What qualifications or certifications should be required for an electrolyzer?

Requirements for local jurisdictions vary, so the AHJ should be consulted, but NFPA 2:2023, Hydrogen Technologies Code, Chapter 13 has requirements for installation of hydrogen generators up to 100 kg H2/h. Section 13.3.1 General says permitted water electrolysis systems are to be listed to ISO 22734:2019, Hydrogen Generators Using Water Electrolysis - Industrial, Commercial, and Residential Applications, or approved by the local authority having jurisdiction (AHJ). 

For laboratory or small demonstration scale of less than 3 kg/day, UL 61010-1, Safety Requirements for Electrical Equipment for Measurement, Control, and Laboratory Use - Part 1: General Requirements, is also permitted. ISO 22734 is a product safety standard for packaged or factory-matched water electrolysis systems, either alkaline or PEM or AEM. It does not address high temperature solid oxide systems, but there are apt product safety standards for those also. Typically, a manufacturer will present certification documents stating the water electrolysis system, either fully packaged or provided as factory matched equipment designed and built to be integrated on site, meets the requirements of ISO 22734 or an equivalent. 

The equipment should bear a marking plate that attests to conformity with the electrolysis safety standard. If a different certification is provided, one may need to show equivalency with ISO 22734, or if small, UL61010-1. In US and Canada, the nationalized equivalent is ANSI/CSA B22734, and there are also nationalized versions in Australia, China, and Great Britain. One can ask the providers to supply a Declaration of Conformity (common in the European Union) or certificate asserting compliance. One can also hire a third party Nationally Recognized Test Lab (NRTL) to review the equipment conformity to the electrolysis safety standards. 

Be aware that there may be an applicable ASME BPVC section VIII code case published in 2023 that will impact how water electrolysis cell stacks are to be labeled and approved - some AHJs may accept listing/labeling at the system level (probably a safer approach), while others may look at individual components. Be aware also of installation requirements for hydrogen equipment regarding piping and integration with gas compressors, storage, fueling dispensers. Hazardous area classification and safe venting of hydrogen and oxygen gases must be carefully reviewed and made compliant with the local building and fire code.