Introduction to Surface Chemistry and Catalysis

The oxygen reduction reaction (ORR) is a key reaction for energy conversion and storage systems such as polymer electrolyte membrane fuel cells (PEMFCs). Studies on Pt and Pt-transition metal alloy single crystals have established that the ORR is best electrocatalyzed on bimetallic alloys and at (111) facets. Combining alloying and ensemble effects recently led to 20-30-fold enhancement of the specific activity (normalized per real cm 2 of catalyst) for the ORR on PtNi/C nanooctahedra relative to Pt/C nanoparticles. However, due to the highly oxidizing conditions of the PEMFC cathode, the stability of PtNi/C octahedra is poor in PEMFC cathode operating conditions, thus compromising their utilization in real devices. Strikingly, it also turned out recently that structurally-disordered PtNi nano-catalysts, such as hollow PtNi/C nanoparticles, dealloyed PtNi/C nanoparticles, PtNi aerogels or PtNi nanowires feature highly desirable and sustainable ORR activity (x 10-12 in specific activity relative to pure Pt/C). However, to date, the mechanisms of this unexpected ORR activity enhancement remain unclear, and prevent further development of this vital technology for a carbon-free energy future. To shed fundamental light onto these issues, state-of-the art PtNi/C nanocatalysts with distinct atomic composition, size, shape and density of disorder were synthesized. Their disorder was quantified experimentally, using the values of microstrain (a parameter accessible by the Rietveld refinement of wide-angle X-ray scattering patterns) that is representative of the local distortion of a crystal lattice. Thanks to ab initio calculations, the contributions of bulk and surface structural disorder were disentangled, and a new parameter, the surface distortion (SD), was established. The SD descriptor was used to rationalize the ORR activity enhancement of the two classes of materials (structurally-ordered and structurally-disordered), and to probe their stability in simulated PEMFC cathode operating conditions.

Surface distortion as a unifying concept and descriptor in oxygen reduction reaction electrocatalysis

Increasing catalytic activity and durability of atomically dispersed metal-nitrogen-carbon (M-N-C) catalysts for the oxygen reduction reaction (ORR) cathode in proton-exchange-membrane fuel cells remains a grand challenge. Here, a high-power and durable Co-N-C nanofiber catalyst synthesized through electrospinning cobalt-doped zeolitic imidazolate frameworks into selected polyacrylonitrile and poly(vinylpyrrolidone) polymers is reported. The distinct porous fibrous morphology and hierarchical structures play a vital role in boosting electrode performance by exposing more accessible active sites, providing facile electron conductivity, and facilitating the mass transport of reactant. The enhanced intrinsic activity is attributed to the extra graphitic N dopants surrounding the CoN4 moieties. The highly graphitized carbon matrix in the catalyst is beneficial for enhancing the carbon corrosion resistance, thereby promoting catalyst stability. The unique nanoscale X-ray computed tomography verifies the well-distributed ionomer coverage throughout the fibrous carbon network in the catalyst. The membrane electrode assembly achieves a power density of 0.40 W cm-2 in a practical H2 /air cell (1.0 bar) and demonstrates significantly enhanced durability under accelerated stability tests. The combination of the intrinsic activity and stability of single Co sites, along with unique catalyst architecture, provide new insight into designing efficient PGM-free electrodes with improved performance and durability.

Single Cobalt Sites Dispersed in Hierarchically Porous Nanofiber Networks for Durable and High-Power PGM-Free Cathodes in Fuel Cells.

Fuel cells are an incredibly powerful renewable energy technology, but their broad applications remains lagging because of the high cost and poor reliability of cathodic electrocatalysts for the oxygen reduction reaction (ORR). This review focuses on the recent progress of ORR electrocatalysts in fuel cells. More importantly, it highlights the fundamental problems associated with the insufficient activity translation from rotating disk electrode to membrane electrode assembly in the fuel cells. Finally, for the atomic-level in-depth information on ORR catalysts in fuel cells, potential perspectives are suggested, including large-scale preparation, unified assessment criteria, advanced interpretation techniques, advanced simulation and artificial intelligence. This review aims to provide valuable insights into the fundamental science and technical engineering for efficient ORR electrocatalysts in fuel cells.

Oxygen Reduction Electrocatalysts toward Practical Fuel Cells: Progress and Perspectives

Restoring the Nitrogen Cycle by Electrochemical Reduction of Nitrate: Progress and Prospects

Clean and efficient energy storage and conversion via sustainable water and nitrogen reactions have attracted substantial attention to address the energy and environmental issues due to the overwhelming use of fossil fuels. These electrochemical reactions are crucial for desirable clean energy technologies, including advanced water electrolyzers, hydrogen fuel cells, and ammonia electrosynthesis and utilization. Their sluggish reaction kinetics lead to inefficient energy conversion. Innovative electrocatalysis, i.e., catalysis at the interface between the electrode and electrolyte to facilitate charge transfer and mass transport, plays a vital role in boosting energy conversion efficiency and providing sufficient performance and durability for these energy technologies. Herein, a comprehensive review on recent progress, achievements, and remaining challenges for these electrocatalysis processes related to water (i.e., oxygen evolution reaction, OER, and oxygen reduction reaction, ORR) and nitrogen (i.e., nitrogen reduction reaction, NRR, for ammonia synthesis and ammonia oxidation reaction, AOR, for energy utilization) is provided. Catalysts, electrolytes, and interfaces between the two within electrodes for these electrocatalysis processes are discussed. The primary emphasis is device performance of OER-related proton exchange membrane (PEM) electrolyzers, ORR-related PEM fuel cells, NRR-driven ammonia electrosynthesis from water and nitrogen, and AOR-related direct ammonia fuel cells.

Advanced Electrocatalysis for Energy and Environmental Sustainability via Water and Nitrogen Reactions

Effect of Ammonia on the Electrocatalysis of Oxygen Reduction Reaction in Base

https://onlinelibrary.wiley.com/doi/pdf/10.1002/adma.202170042

Huanhuan Wang

Papers

Advanced Electrocatalysis for Energy and Environmental Sustainability via Water and Nitrogen Reactions

Effect of Ammonia on the Electrocatalysis of Oxygen Reduction Reaction in Base

Electrocatalysis: Advanced Electrocatalysis for Energy and Environmental Sustainability via Water and Nitrogen Reactions (Adv. Mater. 6/2021)