3. It is possible to purchase small demonstration PEM fuel cells. These nearly all run at ambient pressure. A good place to start for details of these cells is https://www.fuelcellstore.com/ which stocks a good many. Download details of about three of these. Try and describe these cells with regard to: (1) The cooling system used, (2) Any humidification recommended, (3) The power density (kW/L) and specific power (kW/kg). How do your answers compare to the power density figures given for the cells in Figure 4.1? Why do these Ballard cells have such a higher power density? 3. It is possible to purchase small demonstration PEM fuel cells. These nearly all run at ambient pressure. A good place to start for details of these cells is https://www.fuelcellstore.com/ which stocks a good many. Download details of about three of these. Try and describe these cells with regard to: (1) The cooling system used, (2) Any humidification recommended, (3) The power density (kW/L) and specific power (kW/kg). How do your answers compare to the power density figures given for the cells in Figure 4.1? Why do these Ballard cells have such a higher power density