Highly Efficient Trivalent Chromium Electroplating Enabled by the Hydrogen Isotope Effect

Chromium (Cr) electroplating is an industrially important surface finishing technology. However, conventional Cr plating processes suffer from a pronounced hydrogen evolution reaction (HER), resulting in extremely low current efficiencies of 10-20%. Because the HER leads to energy loss and can cause embrittlement of the deposited films, controlling the HER is crucial for establishing sustainable electroplating processes. In this study, we achieved a markedly high current efficiency of approximately 90% for crystalline Cr electrodeposition by selectively suppressing the HER using deuterated Cr(III) electrolytes based on a highly concentrated CaCl₂ system. This selective suppression of the HER originates from the lower acidity of the Cr(III) electrolytes as a result of the weaker dissociation of D₂O, alongside inhibition of typical proton/deuteron hopping diffusion as a result of the addition of excess CaCl₂. Notably, we succeeded in tailoring the water dissociation around Cr(III) ions without relying on organic additives, thereby enabling the efficient crystalline Cr electrodeposition. This study provides a design principle for aqueous electroplating systems that exploits the isotope effect of water.