spacing can be easily exfoliated by mechanical exfoliation like share force milling or ultra-sonication. Since the LDH can slowly dissolve at high pH (e.g. containing Al3+) or at lower pH, neutral solvents like water, alcohols or hydrocarbons are used. The presence of catalytically active transition metals, e.g. Mn, Fe, Ni and Co, in the LDH structure have made these materials an attractive option to study as OER and ORR catalysts throughout literature.
In recent years the amount of literature reported on the OER and ORR using layered TMOs has steadily increased however the impact may be over shadowed by the more popular Transition Metal Dichalcogenides (TMDs) for the HER. This review highlights and discusses recent research in the area of OER and ORR which utilise layered and 2D TMO materials as catalysts. With particular emphasis on how the fabrication route and additives/enhancement aids e.g. carbon nanotube as support for increased conductivity, can affect the performance of these important O2 reactions that are central to the implementation of a future hydrogen economy.
It is widely accepted that variations in the synthetic route and/or electrode fabrication technique can greatly affect the performance and stability of catalysts for electrochemical energy processes.93,94 Therefore, and more specifically, investigating appropriate pathways to produce and fixate layered TMO catalysts on suitable current collectors for the optimisation of the OER is a major area of research that is currently on-going.95 Moreover, further processing techniques and/or conductive aids have been reported to further enhance ‘bare’ layered TMOs for the OER.76 Multiple studies on layered TMO catalysts will be compared and discussed herein to evaluate and understand the optimum fabrication parameters in respect to their OER performance.
In a subsequent report by the same group, the exfoliated Co(OH)2 was deposited onto two different types of high area supports; glassy carbon (GC) foam and nickel (Ni) foam.79 The results reveal the exfoliated Co(OH)2 on the Ni foam was a superior electrode when compared to the same material on the GC foam in terms of absolute OER overpotetnial i.e. the overpotential at 10 mA cm−2. This value was 280 and 380 mV for the Co(OH)2 on the Ni and GC foams, respectively, in 1 M NaOH. However, when comparing the activity of the Co(OH)2 catalysts/substrate combination to the bare support in relative terms the Co(OH)2 on the GC support was the more appropriate relationship. The Co(OH)2 on the GC had an improved OER performance by 57% compared to the bare GC, while the Co(OH)2/Ni achieved a 30% increase over the bare Ni support.79 Interestingly, the Co(OH)2 on the Ni foam behaved as a better OER electrocatalyst when compared to the exfoliated Ni(OH)2 on the same Ni foam support previously discussed in this review by the same group.78 However, regardless of the actual OER performance values, it is clear that the nature of the support plays a role in the activity of layered TMO material.
2D δ-MnO2 on Ni foam produced by an in situ hydrothermal synthesis using KMnO4 and H2O was investigated for its OER characteristics in 0.1 M KOH, Fig. 9.104 The OER measurements of the 2D δ-MnO2 and bulk MnO2 on Ni foam (not in figure) showed that the 2D δ-MnO2 was a much superior catalyst; the overpotential at 10 mA cm−2 for the 2D MnO2 was 0.32 V for the exfoliated material while the bulk MnO2 did not reach this current density at all to allow for the authors to report an overpotential. Interestingly, the 2D δ-MnO2 material even proved to be better than the OER state of the art OER catalysts, IrO2, in regards to the overpotetnial at 10 mA cm−2 and Tafel slope values, see Fig. 9(A) and (B). The enhanced performance of the 2D δ-MnO2 was rationalised to be due to the larger electrochemical surface area determined for the 2D material over the bulk; 19.58 and 0.80 mF cm−2 respectively, which could be rationalised due to the larger surface area of the 2D nanosheets. Another reason for the increase in the OER activity could be due to the lower oxidation state (Mn3+), revealed by the XANES measurement, that is only observed in the 2D structure of MnO2 and gives rise to a half-metallic state not observed in bulk MnO2.
The majority of layered TMO materials which undergo anion exchange consist of more than one metal therefore this section of the review will focus on bi- and tri-metal TMOs. Recently, in a research setting, NiFe based hydroxides/oxides have emerged as a cheap alternative for the OER in alkaline media compared to PGM based oxides i.e. RuO2 and IrO2. These materials also exhibit a layered structure, with some research groups having exploited this by changing the interlayer spacings of the NiFe by anion exchange.
Another recent study on the OER properties of an anodic borate doped Ni(OH)2 has also indicated that the presence of borate enhances the proton accepting properties during the OER compared to a non-doped Ni(OH)2 catalyst, Fig. 11(A and B).112 The overpotential of the borate doped Ni catalyst is increased at 10 mA cm−2 compared to the non-doped Ni material. However, as the Tafel slope values are similar the mechanism in which the OER proceeds is assumed to be the same; the rate determining step involves the formation of adsorbed peroxide intermediates (–OOH). Therefore, the enhancement in the OER was attributed to the reversible transformation of the BO33− to a BO33-OH. The presence of the four coordinated borate was confirmed by NMR spectroscopy. This BO33-OH can accept a proton from the Ni–O–OH2 intermediate and subsequently release H2O, and an electron, leaving a Ni-OOH site.112 This step may require less energy to proceed compared to the equivalent step during the O2 generation of the bare Ni(OH)2, Fig. 11(C), resulting in the enhanced activity, Fig. 11(D).
By comparing recent studies, it has been further shown that the absolute OER overpotentials of NiFe LDH based catalysts fluctuate depending on the ions utilised during the anion exchange process.113 Similarly to the previous study on intercalated NiFe with BO33− ions, the OER performance of a NiFe LDH intercalated with Mo ions during anion exchange, also exhibited a more cathodic behavior favoring the OER.111 For this particular study, the OER optimisation as a result of the anion exchange process was clear as the overpotential at 10 mA cm−2 for the NiFe Mo treated LDH increased by 35 mV compared to the untreated NiFe LDH. The authors attributed this observation to an increase in the electrochemical active sites as a result of the ultrathin thickness of the anion exchanged NiFe LDH.
More interestingly when comparing the two aforementioned studies, the choice of ion for the anion exchange process is evidently important in terms of the resulting electrochemistry. The two previous works use different layer intercalating anions, e.g. Mo or BO33− ions, in the anion exchange process but a similar hydrothermal synthetic route to make the NiFe LDHs. Therefore it can be noted that the OER activity changes depending on the ion utilised. The OER overpotentials for the BO33− ion intercalated NiFe LDH exhibited a larger increase when compared to the Mo ions; a difference of 50 mV was observed for the borate intercalated material and only 35 mV for the Mo intercalated material when compared to their relevant untreated counterpart. Perhaps indicting the BO33− ions may be a more suitable choice of anion over Mo ions for the anion exchange process for NiFe based materials.
Furthermore, the OER activity of the NiFe LDH can be readily tuned by adopting different phosphorus based anions; phosphate, phosphite and hypophosphite, during the anion exchange process of an carbonate intercalated NiFe LDHs.114 The OER performance of the NiFe LDH reveals that the choice of intercalation anion has an effect on the electrocatalytic performance, Fig. 12(a–c). From the LSV curves, Fig. 12(a), it is evident that the phosphorous based anions significantly improves the OER when compared to the carbonate intercalated NiFe LDH, however the Tafel slope was not affected; indicating that O2 evolution proceeds by the same mechanism for all of the NiFe LDH, Fig. 12(b). Interestingly, the electrochemical surface area (ECSA), Fig. 12(c), exhibited the same trend to the overpotential values at 10 mA cm−2i.e. the hypophosphite NiFe showed the best OER overpotentials and the highest ECSA, followed by the phosphite NiFe, then the phosphate NiFe and, finally, the carbonate NiFe.
The rationale for the trend observed for the NiFe LDH based phosphorus catalysts originates from a previous observation that the OER activity of NiFe based catalysts is strongly influenced by the Ni sites.115 From XPS analysis, Fig. 12(d and e), it is clear the lowest Ni valence states are present for the NiFe/hypophosphite and can be correlated to this anion (H2PO2−) possessing the strongest reducibility compared to the other anions. Hence, more electron rich Ni sites would be available for oxidation during the OER i.e. more active sites.
Additionally, the H2PO2− NiFe possess the largest ECSA/double layer capacitance value of 3.8 μF cm−2 when compared to 3.2 and 3.6 μF cm−2 for the PO43− and the HPO32− NiFe based LDH, respectively. This is an interesting example of when the OER activity of a material (NiFe LDH) can be tuned for the OER by using various anions in the synthetic process.
In the literature, it has been shown that a combination of both anion exchange and liquid phase exfoliation can improve the oxygen evolution properties of various bi-metallic LDH materials such as CoCo, NiCo and NiFe LDHs when compared to its bulk LDH and to the state-of-the-art IrO2 for OER.67 Unlike other studies previously mentioned in this review, this work utilised anion exchange as a pre-conditioning step before the exfoliation of the 2D materials by liquid phase exfoliation, rather than the main treatment step. The anion exchange process facilitates the delamination of the 2D structures during the exfoliation process as the interlayer spacings were increased prior to exfoliation.
The anion exchange process was carried on the topochemical fabricated CoCo and NiCo LDH by exchanging the Br− ions with NO3− ions and for the hydrothermally produced NiFe by switching the CO3− ions with ClO4− ions which was successful tracked by XRD Fig. 13(a) and imaged by TEM-EDX, Fig. 13(b–e). The interlayer spacing of the CoCo and NiCo increase from 7.8 Å to 8.7 Å, while the initial interlayer spacing of the NiFe LDH increased from 7.7 Å to 9.1 Å. Subsequently, the chemical composition and morphology of the initial LDHs were maintained. Liquid phase exfoliation was subsequently carried out on the LDH to form 2D structures and was confirmed by a combination of using the Tyndall effect to prove the colloidal nature of the exfoliated suspension, the determination of the 2D layers by TEM analysis, and the absence of the diffraction peaks in the XRD analysis, Fig. 13(a), when compared to the bulk LDHs.
The OER performance of the bulk and exfoliated LDHs were determined in 1 M KOH and subjected to the same experimental conditions and can be seen in Fig. 13(f). The results show that all of the exfoliated LDH was significantly enhanced when compared to their bulk counterpart in respect to the measured overpotetnial at a current density of 10 mA cm−2. The NiCo and NiFe also out-performed IrO2; a state-of-the-art OER catalyst.
The authors attribute the dramatic difference in activity to a greater exposure of the MO6 sites after exfoliation, see Fig. 13. It is evident, that these MO6 sites are the active site for OER in these LDH materials therefore a greater number of these sites will be readily accessible during water oxidation when compared to the bulk LDH, where a portion of these MO6 sites will be blocked by the charge neutralising anion between the 2D layers (Fig. 14). Furthermore, it is quite clear that this increase in activity is due to the rise in active site density and not due to a change in the ECSA as the increase in the ECSA 2D materials is not sufficient to explain the improvement seen in the OER performance.
Finding the true reason for the enhanced catalytic effect of LDH materials when Fe3+ is utilised as the trivalent ion could lead to better and more active OER catalysts as the fabrication of LDH materials could be specifically designed to mimic these findings.
This study and all of the previously highlighted reports in this review, summarised in Table 2, show that the fabrication of LDH from the choice of trivalent ions to the treatments (anion exchange or exfoliation to 2D phase) applied after synthesis are critical to the performance of these materials as OER catalysts. It is evident that aforementioned treatments greatly improve the OER properties of the ‘bare’ LDH and more studies into these fabrication methods will undoubtedly see a further increase over the next number of years. These advances could provide an avenue to discover a cheap and active catalyst based on an LDH materials for electrolysis. This would greatly benefit the current energy crisis in finding an alternative energy conversion catalyst as an alternative to fossil fuels.
The electrically and mechanical properties of the catalyst was enhanced by adding CNT which increased the OER activity of the Co(OH)2 NS materials.
Recently, various reports on the ORR and the use of layered transition metal oxide catalysts have emerged. Similar to the OER, post-fabrication treatment of these layered TMO based materials can also show enhancements toward the ORR, see Table 3. For example MnO2 layered nanosheet materials exhibited improved ORR activity after a so called ‘sulfurisation process’. This MnO2 based nanosheet was prepared by adding KMnO4− into a solution of graphene oxide and water at 80 °C and mixed for 24 hours.74 The product was filtered, washed and dried, then subjected to the ‘sulfurisation process’. This entailed mixing sulfur powder with the layered MnO2 nanosheet product in a crucible and exposing the powder mixture to 155 °C to introduce nanosize pores into the MnO2 nanosheets, see Table 1 for more details and Fig. 16(a) for fabrication schematic.
The ORR performance, Fig. 16(b), reveals that the post-fabrication sulfurisation process improved its catalytic activity; E1/2vs. RHE by 40 mV. The E1/2vs. RHE values for the pre-treated and nonporous MnO2 sheets were 0.69 and 0.73 V, respectively. Interestingly, the nanoporous MnO2 nanosheets is only a mere 100 mV less than Pt/C, the optimum ORR catalysts currently used in fuel cells. The improvement in the ORR activity was related, by the authors, to the evidence observed by the XPS analysis which indicated that more oxygen vacancies where present for the post treated MnO2, Fig. 16(c). The oxygen vacancies may help to facilitate oxygen absorption and reduce kinetic barriers by exposing the Mn sites which was induced by the post synthetic sulfurisation step. This report illustrated that a simple post-fabrication step can help enhance the ORR performance of a mono-metallic LDH and can be easily adapted in future studies by others.
We have already shown previously in this review that NiFe LDHs are researched extensively for the OER however, NiFe LDH based materials have also been explored in the literature as a catalyst for the ORR.120 Herein, we will show how simple modification of these NiFe LDH can change the ORR performance of these popular LDH. The addition of both graphene and rGO during the fabrication method can alter the ORR response of NiFe LDH materials. In one particular study, ‘bare’ NiFe was produced by a one-pot solvothermal synthesis and the graphene oxide was obtained by using the well-known Hummer method.121,122 Subsequently, to fabricate the NiFe/rGO, the graphene oxide was also added to the Teflon reactor with the Ni and Fe metal salts for solvothermal synthesis.
After which, the product was subjected to hydrazine hydrate and ammonia for 1 hour at 90 °C in order to allow for the graphene oxide to be reduced. The NiFe/GO catalysts was prepared in the same manner but the reduction step was omitted.120 The authors examined the ORR properties of these three NiFe based materials in 1 M KOH and by using a high surface area Ni foam support. The ORR activity increase with the addition of the graphene oxide to the NiFe which further increases when rGO is substituted for the graphene oxide. The improved activity of the NiFe/rGO was a result of more exposed active ORR sites which arose from the strong interactions between the NiFe LDH and the rGO observed from XRD analysis.
Another interesting fabrication concept leading to an enhancement in ORR regarding NiFe based LDHs involves the anchoring of NiFe LDH onto N-doped graphene-like 3D macro–meso-porous carbon (denoted as nNiFe LDH/3D MPC).123 The fabrication technique consisted of two steps; the first step was the anchoring of the metal salt precursors onto the 3D MPC platform then growth of the NiFe LDH by a co-precipitation method, see Fig. 17(a). The authors proposed that the carbon based platform would promote the activity of the NiFe towards ORR by manipulating defect sites which would increase the catalytic activity of the overall material.
The ORR performance of the nNiFe LDH/3D MPC was also compared to the 3D MPC platform and a NiFe LDH catalyst with the 3D MPC added after synthesis. The ORR performance of the nNiFe LDH/3D MPC was indeed better than the bNiFe LDH + 3D MPC, again indicating that the route taken to yield LDH catalysts for the ORR have a significant influence upon it's catalytic properties, Fig. 17(b). Unfortunately, and also stated by the authors, the LDH based materials under-performed when compared to the 3D MPC platform. Perhaps with further aids, such as CNT or other conductive aids, this NiFe based LDH could achieve better ORR potentials.
The use of ternary metal LDH catalysts for the ORR have been explored. Two studies based on different ternary LDH catalysts are reviewed herein to highlight the effect of the fabrication techniques on these ternary LDH materials toward the ORR.124,125 In the first study, the effect of varying the molar ratio of two of the three metals present in a CoNiFe material on the ORR was investigated. In this study, three CoNiFe LDHs was synthesis by a two-step method involving a co-precipitation process and then a thermal annealing process. During the fabrication procedure the molar ratio of the Co2+ : Ni2+ (Ni2+ molar percentages (sample name): 33% (M1), 27% (M2) and 47% (M3)) was varied and the molar percentage of the Fe3+ remained constant and as a result a reverse spinel structure for all of the CoNiFe LDH catalysts were produced. The ORR activity of the three CoNiFe LDH materials were probed on a GC disk in 0.5 M KOH. The ORR evaluation revealed a trend with respect to the percentage of Ni2+ in the LDH; the larger percentage of Ni2+ yielded the best performing catalysts while the smallest amount of Ni2+ (or the largest amount of Co2+) produced the worst ORR catalyst. Hence the trend observed toward the ORR performance of the catalysts, from best to worst, is M3 < M1 < M2. XPS analysis provided a rational explanation for this outcome; the authors correlated the higher amount of NiO and octahedral Co3+ sites in the spinal structure of the M3 material to be the cause of the increased ORR activity i.e. the active ORR site. The authors suggested that this result indicates more oxygen vacancies are present on the surface of the material for ORR to proceed when compared to the M2 and M1 materials as these materials contained less NiO/Co3+ sites. This study suggests that approx. a 50 : 50 ratio of Ni : Co is optimum in the ternary CoNiFe LDH is optimum for the ORR.
In the second study, a similar co-precipitation fabrication process to produce the ternary metal LDH materials was utilised compared to the aforementioned study however this time a CoNiMn LDH based catalysts was fabricated. Additionally this work set out to investigate the effect induced by reduced graphene oxide and poly-pyrrole on the CoNiMn LDH towards ORR.125 The reduced graphene oxide (rGO) component of the composite LDH material was prepared by the Hummers method while the poly-pyrrole was fabricated by polymerising pyrrole. Subsequently, the Co, Ni and Mn metal salts were then added to the rGO in deionised water and finally the poly-pyrrole (PPY). To produce the composite LDH the three components were added together with NaOH to induce a co-precipitation reaction, the composite was then aged for 24 hours under stirring, Fig. 18(a). Additionally, for comparison a PPy/rGO, a CoNiMn–rGO and a CoNiMn-LDH were also fabricated by this method; with the missing component in the materials name omitted from the fabrication route.
The ORR performance of the CoNiMn–PPy/rGO, PPy/rGO, and CoNiMn–rGO was evaluated alongside state of the art ORR materials, Fig. 18(b). The CoNiMn–rGO mixed with the PPy/rGO post fabrication was also evaluated to determine if the presence of the PPy/rGO was enough to effect the ORR activity of the LDH or if the PPy/rGO needed to be fabricated directly with the LDH. The results showed that the CoNiMn–PPy/rGO LDH composite made directly during synthesis out-performed all of the aforementioned materials with an E1/2 value vs. RHE of ∼0.78 V in 0.1 M KOH. Additionally, the CoNiMn–PPy/rGO LDH composite proved to be a better ORR catalyst than the more expensive and PGM based RuO2/C however, yet again, the state-of-the-art ORR material, Pt/C, was the optimum catalyst. Interesting, the CoNiMn–rGO mixed with the PPy/rGO post fabrication exhibits a huge decrease in its E1/2 value. This study indicates that the fabrication of the composite in situ or post fabrication changes the properties associated with the ORR and that the synthesis of the CoNiMn–PPy/rGO LDH composite in a one-pot synthesis creates synergistic effects toward the ORR which are not observed for the CoNiMn–rGO mixed with the PPy/rGO.
There has been a significant increase in the utilisation of LDHs as electrocatalysts for O2 electrode reactions; the OER and ORR, in alkaline electrolyte in the recent years. It is clear from this review that even small quantities of simple additives, i.e. rGO or CNTs, during the synthetic process, the use of high surface area supports and/or exfoliation of these layered materials changes the resulting catalytic properties. As observed from the various articles reviewed here, these modifications during the synthesis of the LDHs/LDH-based composites can propel the activity exhibited close to the expensive and scarce state-of-the-art catalysts e.g. RuO2 for OER and Pt for ORR. Further research into the optimisation and/or modification of these interesting materials may open avenues to lead the scientific community in finding a promising cheap and highly active materials for the two aforementioned O2 reactions; a vital step towards the efficient running of a future hydrogen economy.
This work emanated from financial support from the Advanced Functional Nanorobots project (reg. no. CZ.02.1.01/0.0/0.0/15_003/0000444 financed by the EFRR). M. P. B. would also like to acknowledge the European Structural and Investment Funds, OP RDE-funded project ‘ChemJets’ (No. CZ.02.2.69/0.0/0.0/16_027/0008351). Z. S. was supported by Czech Science Foundation (GACR No. 17-11456S) and by the financial support of the Neuron Foundation for science support.
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