Two former General Motors Corp. executives are doing quite well since leaving the automaker. Robert C. Stempel, former GM chief executive, and Roy S. Roberts, former chief of GM's Vehicle Sales, Service and Marketing organization, have found new niches in the automotive industry. Mr. Stempel now serves as chairman of Energy Conversion Devices Inc., which recently rolled out its latest vision for automotive powertrains. ECD debuted a fuel cell that operates on solid hydrogen rather than liquid hydrogen or methanol. Meanwhile, Mr. Roberts now is devoting his time to helping companies fulfill minority contract quotas. Mr. Roberts plans to launch M-Xchange, an online marketplace that will help facilitate minority procurement for companies in all industries. Mr. Roberts says companies around the country often don't know how to find minority businesses to work with. He hopes to change that.