When hydrogen is produced by PEM electrolysis, the generation pressure can be increased internally to the cell through the process of electrochemical compression. In a PEM electrolyzer, there is very little voltage penalty to compressing the hydrogen this way, and it is much more energy efficient than mechanically compressing hydrogen. In addition to compressing hydrogen in the electrolysis process, you can also electrochemically “pump” hydrogen across a similar PEM cell stack to achieve the same pressure increase.