The surface and interfaces of heterogeneous catalysts are essential to their performance as they are often considered to be active sites for catalytic reactions. With the development of nanoscience, the ability to tune surface and interface of nanostructures has provided a versatile tool for the development and optimization of a heterogeneous catalyst. In this Review, we present the surface and interface control of nanoparticle catalysts in the context of oxygen reduction reaction (ORR), electrochemical CO2 reduction reaction (CO2 RR), and tandem catalysis in three sections. In the first section, we start with the activity of ORR on the nanoscale surface and then focus on the approaches to optimize the performance of Pt-based catalyst including using alloying, core-shell structure, and high surface area open structures. In the section of CO2 RR, where the surface composition of the catalysts plays a dominant role, we cover its reaction fundamentals and the performance of different nanosized metal catalysts. For tandem catalysis, where adjacent catalytic interfaces in a single nanostructure catalyze sequential reactions, we describe its concept and principle, catalyst synthesis methodology, and application in different reactions.

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Surface and Interface Control in Nanoparticle Catalysis.

Due to the growing demand for energy and impending environmental issues, fuel cells have attracted significant attention as an alternative to conventional energy technologies. As cost is the main inhibitor of this technology, low cost catalysts with high activity and stable catalytic performance are the key to large scale application of fuel cells. The development and recent advancements of Pt and Pt-based electrocatalysts are discussed in this review. Discussion is mainly focused on the structure and composition of Pt and Pt-based electrocatalysts, which significantly affect the catalytic activities and durability of fuel cell catalysts. The electrocatalysts including Pt single metal, Pt-based alloys (including noble alloys, non-noble alloys, metal oxide alloys, and non-metal alloys), and structure-controlled alloys (nanopolyhedra, nanodendrites, and hollow and core–shell structures) were discussed. The activity, stability, and efficiency of Pt and Pt-based catalysts for both the oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR) as well as the correlation between the catalytic performance, structure optimization, and composition modulation of catalysts are also discussed.

Current progress of Pt and Pt-based electrocatalysts used for fuel cells

In the face of the global energy challenge and progressing global climate change, renewable energy systems and components, such as fuel cells and electrolyzers, which close the energetic oxygen and carbon cycles, have become a technology development priority. The electrochemical oxygen reduction reaction (ORR) and the direct electrochemical carbon dioxide reduction reaction (CO2 RR) are important electrocatalytic processes that proceed at gas diffusion electrodes of hydrogen fuel cells and CO2 electrolyzers, respectively. However, their low catalytic activity (voltage efficiency), limited long-term stability, and moderate product selectivity (related to their Faradaic efficiency) have remained challenges. To address these, suitable catalysts are required. This review addresses the current state of research on Pt-based and Cu-based nanoalloy electrocatalysts for ORR and CO2 RR, respectively, and critically compares and contrasts key performance parameters such as activity, selectivity, and durability. In particular, Pt nanoparticles alloyed with transition metals, post-transition metals and lanthanides, are discussed, as well as the material characterization and their performance for the ORR. Then, bimetallic Cu nanoalloy catalysts are reviewed and organized according to their main reaction product generated by the second metal. This review concludes with a perspective on nanoalloy catalysts for the ORR and the CO2 RR, and proposes future research directions.

Alloy Nanocatalysts for the Electrochemical Oxygen Reduction (ORR) and the Direct Electrochemical Carbon Dioxide Reduction Reaction (CO2 RR).

Currently, Pt-based electrocatalysts are adopted in the practical proton exchange membrane fuel cell (PEMFC), which converts the energy stored in hydrogen and oxygen into electrical power. However, the broad implementation of the PEMFC, like replacing the internal combustion engine in the present automobile fleet, sets a requirement for less Pt loading compared to current devices. In principle, the requirement needs the Pt-based catalyst to be more active and stable. Two main strategies, engineering of the electronic (d-band) structure (including controlling surface facet, tuning surface composition, and engineering surface strain) and optimizing the reactant adsorption sites are discussed and categorized based on the fundamental working principle. In addition, general routes for improving the electrochemical surface area, which improves activity normalized by the unit mass of precious group metal/platinum group metal, and stability of the electrocatalyst are also discussed. Furthermore, the recent progress of full fuel cell tests of novel electrocatalysts is summarized. It is suggested that a better understanding of the reactant/intermediate adsorption, electron transfer, and desorption occurring at the electrolyte-electrode interface is necessary to fully comprehend these electrified surface reactions, and standardized membrane electrode assembly (MEA) testing protocols should be practiced, and data with full parameters detailed, for reliable evaluation of catalyst functions in devices.

Pt-Based Nanocrystal for Electrocatalytic Oxygen Reduction.

Atomically ordered intermetallic nanoparticles are promising for catalytic applications but are difficult to produce because the high-temperature annealing required for atom ordering inevitably acc...

Sulfur-anchoring synthesis of platinum intermetallic nanoparticle catalysts for fuel cells.

Structure Sensitivity of Pd Facets for Enhanced Electrochemical Nitrate Reduction to Ammonia

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 mgPt–1) and a 17.3-fold improvement in the specific activity (2.53 mA cm–2) compared to the commercial Pt/C (0.106 A mgPt–1 and 0.146 mA cm–2). 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...

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Ga-doped Pt-Ni Octahedral Nanoparticles as a Highly Active and Durable Electrocatalyst for Oxygen Reduction Reaction

Ammonia and Nitric Acid Demands for Fertilizer Use in 2050

Recently, Pt3M (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, Pt3Fe L12 structure was obtained without agglomeration of the nanoparticles; the ECSA of Pt-Fe alloy and intermetallic Pt3Fe/C was 37.6 and 33.3 m2 gPt-1, respectively. Pt3Fe/C exhibited excellent mass activity (0.454 A mgPt-1) 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 Pt3Fe surface than those on the Pt-Fe alloy, Pt3Fe/C had better stability than Pt-Fe/C. A single cell fabricated with Pt3Fe/C showed higher initial performance and superior durability, compared to that with commercial Pt/C. We suggest that Pt3M chemically ordered electrocatalysts are excellent candidates that may become the most active and durable ORR catalysts available.

Synthesis of Chemically Ordered Pt3Fe/C Intermetallic Electrocatalysts for Oxygen Reduction Reaction with Enhanced Activity and Durability via a Removable Carbon Coating.

Sodium‐metal batteries (SMBs) are considered as a compliment to lithium‐metal batteries for next‐generation high‐energy batteries because of their low cost and the abundance of sodium (Na). Herein, a 3D nanostructured porous carbon particle containing carbon‐shell‐coated Fe nanoparticles (PC‐CFe) is employed as a highly reversible Na‐metal host. PC‐CFe has a unique 3D hierarchy based on sub‐micrometer‐sized carbon particles, ordered open channels, and evenly distributed carbon‐coated Fe nanoparticles (CFe) on the surface. PC‐CFe achieves high reversibility of Na plating/stripping processes over 500 cycles with a Coulombic efficiency of 99.6% at 10 mA cm–2 with 10 mAh cm–2 in Na//Cu asymmetric cells, as well as over 14 400 cycles at 60 mA cm–2 in Na//Na symmetric cells. Density functional theory calculations reveal that the superior cycling performance of PC‐CFe stems from the stronger adsorption of Na on the surface of the CFe, providing initial nucleation sites more favorable to Na deposition. Moreover, the full cell with a PC‐CFe host without Na metal and a high‐loading Na3V2(PO4)3 cathode (10 mg cm–2) maintains a high capacity of 103 mAh g–1 at 1 mA cm–2 even after 100 cycles, demonstrating the operation of anode‐free SMBs.

JeongHoon Lim

Papers

Structure Sensitivity of Pd Facets for Enhanced Electrochemical Nitrate Reduction to Ammonia

Ga-doped Pt-Ni Octahedral Nanoparticles as a Highly Active and Durable Electrocatalyst for Oxygen Reduction Reaction

Ammonia and Nitric Acid Demands for Fertilizer Use in 2050

Synthesis of Chemically Ordered Pt3Fe/C Intermetallic Electrocatalysts for Oxygen Reduction Reaction with Enhanced Activity and Durability via a Removable Carbon Coating.

A 3D Hierarchical Host with Enhanced Sodiophilicity Enabling Anode‐Free Sodium‐Metal Batteries