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The d-Band Theory for the ORR
Investigations on Extended Surfaces
As introduced in the previous section, Pt is the most active and stable electrocatalysts at a PEMFC cathode. Figure 2.1 shows that metals having either stronger or weaker binding of oxygen than Pt poorly catalyse the ORR. In fact, this can be explained according to the Sabatier principle: if the interaction between the catalyst and the ORR intermediates (adsorbed O, OH and OOH species) is too weak (for example Au, Ag), no reaction takes place (right part of the volcano plot, the ORR kinetics is limited by the rate of electron and proton transfer). On the other hand, if the interaction is too strong (Fe, Ni), the catalyst surface gets blocked by adsorbed ORR intermediates (left part of the volcano, ORR rate is limited by oxide and anion removal). If it was established since the late 1980s-1990s, 57 that Pt alloyed with 3d transition metals (Co, Ni or Cu) better electrocatalyse the ORR, the DFT calculations from Nørskov et al 45.provided the reason why Pt better performs than other metals, and how its electrocatalytic activity can be improved (establish a delicate balance between OOH adsorption (step 1, Equation (1.22) and OH desorption (step 4 Equation (1.24)). Because the binding energies of OOH and OH species are proportional to each other, decreasing the binding energy of OH species (i.e. lowering ∆G4) also results in decreased binding of OOH intermediates, and renders step 1 the limiting step: this is known as scaling relations and means that the two limiting steps of the Volcano plots cannot be optimized separately. Because of these relations, the optimal binding energy of oxygenated species is when ∆G1 = ∆G4. This optimal theoretical point may be reached for a surface binding oxygen about 0.2 eV weaker than Pt(111), and paves the way for ORR electrocatalysis optimization.
Following the work from Hammer et al. from 1995 58 on the key parameters determining the reactivity of metal and alloy surfaces, Stamenkovic et al. investigated both experimentally and theoretically (DFT) the ORR activity of Pt3M alloys with (111) crystallographic orientation (where M= Ni, Co, Fe, and Ti 59). Alloying Pt with a transition metal is known to modify the surface reactivity according to the following effects:
Alloying Pt with smaller atomic size elements results in a contraction of the Pt lattice and a downshift of the Pt d-band centre. As the d-band centre of Pt surface atoms and the chemisorption energy of oxygen are correlated 59, 0enhanced ORR kinetics are observed on PtxM alloys (0 < x < 1) relative to pure Pt. The degree of contraction of the PtxM lattice can be controlled by the nature of the alloying element, the chemical composition of the alloy, the degree of alloying, but also by post-treatments 60,61.
The ligand effect is due to the change in the electronic structure of the catalytic sites by the neighbouring M atoms. The strength of this interaction depends on the electro-negativity of the alloying element (the more electronegative, the more the Pt d-band vacancies increase due to a better affinity of electrons with the alloying element) 62.
In practice, because transition metals are easily leached out in acidic medium, Pt3M single crystal surfaces from Ref. 59 consist of a pure Pt overlayer, with a M-enriched second layer on the top of the bulk Pt3M composition (‘Pt skin’ structure). Both the computed and physical Pt3M surfaces resulted in a wide range of d-band centres and ORR activities. Surprisingly, while the Pt3Ni alloy was predicted to feature the highest ORR activity according to DFT calculations, Pt3Co was best performing for the ORR. However, one year later, Stamenkovic et al. reported that a Pt3Ni(111)-skin surface performed 10 times and 90 times better compared to Pt(111) and a commercial Pt/C catalyst, respectively, in agreement with their previous theoretical work 59,63. Although this surface has become the model surface for the ORR, the authors also reported the influence of the low-index surface orientation on the structure-sensitive adsorption of OH, establishing a hierarchy for Pt(hkl) (activities increasing in the order Pt(100) << Pt(111) < Pt(110) ) or Pt3Ni(hkl)-skin (Pt3Ni(100)-skin < Pt3Ni(110)-skin << Pt3Ni(111)-skin) surfaces. Such effect is referred to as ‘structural effect’ or ‘ensemble effect’ in the literature, and interacts synergistically with the previously introduced ‘strain’ and ‘ligand’ effects.
It worth mentioning that late transition metals are not the only candidates for bimetallic PtM ORR catalyst. Using a computational screening, Greeley et al. identified various promising bimetallic alloys based on Pt and early transition metals 64. In the screening process, the main parameters were the difference in O binding energy compared to Pt(111) (to reach higher ORR activity) but also the heat of formation of the PtM alloy. Indeed, since late transition metals rapidly interdiffuse in PtM alloys 65–67, more negative alloying energy and higher dissolution potential are believed to result in more stable alloys due to the slower interdiffusion of the M metal, and thus to enhance its durability under harsh electrochemical environment 68. Among the identified alloys, Pt3Sc and Pt3Y featured heat of formation per atom about 0.9-1 eV more negative than the late transition metals such as Pt3Ni, Pt3Co or Pt3Cu, and the electrochemical measurement revealed an excellent specific activity for the ORR on bulk polycrystalline Pt3Y. This surface also performed 6 times better than polycrystalline Pt at 0.9 V vs. RHE. However the prepared extended surface were not designed according to the model used for the screening: instead of a Pt monolayer on the top of the alloy, the experimental surfaces were shown to feature a ~1 nm thick Pt overlayer, which suggested that the lattice strain more than the ligand effect indeed controlled the ORR activity 69–71. More PtxM were also tested, notably derived from Pt5Ln, where Ln is in the lanthanide series (Tb and Gb) and featured 5 to 6-fold enhancement of the ORR kinetics over pure Pt 72.
Figure 2.2: Volcano plot showing the rate of the oxygen reduction reaction on Pt-based alloys as a function of the calculated oxygen adsorption energy relative to Pt. From Ref. 69.
Finally, as represented in Figure 2.2, the intensive work on extended crystal surfaces has allowed the comprehension and further optimisation of Pt-based surfaces, leading to experimentally reach almost the optimal theoretical value in activity for the oxygen reduction reaction. However, in view of practical PEMFC systems, the findings obtained on extended surfaces must be transposed to nanomaterials: this is the aim of the following section.
From Single Crystals to Nanoparticles
Because ORR occurs at the surface (heterogeneous catalysis) of the rare and precious metal that is Pt, a catalyst with the highest possible surface / volume (i.e. mass) ratio is desirable. To take into account the high price of Pt, the efficiency of PGM catalyst is usually expressed as the kinetic current associated to the reaction normalized by the mass of precious metal used:
Where pt is the Pt specific surface area of the catalyst and MA and SA are its mass activity and specific activity for the reaction, respectively.
To maximize MA, both the SPt and the SA must be optimized. Whereas the most active surfaces (best SA values) for the ORR are extended surfaces presented in the previous section (Figure 2.2), a dramatic increase of SPt is observed on nanomaterials. In fact, for a spherical nanoparticle of radius and density , the surface to volume ratio and SPt are inversely proportional to its size according to:
As an example, Pt marbles (1.5 cm diameter) and 3 nm nanoparticles have specific surface areas of ~19 mm² gPt−1 and 93 m² gPt−1 respectively, meaning a ~5 million-fold enhancement of the specific surface area in favour of the nanoparticles.
Consequently, lot of efforts have been made over the past decade to transpose the optimized electrocatalytic properties for the ORR from single crystals to nanocatalysts in order to reach high mass activity values and thus increase the electrical performance of the PEMFC at a reduced cost. However, because of the small length scale in nanoparticles (quantum confinement), the usual energy band structure observed for bulk materials tends to disappear for discrete energy levels 73. These changes in system energy have important consequences on its thermodynamic stability: nanoparticles can for example adopt a crystal structure different from that of the normal bulk materials. This modified electronic structure may also change the chemical reactivity of the particles, as well as their magnetic (supraparamagnetism), optical (plasmon resonance), thermal (diminution of the fusion point) or electrical (superconductivity) properties which all depend on the occupation of the outermost energy levels 74. Additionally, surface atoms (in increased proportion in nanomaterials) feature a reduced number of nearest-neighbour atoms, which lead to differences in bounding and electronic structure, giving a rise in the surface energy and so chemical reactivity. The drawback of these metastable structures is their low stability under variable potential and temperature conditions as exposed in the general introduction. The following sections will focus on the recent advances and challenges toward active and stable nanomaterials for the ORR.
Recent Advances in PEMFC Cathode Electrocatalysts
Shape-Controlled PtM Nanoparticles
The direct application of the conclusions obtained from single crystals studies leads to the idea that octahedral nanoparticles with a Pt3Ni-skin composition would be the optimal catalyst for the ORR. Indeed, in the face centred cubic crystallographic system (which is the case for PtxNi1-x alloys), the octahedral shape exhibits only (111) oriented facets, and so, this morphology is well-adapted to the maximal deployment of the ideal Pt3Ni(111)-skin surface.
In 2010, Zhang et al. 75 reported the synthesis of very active Pt3Ni nanoctahedra for the ORR, using oleylamine and oleic acid as both solvent and capping agents, and tungsten hexacarbonyl (W(CO)6) as reducing agent. According to the authors, the presence of W(CO)6 in the synthesis medium is key to reach high shape control of the nanocrystal, not because it stabilizes the (111) facets (that would be more the role of oleylamine and oleic acid, even if this facet orientation is naturally more stable) but because it facilitates fast Pt nucleation. By changing the order of addition of the synthesis precursors, nanocubes of same size and composition but exhibiting different surface orientation were produced. Surprisingly, the PtNi octahedra achieved ‘only’ 5 and 7-fold enhancement in specific activity (2.8 and 4-fold enhancement in mass activity) relative to the nanocubes and commercial Pt/C, respectively (where an enhancement of the ORR kinetics up to 90 vs. Pt/C could have been expected with resect to single crystal studies), as shown in Figure 2.3. Nevertheless, the significant difference of ORR kinetics on various particle shapes (octahedra vs. cubes vs. spheres) highlighted the viability of the approach.
Table of contents :
1 GENERAL INTRODUCTION
1.1 ENERGETIC CONTEXT
1.1.1 Global Warming, is There Time for Scepticism?
1.1.2 The Energetic Transition
1.2 PEMFC OVERVIEW
1.2.2 The Proton Exchange Membrane
1.2.3 The Catalyst Layers.
2 STATE-OF-THE-ART ON ORR ELECTROCATALYSIS
2.1 THE D-BAND THEORY FOR THE ORR
2.1.1 Investigations on Extended Surfaces
2.1.2 From Single Crystals to Nanoparticles
2.2 RECENT ADVANCES IN PEMFC CATHODE ELECTROCATALYSTS
2.2.1 Shape-Controlled PtM Nanoparticles
2.2.2 Pt-Based Core-Shell Nanoparticles
2.2.3 Hollow PtM Nanoparticles
2.3 PHD THESIS OUTLINE
3 HOLLOW PTNI/C NANOPARTICLES AS ELECTROCATALYST FOR THE ORR
3.2 UNVEILING THE SYNTHESIS OF HOLLOW PTNI/C NANOPARTICLES
3.2.2 Atomic-Scale Morphological Changes Occurring During the Synthesis of Hollow PtNi/C NPs
3.2.3 Structural Changes Occurring During the Synthesis of Hollow PtNi/C Nanoparticles
3.3 ELECTROCATALYTIC PERFORMANCE OF THE HOLLOW PTNI/C CATALYST
3.3.1 Something More than Strain and Ligand Effects
3.3.2 Structure-ORR Activity Causal Relationship
4 BEYOND ALLOYING EFFECTS: MICROSTRAIN-INDUCED ENHANCEMENT OF ELECTROCATALYTIC PROPERTIES ON VARIOUS PTNI/C NANOSTRUCTURES
4.2 VARIOUS NANOCATALYSTS FROM PTNI NANO-BRICKS
4.2.1 Synthesis and Characterization of PtNi/C Nanoparticles
4.2.2 Structure and Chemical Composition of the Different Electrocatalysts
4.3 MICROSTRAIN AND ELECTROCATALYSIS
4.3.1 COads Stripping – ORR Catalytic Activity Relationships
4.3.2 DFT and Extension to Methanol and Ethanol Oxidation Reactions
5 FROM ‘PERFECT’ TO ‘DEFECTIVE’ PTNI ELECTROCATALYSTS: A MATTER OF TIME?
5.2 PTNI MATERIALS SAMPLING FROM THE ORR ELECTROCATALYSIS LANDSCAPE 116
5.2.1 PtNi Materials Introduction and Synthesis
5.2.2 Materials Characterization
5.3 TOWARD UNPRECEDENTED ELECTROCATALYTIC TRENDS
5.3.1 From Microstrain to Surface Distortion
5.3.2 Facing the COads Stripping Puzzle
5.3.3 Apprehending ORR Catalysis Differently
5.4 INITIAL TRENDS VS. (ELECTRO)CHEMICAL AGEING.
5.4.1 Acidic Treatment
5.4.2 Accelerated Stress Tests
6 GENERAL CONCLUSION AND OUTLOOK