from functools import reduce
import pandas as pd

dfs = [df1, df2, df3, ...]
nan_value = 0

# solution 1 (fast)
result_1 = pd.concat(dfs, join='outer', axis=1).fillna(nan_value)

# solution 2
result_2 = reduce(lambda df_left,df_right: pd.merge(df_left, df_right, 
                                              left_index=True, right_index=True, 
                                              how='outer'), 
                  dfs).fillna(nan_value)