The electrodeposition of metals and alloys is explored with a focus on solvents and additives capable of reducing or eliminating hydrogen evolution while operating at highly cathodic potentials. The nucleation and growth behavior of binary codepositing systems are modelled in Chapter 2. Deep eutectic solvents based on choline chloride and urea are demonstrated to be capable of electrodepositing metallic manganese for the first time in Chapter 3. Chapter 4 describes the first time manganese has been incorporated into an electrodeposited magnetic iron-group alloy. Water-in-salt electrolytes are applied to the electrodeposition of metals in Chapters 5 and 6. These electrolytes are shown to suppress the proton reduction reaction and subsequent hydrogen evolution in aqueous systems. The tetrabutylammonium ion is also shown to be capable of suppression of proton reduction. The origins of this suppression are examined in Chapter 6, and it is determined that the additive adsorbs onto the electrode surface, blocking proton access. The suppressing behaviors of tetrabutylammonium and water-in-salt electrolytes are combined to achieve significant suppression of proton reduction and the ability to electrodeposit metals at highly negative cathodic potentials. Chapter 6 describes the use of these solvents to electrodeposit ruthenium for interconnect applications. The origin of enhanced superconductivity in rhenium electrodeposited from water-in-salt electrolytes is explored in Chapter 5. A disordered atomic structure is found to be highly correlated with enhanced superconductivity.