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oxidizing agent (oxygen). [4,5] Fuel cells are considered likely by many to play a central and integral role in future renewable energy developments. However, reducing their cost has been a significant challenge, and is critical for promoting future studies and accelerating market growth. [6-8] Currently, noble Pt materials are the most favored electrocatalysts for the electrochemical reactions at both the anode and cathode in fuel cells. The cathodic oxygen reduction reaction (ORR) rate is substantially slower than the anodic hydrogen oxidation reaction by several orders of magnitude. When properly tailored in electrodes, Pt materials provide fast reaction kinetics, good electrical conductivity, and high chemical stability in strongly acidic electrolytes. [9-11] Conventional electrocatalyst structures generally employ fine Pt powders (2-3 nm) supported on high surface area carbon (Pt/C). However, as the number of large scale energy applications continues to increase, the high required Pt loading (typically ≈0.5 mg cm −2 for commercial electrodes) and high cost of this precious metal (≈$1000 per troy ounce) pose significant barriers to the widespread use of fuel cells containing Pt materials. The quantity of Pt required to achieve the targeted electrode performance significantly contributes to the high cost of fuel cell systems. [12-14] In recent decades, significant efforts have been devoted to investigating the principles underlying the electrocatalyst layer, A new direction for developing electrocatalysts for hydrogen fuel cell systems has emerged, based on the fabrication of 3D architectures. These new architectures include extended Pt surface building blocks, the strategic use of void spaces, and deliberate network connectivity along with tortuosity, as design components. Various strategies for synthesis now enable the functional and structural engineering of these electrocatalysts with appropriate electronic, ionic, and electrochemical features. The new architectures provide efficient mass transport and large electrochemically active areas. To date, although there are few examples of fully functioning hydrogen fuel cell devices, these 3D electrocatalysts have the potential to achieve optimal cell performance and durability, exceeding conventional Pt powder (i.e., Pt/C) electrocatalysts. This progress report highlights the various 3D architectures proposed for Pt electrocatalysts, advances made in the fabrication of these structures, and the remaining technical challenges. Attempts to develop design rules for 3D architectures and modeling, provide insights into their achievable and potential performance. Perspectives on future developments of new multiscale designs are also discussed along with future study directions.

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