Approximately 80% of the world's current energy supply is solely reliant on fossil fuels, the combustion of which is attributed as the most significant contributor to global warming, thus motivating a search for renewable alternatives to fossil fuels with “zero” CO₂ emissions. For the future energy portfolio, electrolytic water splitting, driven by renewable electricity, is one of the most promising sources of renewable energy as it produces H₂ as the fuel with O₂ as the reaction byproduct and is facilitated by the utilization of a catalyst. Some precious group metals or their oxides, for example, IrO₂, RuO₂, and Pt show extremely good catalytic activity toward the water splitting reaction. However, the need for large-scale production of H₂ necessitates the search for inexpensive and widely available materials, like the first-row transition metals. Their catalytic properties can be fine-tuned by incorporating a nonmetal into their crystal structure, such as phosphorus, which can draw electron density from metals while trapping protons during water splitting. The availability of catalytic sites can be increased by producing materials on the nanoscale. Recent investigations into various monometallic phosphide nanoparticles, including copper phosphide (Cu_3-xP), have unveiled their promising capabilities as effective catalysts for water splitting. Cu_3-xP, distinguished by its inherent vacancies and partial ionic character, serves as an ideal material platform for the introduction of other metal ions, creating metastable, bimetallic phosphide systems. We and others have found that the bimetallic phosphide system is more catalytically active than the monometallic systems towards water splitting. Therefore, this study focuses on copper phosphide nanoparticles, which will serve as templates, undergoing cation exchange with late, first-row transition metal (M) ions (M = Mn, Fe, Co, Ni). This approach aims to create a series of metastable Cu_xM_yP_z bimetallic phosphide systems and comprehensively evaluate the structure-property relationship of these innovative materials.