SEOUL -- SK Group, one of South Korea's biggest oil refiners, has a made a cross-border move to grow its hydrogen energy business with an investment in American fuel cell developer Plug Power. The South Korean conglomerate is set to purchase a 9.9% stake in Latham, New York-based Plug Power for roughly $1.5 billion by the end of March, becoming its top shareholder. The deal shows SK starting to shift from fossil fuels to cleaner energy sources in line with growing international commitments to slash greenhouse gas emissions.