TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Method: The method involves cooling the chemical etchant to below room temperature before it is contacted with the elemental particulate silicon. In the method, the temperature is maintained below 0 degrees C. The etchant is cooled to less than or equal to 5 degrees C (preferably less than or equal to 0 degrees C, especially less than or equal to -5 degrees C), or the etchant is cooled to -25 to 5 degrees C (preferably -25 to less than or equal to 0 degrees C, especially -25 to -15 degrees C, particularly -15 degrees C). During porosification, the temperature is maintained below 10 degrees C, and following porosification of the elemental particulate silicon, water is added and the mixture is filtered and dried. Preferred Components: The metal salt is ferric chloride (FeCl3). The chemical etchant further comprises sulfuric acid. The elemental particulate silicon has a d90 value of less than or equal to 11 (preferably less than or equal to 10) mu m. The porous silicon has a surface area of greater than or equal to 200 (preferably greater than or equal to 400) m2/g; and has a pore volume of greater than 0.3 (preferably greater than 0.5) ml/g. The stain-etched porous silicon has a surface area of greater than or equal to 400m2/g. EXAMPLE - Aqueous ferric chloride solution (600 ml, 1.85M) was added to 40% aqueous hydrofluoric acid (120 ml). The resulting solution was chilled to -25 degrees C and poured into a 2l polyethylene bottle containing metallurgical-grade silicon powder (10 g). The bottle was returned to the chilling bath (-25 degrees C), covered and stirred with an acid-resistant screw-propeller stirring rod ( approximately 400 revolutions per minute). The temperature was monitored. At time of approximately 30 minutes, de-ionized water (500 ml) was added to the mixture and the diluted mixture immediately poured into a large-area polytetrafluoroethylene (PTFE) vacuum-filtration vessel containing a 10 mu m of MITEX (RTM: PTFE filter membrane). The mixture was filtered until a damp powder remained on the membrane (the filter time was 19 minutes). The membrane/powder was removed and dried on a hotplate at 45 degrees C for 24 hours in air and the resulting powder was weighed. This gave a 23.7% yield of porous silicon which possessed: a surface area of 479.7 m2/g, an average pore volume of 0.52 ml/g; and an average pore diameter of 3.8 nm.