Highly active, selective, and stable Pd single-atom catalyst anchored on N-doped hollow carbon sphere for electrochemical H₂O₂ synthesis under acidic conditions

Highlights

• Pd single-atom stabilized on N-doped carbon hollow nanosphere (Pd1/N-C).

• Remarkable electrochemical H₂O₂ production rate of Pd1/N-C under acidic conditions.

• The ORR volcano relationship rationalizes activity and selectivity of Pd1/N-C.

• Synergy of atomically dispersed Pd and N-Pd coordination configuration.

Abstract

Single-atom catalysts (SACs) have recently attracted broad scientific interests due to their unique structural feature, the single-atom dispersion. Optimized electronic structure as well as high stability are required for single-atom catalysts to enable efficient electrochemical production of H₂O₂. Herein, we report a facile synthesis method that stabilizes atomic Pd species on the reduced graphene oxide/N-doped carbon hollow carbon nanospheres (Pd1/N-C). Pd1/N-C exhibited remarkable electrochemical H₂O₂ production rate with high faradaic efficiency, reaching 80%. The single-atom structure and its high H₂O₂ production rate were maintained even after 10,000 cycle stability test. The existence of single-atom Pd as well as its coordination with N species is responsible for its high activity, selectivity, and stability. The N coordination number and substrate doping around Pd atoms are found to be critical for an optimized adsorption energy of intermediate *OOH, resulting in efficient electrochemical H₂O₂ production.

Introduction

Hydrogen peroxide, H₂O₂, is an environmentally friendly oxidant with annual production over 5 million tons and its wide usage covers bleaching, chemicals synthesis, and waste water treatment [1], [3], [4]. Current production of H₂O₂ is limited by the anthraquinone process which is a viable batch process that requires a centralized plant [4]. Centralized production requires additional costs for distillation, transportation, and dilution at the point of use [1]. Thermal catalysis for H₂O₂ synthesis has potential for the decentralized production with high yields in batch reactors, but developments in flow reactors remain a challenge in this technology’s applicability [2], [5], [6], [7], [8], [9], [10], [11], [12]. Electrochemical synthesis of H₂O₂ by the oxygen reduction reaction in acidic conditions holds promise for decentralized, on-site production, where the electrochemical device requires only water, air, and electricity [1], [13], [14], [15], [16]. Designing a selective and active cathode catalyst with high stability in acidic conditions is critical for realizing electrochemical H₂O₂ production [17]. Single atom catalysts (SACs), particularly single metal atoms stabilized in a porphyrin-like support, are promising as selective, active, and stable catalysts for various electrochemical reactions including the oxygen reduction reaction (ORR) resulting in the scalable electrochemical production of H₂O₂ [15], [18], [19], [20], [21].

Acidic conditions have an advantage over alkaline conditions for H₂O₂ production. In alkaline conditions, H₂O₂ can be easily produced by using a glassy carbon electrode, but the product HO₂⁻, is unstable, thus its applicability is limited to processes where the H₂O₂ must be used immediately. In contrast, acidic conditions stabilize H₂O₂, and allow the use of durable proton-exchange membranes resulting in the production of pure aqueous H₂O₂ solution, which has broader applications. The overall reactions of oxygen reduction and their thermodynamic equilibrium potentials versus reversible hydrogen electrode (RHE) in acidic conditions are as follows:

$${\mathrm{O}}_2+2{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}\to {\mathrm{H}}_2{\mathrm{O}}_2$$

$$(U_0=+0.7\mathrm{V}\mathrm{v}\mathrm{s}\mathrm{R}\mathrm{H}\mathrm{E})$$

$${\mathrm{O}}_2+4{\mathrm{H}}^{+}+4{\mathrm{e}}^{-}\to {2\mathrm{H}}_2\mathrm{O}$$

$$U_0=+1.23\mathrm{V}\mathrm{v}\mathrm{s}\mathrm{R}\mathrm{H}\mathrm{E})$$

The oxygen reduction reaction can either produce H₂O₂ via 2-electron (2e) pathway (1) or H₂O via 4-electron (4e) pathway (2). Efficient catalyst for electrochemical H₂O₂ production should produce H₂O₂ via the 2e pathway with high selectivity at a high rate. Adsorption energies of reaction intermediates are often good descriptors to rationalize given catalytic reaction. The only intermediate in the 2e pathway is *OOH, and the 4e pathway has additional intermediates *OH and *O. There are two principles determining catalytic performance for electrochemical H₂O₂ production. The first principle is electronic effect. Adsorption energy of the intermediate *OOH is tuned by the electronic structure of catalyst, and it should neither be too strong nor too weak, as stated in the Sabatier principle. A weak affinity for the reaction intermediates is preferred to inhibit O-O bond dissociation, limiting the 4e pathway that yields H₂O production. The second principle is geometric effect. On a metal surface, the most stable adsorption site for *OOH and *OH is on-top, while *O prefer to sit in a hollow site. On single site catalysts, isolated atoms cannot provide a hollow adsorption site, therefore *O is forced to bind on-top site, weakening its adsorption energy as compared to the other reaction intermediates. The selective destabilization of *O as compared to *OOH is the reason why single site catalysts are frequently studied for the 2e pathway [1], [16], [22].

Single-atom catalysts (SAC) have the potential to provide optimal active site structures for state-of-the-art electrochemical H₂O₂ production in acidic conditions [1], [13], [14], [15]. Siahrostami et al. (2013) provided a rational background for designing new catalysts for electrochemical H₂O₂ production by alloying a strong oxygen adsorbing element, Pt, with a weaker oxygen adsorbing element, Hg. Alloying these two elements resulted in the isolation of a single active Pt atom surrounded by less active Hg atoms, providing an optimized electronic structure, electronic effect, and geometric effect [22]. Following this idea, Verdaguer-Casadevall et al. (2014) examined a set of Metal-Hg surfaces and Pd-Hg was found to be the best catalyst providing the highest activity up to this date [16]. A similar strategy was applied for Pd-Au alloy [23], [24], cobalt-porphyrin (Co-N₄) like structures [13], [15], [25], [26], and more recently sulfide type catalysts, Pt₁-CuS_X [27] and CoS₂ [28], have been proposed where S atoms serving as weak *OOH adsorbing element. Atomically dispersed active sites satisfy the geometric requirements; and maximized utilization of noble metal atoms may dramatically reduce the materials cost. Previous work reported that Pt single-atoms on TiN [29], TiC [30], S doped carbon [31] provided high selectivity for 2e pathway with appreciable activity. Tuning the electronic structure of single-atom catalysts (SAC) can lead to even higher activity through an optimized electronic effect.

The anchoring site plays a crucial role in improving the activity, stability and selectivity of the SAC via a direct coordination effect between atomic metal and support [30], [32], [33]. The metal-coordination not only alters the electronic effect, but also, very importantly, determines the stability of the atomic structure. The interaction between support and metal atom can be strengthened by modifying support materials and introducing coordination sites for atomic metal [34], [35], [36]. Recent findings also showed that selectivity can be tuned through electronic effect and/or heteroatoms (e.g., N, O, S) electron donation between metal and heteroatom containing supports [37], [38], [39]. In line with this, a suitable support material that provides anchoring sites and coordination environment for single atoms are important to achieve satisfactory catalytic performance.

In this work, we propose a strategy to overcome the stability issue and improve the activity and the selectivity of SACs by introducing suitable coordination sites for Pd single-atoms. Pd atoms are anchored by doped N species on a hollow carbon nanosphere via dative coordination. We have previously synthesized hollow carbon nanospheres by taking a graphene oxide (GO) shell and coated it with amorphous carbon. These nanospheres anchored Pd single-atom sites and exhibited high stability towards 4-nitrophenol reduction reaction [40]. In this work we introduced N sites into the carbon support to further modify the coordination environment for Pd single-atom. Here we present the preparation of Pd single-atoms anchored on N-doped hollow carbon nanospheres (Pd1/N-C) and its enhanced electrochemical performance for the oxygen reduction reaction. The mass activity of Pd1/N-C was comparable to the best performing catalysts with 78.9 ± 2.5% faradaic efficiency. Single-atom dispersion of Pd1/N-C was maintained even after 10,000 cycle “on-off” test with little activity degradation showing its exceptional stability. The Pd single site surrounded by six coordinating pyridinic N atoms and moderate additional graphitic N doping was suggested as a possible explanation of the observed high activity by density functional theory (DFT) calculations.