Electrocatalytic conversion of nitrogen oxides to value-added chemicals is a promising strategy for mitigating the human-caused unbalance of the global nitrogen-cycle, but controlling product selectivity remains a great challenge. Here we show iron–nitrogen-doped carbon as an efficient and durable electrocatalyst for selective nitric oxide reduction into hydroxylamine. Using in operando spectroscopic techniques, the catalytic site is identified as isolated ferrous moieties, at which the rate for hydroxylamine production increases in a super-Nernstian way upon pH decrease. Computational multiscale modelling attributes the origin of unconventional pH dependence to the redox active (non-innocent) property of NO. This makes the rate-limiting NO adsorbate state more sensitive to surface charge which varies with the pH-dependent overpotential. Guided by these fundamental insights, we achieve a Faradaic efficiency of 71% and an unprecedented production rate of 215 μmol cm−2 h−1 at a short-circuit mode in a flow-type fuel cell without significant catalytic deactivation over 50 h operation.

Electrocatalytic conversion of CO2 into value-added products offers a new paradigm for a sustainable carbon economy. For active CO2 electrolysis, the single-atom Ni catalyst has been proposed as promising from experiments, but an idealized Ni–N4 site shows an unfavorable energetics from theory, leading to many debates on the chemical nature responsible for high activity. To resolve this conundrum, here we investigated CO2 electrolysis of Ni sites with well-defined coordination, tetraphenylporphyrin (N4–TPP) and 21-oxatetraphenylporphyrin (N3O–TPP). Advanced spectroscopic and computational studies revealed that the broken ligand-field symmetry is the key for active CO2 electrolysis, which subordinates an increase in the Ni redox potential yielding NiI. Along with their importance in activity, ligand-field symmetry and strength are directly related to the stability of the Ni center. This suggests the next quest for an activity–stability map in the domain of ligand-field strength, toward a rational ligand-field engineering of single-atom Ni catalysts for efficient CO2 electrolysis.

Single-atom catalysts (SACs) have quickly emerged as a new class of catalytic materials. When confronted with classical carbon-supported nanoparticulated catalysts (Pt/C), SACs are often claimed to have superior electrocatalytic properties, e.g., stability. In this study, we critically assess this statement by investigating S-doped carbon-supported Pt SACs as a representative example of noble-metal-based SACs. We use a set of complementary techniques, which includes online inductively coupled plasma mass spectrometry (online ICP-MS), identical location transmission electron microscopy (IL-TEM), and X-ray photoelectron spectroscopy (XPS). It is shown by online ICP-MS that the dissolution behavior of as-synthesized Pt SACs is significantly different from that of metallic Pt/C. Moreover, Pt SACs are, indeed, confirmed to be more stable toward Pt dissolution. When cycled to potentials of up to 1.5 VRHE, however, the dissolution profiles of Pt SACs and Pt/C become similar. IL-TEM and XPS show that this transition is due to morphological and chemical changes caused by cycling. The latter, in turn, is a consequence of the relatively poor stability of S ligands. As monitored by online ICP-MS and XPS, significant amounts of sulfur leave the catalyst during oxidation. Hence, in case catalysts with improved stability in the anodic potential region are desired, more robust supports and ligands must be developed.

Single-atom catalysts (SACs) featuring atomically dispersed metal cations covalently embedded in a carbon matrix show significant potential to achieve high catalytic performance in various electrocatalytic reactions. Although considerable advances have been achieved in their syntheses and electrochemical applications, further development and fundamental understanding are limited by a lack of strategies that can allow the quantitative analyses of their intrinsic catalytic characteristics, that is, active site density (SD) and turnover frequency (TOF). Here we show an in situ SD quantification method using a cyanide anion as a probe molecule. The decrease in cyanide concentration triggered by irreversible adsorption on metal-based active sites of a model Fe–N–C catalyst is precisely measured by spectrophotometry, and it is correlated to the relative decrease in electrocatalytic activity in the model reaction of oxygen reduction reaction. The linear correlation verifies the surface-sensitive and metal-specific adsorption of cyanide on Fe–N x sites, based on which the values of SD and TOF can be determined. Notably, this analytical strategy shows versatile applicability to a series of transition/noble metal SACs and Pt nanoparticles in a broad pH range (1–13). The SD and TOF quantification can afford an improved understanding of the structure–activity relationship for a broad range of electrocatalysts, in particular, the SACs, for which no general electrochemical method to determine the intrinsic catalytic characteristics is available.

Carbon monoxide is widely known to poison Pt during heterogeneous catalysis owing to its strong donor−acceptor binding ability. Herein, we report a counterintuitive phenomenon of this general paradigm when the size of Pt decreases to an atomic level, namely, the CO-promoting Pt electrocatalysis toward hydrogen evolution reactions (HER). Compared to pristine atomic Pt catalyst, reduction current on a CO-modified catalyst increases significantly.Operando mass spectroscopy and electrochemical analyses demonstrate that the increased current arises due to enhanced H 2 evolution, not additional CO reduction. Through structural identification of catalytic sites and computational analysis, we conclude that CO-ligation on the atomic Pt facilitates H ads formation via water dissociation. This counterintuitive effect exemplifies the fully distinct characteristics of atomic Pt catalysts from those of bulk Pt, and offers new insights for tuning the activity of similar classes of catalysts.

The free energy of H adsorption (ΔG H ) on a metallic catalyst has been taken as a descriptor to predict the hydrogen evolution reaction (HER) kinetics but has not been well applied in alkaline media. To assess this, we prepare Pd@Pt and PdH@Pt core−shell octahedra enclosed by Pt(111) facets as model catalysts for controlling the ΔG H affected by the ligand, the strain, and their ensemble effects. The Pt shell thickness is adjusted from 1 to 5 atomic layers by varying the amount of Pt precursor added during synthesis. In an alkaline electrolyte, the HER activity of core−shell models is improved either by the construction of core−shell structures or by the increased number of Pt shells. These experimental results are in good agreement with the ΔG H values calculated by the first-principles density functional theory with a complex surface strained core−shell slab model. However, enhanced HER activities of Pd@Pt and PdH@Pt core−shell nanocrystals over the Pt catalyst are inconsistent with the thermodynamic ΔG H scaling relationship only but can be explained by the work function and apparent ΔG H models that predict the interfacial electric field for the HER.

Ammonia has recently received considerable attention as an alternative energy carrier and a carbon-neutral fuel. In this future energy scenario, the ammonia oxidation reaction (AOR) is a pivotal process for onsite hydrogen production and/or electricity generation. However, its implementation is hindered by the nondurable nature of AOR catalysis by platinum. Accordingly, securement of a durable Pt electrocatalysis for the AOR is critical but has been hampered by the well-known chemical deactivation (i.e., poisoning). Additionally, the structural stability, which could also affect durable AOR operation, has scarcely been investigated. Herein, the degradation of Pt catalysts under AOR conditions has been investigated with various operando and in/ex situ spectroscopies. We demonstrate that NH3 (or AOR intermediates/byproducts) modifies the chemical structures of both the Pt surface and dissolved Pt ions, specifically by passivation of the Pt surface with NH3-derived adsorbates and complexation of the dissolved Pt ions, respectively. These modifications lead to a significant acceleration in Pt dissolution but a deceleration in its redeposition, resulting in the augmented structural degradation of Pt catalysts in NH3-containing electrolyte after the Pt has experienced a potential excursion above ca. 1 VRHE. With these understandings, a quasi-stable operation potential window and operational strategy are suggested. The tentative AOR protocol allows prolonged NH3 electrolysis with alleviated Pt dissolution (<0.02 ng cmPt –2 s–1), suggesting that NH3 will be a viable future energy carrier if the rational operational strategy proposed herein is developed further.

Corrosion of carbon support is one of the most crucial causes of the degradation of polymer electrolyte membrane fuel cells (PEMFCs) utilizing carbon-supported platinum nanoparticles (Pt/C) as a catalyst. To mitigate carbon corrosion, Pt is alloyed with iridium (Ir), which is catalytically active for the oxygen evolution reaction (OER), with various compositions of Pt x Ir y . The carbon-supported Pt x Ir y alloy catalysts (Pt x Ir y /C) show slightly lower initial activity for the oxygen reduction reaction (ORR) than Pt/C. However, the ORR activities of the Pt x Ir y /C catalysts increase with repeating potential cycles from 1.0 to 1.5 V RHE , while Pt/C exhibits a rapid decay in the ORR activity and a mixture of Pt/C and Ir/C (Pt/C + Ir/ C, Pt-to-Ir ratio of 85:15) maintains its initial activity. After 5k potential cycles, the mass activity of Pt 85 Ir 15 was 0.071 A mg PGM −1 , which is significantly higher than that of Pt/C (0.017 A mg PGM −1 ) and Pt/C + Ir/C (0.039 A mg PGM −1 ). These results can be attributed to the atomically distributed Ir in Pt 85 Ir 15 . Clearly, carbon corrosion occurs in Pt/C and in Pt-rich regions of Pt/C + Ir/C, whereas the carbon support in Pt 85 Ir 15 /C is effectively protected from corrosion. As a result, the greatest amount of CO 2 emission is detected as coming from Pt/C, followed by Pt/C + Ir/C and Pt 85 Ir 15 /C. During the potential cycles, high-index Pt facets are formed on the surface of Pt 85 Ir 15 /C, leading to an increase in the ORR activity. When employed as cathode catalysts of a PEMFC, Pt 85 Ir 15 /C exhibits improved durability compared to Pt/C and Pt/C + Ir/C under high-voltage cycles to 1.5 V (5k cycles). This work demonstrates that the atomic distribution of Ir in Pt is an effective strategy for mitigating corrosion of the carbon support and to enhance the durability of PEMFCs exposed to high potentials.