Electrolysis splits water molecules into hydrogen and oxygen using an electrical current in a device called an electrolyzer. This reaction is a primary method for producing clean hydrogen gas. While the maximum output is governed by strict scientific laws, practical production is limited by efficiency challenges and industrial system specifications. Understanding the quantitative limits and yields of this process clarifies hydrogen’s role as a potential future energy carrier.
The Theoretical Limit of Hydrogen Production
The maximum amount of hydrogen that can be produced by electrolysis is determined by Michael Faraday’s first law of electrolysis. This law establishes a direct relationship between the electrical charge passed through the water and the mass of the substance liberated at the electrode. The amount of hydrogen produced is directly proportional to the total electric charge flowing through the system.
This theoretical maximum yield is known as 100% Faradaic efficiency, meaning every electron contributes solely to splitting the water. Faraday’s constant (approximately 96,485 coulombs per mole of electrons) links the electrical input to the chemical output. This theoretical limit serves as the scientific benchmark against which all real-world electrolyzer performance is measured.
The theoretical minimum energy required to split one kilogram of water at 100% efficiency is approximately 39.4 kilowatt-hours (kWh) of electricity. This value represents the energy content of the hydrogen itself, based on its higher heating value. Ideally, an electrolyzer would consume exactly this amount of energy to produce one kilogram of hydrogen.
Variables Influencing Actual Output Efficiency
Practical hydrogen production systems never reach 100% Faradaic efficiency because of various factors that introduce electrical and chemical losses. These variables determine the actual, achievable output, which is always less than the theoretical maximum. Electrolyzer efficiency is measured by how much energy, exceeding the theoretical 39.4 kWh/kg, is consumed to produce one kilogram of hydrogen.
Overpotential and Heat Loss
One significant energy drain is overpotential, the excess voltage required beyond the theoretical minimum to drive the water-splitting reaction at a practical rate. This extra voltage is not converted into chemical energy but is released as waste heat within the cell. This heat generation directly lowers the system’s overall electrical efficiency.
Electrolyte Choice and Resistance
The choice and concentration of the electrolyte impact the required energy input. Since pure water is a poor conductor, electrolytes like potassium hydroxide (KOH) or sulfuric acid are added to increase conductivity. The electrolyte’s ionic conductivity influences the cell’s internal resistance. Higher resistance requires greater voltage input to maintain the current, leading to greater heat losses.
Current Density and Operating Conditions
Current density is the amount of electrical current flowing through a specific electrode area. While increasing current density boosts the production rate, pushing the system too hard increases internal losses and reduces energy efficiency. Operating the system at higher temperatures can improve efficiency by increasing electrolyte conductivity and reducing overpotential. Operating at elevated pressure also reduces the size of gas bubbles on the electrodes, which lowers ohmic resistance and slightly reduces power consumption.
Measuring Production Capacity in Industrial Systems
Industrial capacity is measured by standardized metrics that reflect real-world performance and energy consumption, rather than theoretical limits. The primary metric defining an electrolyzer’s size is its maximum operational power, measured in megawatts (MW) or gigawatts (GW). This power rating indicates the maximum electrical input the system can handle.
The practical output is quantified by mass or volume, typically as kilograms of hydrogen per hour or day (kg/h or kg/day), or as normal cubic meters per hour (Nm³/h). The system-specific electricity consumption, expressed in kWh/kg of hydrogen, is the most direct measure of operational efficiency. Modern commercial electrolyzers typically require between 47 and 77 kWh of electricity to produce one kilogram of hydrogen, corresponding to an overall energy efficiency range of approximately 62% to 82%.
The choice of technology also dictates the practical limits of production for a facility. For instance, Alkaline electrolyzers (AEL) are well-established and can scale to the MW range, operating at temperatures around 70–90°C and pressures up to 30 bar. Proton Exchange Membrane (PEM) electrolyzers offer high purity and flexibility, making them suitable for coupling with intermittent renewable energy sources, and their efficiency is projected to increase further in the coming years. A 10 MW electrolyzer operating at a consumption rate of 50 kWh/kg would practically produce 200 kg of hydrogen per hour.