Introduction:

The design of efficient small-scale hydrogen energy storage systems requires balancing hydrogen production rate, electrical efficiency, and system simplicity. This study investigates the energetic trade-off between kinetic performance and energy conversion efficiency imposed by electrolyzer architecture and separator materials.

Methods:

A comparative experimental analysis was conducted using finite-gap (H-cell) and zero-gap electrolyzer configurations for both alkaline and proton exchange membrane (PEM) systems. Zirfon® Pearl 500 diaphragms were employed in alkaline electrolyzers (25 wt% KOH), while Nafion™ 117 membranes were used in PEM systems, operating in an acidic medium (2.55 M H₂SO₄) for finite-gap and deionized water for zero-gap configurations. The polarization behavior, hydrogen production rate, Faradaic efficiency, and apparent cell resistance were evaluated under comparable operating conditions to assess the trade-offs associated with separator properties and electrolyte conductivity.

Results:

Zero-gap PEM electrolyzers exhibited significantly reduced internal resistance and superior kinetic performance, achieving currents from 1.5 to 3.8 A at voltages below 2.5 V, and up to 10 A at 2.8 V. In contrast, finite-gap PEM systems required higher voltages, delivering only 0.73 A at 4 V and 2.86 A at 18 V. Alkaline H-cell electrolyzers using Zirfon® diaphragms showed lower current densities (0.24–0.27 A at 4 V and <2.35 A at 18 V), with improved performance observed using nickel electrodes. At 4 V, acidic H-cell systems with Nafion™ 117 membranes exhibited internal resistance (~5.5 Ω) approximately three times lower than alkaline counterparts (~14.81 Ω), resulting in hydrogen production rates up to 2.3 times higher. However, alkaline Ni/Zirfon configurations achieved higher Faradaic efficiency (~98%) and energy efficiency (~36%), attributed to reduced parasitic reactions and improved gas separation.

Conclusions:

The results demonstrate a clear trade-off between kinetic performance and energy efficiency depending on electrolyzer architecture and separator choice. While acidic PEM systems favor higher reaction rates due to lower internal resistance, alkaline systems provide superior energy utilization. This study establishes a comparative framework for selecting electrolyzer configurations tailored to decentralized hydrogen storage, highlighting the critical role of separator properties, ionic path length, and electrolyte conductivity in determining system performance.