Propulsion system for an aircraft , the propulsion system comprising: - a nacelle having an air channel which is annular around a longitudinal direction (X), and which opens at the front at an inlet opening and at the rear at an ejection opening , - an electric motor fixed inside the nacelle and having an output on which is mounted a shaft parallel to the longitudinal direction (X), a first electrical terminal, and a second electrical terminal, - a propeller placed in front of the inlet opening and driven in rotation by said shaft , - a fuel cell , comprising: - a core around the longitudinal direction (X) and outside the air channel , - open channels around the core , each open channel having an open inlet in the air channel and an open outlet in the air channel downstream of the open inlet, relative to the direction of the air flow in the air channel , - for each open channel , a fuel chamber supplied with fuel, - for each pair consisting of an open channel and a fuel chamber , an electrolyte between the open channel and the fuel chamber , - between each open channel and the neighbouring electrolyte , a cathode , - between each fuel chamber and the neighbouring electrolyte , an anode , the anodes being electrically connected to the first electrical terminal, and the cathodes being electrically connected to the second electrical terminal, each open channel having an inlet surface area which is less than the surface area of an intermediate area between the inlet and the outlet, the surface area of the outlet being smaller than the surface area of the intermediate area. - a nacelle having an air channel which is annular around a longitudinal direction (X), and which opens at the front at an inlet opening and at the rear at an ejection opening , - an electric motor fixed inside the nacelle and having an output on which is mounted a shaft parallel to the longitudinal direction (X), a first electrical terminal, and a second electrical terminal, - a propeller placed in front of the inlet opening and driven in rotation by said shaft , - a fuel cell , comprising: - a core around the longitudinal direction (X) and outside the air channel , - open channels around the core , each open channel having an open inlet in the air channel and an open outlet in the air channel downstream of the open inlet, relative to the direction of the air flow in the air channel , - for each open channel , a fuel chamber supplied with fuel, - for each pair consisting of an open channel and a fuel chamber , an electrolyte between the open channel and the fuel chamber , - between each open channel and the neighbouring electrolyte , a cathode , - between each fuel chamber and the neighbouring electrolyte , an anode , - a core around the longitudinal direction (X) and outside the air channel , - open channels around the core , each open channel having an open inlet in the air channel and an open outlet in the air channel downstream of the open inlet, relative to the direction of the air flow in the air channel , - for each open channel , a fuel chamber supplied with fuel, - for each pair consisting of an open channel and a fuel chamber , an electrolyte between the open channel and the fuel chamber , - between each open channel and the neighbouring electrolyte , a cathode , - between each fuel chamber and the neighbouring electrolyte , an anode , <EOS>