Petrol is a hydrocarbon made from crude oil. Burning it releases carbon dioxide, and it can also release carbon monoxide, soot, sulfur dioxide and oxides of nitrogen. Crude oil is a non-renewable resource that will run out. Hydrogen is a possible alternative fuel for cars.
Advantages of hydrogen. When hydrogen burns the only product is water, so there is no carbon dioxide, carbon monoxide or soot, because hydrogen contains no carbon. Hydrogen can be made from water by electrolysis, so it is not limited by crude oil supplies.
Disadvantages of hydrogen. Hydrogen is a gas that takes up a large volume, so it is hard to store and must be kept under high pressure or cooled to a very low temperature. It is highly flammable. There are few filling stations, and engines have to be changed to use it. Making hydrogen needs energy, and if the electricity comes from fossil fuels then carbon dioxide is still produced.