How to calculate the risk-free rate of cryptocurrencies?
Bitcoin itself has no returns: 1 Bitcoin is worth 1 Bitcoin, always.  Presumably you actually want to discuss the returns of a Bitcoin plus ordinary currency pair, such as BTC/USD.  In that case you'd would treat it the same as you would treat any other asset trading against USD and use some presumed near zero risk bond rate, like US treasuries, in your analysis.