A high-power, high-energy pulsed-power generator, based on a 415 kJ/22 kV capacitor bank, has recently been developed at Loughborough University [1]. The generator can drive a high-impedance system, generating a total electrical power of many GW, while depositing a Joule energy of many tens of kJ in the load. To remove the unwanted negative-polarity voltage precursor and at the same time sharpen the output, the arrangement requires the use of a high-power `diode' in the transformer secondary winding circuit. The paper describes the successful development of a polarity-dependent spark-gap which can be used as a high-voltage high-power diode. Operated in air at atmospheric pressure, the unit is capable of removing unwanted negative polarity voltage impulse precursors up to -300 kV, while at the same time capable of closing at +140 kV, thus reducing the rise time of the positive polarity voltage impulse down to about two microseconds. After a short description of the high-power, high-energy pulsed power system, the design of the high-power diode is outlined. Using an electrostatic simulation of the special electrode configuration, an explanation is provided for the polarity-dependent breakdown phenomena. Preliminary experimental tests, using the Pau University Marx generators to confirm the diode effect, are also presented Finally, high-energy high-power tests using the generator are briefly presented, highlighting the impeccable functioning of the high-power diode.