TECHNOLOGY FOCUS - BIOTECHNOLOGY - Preferred Enzyme: (A) is isolated from (i) a microorganism that activates hydrogen (e.g. Desulfovibrio or other sulfur reducers; Rhizobium or other nitrogen fixers; and Methanobacterium or other methane producers) or (ii) algae (prokaryotic or eukaryotic) that metabolize hydrogen (e.g. Scenedesmus obliquus, Anabaena cilindrica or Anacystis nidulans). Optionally (A) is modified by attachment of affinity motifs (particularly a series of His residues) at selected locations for oriented immobilization. The most preferred enzyme is from D. gigas and its sequence has been deposited as PDB 1H2A.    TECHNOLOGY FOCUS - ELECTRICAL POWER AND ENERGY - Preferred Electrode: This is made of metal (specifically gold, copper, silver or platinum) or a carbonaceous material (e.g. vitreous, pyrolytic or microporous carbon; nanotubes or (nano)fibers). Most preferred are gold and 'edge' pyrolytic carbon. The carbon is modified with carboxy groups, introduced as N,N-bis(carboxymethyl)-L-lysine, able to complex with Ni, Cu or Co and the complex formed has affinity for a polyhistidine tag engineered into (A). Preparation: (1) Where the electrode is made of carbonaceous material, it is modified by electroreduction of an aryl derivative (X) of diazonium salts in aproptic or acidic aqueous medium to form primary amino groups; then covalent attachment of (A) in phosphate buffer (0.01-1 M; pH 5-9), through carboxy groups activated with N-hydroxysuccinimide in presence of a soluble carbodiimide, finally washing with phosphate buffer. Particularly the electrode surface is modified by plasma irradiation to generate carboxy groups. (2) Where the electrode is made of metal, it is covered by a monolayer of bifunctional alkyl compound with a thiol group at one end (for attachment to the metal) and a functional group, reactive with an exposed group in (A), at the other end. EXAMPLE - A cleaned gold electrode was incubated in a 1 mM solution of cystamine in aqueous ethanol for 20 hours, washed with pH 7.6 buffer then immersed for 90 minutes at 25degreesC in a solution of Desulfovibrio gigas hydrogenase (0.8 muM); N-(3-dimethylaminopropyl)-N'-ethylcarbodimide and N-hydroxysuccinimide. The electrode was mounted in an electrolytic cell (Ag/AgCl reference electrode; platinum counter electrode) and the cell was then able (a) to oxidize hydrogen in solution, with generation of an electrical current and (b) to produce hydrogen (from protons and electrons supplied to it).