Currently, supported platinum (Pt) and Pt-based alloys are used as oxygen reduction reaction (ORR) electrocatalysts for commercial proton exchange membrane fuel cells (PEMFCs) for vehicles, back-up power, and other applications. Carbon black is a traditional support material because of its high surface area and electronic conductivity, however carbon corrosion highly affects the long-term stability compromising fuel cell performance under operational conditions. To improve stability, the replacement of carbon with more resistant supports or the use of unsupported catalysts can contribute to increase the stability and performance of these materials. Here, we present the synthesis and characterization of two-dimensional (2D) bimetallic NiPt nanostructures that function as unsupported (carbon-free) electrocatalysts with both significantly higher catalytic activity and stability compared with conventional Pt/carbon electrocatalysts for the ORR. The nanoframes consist of a hierarchical 2D framework composed of a highly catalytically active Pt-Ni alloy phase with an interconnected solid and pore network that results in three-dimensional molecular accessibility. The physicochemical properties were modulated by controlling the synthesis parameters and atmospheric and thermal treatment conditions. The unsupported metallic nanoframes allowed the improvement of ORR specific activity by almost one order of magnitude compared to commercial Pt/C. In addition to higher activity, accelerated stability testing showed that the metallic Ni-Pt nanoframes exhibit significantly improved (86% higher) stability compared with Pt/carbon catalysts (43%). Analysis of the electrochemical reaction kinetics using the Tafel slopes and rotating ring-disk electrode (RRDE) experiments supports that at low overpotentials the first electron transfer is the rate determining step and the oxygen reduction reaction proceeds mainly by four electron transfer on the Ni-Pt nanoframes. The ability to create metallic 2D structures with 3D molecular accessibility opens up new opportunities for the design of high activity and stability carbon-free catalyst nanoarchitectures for numerous electrocatalytic and catalytic applications.