The design of electrocatalysts capable of selectively reducing nitrate to ammonia is gaining interest as a means of transforming waste into fertilizers. However, most prior investigations of prototypical electrocatalysts, such as polycrystalline Pd and Pt, have focused on unraveling the mechanisms responsible for the selective reduction of nitrate to nitrogen gas. Such polycrystalline noble metals demonstrate notoriously low activity for nitrate reduction (nitrate to nitrite) and high activity for nitrite reduction (nitrite to nitrogen). Here, we aim to elucidate the effect Pd surface structure has on nitrate and nitrite reduction and to determine what role catalyst structural design can play in enabling selective reduction of nitrate to ammonia. Through synthesizing nanocatalysts with controlled facets (e.g., nanocubes, cuboctahedrons, octahedrons, and concave nanocubes), we demonstrate that Pd(111) > Pd(100) > Pd­(hk0) for nitrate reduction activity and Pd(100) > Pd­(hk0) > Pd(111) for nitrite reduction activity in an alkaline electrolyte. Octahedrons without Pd (100) facets exhibited nearly selective production of NO2 – with little to no measurable NH3 or N2. However, nanocubes that expose only the Pd(100) facet exhibited high activity for NO2 – reduction to NH3. Cuboctahedrons that expose both Pd(111) and Pd(100) facets demonstrated the highest production of ammonia (306.8 μg h–1 mgPd –1) with a faradaic efficiency of 35%. Density functional theory (DFT) simulations reveal that *NO3 dissociation to *NO2 + O* is more favorable on Pd(111) than Pd(100), explaining the faster nitrate reduction kinetics on the Pd(111) facet observed in the experiments. The simulations also show that *NO2 binds less strongly to Pd(111) compared to Pd(100). Thus, nitrite formed via nitrate dissociation readily desorbs from the Pd(111) surface, which explains why Pd(111) selectively reduces nitrate to nitrite. The results show that cuboctahedron is bifunctional in nature, with the (111) facet catalyzing the conversion of NO3 – to NO2 – and the (100) facet catalyzing the conversion of NO2 – to NH3.

Access to nitrogen-based fertilizers is critical to maximize agricultural yield, as nitrogen is the most common rate-limiting nutrient. Nearly all nitrogenbased fertilizers rely on ammonia and nitric acid as feedstocks, and thus the demand for these chemicals is heavily dependent on the global population and food demand. Over the next three decades, the global population will continue to dictate the market size and value for ammonia and nitric acid, which consequently will have a significant impact on our energy infrastructure. Here, we discuss the potential for carbon-free electrocatalytic nitrogen reduction, nitrogen oxidation, and nitrate reduction to meet fertilizer manufacturing demands. We also explore various growth scenarios to predict the 2050 market size and value for ammonia and nitric acid. We highlight that if the current approaches for manufacturing ammonia and nitric acid remain constant, carbon emissions from the production of fixed fertilizer feedstocks could exceed 1300 Mt CO 2eq /yr, prompting a strong need for green alternatives.

Bimetallic PtNi nanoparticles have been considered as a promising electrocatalyst for oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells (PEMFCs) owing to their high catalytic activity. However, under typical fuel cell operating conditions, Ni atoms easily dissolve into the electrolyte, resulting in degradation of the catalyst and the membrane-electrode assembly (MEA). Here, we report gallium-doped PtNi octahedral nanoparticles on a carbon support (Ga-PtNi/C). The Ga-PtNi/C shows high ORR activity, marking an 11.7-fold improvement in the mass activity (1.24 A mg) and a 17.3-fold improvement in the specific activity (2.53 mA cm) compared to the commercial Pt/C (0.106 A mg and 0.146 mA cm). Density functional theory calculations demonstrate that addition of Ga to octahedral PtNi can cause an increase in the oxygen intermediate binding energy, leading to the enhanced catalytic activity toward ORR. In a voltage-cycling test, the Ga-PtNi/C exhibits superior stability to PtNi/C and the commercial Pt/C, maintaining the initial Ni concentration and octahedral shape of the nanoparticles. Single cell using the Ga-PtNi/C exhibits higher initial performance and durability than those using the PtNi/C and the commercial Pt/C. The majority of the Ga-PtNi nanoparticles well maintain the octahedral shape without agglomeration after the single cell durability test (30,000 cycles). This work demonstrates that the octahedral Ga-PtNi/C can be utilized as a highly active and durable ORR catalyst in practical fuel cell applications.

Recently, PtM (M = Fe, Ni, Co, Cu, etc.) intermetallic compounds have been highlighted as promising candidates for oxygen reduction reaction (ORR) catalysts. In general, to form those intermetallic compounds, alloy phase nanoparticles are synthesized and then heat-treated at a high temperature. However, nanoparticles easily agglomerate during the heat treatment, resulting in a decrease in electrochemical surface area (ECSA). In this study, we synthesized Pt-Fe alloy nanoparticles and employed carbon coating to protect the nanoparticles from agglomeration during heat treatment. As a result, PtFe L1 structure was obtained without agglomeration of the nanoparticles; the ECSA of Pt-Fe alloy and intermetallic PtFe/C was 37.6 and 33.3 m g, respectively. PtFe/C exhibited excellent mass activity (0.454 A mg) and stability with superior resistances to nanoparticle agglomeration and iron leaching. Density functional theory (DFT) calculation revealed that, owing to the higher dissolution potential of Fe atoms on the PtFe surface than those on the Pt-Fe alloy, PtFe/C had better stability than Pt-Fe/C. A single cell fabricated with PtFe/C showed higher initial performance and superior durability, compared to that with commercial Pt/C. We suggest that PtM chemically ordered electrocatalysts are excellent candidates that may become the most active and durable ORR catalysts available.

Metal-free carbon materials have emerged as cost-effective and high-performance catalysts for the production of hydrogen peroxide (H2O2) through the two-electron oxygen reduction reaction (ORR). Here, we show that 3D crumpled...