(a) What is the difference between a battery and a fuel cell? (b) Can the "fuel" of a fuel cell be a solid? Explain.

A battery is an electrochemical device that stores chemical energy in its electrodes and converts it into electrical energy when connected to an external circuit. The chemical reactions taking place in the battery are not reversible; once the chemicals have been depleted, the battery can no longer produce electricity and must be either discarded or recharged. A fuel cell, on the other hand, is an electrochemical energy conversion device that converts the chemical energy stored in a fuel (like hydrogen) and an oxidant (like oxygen) into electricity through a chemical reaction. Unlike a battery, the reaction is continuous as long as the fuel source is available, and the fuel cell can generate electricity without the need for recharging or replacement.

Yes, the fuel of a fuel cell can be a solid. A solid-oxide fuel cell (SOFC) is a type of fuel cell that uses a solid oxide (typically a ceramic material) as the electrolyte. The most common fuel for SOFC is hydrogen, and it can also use other gaseous fuels such as methane or natural gas. However, it is important to note that a solid fuel, like coal or biomass, can also be used in these types of fuel cells, as long as the solid fuel can be converted into a suitable gaseous form (e.g., synthesis gas) before being fed into the fuel cell. This conversion is usually done through a process called gasification, where the solids react at high temperatures with steam or a controlled amount of air to generate the required gaseous fuels.