Alloying effect in silver-based dilute nanoalloy catalysts for oxygen reduction reactions
Graphical abstract
The density functional theory (DFT) calculations and experiments are combined, and we find that the M1Ag(1 1 1) dilute nanoalloys exhibit high stability when M is on the subsurface of the Ag(1 1 1) facet. The overpotential of as-prepared CuMnAg nanoalloy is close to the predicted overpotential and that of the Pt/C catalyst.
Introduction
Alkaline fuel cells and metal-air batteries that operate in basic media have the potential to exhibit better cost-effective performance than proton exchange membrane fuel cells, which operate in acidic environments. Given that oxygen reduction reaction (ORR) kinetics can be significantly enhanced in alkaline environments, platinum-group-metal (PGM)-free cathode electrocatalysts can be employed to enhance the activity and stability of catalysts, and applying these catalysts at the cathode side of an alkaline fuel cell is advantageous [1], [2], [3], [4].
PGM-free catalysts demonstrate an ORR activity performance that is on par with or even better than that of commercial Pt/C catalysts but also typically with higher long-term stability. Among the promising PGM-free catalysts, metal–nitrogen–carbon catalysts (M−N−C), [5], [6], [7] where M denotes transition metals (often Fe or Co), are potential candidates for driving ORR through the direct 4e− pathway. These catalysts can be easily prepared via high-temperature pyrolysis of inexpensive precursors and show high ORR activities when tested for a rotating disk electrode (RDE) [8], [9], [10]. However, alkaline fuel cell performance using these materials has rarely been demonstrated to be better than the performance of commercial Pt/C catalysts in contact with polymer electrolytes [11], [12], [13], [14]. M−N−C catalysts are always associated with low volume density of metal–nitrogen active sites, which require thick catalyst layers (up to 100 μm) to reach the loading required by the high oxygen reduction activity and consequently reduce the cathode performance. Therefore, other PGM-free electrocatalysts with high volumetric activity [11] must be developed to enable alkaline fuel cells.
Silver metal is precious but abundant and relatively inexpensive compared with nitrogen-doped graphene. When the non-PGM catalyst is silver metal, the thickness of the catalyst layer is not significantly different from the thickness (about 10 μm) of a catalyst layer based on Pt. Consequently, the transport losses in a Ag-catalysed air cathode should not be significantly higher than the transport losses in a Pt-catalysed air cathode. Given their relatively low price and high long-term stability in alkaline conditions, Ag-based catalysts have promoted the development of fuel cells, and the ability to use non-PGM catalysts, such as Ag-Cu [15], [16], [17], [18], [19], [20] and Ag-Sn [21], [22], in alkaline fuel cells and zinc-air batteries have been well documented in our previous work. AgCo [23] electrocatalysts exhibit higher specific activity and long-term stability than those of commercial Pt/C, and the ORR mechanisms of these non-PGM silver electrocatalysts in alkaline media have also been studied [24], [25], [26].
As a kind of impurity-doped alloy, dilute nanoalloys that dope few precious metals to common elements, such as single-atom alloys [27], [28] and single-atomic-site catalysts [29], [30], have attracted the attention of researchers, dilute nanoalloys could provide ultralow metal loading for alkaline fuel cells and metal-air batteries [31], [32], [33], [34], [35]. Several dilute alloy catalysts exhibit the electronic structures of the low-content precious metal element and unprecedented catalytic properties [36], [37], [38]. Greiner et al. [39] reported that in a AgCu single-atom alloy, the Cu 3d states are five times narrower than those of bulk Cu (that is, 0.5 eV for Cu in AgCu single-atom alloy compared with 2.5 eV for bulk Cu), and the Cu–H bond in AgCu is significantly stronger than that in bulk Cu. Wrasman et al. [40] reported that AuPd single-atom alloys can be synthesised experimentally, thereby improving the hydro-oxidation performance of 2-propanol to acetone. Nigam et al. [41] reported that a novel AgPt9 single-atom alloy at the sub-nanometer length scale shows superior catalytic behaviour for SO3 decomposition. However, no work has designed an optimal dilute alloy catalyst for ORR. Inspired by the traditional single-atom alloys, several studies have investigated the effect of a few impurities on the structure and catalytic properties of Ag-based nanoalloys. One of the major results of such catalysis research is the identification of the optimal element in the silver matrix that leads to the highest activity for alkaline fuel cells.
In this study, the catalytic performance of a series of M1Ag(1 1 1) dilute nanoalloys, where M is a 3d transition metal atom (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn), is investigated as ORR catalysts in alkaline media with density functional theory calculations and experiments. The M1Ag(1 1 1) dilute nanoalloy catalysts exhibit higher stability when the M atom is doped on the subsurface (2L) of the Ag(1 1 1) facet than when the M atom is doped on the surface (1L) of Ag(1 1 1). For the first time, the Ag-based dilute nanoalloy is determined to exhibit subsurface stability, which is related to the positive charge transfer of surface Ag atoms to the surrounding atoms. A new ternary Cu1Mn1Ag(1 1 1) dilute nanoalloy with higher activity than Cu1Ag(1 1 1) or Mn1Ag(1 1 1) is designed with DFT then prepared experimentally using the pulse laser deposition (PLD) method for the first time. The resultant CuMnAg and dealloyed DE-CuMnAg ternary alloys exhibit an enhanced catalytic activity compared with pure Ag and CuAg catalysts. These newfound CuMnAg nanoalloys are promising cathode catalysts for alkaline fuel cell and metal-air applications.
Section snippets
Computational and experimental method
Computational model: The periodic Ag(1 1 1) surface is modelled using a slab model with a 2 × 2 supercell of four atomic layers as shown in Fig. 1a. Single atoms from all ten elements in the 3d transition metals M were doped into the Ag(1 1 1) surface (1L), subsurface (2L) or bulk (3L). For the (2x2) model, the surface M atom density is 0 for the 2L-M1Ag(1 1 1) alloy and 12.5% if we assume subsurface M atom is doped in the surface and subsurface layer, which are still in the dilute alloy regime
Stability of the M1Ag(1 1 1) dilute nanoalloy structure with M atom at the surface (1L) or subsurface (2L) site
To confirm the stability of the M1Ag(1 1 1) dilute nanoalloy structure with the M atom at the surface (1L) or subsurface (2L) site, we compared the segregation and mixing energies. The segregation behaviour of the surface atoms was calculated using segregation energy Eseg, which can be defined as the energy difference between the doped atoms on the surface and inside. Surface segregation energy can be calculated aswhere EM1Ag(1 1 1)1L/2L, and EM1Ag(1 1 1)3L
Conclusions
In this work, we investigated the thermodynamic stability of dilute nanoalloy catalysts to guide theoretical ORR catalysts of dilute nanoalloys by considering M1Ag(1 1 1) as a case study. M1Ag(1 1 1) is a 3d transition metal atom M (M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn) doped on the surface and subsurface of Ag(1 1 1). We compared the surface segregation and mixing energies of M1Ag(1 1 1) and found that the M doped on the subsurface (2L) is more stable than doped on the surface (1L) of
Declaration of Competing Interest
The authors declared that there is no conflict of interest.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (grant nos. 51874243, 51271148 and 50971100), the Research Fund of State Key Laboratory of Solidification Processing in China (grant no. 150-ZH-2016), the Aeronautic Science Foundation Program of China (grant no.2012ZF53073), the Project of Transformation of Scientific and Technological Achievements of NWPU (grant no. 19-2017), the Doctoral Fund of Ministry of Education of China (grant no.20136102110013), and the Open
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